2018 122 9 May

Dielectron production in $\mathrm{pp}$ collisions at
$\sqrt{s}=13~{}$\mathrm{TeV}$$

ALICE Collaboration††thanks: See Appendix A for the list of collaboration
members ALICE Collaboration

The measurement of dielectron production is presented as a function of
invariant mass and transverse momentum ($p_{\rm T}$) at midrapidity ($|y_{\rm
e}|<0.8$) in proton-proton (pp) collisions at a centre-of-mass energy of
$\sqrt{s}=13$ TeV. The contributions from light-hadron decays are calculated
from their measured cross sections in pp collisions at $\sqrt{s}=7$ TeV or 13
TeV. The remaining continuum stems from correlated semileptonic decays of
heavy-flavour hadrons. Fitting the data with templates from two different MC
event generators, PYTHIA and POWHEG, the charm and beauty cross sections at
midrapidity are extracted for the first time at this collision energy: ${\rm
d}\sigma_{\rm c\bar{c}}/{\rm d}y|_{y=0}=974\pm 138(\rm{stat.})\pm
140(\rm{syst.})\pm 214(\rm{BR})~{}\mu{\rm b}$ and ${\rm d}\sigma_{\rm
b\bar{b}}/{\rm d}y|_{y=0}=79\pm 14(\rm{stat.})\pm 11(\rm{syst.})\pm
5(\rm{BR})~{}\mu{\rm b}$ using PYTHIA simulations and ${\rm d}\sigma_{\rm
c\bar{c}}/{\rm d}y|_{y=0}=1417\pm 184(\rm{stat.})\pm 204(\rm{syst.})\pm
312(\rm{BR})~{}\mu{\rm b}$ and ${\rm d}\sigma_{\rm b\bar{b}}/{\rm
d}y|_{y=0}=48\pm 14(\rm{stat.})\pm 7(\rm{syst.})\pm 3(\rm{BR})~{}\mu{\rm b}$
for POWHEG. These values, whose uncertainties are fully correlated between the
two generators, are consistent with extrapolations from lower energies. The
different results obtained with POWHEG and PYTHIA imply different kinematic
correlations of the heavy-quark pairs in these two generators. Furthermore,
comparisons of dielectron spectra in inelastic events and in events collected
with a trigger on high charged-particle multiplicities are presented in
various $p_{\rm T}$ intervals. The differences are consistent with the already
measured scaling of light-hadron and open-charm production at high charged-
particle multiplicity as a function of $p_{\rm T}$. Upper limits for the
contribution of virtual direct photons are extracted at 90% confidence level
and found to be in agreement with pQCD calculations.

##  1 Introduction

Heavy-flavour quarks (charm and beauty) are copiously produced by inelastic
partonic scatterings in high-energy proton–proton ($\mathrm{pp}$) collisions
at the CERN Large Hadron Collider (LHC). Their large masses ($m_{\rm Q}$) make
it possible to calculate their production cross sections with perturbative
quantum chromodynamics (pQCD) [1, 2, 3]. Hence, experimental measurements of
heavy-quark production provide an excellent test of pQCD in this energy
regime. Flavour conservation allows heavy quarks to be only produced in pairs.
Charm hadrons and their decay products reflect the initial angular correlation
of the heavy-quark pairs, whereas in the case of decays of beauty hadrons the
correlation is weakened due to their large masses. The contribution from the
simultaneous semileptonic decays of the corresponding heavy-flavour hadron
pairs dominates the dilepton yield in the intermediate mass region (IMR)
$1<m_{\ell\ell}<3~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}^{2}$$. Hence,
dielectron measurements can be used to study charm and beauty production.

The ALICE Collaboration has reported charm and beauty production cross
sections measurements at midrapidity ($|y|<0.5$) in $\mathrm{pp}$ collisions
at centre-of-mass energies of $\sqrt{s}=2.76$ and $7~{}$\mathrm{TeV}$$ [4, 5,
6, 7, 8, 9, 10]. The charm measurement at $\sqrt{s}=7~{}$\mathrm{TeV}$$ is
complemented by ATLAS data extending to higher transverse momentum
($p_{\mathrm{T}}$) and $|y|<2.1$ [11]. Furthermore, the CMS Collaboration has
provided a variety of charm and bottom measurements at midrapidity at
$\sqrt{s}=2.76$, 5 and 7 $\mathrm{TeV}$ [12, 13, 14, 15, 16, 17, 18, 19, 20].
At forward rapidity ($2<y<5$), the LHCb Collaboration has measured charm and
beauty production cross sections in $\mathrm{pp}$ collisions at $\sqrt{s}=5$,
7, 8 and 13 $\mathrm{TeV}$ [21, 22, 23, 24]. These results are generally in
good agreement with pQCD calculations at next-to-leading order (NLO) in the
strong coupling constant ($\alpha_{\rm s}$) with all-order resummation of the
logarithms of $p_{\mathrm{T}}/m_{\rm Q}$ (FONLL) [1, 2, 3]. Though the
measured charm production cross sections consistently lie on the upper edge of
the systematic uncertainties of the theory calculations. Recently, the ALICE
Collaboration has measured the charm and beauty production cross sections in
$\mathrm{pp}$ collisions at $\sqrt{s}=7~{}$\mathrm{TeV}$$ using
electron–positron pairs (dielectrons) from correlated semileptonic decays of
heavy-flavour hadrons [25]. Such an approach was first performed by the PHENIX
Collaboration in $\mathrm{pp}$ and d–Au collisions at
$\sqrt{s_{{}_{\mathrm{NN}}}}=200~{}$\mathrm{GeV}$$ at the Relativistic Heavy
Ion Collider (RHIC) [26, 27, 28]. These measurements have the advantage that
they probe the full $p_{\mathrm{T}}$ range of heavy-quark pairs and contain
complementary information about the initial correlation of charm quarks, i. e.
the underlying production mechanism, which is not accessible in conventional
single heavy-flavour measurements.

The measurement of direct photons, i. e. those produced in hard scatterings
between incoming partons in hadronic collisions, provides another important
test of pQCD. Furthermore, at
$p_{\mathrm{T}}<3~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}$$, where the
applicability of perturbation theory may be questionable, experimental data of
direct-photon production in $\mathrm{pp}$ collisions serve as a crucial
reference to establish the presence of thermal radiation from the hot and
dense medium created in heavy-ion collisions [29, 30, 31, 32]. The measurement
of real (massless) direct photons at low $p_{\mathrm{T}}$ is challenging
because of the large background of hadron decay photons. This background can
be largely reduced by measuring the contribution of virtual direct photons, i.
e. direct $\mathrm{e}^{+}\mathrm{e}^{-}$ pairs, to the dielectron invariant-
mass spectrum above the $\pi^{0}$ mass [29, 30].

Proton–proton collisions in which a large number of charged particles are
produced have recently attracted the interest of the heavy-ion community [33,
34]. These events exhibit features that are similar to those observed in
heavy-ion collisions, e. g. collective effects, such as long-range angular
correlations [35, 36, 37, 38, 39, 40] or enhanced strangeness production [41].
Charged-hadron $p_{\mathrm{T}}$ spectra in $\mathrm{pp}$ collisions at
$\sqrt{s}=13~{}$\mathrm{TeV}$$ show a hardening with increasing multiplicity,
an effect that arises naturally from jets [42]. Also, heavy-quark production
is found to scale faster than linearly with the charged-particle multiplicity
in $\mathrm{pp}$ collisions at $\sqrt{s}=7~{}$\mathrm{TeV}$$ [43, 44]. This
motivates the study of dielectron production in high-multiplicity
$\mathrm{pp}$ collisions. In the low mass region
($m_{\mathrm{ee}}<1~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}^{2}$$),
dielectron measurements provide further insight into possible modifications of
the light vector and pseudo-scalar meson production via their resonance and/or
Dalitz decays, whereas in the IMR they allow for complementary studies of the
heavy-flavour production. At LHC energies, the contribution from open charm
already dominates the dielectron continuum at $m_{\mathrm{ee}}\approx
0.5~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}^{2}$$. Moreover, if a
thermalised system were created in such high-multiplicity $\mathrm{pp}$
collisions, a signal of thermal (virtual) photons should be present.

In this letter, first results of charm and beauty production cross sections at
midrapidity in inelastic (INEL) and high-multiplicity (HM) $\mathrm{pp}$
collisions at $\sqrt{s}=13~{}$\mathrm{TeV}$$ are reported. The paper is
organised as follows: the ALICE apparatus and the data samples used in the
analysis are described in Section 2, the data analysis is discussed in Section
3, Section 4 introduces the cocktail of known hadronic sources, and the
results are presented and discussed in Section 5.

##  2 The ALICE detector and data samples

A detailed description of the ALICE apparatus and its performance can be found
in [45, 46, 47, 48]. The detectors used in this analysis are briefly described
below.

Trajectories of charged particles are reconstructed in the ALICE central
barrel with the Inner Tracking System (ITS) and the Time Projection Chamber
(TPC) that reside within a solenoid, which provides a homogeneous magnetic
field of $0.5\text{~{}}\mathrm{T}$ along the beam direction. The ITS consists
of six cylindrical layers of silicon detectors, with radial distances from the
beam axis between $3.9\text{~{}}\mathrm{cm}$ and $43\text{~{}}\mathrm{cm}$.
The two innermost layers are equipped with Silicon Pixel Detectors (SPD), the
two intermediate layers are composed of Silicon Drift Detectors, and the two
outermost layers are made of Silicon Strip Detectors. The TPC, main tracking
device in the ALICE central barrel, is a $5\text{~{}}\mathrm{m}$ long
cylindrical gaseous detector extending from $85\text{~{}}\mathrm{cm}$ to
$247\text{~{}}\mathrm{cm}$ in radial direction. It provides up to 159 spacial
points per track for charged-particle reconstruction and particle
identification (PID) through the measurement of the specific ionisation energy
loss $\mathrm{d}E/\mathrm{d}x$ in the gas volume.

The PID is complemented by the Time-Of-Flight (TOF) system located at a radial
distance of $3.7\text{~{}}\mathrm{m}$ from the nominal interaction point. It
measures the arrival time of particles relative to the event collision time
provided by the TOF detector itself or by the T0 detectors, two arrays of
Cherenkov-counters located at forward rapidities [49].

Collision events are triggered by the V0 detector that comprises two plastic
scintillator arrays placed on both sides of the interaction point at
pseudorapidities $2.8<\eta<5.1$ and $-3.7<\eta<-1.7$. The V0 is also used to
reject background events like beam-gas interactions, collisions with de-
bunched protons or with mechanical structures of the beam line.

The data samples used in this letter were recorded with ALICE in 2016 during
the LHC $\mathrm{pp}$ run at $\sqrt{s}=13~{}$\mathrm{TeV}$$. For the minimum-
bias event trigger that is used to define the data sample for the analysis of
inelastic $\mathrm{pp}$ collisions, coincident signals in both V0
scintillators are required to be synchronous with the beam crossing time
defined by the LHC clock. Events with high charged-particle multiplicities are
triggered on by additionally requiring the total signal amplitude measured in
the V0 detector to exceed a certain threshold. At the analysis level, the
0.036 percentile of inelastic events with the highest V0 multiplicity (V0M) is
selected to define the high-multiplicity event class. This value is low enough
to avoid inefficiencies due to trigger threshold variations during data
taking. Track segments reconstructed in the SPD are extrapolated back to the
beam line to define the interaction vertex. Events with multiple vertices
identified with the SPD are tagged as pile-up and removed from the analysis
[48]. The vertex information may be improved based on the information provided
by tracks reconstructed in the ITS and TPC. To assure a uniform detector
coverage within $|\eta|<0.8$, the vertex position along the beam direction is
restricted to $\pm$10\text{~{}}\mathrm{cm}$$ around the nominal interaction
point. A total of $455\times 10^{6}$ minimum-bias (MB) $\mathrm{pp}$ events
and $79.2\times 10^{6}$ high-multiplicity $\mathrm{pp}$ events are considered
for further analysis, which corresponds to an integrated luminosity of
$\mathcal{L}_{\mathrm{int}}^{\mathrm{MB}}=7.87\pm 0.40~{}${\mathrm{nb}}^{-1}$$
and $\mathcal{L}_{\mathrm{int}}^{\mathrm{HM}}=2.79\pm
0.15~{}${\mathrm{pb}}^{-1}$$, respectively. The luminosity determination is
based on the visible cross section for the V0-based minimum-bias trigger,
measured in a van der Meer scan carried out in 2015 [50]. A conservative
uncertainty of 5% is assigned to this measurement, to account for possible
variations of the cross section between the two data-taking periods.

##  3 Data analysis

Electron111The term ‘electron’ is used for both electrons and positrons if not
stated otherwise. candidates are selected from charged-particle tracks
reconstructed in the ITS and TPC in the kinematic range
$|\eta_{\mathrm{e}}|<0.8$ and
$p_{\mathrm{T,\,e}}>0.2~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}$$. Basic
track quality criteria are applied, e. g. a sufficient number of space points
measured in the TPC and ITS as well as a good track fit. The contribution from
secondary tracks is reduced by requiring a maximum distance of closest
approach (DCA) to the primary vertex in the transverse plane (DCA${}_{\rm
xy}<$1.0\text{~{}}\mathrm{cm}$$) and in the longitudinal direction
(DCA${}_{\rm z}<$3.0\text{~{}}\mathrm{cm}$$). To further suppress the
contribution from photon conversions in the detector material, electron
candidates are required to have a hit in the first SPD layer and no ITS
clusters shared with other reconstructed tracks.

The electron identification is based on the complementary information provided
by the TPC and TOF. The detector PID response, $n(\sigma_{i}^{\rm DET})$, is
expressed in terms of the deviation between the measured and expected value of
the specific ionisation energy loss in the TPC or time-of-flight in the TOF
for a given particle hypothesis $i$ and momentum, normalised by the detector
resolution $(\sigma^{\rm DET})$. In the TPC, electrons are selected in the
range $\left|n\left(\sigma_{\rm e}^{\rm TPC}\right)\right|<3$ and pions are
rejected by requiring $n\left(\sigma_{\rm\pi}^{\rm TPC}\right)>4$.
Furthermore, kaons and protons are rejected with $\left|n\left(\sigma_{\rm
K}^{\rm TPC}\right)\right|>4$ and $\left|n\left(\sigma_{\rm p}^{\rm
TPC}\right)\right|>4$, unless the candidate is positively identified as an
electron in the TOF, i. e. fulfilling $\left|n\left(\sigma_{\rm e}^{\rm
TOF}\right)\right|<3$. For particles that are outside
$\left|n\left(\sigma_{\rm K}^{\rm TPC}\right)\right|<4$ and
$\left|n\left(\sigma_{\rm p}^{\rm TPC}\right)\right|<4$ the TOF information is
only used to select electron candidates with $\left|n\left(\sigma_{\rm e}^{\rm
TOF}\right)\right|<3$ if the track has an associated hit in the TOF detector.

Since experimentally the origin of each electron or positron is unknown, all
electron candidates are paired considering combinations with opposite
$(N_{+-})$ but also same-sign charge $(N_{\pm\pm})$. Most of the electron
pairs arise from the combination of two electrons originating from different
mother particles. These pairs give rise to the combinatorial background $B$
that is estimated via the geometric mean of same-sign pairs
$\sqrt{N_{++}N_{--}}$ within the same event. Opposite- and same-sign pairs
include correlated background, e. g. originating from $\pi^{0}$ decays with
two $\mathrm{e}^{+}\mathrm{e}^{-}$ pairs in the final state
($\pi^{0}\rightarrow\gamma^{(\ast)}\gamma^{(\ast)}\rightarrow\mathrm{e}^{+}\mathrm{e}^{-}\mathrm{e}^{+}\mathrm{e}^{-}$),
which includes decay channels with real photons and their subsequent
conversion in detector material. Such processes lead to opposite and same-sign
pairs at equal rate. The background estimate needs to be corrected for the
different detector acceptance of opposite and same-sign pairs. This correction
factor is determined by dividing the yields of uncorrelated opposite
$(M_{+-})$ and same-sign pairs $(M_{\pm\pm})$ in mixed events:
$R=M_{+-}/(2\sqrt{M_{++}M_{--}})$. The dielectron signal is then obtained as
$S=N_{+-}-B=N_{+-}-2R\sqrt{N_{++}N_{--}}$. The signal $S$ is shown together
with the opposite-sign spectrum $N_{+-}$ and the combinatorial background $B$
in Fig. 1 for minimum-bias and high-multiplicity events. In the mass interval
$0.2<m_{\mathrm{ee}}<3~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}^{2}$$, the
signal-to-background ratio varies in MB events between 0.3 and 0.04 with a
minimum around $m_{\mathrm{ee}}\approx
0.5~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}^{2}$$ and is roughly constant
at 0.2 in the IMR [51]. In HM events, the minimum reaches 0.01 and is about
0.08 in the IMR.

Figure 1: Opposite-sign spectrum $N_{+-}$, the combinatorial background $B$
and the signal $S$ in minimum-bias (left) and high-multiplicity (right)
events. Only statistical uncertainties are shown.

Electron–positron pairs from photon conversion in the detector material,
contributing to the low mass spectrum below 0.14
$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}^{2}$, are removed by using their
distinct orientation relative to the magnetic field [25].

The data are corrected for the reconstruction efficiencies using detailed
Monte Carlo (MC) simulations. For this, $\mathrm{pp}$ events are generated
with the Monash 2013 tune of Pythia 8 [52] for light-hadron decays and the
Perugia 2011 tune of Pythia 6.4 for heavy-flavour decays [53] and the
resulting particles are propagated through a detector simulation using Geant 3
[54]. The choice of the different Pythia versions is motivated by the fact
that the Perugia 2011 tune describes reasonably well the transverse momentum
spectra of heavy-flavour hadrons while the Monash 2013 tune reproduces many of
the relevant light-hadron multiplicities [55, 56]. The signal reconstruction
efficiencies were studied as a function of $m_{\mathrm{ee}}$ and pair
transverse momentum $p_{\mathrm{T,\,ee}}$ separately for the different
$\mathrm{e}^{+}\mathrm{e}^{-}$ sources: resonance and Dalitz decays of
relevant mesons as well as correlated semileptonic decays of charm and beauty
hadrons. The total signal reconstruction efficiency is obtained by weighting
the efficiency of each dielectron source by its expected contribution and is
found to be about $20\%$ in
$0.7<m_{\mathrm{ee}}<1.2~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}^{2}$$
and approaches 30% at lower and higher masses.

Different aspects of the analysis are considered as possible sources of
systematic uncertainties, which are summarised in Table 1. The systematic
uncertainties due to the track reconstruction are estimated by comparing the
efficiency of the ITS–TPC matching, the requirement of a hit in the first SPD
layer, and the requirement of no shared ITS clusters in MC simulations and
data. The residual disagreements between data and MC add to a 6.5% uncertainty
on the single track level, which leads to a 13% uncertainty for pairs. The MC
simulations were also checked to reproduce all details of the PID selection
within a systematic uncertainty of 2% for $\mathrm{e}^{+}\mathrm{e}^{-}$
pairs. The purity of the electron sample is estimated to be ${>}93\%$ over the
relevant $p_{\mathrm{T}}$ range, with a $p_{\mathrm{T}}$-integrated hadron
contamination of about 4%. The resulting hadron contamination on the
dielectron signal is found to be negligible. For
$m_{\mathrm{ee}}<0.14~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}^{2}$$, a 2%
uncertainty on the conversion rejection was estimated from the yield change
when tightening the selection to reject photon conversions. A 2% uncertainty
on the signal yield due to the correction factor $R$ is obtained by repeating
the event mixing in different event classes, defined by the position of the
reconstructed primary vertex and by the charged-particle multiplicity. The
efficiency of the minimum-bias trigger to select inelastic events with an
$\mathrm{e}^{+}\mathrm{e}^{-}$ pair in the ALICE acceptance
($|\eta_{\mathrm{e}}|<0.8$ and
$p_{\mathrm{T,\,e}}>0.2~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}$$) is
estimated to be $(99\pm 1)\%$ from the Monash 2013 tune of Pythia 8\. This and
the luminosity uncertainty of 5% [50] are global uncertainties, which are not
included in the point-to-point uncertainties. No significant variation of
systematic uncertainties on mass or $p_{\mathrm{T,\,ee}}$ is observed in the
analysis, and the same total uncertainty of 14% is assigned as point-to-point
correlated uncertainties on the differential dielectron cross section in
inelastic $\mathrm{pp}$ collisions.

The analysis of the high-multiplicity data has additional systematic
uncertainties. First, no dedicated high-multiplicity MC simulation was
performed. In such events the vertex distribution is biased more than in MB
events by the asymmetric pseudorapidity coverage of the two V0 detectors. The
change of the detector acceptance with vertex position could lead to a
difference in the number of reconstructed electrons of up to 3%, which results
in an uncertainty of 6% for $\mathrm{e}^{+}\mathrm{e}^{-}$ pairs. Second, a
possible multiplicity dependence of the reconstruction and PID efficiency is
covered by an uncertainty of 6% [57]. Added in quadrature, this amounts to a
total uncertainty of 15%.

Source | Minimum bias | High multiplicity
---|---|---
Track reconstruction | 13% | 13%
Electron identification | 2% | 2%
Conversion rejection ($m_{\mathrm{ee}}<0.14~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}^{2}$$) | 2% | 2%
Acceptance correction factor ($R$) | 2% | 2%
Vertex distribution bias | – | 6%
Multiplicity dependence of tracking and PID | – | 6%
Total | 14% | 15%
Table 1: Sources of systematic uncertainties.

##  4 Cocktail of known hadronic sources

The dielectron spectrum measured in $\mathrm{pp}$ collisions at
$\sqrt{s}=13~{}$\mathrm{TeV}$$ is compared with the expectations from all
known hadron sources, i. e. the hadronic cocktail, contributing to the
dielectron spectrum in the ALICE central barrel acceptance
($|\eta_{\mathrm{e}}|<0.8$ and
$p_{\mathrm{T,\,e}}>0.2~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}$$). A
fast MC simulation is used to estimate the contribution from $\pi^{0}$,
$\eta$, $\eta^{\prime}$, $\rho$, $\omega$ and $\phi$ decays in $\mathrm{pp}$
collisions, as detailed in [25].

Following the approach outlined in [58], the pion $p_{\mathrm{T}}$-spectrum at
$\sqrt{s}=13~{}$\mathrm{TeV}$$ is approximated by scaling the
$p_{\mathrm{T}}$-spectrum of charged hadrons [42] by the pion-to-hadron ratio
measured at $\sqrt{s}=7~{}$\mathrm{TeV}$$ [59, 60]. The difference with
respect to the same procedure based on the pion-to-hadron ratio measured at
$\sqrt{s}=2.76~{}$\mathrm{TeV}$$ [59, 61] is smaller than 1% at low
$p_{\mathrm{T}}$ and reaches 5% at high $p_{\mathrm{T}}$. The charged hadron
$p_{\mathrm{T}}$-spectra at $\sqrt{s}=13~{}$\mathrm{TeV}$$ are normalised to
INEL${>}0$ events, i. e. inelastic collisions that produce at least one
charged particle in $|\eta|<1$, rather than INEL events. This is corrected
taking the 21% difference in the $p_{\mathrm{T}}$ integrated $\mathrm{d}N_{\rm
ch}/\mathrm{d}\eta$ values for these two event classes [42]. A conservative
uncertainty of 10% is assigned on this extrapolation.

A fit of the obtained charged-pion $p_{\mathrm{T}}$-spectrum with a modified
Hagedorn function is then taken as proxy for the neutral-pion
$p_{\mathrm{T}}$-distribution. The simulated cross section per unit rapidity
of the $\pi^{0}$ is
$\mathrm{d}\sigma/\mathrm{d}y|_{y=0}=155.2~{}$\mathrm{mb}$$. For the $\eta$
meson a fit of the measured $\eta/\pi^{0}$ ratio in $\mathrm{pp}$ collisions
at $\sqrt{s}=7~{}$\mathrm{TeV}$$ is used [62]. The Monash 2013 tune of Pythia
8 describes the $\rho/\pi^{0}$ and $\omega/\pi^{0}$ ratios measured in
$\mathrm{pp}$ collisions at $\sqrt{s}=2.76$ and 7 $\mathrm{TeV}$, respectively
[55, 56]. Therefore, MC simulations obtained with this tune at
$\sqrt{s}=13~{}$\mathrm{TeV}$$ are used to obtain the $\rho/\pi^{0}$ and
$\omega/\pi^{0}$ ratios. Based on the $\eta/\pi^{0}$, $\rho/\pi^{0}$ and
$\omega/\pi^{0}$ data, the ratios at high $p_{\mathrm{T}}$ are $0.5\pm 0.1$,
$1.0\pm 0.2$ and $0.85\pm 0.17$, respectively. The $\eta^{\prime}$ and $\phi$
mesons are generated assuming $m_{\mathrm{T}}$ scaling, replacing
$p_{\mathrm{T}}$ with $\sqrt{m^{2}-m_{\pi}^{2}+(p_{\mathrm{T}}/c)^{2}}$ [63].
For the $m_{\mathrm{T}}$ scaling, particle yields are normalised at high
$p_{\mathrm{T}}$ relative to the $\pi^{0}$ yield: $0.40\pm 0.08$ for
$\eta^{\prime}$ (from Pythia 6 calculations) and $0.13\pm 0.04$ for $\phi$
[64]. The detector response, including momentum and angular resolutions, as
well as Bremsstrahlung effects obtained from full MC simulations, is applied
to the decay electrons as a function of $p_{\mathrm{T,\,e}}$, $\eta_{\rm e}$
and the azimuth $\varphi_{\rm e}$. This results in a mass resolution of
approximately 1%. The following sources of systematic uncertainties were
evaluated: the input parameterisations of the measured spectra as a function
of $p_{\mathrm{T}}$ ($\pi^{\pm}$, $\eta/\pi^{0}$ and $\omega/\pi^{0}$), the
branching fractions of all included decay modes, the $m_{\mathrm{T}}$ scaling
parameters and the resolution smearing. For the high-multiplicity cocktail,
the input hadron $p_{\mathrm{T}}$-distributions are adjusted according to the
measured modifications of the charged-hadron $p_{\mathrm{T}}$ spectra [42].
The uncertainties of the cocktail from light-hadron decays are about $\pm
15\%$, reaching up to $+50\%$ in the region dominated by the $\eta$ meson due
to uncertainties in the extrapolation to low $p_{\mathrm{T}}$. The
multiplicity dependence has an uncertainty that varies between about 12% at
low $p_{\mathrm{T}}$ and 40% at high $p_{\mathrm{T}}$.

The Perugia 2011 tune of Pythia 6.4, which includes NLO parton showering
processes, is used to estimate the contributions of correlated semileptonic
decays of open charm and beauty hadrons [65, 53]. As an alternative, the NLO
event generator Powheg is also considered [66, 67, 68, 69]. The resulting
same-sign spectrum is subtracted from the opposite-sign distribution as in the
data analysis. Detector effects are implemented as for the light-hadron
cocktail. The spectra are normalised to cross sections at midrapidity that are
based on FONLL [1, 2, 3] extrapolations of the ALICE measurements at 7
$\mathrm{TeV}$ [8, 9, 10]. Following the description in [70], this leads to
cross sections per unit rapidity of
$\mathrm{d}\sigma_{\mathrm{c}\overline{\mathrm{c}}}/\mathrm{d}y|_{y=0}=1296^{+172}_{-162}~{}$\mathrm{\SIUnitSymbolMicro
b}$$ and
$\mathrm{d}\sigma_{\mathrm{b}\overline{\mathrm{b}}}/\mathrm{d}y|_{y=0}=68^{+15}_{-16}~{}$\mathrm{\SIUnitSymbolMicro
b}$$ at $\sqrt{s}=13~{}$\mathrm{TeV}$$. The quoted uncertainties take into
account both the measured uncertainty and the FONLL extrapolation
uncertainties. The latter (dominated by scale uncertainties but also including
PDF and mass uncertainties) are considered to be fully correlated between the
two energies [71]. For the high-multiplicity cocktail, the open charm
contribution is weighted as a function of $p_{\mathrm{T}}$ according to the
measured enhancement of $\mathrm{D}$ mesons with
$p_{\mathrm{T}}>1~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}$$ at
$\sqrt{s}=7~{}$\mathrm{TeV}$$ [43]. The same weights are applied to the open
beauty contribution as no significant difference between the production of
$\mathrm{D}$ mesons and $\mathrm{J}/\psi$ from beauty-hadron decays is
observed [43]. For electrons originating from charm or beauty hadrons with
$p_{\mathrm{T}}<1~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}$$, the same
weight as for
$1<p_{\mathrm{T}}<2~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}$$ is assumed
in the absence of a measurement. This leads to an uncertainty on the
multiplicity dependence of about 40% at low $p_{\mathrm{T}}$ decreasing to 20%
at high $p_{\mathrm{T}}$.

The $\mathrm{J}/\psi$ contribution is simulated with Pythia 6.4 and normalised
to the cross section at $\sqrt{s}=13~{}$\mathrm{TeV}$$, extrapolated with
FONLL [9] from the measurement at $\sqrt{s}=7~{}$\mathrm{TeV}$$ by the ALICE
Collaboration [72]. In the high-multiplicity cocktail, the $\mathrm{J}/\psi$
is scaled according to a dedicated, $p_{\mathrm{T}}$-integrated measurement
[44]. The $\psi\mathrm{(2S)}$ contribution is normalised to the
$\mathrm{J}/\psi$ based on a cross section ratio of
$\sigma_{\psi\mathrm{(2S)}\rightarrow\mathrm{e}^{+}\mathrm{e}^{-}}/\sigma_{\mathrm{J}/\psi\rightarrow\mathrm{e}^{+}\mathrm{e}^{-}}=(1.59\pm
0.17)\%$ [73].

##  5 Results

The dielectron cross sections are reported within the ALICE central barrel
acceptance $|\eta_{\rm e}|<0.8$ and
$p_{\mathrm{T,\,e}}>0.2~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}$$, i. e.
without correction to full phase space. The result, integrated over
$p_{\mathrm{T,\,ee}}<6~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}$$, is
shown as a function of $m_{\mathrm{ee}}$ in the left panel of Fig. 2. The data
are compared with the expectation from the hadronic decay cocktail, using
Pythia for the heavy-flavour components, and found to be in agreement within
uncertainties. Good agreement between data and cocktail calculations is also
found as a function of $p_{\mathrm{T,\,ee}}$, which is shown for three
$m_{\mathrm{ee}}$ intervals in the right panel of Fig. 2.

Figure 2: The dielectron cross section in inelastic $\mathrm{pp}$ collisions
at $\sqrt{s}=13~{}$\mathrm{TeV}$$ as a function of invariant mass (left) and
of pair transverse momentum in different mass intervals (right). The global
scale uncertainty on the $\mathrm{pp}$ luminosity (5%) is not shown. The
statistical and systematic uncertainties of the data are shown as vertical
bars and boxes. The expectation from the hadronic decay cocktail is shown as a
band, and the bottom left plot shows the ratio data to cocktail together with
the cocktail uncertainty.

Figures 3 and 4 show the ratios of the dielectron spectra in high-multiplicity
over inelastic events as a function of $m_{\mathrm{ee}}$ for different
$p_{\mathrm{T,\,ee}}$ intervals. To account for the trivial scaling with
charged-particle multiplicity, the ratio is scaled by the factor
$\mathrm{d}N_{\rm ch}/\mathrm{d}\eta({\rm HM})/\langle\mathrm{d}N_{\rm
ch}/\mathrm{d}\eta({\rm INEL})\rangle=6.27\pm 0.22$, where $\mathrm{d}N_{\rm
ch}/\mathrm{d}\eta({\rm HM})=33.29\pm 0.39$ and $\langle\mathrm{d}N_{\rm
ch}/\mathrm{d}\eta({\rm INEL})\rangle=5.31\pm 0.18$ are the charged-particle
multiplicities in $|\eta_{\rm ch}|<0.5$ measured in high-multiplicity and
inelastic $\mathrm{pp}$ collisions, respectively [42]. In this ratio, the
multiplicity-independent uncertainties cancel and the total systematic
uncertainty reduces to 9%. The ratio is in good agreement with the hadronic
decay cocktail calculations over the whole measured $m_{\mathrm{ee}}$ and
$p_{\mathrm{T,\,ee}}$ range. This is the first measurement sensitive to the
production of $\pi^{0}$, $\eta$, $\omega$ and $\phi$ in high-multiplicity
$\mathrm{pp}$ collisions. The result confirms the hypothesis that these light
mesons have the same multiplicity dependence as a function of
$m_{\mathrm{T}}$, which was used in the construction of the high-multiplicity
hadron cocktail. From the agreement between data and cocktail in the
high-$p_{\mathrm{T}}$ range
($3<p_{\mathrm{T,\,ee}}<6~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}$$),
which is dominated by open beauty, it can be also concluded for the first time
that the open beauty production has a multiplicity dependence similar to that
of open charm. This puts additional constraints on mechanisms used to describe
heavy-flavour production in high-multiplicity $\mathrm{pp}$ collisions, such
as multiple parton interactions, percolation or hydrodynamic models.

Figure 3: Ratio of dielectron spectra in HM and INEL events scaled by the
charged-particle multiplicity. The statistical and systematic uncertainties of
the data are shown as vertical bars and boxes. The expectation from the
hadronic decay cocktail calculation is shown as a grey band.

Figure 4: Ratio of dielectron spectra in HM and INEL events scaled by the
charged-particle multiplicity in different $p_{\mathrm{T,\,ee}}$ intervals.
The statistical and systematic uncertainties of the data are shown as vertical
bars and boxes. The expectation from the hadronic decay cocktail calculation
is shown as a grey band.

In the intermediate mass region
($1.03<m_{\mathrm{ee}}<2.86~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}^{2}$$),
which is dominated by open heavy-flavour decays, the data are fitted
simultaneously in $m_{\mathrm{ee}}$ and $p_{\mathrm{T,\,ee}}$ (for
$p_{\mathrm{T,\,ee}}<6~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}$$) with
Pythia and Powheg templates of open charm and beauty production, keeping the
light-flavour and $\mathrm{J}/\psi$ contributions fixed, which introduces
negligible uncertainties on the heavy-flavour cross section. The Pythia and
Powheg least-square fits of dielectron spectra in inelastic events projected
over $p_{\mathrm{T,\,ee}}$ and $m_{\mathrm{ee}}$ are shown in the left and
right panels of Fig. 5, respectively. The resulting cross sections are
summarised in Table 2. The first uncertainty is the statistical uncertainty
resulting from the fits and the second is the systematic uncertainty, which is
determined by moving the data points coherently upward and downward by their
systematic uncertainties and repeating the fits. The branching fraction of
charm-hadron decays to electrons is taken as $(9.6\pm 0.4)\%$ [74]. An
additional uncertainty of of 9.3% is added in quadrature to account for
differences in the $\Lambda_{c}/\mathrm{D}^{0}$ ratio measured by ALICE in
$\mathrm{pp}$ collisions at $\sqrt{s}=7~{}$\mathrm{TeV}$$, which is $0.543\pm
0.061\,(\text{stat.})\pm 0.160\,(\text{syst})$ for
$p_{\mathrm{T}}>1~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}$$ [75], and the
LEP average of $0.113\pm 0.013\pm 0.006$ [76]. This translates into a 22%
uncertainty at the pair level. The branching fraction of beauty hadrons
decaying into electrons, including via intermediate charm hadrons, is
$(21.53\pm 0.63)\%$ [74], which leads to a 6% uncertainty on the dielectron-
based cross section measurement. Like the statistical and systematic
uncertainties, these branching fraction uncertainties are fully correlated
between the Pythia and Powheg based results.

The results are consistent with extrapolations from lower energies based on
pQCD calculations discussed in the previous section. There is a strong anti-
correlation between the fitted charm and beauty cross sections. The sizeable
difference in the cross sections between the two MC event generators are
comparable to what is observed at $\sqrt{s}=7~{}$\mathrm{TeV}$$ [25]. The
different cross sections obtained from fits with Pythia and Powheg simulations
are caused by acceptance differences of $\mathrm{e}^{+}\mathrm{e}^{-}$ pairs
from heavy-flavour hadron decays in these two event generators because of
different kinematic correlations of the heavy quark pairs, in particular in
rapidity. The fraction of $\mathrm{e}^{+}\mathrm{e}^{-}$ pairs that fall into
the ALICE acceptance and the intermediate mass region originating from
$\mathrm{c}\overline{\mathrm{c}}$ pairs at midrapidity is 14% in Pythia and
10% in Powheg. This points to important differences in the heavy quark
production mechanisms between the two generators. It should be stressed that
single heavy-flavour measurements appear insensitive to these differences as
the cross sections obtained from such measurements agree between Pythia and
Powheg based extrapolations [22, 11, 7]. Therefore, dielectrons provide
complementary information on heavy-flavour production that, if properly
modelled, should lead to consistent cross sections with Pythia and Powheg.

Table 2 also summarises the corresponding cross sections for the high-
multiplicity data. In case of Pythia, the measured charm cross section
translates into an enhancement of $1.86\pm 0.40\,(\text{stat.})\pm
0.40\,(\text{syst.})$ relative to the charged-particle multiplicity increase.
This is consistent with the modelled multiplicity dependence used as input for
the cocktail in Figs. 3 and 4. For the beauty cross section the observed
enhancement is $1.63\pm 0.50\,(\text{stat.})\pm 0.35\,(\text{syst.})$. This is
consistent with the multiplicity dependence observed for open charm, but a
scaling with charged-particle multiplicity cannot be excluded.

Figure 5: Projection of the heavy-flavour dielectron fit (grey line) in inelastic $\mathrm{pp}$ collisions at $\sqrt{s}=13~{}$\mathrm{TeV}$$ onto the dielectron mass (left) and $p_{\mathrm{T,\,ee}}$ (right) using the Pythia and Powheg event generators. The lines show the charm (red) and beauty (magenta) contributions after the fit. The global scale uncertainty on the $\mathrm{pp}$ luminosity (5%) is not shown. The statistical and systematic uncertainties of the data are shown as vertical bars and boxes. The fits with Pythia and Powheg result in a $\chi^{2}/{\rm ndf}$ of $57.8/66$ and $52.6/66$, respectively. | Pythia | Powheg
---|---|---
$\mathrm{d}\sigma_{\mathrm{c}\overline{\mathrm{c}}}/\mathrm{d}y|_{y=0}$ | $974\pm 138\,(\text{stat.})\pm 140\,(\text{syst.})~{}$\mathrm{\SIUnitSymbolMicro b}$$ | $1417\pm 184\,(\text{stat.})\pm 204\,(\text{syst.})~{}$\mathrm{\SIUnitSymbolMicro b}$$
$\mathrm{d}\sigma_{\mathrm{b}\overline{\mathrm{b}}}/\mathrm{d}y|_{y=0}$ | $79\pm 14\,(\text{stat.})\pm 11\,(\text{syst.})~{}$\mathrm{\SIUnitSymbolMicro b}$$ | $48\pm 14\,(\text{stat.})\pm\,7(\text{syst.})~{}$\mathrm{\SIUnitSymbolMicro b}$$
$\mathrm{d}\sigma_{\mathrm{c}\overline{\mathrm{c}}}/\mathrm{d}y|_{y=0}^{\rm HM}$ | $4.14\pm 0.67\,(\text{stat.})\pm 0.66\,(\text{syst.})~{}$\mathrm{\SIUnitSymbolMicro b}$$ | $5.95\pm 0.91\,(\text{stat.})\pm 0.95\,(\text{syst.})~{}$\mathrm{\SIUnitSymbolMicro b}$$
$\mathrm{d}\sigma_{\mathrm{b}\overline{\mathrm{b}}}/\mathrm{d}y|_{y=0}^{\rm HM}$ | $0.29\pm 0.07\,(\text{stat.})\pm 0.05\,(\text{syst.})~{}$\mathrm{\SIUnitSymbolMicro b}$$ | $0.17\pm 0.07\,(\text{stat.})\pm 0.03\,(\text{syst.})~{}$\mathrm{\SIUnitSymbolMicro b}$$
Table 2: Heavy-flavour cross sections in inelastic and high-multiplicity
$\mathrm{pp}$ collisions at $\sqrt{s}=13~{}$\mathrm{TeV}$$. The 22% (6%)
branching fraction uncertainty for charm (beauty) decays into electrons is not
listed. Like statistical and systematic uncertainties, it is fully correlated
between the Pythia and Powheg based results.

The fraction of real direct photons to inclusive photons can be extracted from
the dielectron spectrum at small invariant masses assuming the equivalence
between this fraction and the ratio of virtual direct photons to inclusive
photons. The data are fitted minimising the $\chi^{2}$, in bins of
$p_{\mathrm{T,\,ee}}$, with the sum of the light-flavour cocktail
$\left(f_{\rm LF}(m_{\mathrm{ee}})\right)$, open heavy-flavour contribution
$\left(f_{\rm HF}(m_{\mathrm{ee}})\right)$ and a virtual direct photon
component $\left(f_{\rm direct}(m_{\mathrm{ee}})\right)$, whose shape is
described by the Kroll-Wada equation [77, 78] in the quasi-real virtual photon
regime $(p_{\mathrm{T,\,ee}}\gg m_{\mathrm{ee}})$. The normalisation of the
open heavy-flavour component is fixed to the measured open charm and beauty
cross sections presented above, using the Pythia simulations for the nominal
fit. As systematic uncertainty estimate, the Powheg simulation is used
instead. The light-flavour cocktail and virtual direct photon templates are
normalised independently to the data in
$m_{\mathrm{ee}}<0.04~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}^{2}$$, i.
e. in a mass window in which both Dalitz decays and direct photons have the
same $1/m_{\mathrm{ee}}$ dependence. The direct-photon fraction $r$ is then
extracted by fitting the data in the mass interval
$0.14<m_{\mathrm{ee}}<0.32~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}^{2}$$,
i. e. above the $\pi^{0}$ mass to suppress the most dominant hadron
background, with the following expression:
$\mathrm{d}\sigma/\mathrm{d}m_{\mathrm{ee}}=rf_{\rm
dir}(m_{\mathrm{ee}})+(1-r)f_{\rm LF}(m_{\mathrm{ee}})+f_{\rm
HF}(m_{\mathrm{ee}})$.

No significant direct photon contribution is observed in neither the inelastic
nor the high-multiplicity events [51]. Upper limits at 90% confidence level
(C.L.) are extracted with the Feldman–Cousins method [79] and summarised in
Table 3 together with predictions from perturbative QCD calculations for
inelastic events [80]. The current uncertainties prevent any conclusions on
the scaling of direct-photon production with charged-particle multiplicity.

Data sample | $1<p_{\mathrm{T,\,ee}}<2~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}$$ | $2<p_{\mathrm{T,\,ee}}<3~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}$$ | $3<p_{\mathrm{T,\,ee}}<6~{}$\mathrm{G}\mathrm{eV}\mathrm{/}\mathit{c}$$
---|---|---|---
Minimum bias | 0.057 | 0.072 | 0.023
High multiplicity | 0.060 | 0.083 | 0.055
pQCD | 0.003 | 0.007 | 0.013
Table 3: Upper limits at 90% C.L. on the direct-photon fractions in comparison
with the expectation in inelastic $\mathrm{pp}$ collisions based on a NLO pQCD
calculation for a factorisation and renormalisation scale choice of
$\mu=p_{\mathrm{T}}$ [80].

##  6 Summary and conclusion

We have presented the first measurement of dielectron production at
midrapidity ($|y_{\mathrm{e}}|<0.8$) in proton–proton collisions at
$\sqrt{s}=13~{}$\mathrm{TeV}$$. The dielectron continuum can be well described
by the expected contributions from decays of light- and heavy-flavour hadrons.
The charm and beauty cross sections are extracted for the first time at
midrapidity at $\sqrt{s}=13~{}$\mathrm{TeV}$$ and are consistent with
extrapolations from lower energies based on pQCD calculations. The differences
observed between Powheg and Pythia imply different kinematic correlations of
the heavy-quark pairs in these two event generators. Therefore dielectrons are
uniquely sensitive to the heavy quark production mechanisms. The comparison of
the dielectron spectra in inelastic events and in events with high charged-
particle multiplicities does not reveal modifications of the spectrum beyond
the already established ones of light and open charm hadrons. The upper limits
on the direct-photon fractions are consistent with predictions from
perturbative quantum chromodynamics calculations.

## Acknowledgements

The ALICE Collaboration would like to thank Werner Vogelsang for providing the
NLO pQCD calculations for direct photon production.

The ALICE Collaboration would like to thank all its engineers and technicians
for their invaluable contributions to the construction of the experiment and
the CERN accelerator teams for the outstanding performance of the LHC complex.
The ALICE Collaboration gratefully acknowledges the resources and support
provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG)
collaboration. The ALICE Collaboration acknowledges the following funding
agencies for their support in building and running the ALICE detector: A. I.
Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation
(ANSL), State Committee of Science and World Federation of Scientists (WFS),
Armenia; Austrian Academy of Sciences and Nationalstiftung für Forschung,
Technologie und Entwicklung, Austria; Ministry of Communications and High
Technologies, National Nuclear Research Center, Azerbaijan; Conselho Nacional
de Desenvolvimento Científico e Tecnológico (CNPq), Universidade Federal do
Rio Grande do Sul (UFRGS), Financiadora de Estudos e Projetos (Finep) and
Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil;
Ministry of Science & Technology of China (MSTC), National Natural Science
Foundation of China (NSFC) and Ministry of Education of China (MOEC) , China;
Ministry of Science and Education, Croatia; Ministry of Education, Youth and
Sports of the Czech Republic, Czech Republic; The Danish Council for
Independent Research — Natural Sciences, the Carlsberg Foundation and Danish
National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics
(HIP), Finland; Commissariat à l’Energie Atomique (CEA) and Institut National
de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre
National de la Recherche Scientifique (CNRS), France; Bundesministerium für
Bildung, Wissenschaft, Forschung und Technologie (BMBF) and GSI
Helmholtzzentrum für Schwerionenforschung GmbH, Germany; General Secretariat
for Research and Technology, Ministry of Education, Research and Religions,
Greece; National Research, Development and Innovation Office, Hungary;
Department of Atomic Energy Government of India (DAE), Department of Science
and Technology, Government of India (DST), University Grants Commission,
Government of India (UGC) and Council of Scientific and Industrial Research
(CSIR), India; Indonesian Institute of Science, Indonesia; Centro Fermi -
Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi and Istituto
Nazionale di Fisica Nucleare (INFN), Italy; Institute for Innovative Science
and Technology , Nagasaki Institute of Applied Science (IIST), Japan Society
for the Promotion of Science (JSPS) KAKENHI and Japanese Ministry of
Education, Culture, Sports, Science and Technology (MEXT), Japan; Consejo
Nacional de Ciencia (CONACYT) y Tecnología, through Fondo de Cooperación
Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de
Asuntos del Personal Academico (DGAPA), Mexico; Nederlandse Organisatie voor
Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway,
Norway; Commission on Science and Technology for Sustainable Development in
the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú, Peru;
Ministry of Science and Higher Education and National Science Centre, Poland;
Korea Institute of Science and Technology Information and National Research
Foundation of Korea (NRF), Republic of Korea; Ministry of Education and
Scientific Research, Institute of Atomic Physics and Romanian National Agency
for Science, Technology and Innovation, Romania; Joint Institute for Nuclear
Research (JINR), Ministry of Education and Science of the Russian Federation
and National Research Centre Kurchatov Institute, Russia; Ministry of
Education, Science, Research and Sport of the Slovak Republic, Slovakia;
National Research Foundation of South Africa, South Africa; Centro de
Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía, Cuba and
Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas
(CIEMAT), Spain; Swedish Research Council (VR) and Knut & Alice Wallenberg
Foundation (KAW), Sweden; European Organization for Nuclear Research,
Switzerland; National Science and Technology Development Agency (NSDTA),
Suranaree University of Technology (SUT) and Office of the Higher Education
Commission under NRU project of Thailand, Thailand; Turkish Atomic Energy
Agency (TAEK), Turkey; National Academy of Sciences of Ukraine, Ukraine;
Science and Technology Facilities Council (STFC), United Kingdom; National
Science Foundation of the United States of America (NSF) and United States
Department of Energy, Office of Nuclear Physics (DOE NP), United States of
America.

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##  Appendix A The ALICE Collaboration

S. Acharyaorg139&F.T.-. Acostaorg20&D. Adamováorg93&J. Adolfssonorg80&M.M.
Aggarwalorg98&G. Aglieri Rinellaorg34&M. Agnelloorg31&N. Agrawalorg48&Z.
Ahammedorg139&S.U. Ahnorg76&S. Aiolaorg144&A. Akindinovorg64&M. Al-
Turanyorg104&S.N. Alamorg139&D.S.D. Albuquerqueorg121&D. Aleksandrovorg87&B.
Alessandroorg58&R. Alfaro Molinaorg72&Y. Aliorg15&A. Aliciorg10,org27,org53&A.
Alkinorg2&J. Almeorg22&T. Altorg69&L. Altenkamperorg22&I. Altsybeevorg111&M.N.
Anaamorg6&C. Andreiorg47&D. Andreouorg34&H.A. Andrewsorg108&A.
Andronicorg142,org104&M. Angelettiorg34&V. Anguelovorg102&C. Ansonorg16&T.
Antičićorg105&F. Antinoriorg56&P. Antonioliorg53&R. Anwarorg125&N.
Apadulaorg79&L. Aphecetcheorg113&H. Appelshäuserorg69&S. Arcelliorg27&R.
Arnaldiorg58&O.W. Arnoldorg103,org116&I.C. Arseneorg21&M. Arslandokorg102&A.
Augustinusorg34&R. Averbeckorg104&M.D. Azmiorg17&A. Badalàorg55&Y.W.
Baekorg60,org40&S. Bagnascoorg58&R. Bailhacheorg69&R. Balaorg99&A.
Baldisseriorg135&M. Ballorg42&R.C. Baralorg85&A.M. Barbanoorg26&R.
Barberaorg28&F. Barileorg52&L. Barioglioorg26&G.G. Barnaföldiorg143&L.S.
Barnbyorg92&V. Barretorg132&P. Bartaliniorg6&K. Barthorg34&E. Bartschorg69&N.
Bastidorg132&S. Basuorg141&G. Batigneorg113&B. Batyunyaorg75&P.C.
Batzingorg21&J.L. Bazo Albaorg109&I.G. Beardenorg88&H. Beckorg102&C.
Beddaorg63&N.K. Beheraorg60&I. Belikovorg134&F. Belliniorg34&H. Bello
Martinezorg44&R. Bellwiedorg125&L.G.E. Beltranorg119&V. Belyaevorg91&G.
Bencediorg143&S. Beoleorg26&A. Bercuciorg47&Y. Berdnikovorg96&D.
Berenyiorg143&R.A. Bertensorg128&D. Berzanoorg34,org58&L. Betevorg34&P.P.
Bhaduriorg139&A. Bhasinorg99&I.R. Bhatorg99&H. Bhattorg48&B.
Bhattacharjeeorg41&J. Bhomorg117&A. Bianchiorg26&L. Bianchiorg125&N.
Bianchiorg51&J. Bielčíkorg37&J. Bielčíkováorg93&A. Bilandzicorg116,org103&G.
Biroorg143&R. Biswasorg3&S. Biswasorg3&J.T. Blairorg118&D. Blauorg87&C.
Blumeorg69&G. Bocaorg137&F. Bockorg34&A. Bogdanovorg91&L. Boldizsárorg143&M.
Bombaraorg38&G. Bonomiorg138&M. Bonoraorg34&H. Borelorg135&A.
Borissovorg142&M. Borriorg127&E. Bottaorg26&C. Bourjauorg88&L. Bratrudorg69&P.
Braun-Munzingerorg104&M. Bregantorg120&T.A. Brokerorg69&M. Brozorg37&E.J.
Bruckenorg43&E. Brunaorg58&G.E. Brunoorg34,org33&D. Budnikovorg106&H.
Bueschingorg69&S. Bufalinoorg31&P. Buhlerorg112&P. Buncicorg34&O.
Buschorg131org*&Z. Butheleziorg73&J.B. Buttorg15&J.T. Buxtonorg95&J.
Cabalaorg115&D. Caffarriorg89&H. Cainesorg144&A. Calivaorg104&E. Calvo
Villarorg109&R.S. Camachoorg44&P. Cameriniorg25&A.A. Caponorg112&F.
Carenaorg34&W. Carenaorg34&F. Carnesecchiorg27,org10&J. Castillo
Castellanosorg135&A.J. Castroorg128&E.A.R. Casulaorg54&C. Ceballos
Sanchezorg8&S. Chandraorg139&B. Changorg126&W. Changorg6&S. Chapelandorg34&M.
Chartierorg127&S. Chattopadhyayorg139&S. Chattopadhyayorg107&A.
Chauvinorg103,org116&C. Cheshkovorg133&B. Cheynisorg133&V. Chibante
Barrosoorg34&D.D. Chinellatoorg121&S. Choorg60&P. Chochulaorg34&T.
Chowdhuryorg132&P. Christakoglouorg89&C.H. Christensenorg88&P.
Christiansenorg80&T. Chujoorg131&S.U. Chungorg18&C. Cicaloorg54&L.
Cifarelliorg10,org27&F. Cindoloorg53&J. Cleymansorg124&F. Colamariaorg52&D.
Colellaorg65,org52&A. Colluorg79&M. Colocciorg27&M. Concasorg58orgI&G. Conesa
Balbastreorg78&Z. Conesa del Valleorg61&J.G. Contrerasorg37&T.M.
Cormierorg94&Y. Corrales Moralesorg58&P. Corteseorg32&M.R. Cosentinoorg122&F.
Costaorg34&S. Costanzaorg137&J. Crkovskáorg61&P. Crochetorg132&E.
Cuautleorg70&L. Cunqueiroorg142,org94&T. Dahmsorg103,org116&A. Daineseorg56&S.
Daniorg66&M.C. Danischorg102&A. Danuorg68&D. Dasorg107&I. Dasorg107&S.
Dasorg3&A. Dashorg85&S. Dashorg48&A. Dashiorg103,org116&S. Deorg49&A. De
Caroorg30&G. de Cataldoorg52&C. de Contiorg120&J. de Cuvelandorg39&A. De
Falcoorg24&D. De Gruttolaorg10,org30&N. De Marcoorg58&S. De Pasqualeorg30&R.D.
De Souzaorg121&H.F. Degenhardtorg120&A. Deistingorg104,org102&A.
Delofforg84&S. Delsantoorg26&C. Deplanoorg89&P. Dhankherorg48&D. Di
Bariorg33&A. Di Mauroorg34&B. Di Ruzzaorg56&R.A. Diazorg8&T. Dietelorg124&P.
Dillensegerorg69&Y. Dingorg6&R. Diviàorg34&Ø. Djuvslandorg22&A. Dobrinorg34&D.
Domenicis Gimenezorg120&B. Dönigusorg69&O. Dordicorg21&L.V.R.
Doremalenorg63&A.K. Dubeyorg139&A. Dublaorg104&L. Ducrouxorg133&S.
Dudiorg98&A.K. Duggalorg98&M. Dukhishyamorg85&P. Dupieuxorg132&R.J.
Ehlersorg144&D. Eliaorg52&E. Endressorg109&H. Engelorg74&E. Eppleorg144&B.
Erazmusorg113&F. Erhardtorg97&M.R. Ersdalorg22&B. Espagnonorg61&G.
Eulisseorg34&J. Eumorg18&D. Evansorg108&S. Evdokimovorg90&L.
Fabbiettiorg103,org116&M. Fagginorg29&J. Faivreorg78&A. Fantoniorg51&M.
Faselorg94&L. Feldkamporg142&A. Felicielloorg58&G. Feofilovorg111&A. Fernández
Téllezorg44&A. Ferrettiorg26&A. Festantiorg34&V.J.G. Feuillardorg102&J.
Figielorg117&M.A.S. Figueredoorg120&S. Filchaginorg106&D. Finogeevorg62&F.M.
Fiondaorg22&G. Fiorenzaorg52&F. Flororg125&M. Florisorg34&S. Foertschorg73&P.
Fokaorg104&S. Fokinorg87&E. Fragiacomoorg59&A. Francesconorg34&A.
Franciscoorg113&U. Frankenfeldorg104&G.G. Fronzeorg26&U. Fuchsorg34&C.
Furgetorg78&A. Fursorg62&M. Fusco Girardorg30&J.J. Gaardhøjeorg88&M.
Gagliardiorg26&A.M. Gagoorg109&K. Gajdosovaorg88&M. Gallioorg26&C.D.
Galvanorg119&P. Ganotiorg83&C. Garabatosorg104&E. Garcia-Solisorg11&K.
Gargorg28&C. Gargiuloorg34&P. Gasikorg116,org103&E.F. Gaugerorg118&M.B. Gay
Ducatiorg71&M. Germainorg113&J. Ghoshorg107&P. Ghoshorg139&S.K. Ghoshorg3&P.
Gianottiorg51&P. Giubellinoorg104,org58&P. Giubilatoorg29&P.
Glässelorg102&D.M. Goméz Coralorg72&A. Gomez Ramirezorg74&V. Gonzalezorg104&P.
González-Zamoraorg44&S. Gorbunovorg39&L. Görlichorg117&S. Gotovacorg35&V.
Grabskiorg72&L.K. Graczykowskiorg140&K.L. Grahamorg108&L. Greinerorg79&A.
Grelliorg63&C. Grigorasorg34&V. Grigorievorg91&A. Grigoryanorg1&S.
Grigoryanorg75&J.M. Gronefeldorg104&F. Grosaorg31&J.F. Grosse-
Oetringhausorg34&R. Grossoorg104&R. Guernaneorg78&B. Guerzoniorg27&M.
Guittiereorg113&K. Gulbrandsenorg88&T. Gunjiorg130&A. Guptaorg99&R.
Guptaorg99&I.B. Guzmanorg44&R. Haakeorg34&M.K. Habiborg104&C.
Hadjidakisorg61&H. Hamagakiorg81&G. Hamarorg143&M. Hamidorg6&J.C.
Hamonorg134&R. Hanniganorg118&M.R. Haqueorg63&A. Harlenderovaorg104&J.W.
Harrisorg144&A. Hartonorg11&H. Hassanorg78&D. Hatzifotiadouorg53,org10&S.
Hayashiorg130&S.T. Heckelorg69&E. Hellbärorg69&H. Helstruporg36&A.
Herghelegiuorg47&E.G. Hernandezorg44&G. Herrera Corralorg9&F.
Herrmannorg142&K.F. Hetlandorg36&T.E. Hildenorg43&H. Hillemannsorg34&C.
Hillsorg127&B. Hippolyteorg134&B. Hohlwegerorg103&D. Horakorg37&S.
Hornungorg104&R. Hosokawaorg131,org78&J. Hotaorg66&P. Hristovorg34&C.
Huangorg61&C. Hughesorg128&P. Huhnorg69&T.J. Humanicorg95&H. Hushnudorg107&N.
Hussainorg41&T. Hussainorg17&D. Hutterorg39&D.S. Hwangorg19&J.P.
Iddonorg127&S.A. Iga Buitronorg70&R. Ilkaevorg106&M. Inabaorg131&M.
Ippolitovorg87&M.S. Islamorg107&M. Ivanovorg104&V. Ivanovorg96&V.
Izucheevorg90&B. Jacakorg79&N. Jacazioorg27&P.M. Jacobsorg79&M.B.
Jadhavorg48&S. Jadlovskaorg115&J. Jadlovskyorg115&S. Jaelaniorg63&C.
Jahnkeorg120,org116&M.J. Jakubowskaorg140&M.A. Janikorg140&C. Jenaorg85&M.
Jercicorg97&O. Jevonsorg108&R.T. Jimenez Bustamanteorg104&M. Jinorg125&P.G.
Jonesorg108&A. Juskoorg108&P. Kalinakorg65&A. Kalweitorg34&J.H. Kangorg145&V.
Kaplinorg91&S. Karorg6&A. Karasu Uysalorg77&O. Karavichevorg62&T.
Karavichevaorg62&P. Karczmarczykorg34&E. Karpechevorg62&U. Kebschullorg74&R.
Keidelorg46&D.L.D. Keijdenerorg63&M. Keilorg34&B. Ketzerorg42&Z.
Khabanovaorg89&A.M. Khanorg6&S. Khanorg17&S.A. Khanorg139&A.
Khanzadeevorg96&Y. Kharlovorg90&A. Khatunorg17&A. Khuntiaorg49&M.M.
Kielbowiczorg117&B. Kilengorg36&B. Kimorg131&D. Kimorg145&D.J. Kimorg126&E.J.
Kimorg13&H. Kimorg145&J.S. Kimorg40&J. Kimorg102&M. Kimorg60,org102&S.
Kimorg19&T. Kimorg145&T. Kimorg145&S. Kirschorg39&I. Kiselorg39&S.
Kiselevorg64&A. Kisielorg140&J.L. Klayorg5&C. Kleinorg69&J.
Kleinorg34,org58&C. Klein-Bösingorg142&S. Klewinorg102&A. Klugeorg34&M.L.
Knichelorg34&A.G. Knospeorg125&C. Kobdajorg114&M. Kofaragoorg143&M.K.
Köhlerorg102&T. Kolleggerorg104&N. Kondratyevaorg91&E. Kondratyukorg90&A.
Konevskikhorg62&P.J. Konopkaorg34&M. Konyushikhinorg141&O. Kovalenkoorg84&V.
Kovalenkoorg111&M. Kowalskiorg117&I. Králikorg65&A. Kravčákováorg38&L.
Kreisorg104&M. Krivdaorg65,org108&F. Krizekorg93&M. Krügerorg69&E.
Kryshenorg96&M. Krzewickiorg39&A.M. Kuberaorg95&V. Kučeraorg93,org60&C.
Kuhnorg134&P.G. Kuijerorg89&J. Kumarorg48&L. Kumarorg98&S. Kumarorg48&S.
Kunduorg85&P. Kurashviliorg84&A. Kurepinorg62&A.B. Kurepinorg62&A.
Kuryakinorg106&S. Kushpilorg93&J. Kvapilorg108&M.J. Kweonorg60&Y.
Kwonorg145&S.L. La Pointeorg39&P. La Roccaorg28&Y.S. Laiorg79&I.
Lakomovorg34&R. Langoyorg123&K. Lapidusorg144&A. Lardeuxorg21&P.
Larionovorg51&E. Laudiorg34&R. Lavickaorg37&R. Leaorg25&L. Leardiniorg102&S.
Leeorg145&F. Lehasorg89&S. Lehnerorg112&J. Lehrbachorg39&R.C. Lemmonorg92&I.
León Monzónorg119&P. Lévaiorg143&X. Liorg12&X.L. Liorg6&J. Lienorg123&R.
Lietavaorg108&B. Limorg18&S. Lindalorg21&V. Lindenstruthorg39&S.W.
Lindsayorg127&C. Lippmannorg104&M.A. Lisaorg95&V. Litichevskyiorg43&A.
Liuorg79&H.M. Ljunggrenorg80&W.J. Llopeorg141&D.F. Lodatoorg63&V.
Loginovorg91&C. Loizidesorg94,org79&P. Loncarorg35&X. Lopezorg132&E. López
Torresorg8&A. Loweorg143&P. Luettigorg69&J.R. Luhderorg142&M. Lunardonorg29&G.
Luparelloorg59&M. Lupiorg34&A. Maevskayaorg62&M. Magerorg34&S.M.
Mahmoodorg21&A. Maireorg134&R.D. Majkaorg144&M. Malaevorg96&Q.W. Malikorg21&L.
Malininaorg75orgII&D. Mal’Kevichorg64&P. Malzacherorg104&A. Mamonovorg106&V.
Mankoorg87&F. Mansoorg132&V. Manzariorg52&Y. Maoorg6&M.
Marchisoneorg129,org73,org133&J. Marešorg67&G.V. Margagliottiorg25&A.
Margottiorg53&J. Marguttiorg63&A. Marínorg104&C. Markertorg118&M.
Marquardorg69&N.A. Martinorg104&P. Martinengoorg34&J.L. Martinezorg125&M.I.
Martínezorg44&G. Martínez Garcíaorg113&M. Martinez Pedreiraorg34&S.
Masciocchiorg104&M. Maseraorg26&A. Masoniorg54&L. Massacrierorg61&E.
Massonorg113&A. Mastroserioorg52,org136&A.M. Mathisorg116,org103&P.F.T.
Matuokaorg120&A. Matyjaorg117,org128&C. Mayerorg117&M. Mazzilliorg33&M.A.
Mazzoniorg57&F. Meddiorg23&Y. Melikyanorg91&A. Menchaca-Rochaorg72&E.
Meninnoorg30&J. Mercado Pérezorg102&M. Meresorg14&C.S. Mezaorg109&S.
Mhlangaorg124&Y. Miakeorg131&L. Michelettiorg26&M.M. Mieskolainenorg43&D.L.
Mihaylovorg103&K. Mikhaylovorg64,org75&A. Mischkeorg63&A.N. Mishraorg70&D.
Miśkowiecorg104&J. Mitraorg139&C.M. Mituorg68&N. Mohammadiorg34&A.P.
Mohantyorg63&B. Mohantyorg85&M. Mohisin Khanorg17orgIII&D.A. Moreira De
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Morschorg34&T. Mrnjavacorg34&V. Mucciforaorg51&E. Mudnicorg35&D.
Mühlheimorg142&S. Muhuriorg139&M. Mukherjeeorg3&J.D. Mulliganorg144&M.G.
Munhozorg120&K. Münningorg42&M.I.A. Munozorg79&R.H. Munzerorg69&H.
Murakamiorg130&S. Murrayorg73&L. Musaorg34&J. Musinskyorg65&C.J.
Myersorg125&J.W. Myrchaorg140&B. Naikorg48&R. Nairorg84&B.K. Nandiorg48&R.
Naniaorg53,org10&E. Nappiorg52&A. Narayanorg48&M.U. Naruorg15&A.F.
Nassirpourorg80&H. Natal da Luzorg120&C. Nattrassorg128&S.R. Navarroorg44&K.
Nayakorg85&R. Nayakorg48&T.K. Nayakorg139&S. Nazarenkoorg106&R.A. Negrao De
Oliveiraorg69,org34&L. Nellenorg70&S.V. Nesboorg36&G. Neskovicorg39&F.
Ngorg125&M. Nicassioorg104&J. Niedzielaorg140,org34&B.S. Nielsenorg88&S.
Nikolaevorg87&S. Nikulinorg87&V. Nikulinorg96&F. Noferiniorg10,org53&P.
Nomokonovorg75&G. Noorenorg63&J.C.C. Norisorg44&J. Normanorg78&A.
Nyaninorg87&J. Nystrandorg22&H. Ohorg145&A. Ohlsonorg102&J.
Oleniaczorg140&A.C. Oliveira Da Silvaorg120&M.H. Oliverorg144&J.
Onderwaaterorg104&C. Oppedisanoorg58&R. Oravaorg43&M. Oravecorg115&A. Ortiz
Velasquezorg70&A. Oskarssonorg80&J. Otwinowskiorg117&K. Oyamaorg81&Y.
Pachmayerorg102&V. Pacikorg88&D. Paganoorg138&G. Paićorg70&P. Palniorg6&J.
Panorg141&A.K. Pandeyorg48&S. Panebiancoorg135&V. Papikyanorg1&P.
Pareekorg49&J. Parkorg60&J.E. Parkkilaorg126&S. Parmarorg98&A.
Passfeldorg142&S.P. Pathakorg125&R.N. Patraorg139&B. Paulorg58&H. Peiorg6&T.
Peitzmannorg63&X. Pengorg6&L.G. Pereiraorg71&H. Pereira Da Costaorg135&D.
Peresunkoorg87&E. Perez Lezamaorg69&V. Peskovorg69&Y. Pestovorg4&V.
Petráčekorg37&M. Petroviciorg47&C. Pettaorg28&R.P. Pezziorg71&S. Pianoorg59&M.
Piknaorg14&P. Pillotorg113&L.O.D.L. Pimentelorg88&O. Pinazzaorg53,org34&L.
Pinskyorg125&S. Pisanoorg51&D.B. Piyarathnaorg125&M. Płoskońorg79&M.
Planinicorg97&F. Pliquettorg69&J. Plutaorg140&S. Pochybovaorg143&P.L.M.
Podesta-Lermaorg119&M.G. Poghosyanorg94&B. Polichtchoukorg90&N. Poljakorg97&W.
Poonsawatorg114&A. Poporg47&H. Poppenborgorg142&S. Porteboeuf-
Houssaisorg132&V. Pozdniakovorg75&S.K. Prasadorg3&R. Preghenellaorg53&F.
Prinoorg58&C.A. Pruneauorg141&I. Pshenichnovorg62&M. Puccioorg26&V.
Puninorg106&J. Putschkeorg141&S. Rahaorg3&S. Rajputorg99&J. Rakorg126&A.
Rakotozafindrabeorg135&L. Ramelloorg32&F. Ramiorg134&R. Raniwalaorg100&S.
Raniwalaorg100&S.S. Räsänenorg43&B.T. Rascanuorg69&V. Ratzaorg42&I.
Ravasengaorg31&K.F. Readorg128,org94&K. Redlichorg84orgIV&A. Rehmanorg22&P.
Reicheltorg69&F. Reidtorg34&X. Renorg6&R. Renfordtorg69&A. Reshetinorg62&J.-P.
Revolorg10&K. Reygersorg102&V. Riabovorg96&T. Richertorg63&M. Richterorg21&P.
Riedlerorg34&W. Rieglerorg34&F. Riggiorg28&C. Risteaorg68&S.P. Rodeorg49&M.
Rodríguez Cahuantziorg44&K. Røedorg21&R. Rogalevorg90&E. Rogochayaorg75&D.
Rohrorg34&D. Röhrichorg22&P.S. Rokitaorg140&F. Ronchettiorg51&E.D.
Rosasorg70&K. Roslonorg140&P. Rosnetorg132&A. Rossiorg29&A. Rotondiorg137&F.
Roukoutakisorg83&C. Royorg134&P. Royorg107&O.V. Ruedaorg70&R. Ruiorg25&B.
Rumyantsevorg75&A. Rustamovorg86&E. Ryabinkinorg87&Y. Ryabovorg96&A.
Rybickiorg117&S. Saarinenorg43&S. Sadhuorg139&S. Sadovskyorg90&K.
Šafaříkorg34&S.K. Sahaorg139&B. Sahooorg48&P. Sahooorg49&R. Sahooorg49&S.
Sahooorg66&P.K. Sahuorg66&J. Sainiorg139&S. Sakaiorg131&M.A. Salehorg141&S.
Sambyalorg99&V. Samsonovorg96,org91&A. Sandovalorg72&A. Sarkarorg73&D.
Sarkarorg139&N. Sarkarorg139&P. Sarmaorg41&M.H.P. Sasorg63&E.
Scapparoneorg53&F. Scarlassaraorg29&B. Schaeferorg94&H.S. Scheidorg69&C.
Schiauaorg47&R. Schickerorg102&C. Schmidtorg104&H.R. Schmidtorg101&M.O.
Schmidtorg102&M. Schmidtorg101&N.V. Schmidtorg94,org69&J. Schukraftorg34&Y.
Schutzorg34,org134&K. Schwarzorg104&K. Schwedaorg104&G. Scioliorg27&E.
Scomparinorg58&M. Šefčíkorg38&J.E. Segerorg16&Y. Sekiguchiorg130&D.
Sekihataorg45&I. Selyuzhenkovorg104,org91&S. Senyukovorg134&E.
Serradillaorg72&P. Settorg48&A. Sevcencoorg68&A. Shabanovorg62&A.
Shabetaiorg113&R. Shahoyanorg34&W. Shaikhorg107&A. Shangaraevorg90&A.
Sharmaorg98&A. Sharmaorg99&M. Sharmaorg99&N. Sharmaorg98&A.I. Sheikhorg139&K.
Shigakiorg45&M. Shimomuraorg82&S. Shirinkinorg64&Q. Shouorg6,org110&K.
Shtejerorg26&Y. Sibiriakorg87&S. Siddhantaorg54&K.M. Sielewiczorg34&T.
Siemiarczukorg84&D. Silvermyrorg80&G. Simatovicorg89&G.
Simonettiorg34,org103&R. Singarajuorg139&R. Singhorg85&R. Singhorg99&V.
Singhalorg139&T. Sinhaorg107&B. Sitarorg14&M. Sittaorg32&T.B. Skaaliorg21&M.
Slupeckiorg126&N. Smirnovorg144&R.J.M. Snellingsorg63&T.W. Snellmanorg126&J.
Songorg18&F. Soramelorg29&S. Sorensenorg128&F. Sozziorg104&I.
Sputowskaorg117&J. Stachelorg102&I. Stanorg68&P. Stankusorg94&E.
Stenlundorg80&D. Stoccoorg113&M.M. Storetvedtorg36&P. Strmenorg14&A.A.P.
Suaideorg120&T. Sugitateorg45&C. Suireorg61&M. Suleymanovorg15&M.
Suljicorg34,org25&R. Sultanovorg64&M. Šumberaorg93&S. Sumowidagdoorg50&K.
Suzukiorg112&S. Swainorg66&A. Szaboorg14&I. Szarkaorg14&U. Tabassamorg15&J.
Takahashiorg121&G.J. Tambaveorg22&N. Tanakaorg131&M. Tarhiniorg113&M.
Tariqorg17&M.G. Tarzilaorg47&A. Tauroorg34&G. Tejeda Muñozorg44&A.
Telescaorg34&C. Terrevoliorg29&B. Teyssierorg133&D. Thakurorg49&S.
Thakurorg139&D. Thomasorg118&F. Thoresenorg88&R. Tieulentorg133&A.
Tikhonovorg62&A.R. Timminsorg125&A. Toiaorg69&N. Topilskayaorg62&M.
Toppiorg51&S.R. Torresorg119&S. Tripathyorg49&S. Trogoloorg26&G.
Trombettaorg33&L. Tropporg38&V. Trubnikovorg2&W.H. Trzaskaorg126&T.P.
Trzcinskiorg140&B.A. Trzeciakorg63&T. Tsujiorg130&A. Tumkinorg106&R.
Turrisiorg56&T.S. Tveterorg21&K. Ullalandorg22&E.N. Umakaorg125&A.
Urasorg133&G.L. Usaiorg24&A. Utrobicicorg97&M. Valaorg115&J.W. Van
Hoorneorg34&M. van Leeuwenorg63&P. Vande Vyvreorg34&D. Vargaorg143&A.
Vargasorg44&M. Vargyasorg126&R. Varmaorg48&M. Vasileiouorg83&A.
Vasilievorg87&A. Vauthierorg78&O. Vázquez Doceorg103,org116&V.
Vecherninorg111&A.M. Veenorg63&E. Vercellinorg26&S. Vergara Limónorg44&L.
Vermuntorg63&R. Vernetorg7&R. Vértesiorg143&L. Vickovicorg35&J.
Viinikainenorg126&Z. Vilakaziorg129&O. Villalobos Baillieorg108&A. Villatoro
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Vodopyanovorg75&M.A. Völklorg101&K. Voloshinorg64&S.A. Voloshinorg141&G.
Volpeorg33&B. von Hallerorg34&I. Vorobyevorg116,org103&D. Voscekorg115&D.
Vranicorg104,org34&J. Vrlákováorg38&B. Wagnerorg22&H. Wangorg63&M. Wangorg6&Y.
Watanabeorg131&M. Weberorg112&S.G. Weberorg104&A. Wegrzynekorg34&D.F.
Weiserorg102&S.C. Wenzelorg34&J.P. Wesselsorg142&U. Westerhofforg142&A.M.
Whiteheadorg124&J. Wiechulaorg69&J. Wikneorg21&G. Wilkorg84&J.
Wilkinsonorg53&G.A. Willemsorg142,org34&M.C.S. Williamsorg53&E.
Willsherorg108&B. Windelbandorg102&W.E. Wittorg128&R. Xuorg6&S. Yalcinorg77&K.
Yamakawaorg45&S. Yanoorg45&Z. Yinorg6&H. Yokoyamaorg78,org131&I.-K.
Yooorg18&J.H. Yoonorg60&V. Yurchenkoorg2&V. Zaccoloorg58&A. Zamanorg15&C.
Zampolliorg34&H.J.C. Zanoliorg120&N. Zardoshtiorg108&A. Zarochentsevorg111&P.
Závadaorg67&N. Zaviyalovorg106&H. Zbroszczykorg140&M. Zhalovorg96&X.
Zhangorg6&Y. Zhangorg6&Z. Zhangorg6,org132&C. Zhaoorg21&V.
Zherebchevskiiorg111&N. Zhigarevaorg64&D. Zhouorg6&Y. Zhouorg88&Z.
Zhouorg22&H. Zhuorg6&J. Zhuorg6&Y. Zhuorg6&A. Zichichiorg27,org10&M.B.
Zimmermannorg34&G. Zinovjevorg2&J. Zmeskalorg112&S. Zouorg6&

## Affiliation notes

org*Deceased

orgIDipartimento DET del Politecnico di Torino, Turin, Italy

orgIIM.V. Lomonosov Moscow State University, D.V. Skobeltsyn Institute of
Nuclear, Physics, Moscow, Russia

orgIIIDepartment of Applied Physics, Aligarh Muslim University, Aligarh, India

orgIVInstitute of Theoretical Physics, University of Wroclaw, Poland

## Collaboration Institutes

org1A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute)
Foundation, Yerevan, Armenia

org2Bogolyubov Institute for Theoretical Physics, National Academy of Sciences
of Ukraine, Kiev, Ukraine

org3Bose Institute, Department of Physics and Centre for Astroparticle Physics
and Space Science (CAPSS), Kolkata, India

org4Budker Institute for Nuclear Physics, Novosibirsk, Russia

org5California Polytechnic State University, San Luis Obispo, California,
United States

org6Central China Normal University, Wuhan, China

org7Centre de Calcul de l’IN2P3, Villeurbanne, Lyon, France

org8Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana,
Cuba

org9Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City
and Mérida, Mexico

org10Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche
“Enrico Fermi’, Rome, Italy

org11Chicago State University, Chicago, Illinois, United States

org12China Institute of Atomic Energy, Beijing, China

org13Chonbuk National University, Jeonju, Republic of Korea

org14Comenius University Bratislava, Faculty of Mathematics, Physics and
Informatics, Bratislava, Slovakia

org15COMSATS Institute of Information Technology (CIIT), Islamabad, Pakistan

org16Creighton University, Omaha, Nebraska, United States

org17Department of Physics, Aligarh Muslim University, Aligarh, India

org18Department of Physics, Pusan National University, Pusan, Republic of
Korea

org19Department of Physics, Sejong University, Seoul, Republic of Korea

org20Department of Physics, University of California, Berkeley, California,
United States

org21Department of Physics, University of Oslo, Oslo, Norway

org22Department of Physics and Technology, University of Bergen, Bergen,
Norway

org23Dipartimento di Fisica dell’Università ’La Sapienza’ and Sezione INFN,
Rome, Italy

org24Dipartimento di Fisica dell’Università and Sezione INFN, Cagliari, Italy

org25Dipartimento di Fisica dell’Università and Sezione INFN, Trieste, Italy

org26Dipartimento di Fisica dell’Università and Sezione INFN, Turin, Italy

org27Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN,
Bologna, Italy

org28Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN,
Catania, Italy

org29Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN,
Padova, Italy

org30Dipartimento di Fisica ‘E.R. Caianiello’ dell’Università and Gruppo
Collegato INFN, Salerno, Italy

org31Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy

org32Dipartimento di Scienze e Innovazione Tecnologica dell’Università del
Piemonte Orientale and INFN Sezione di Torino, Alessandria, Italy

org33Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari,
Italy

org34European Organization for Nuclear Research (CERN), Geneva, Switzerland

org35Faculty of Electrical Engineering, Mechanical Engineering and Naval
Architecture, University of Split, Split, Croatia

org36Faculty of Engineering and Science, Western Norway University of Applied
Sciences, Bergen, Norway

org37Faculty of Nuclear Sciences and Physical Engineering, Czech Technical
University in Prague, Prague, Czech Republic

org38Faculty of Science, P.J. Šafárik University, Košice, Slovakia

org39Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-
Universität Frankfurt, Frankfurt, Germany

org40Gangneung-Wonju National University, Gangneung, Republic of Korea

org41Gauhati University, Department of Physics, Guwahati, India

org42Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-
Wilhelms-Universität Bonn, Bonn, Germany

org43Helsinki Institute of Physics (HIP), Helsinki, Finland

org44High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico

org45Hiroshima University, Hiroshima, Japan

org46Hochschule Worms, Zentrum für Technologietransfer und Telekommunikation
(ZTT), Worms, Germany

org47Horia Hulubei National Institute of Physics and Nuclear Engineering,
Bucharest, Romania

org48Indian Institute of Technology Bombay (IIT), Mumbai, India

org49Indian Institute of Technology Indore, Indore, India

org50Indonesian Institute of Sciences, Jakarta, Indonesia

org51INFN, Laboratori Nazionali di Frascati, Frascati, Italy

org52INFN, Sezione di Bari, Bari, Italy

org53INFN, Sezione di Bologna, Bologna, Italy

org54INFN, Sezione di Cagliari, Cagliari, Italy

org55INFN, Sezione di Catania, Catania, Italy

org56INFN, Sezione di Padova, Padova, Italy

org57INFN, Sezione di Roma, Rome, Italy

org58INFN, Sezione di Torino, Turin, Italy

org59INFN, Sezione di Trieste, Trieste, Italy

org60Inha University, Incheon, Republic of Korea

org61Institut de Physique Nucléaire d’Orsay (IPNO), Institut National de
Physique Nucléaire et de Physique des Particules (IN2P3/CNRS), Université de
Paris-Sud, Université Paris-Saclay, Orsay, France

org62Institute for Nuclear Research, Academy of Sciences, Moscow, Russia

org63Institute for Subatomic Physics, Utrecht University/Nikhef, Utrecht,
Netherlands

org64Institute for Theoretical and Experimental Physics, Moscow, Russia

org65Institute of Experimental Physics, Slovak Academy of Sciences, Košice,
Slovakia

org66Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India

org67Institute of Physics of the Czech Academy of Sciences, Prague, Czech
Republic

org68Institute of Space Science (ISS), Bucharest, Romania

org69Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt,
Frankfurt, Germany

org70Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México,
Mexico City, Mexico

org71Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS),
Porto Alegre, Brazil

org72Instituto de Física, Universidad Nacional Autónoma de México, Mexico
City, Mexico

org73iThemba LABS, National Research Foundation, Somerset West, South Africa

org74Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik,
Fachbereich Informatik und Mathematik, Frankfurt, Germany

org75Joint Institute for Nuclear Research (JINR), Dubna, Russia

org76Korea Institute of Science and Technology Information, Daejeon, Republic
of Korea

org77KTO Karatay University, Konya, Turkey

org78Laboratoire de Physique Subatomique et de Cosmologie, Université
Grenoble-Alpes, CNRS-IN2P3, Grenoble, France

org79Lawrence Berkeley National Laboratory, Berkeley, California, United
States

org80Lund University Department of Physics, Division of Particle Physics,
Lund, Sweden

org81Nagasaki Institute of Applied Science, Nagasaki, Japan

org82Nara Women’s University (NWU), Nara, Japan

org83National and Kapodistrian University of Athens, School of Science,
Department of Physics , Athens, Greece

org84National Centre for Nuclear Research, Warsaw, Poland

org85National Institute of Science Education and Research, Homi Bhabha
National Institute, Jatni, India

org86National Nuclear Research Center, Baku, Azerbaijan

org87National Research Centre Kurchatov Institute, Moscow, Russia

org88Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark

org89Nikhef, National institute for subatomic physics, Amsterdam, Netherlands

org90NRC Kurchatov Institute IHEP, Protvino, Russia

org91NRNU Moscow Engineering Physics Institute, Moscow, Russia

org92Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United
Kingdom

org93Nuclear Physics Institute of the Czech Academy of Sciences, Řež u Prahy,
Czech Republic

org94Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States

org95Ohio State University, Columbus, Ohio, United States

org96Petersburg Nuclear Physics Institute, Gatchina, Russia

org97Physics department, Faculty of science, University of Zagreb, Zagreb,
Croatia

org98Physics Department, Panjab University, Chandigarh, India

org99Physics Department, University of Jammu, Jammu, India

org100Physics Department, University of Rajasthan, Jaipur, India

org101Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen,
Germany

org102Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg,
Heidelberg, Germany

org103Physik Department, Technische Universität München, Munich, Germany

org104Research Division and ExtreMe Matter Institute EMMI, GSI
Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany

org105Rudjer Bošković Institute, Zagreb, Croatia

org106Russian Federal Nuclear Center (VNIIEF), Sarov, Russia

org107Saha Institute of Nuclear Physics, Homi Bhabha National Institute,
Kolkata, India

org108School of Physics and Astronomy, University of Birmingham, Birmingham,
United Kingdom

org109Sección Física, Departamento de Ciencias, Pontificia Universidad
Católica del Perú, Lima, Peru

org110Shanghai Institute of Applied Physics, Shanghai, China

org111St. Petersburg State University, St. Petersburg, Russia

org112Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria

org113SUBATECH, IMT Atlantique, Université de Nantes, CNRS-IN2P3, Nantes,
France

org114Suranaree University of Technology, Nakhon Ratchasima, Thailand

org115Technical University of Košice, Košice, Slovakia

org116Technische Universität München, Excellence Cluster ’Universe’, Munich,
Germany

org117The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy
of Sciences, Cracow, Poland

org118The University of Texas at Austin, Austin, Texas, United States

org119Universidad Autónoma de Sinaloa, Culiacán, Mexico

org120Universidade de São Paulo (USP), São Paulo, Brazil

org121Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil

org122Universidade Federal do ABC, Santo Andre, Brazil

org123University College of Southeast Norway, Tonsberg, Norway

org124University of Cape Town, Cape Town, South Africa

org125University of Houston, Houston, Texas, United States

org126University of Jyväskylä, Jyväskylä, Finland

org127University of Liverpool, Liverpool, United Kingdom

org128University of Tennessee, Knoxville, Tennessee, United States

org129University of the Witwatersrand, Johannesburg, South Africa

org130University of Tokyo, Tokyo, Japan

org131University of Tsukuba, Tsukuba, Japan

org132Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France

org133Université de Lyon, Université Lyon 1, CNRS/IN2P3, IPN-Lyon,
Villeurbanne, Lyon, France

org134Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg,
France, Strasbourg, France

org135 Université Paris-Saclay Centre d¿Études de Saclay (CEA), IRFU,
Department de Physique Nucléaire (DPhN), Saclay, France

org136Università degli Studi di Foggia, Foggia, Italy

org137Università degli Studi di Pavia, Pavia, Italy

org138Università di Brescia, Brescia, Italy

org139Variable Energy Cyclotron Centre, Homi Bhabha National Institute,
Kolkata, India

org140Warsaw University of Technology, Warsaw, Poland

org141Wayne State University, Detroit, Michigan, United States

org142Westfälische Wilhelms-Universität Münster, Institut für Kernphysik,
Münster, Germany

org143Wigner Research Centre for Physics, Hungarian Academy of Sciences,
Budapest, Hungary

org144Yale University, New Haven, Connecticut, United States

org145Yonsei University, Seoul, Republic of Korea