# Measuring dilepton and heavy quark production
at large $\mu_{\rm B}$: the NA60+ experiment at the CERN SPS ††thanks:
Presented at the 29th International Conference on Ultrarelativistic Nucleus
Nucleus Collisions, April 4-10 2022, Kraków, Poland

Enrico Scomparin  for the NA60+ Collaboration  INFN Torino, Italy

###### Abstract

The high-$\mu_{\rm B}$ region of the QCD phase diagram has become the object
of several studies, focused on the investigation of the order of the phase
transition and the search for the critical point. Accessing rare probes is
experimentally challenging as it requires large integrated luminosities, and a
fixed-target environment represents an ideal solution for these studies. The
CERN SPS covers, with large beam intensity, the collision energy region
$5<\sqrt{s_{\rm NN}}<17$ GeV. A future experiment, NA60+, is being proposed to
access this region and perform accurate measurements of the dimuon spectrum
from threshold up to the charmonium mass region, and of hadronic decays of
charm and strange hadrons. The experiment, which is also part of the Physics
Beyond Colliders CERN initiative, aims at taking its first data with Pb and
proton beams around 2029.

##  1 Introduction

Studies of electromagnetic and hard probes have represented one of the most
important sources of information on the early stages of the strongly
interacting partonic system created in ultra-relativistic heavy-ion
collisions. Virtual photons, which can be detected as lepton pairs, are
emitted all along the collision history but their yield is dominated by the
high-temperature phase. The $T_{\rm slope}$ parameter of the spectral shape of
the dilepton invariant mass spectrum can be considered as a space-time average
of the thermal temperature $T$ over the fireball evolution. Measurements
carried out by the HADES and NA60 experiments, respectively in Au–Au
collisions at $\sqrt{s_{\rm NN}}=2.42$ GeV and In–In at $\sqrt{s_{\rm
NN}}=17.3$ GeV, allowed experimental estimates of the medium temperature.
Values increasing from $T_{\rm slope}=71.8\pm 2.1$ MeV to $205\pm 12$ MeV were
obtained, showing that the crossing of the pseudo-critical temperature should
be reached in the collision energy interval defined by the two existing
measurements. In such a range, measurements of lepton pairs allow a study of
modifications of the spectral function of the $\rho_{0}$ and of its chiral
partner $a_{\rm 1}$, related to chiral symmetry restoration. The latter
resonance is not directly coupled to the dilepton channel, but its
modification leads to an enhancement of the continuum in the region $1<m_{\rm
ll}<1.4$ GeV/c2. Precise measurements of the dilepton observables outlined in
this paragraph are missing below top SPS energy ($\sqrt{s_{\rm NN}}=17.3$ GeV)
and represent a solid motivation for a dedicated experiment at this facility,
to scan the range $5<\sqrt{s_{\rm NN}}<17$ GeV.

Such a dilepton experiment can also access quarkonium production. A
measurement of the J/$\psi$ suppression, and possibly of excited states
($\psi({\rm 2S})$, $\chi_{\rm c}$) is of utmost importance, seen its relation
with deconfinement, studied in great detail from top SPS to RHIC and LHC
energy. In addition to hidden charm, open charm production represents a
crucial measurement to access information on the transport coefficients of the
medium, as well as its hadronization mechanisms. Accurate measurements of open
charm hadrons can only be performed by accessing their hadronic decays, and
therefore a complete study of heavy quark production at SPS energy requires
the capability of tracking charged particles in addition to dilepton
measurements.

This situation has led to studies for NA60+, a new experiment at the CERN SPS
[1] based on a muon spectrometer coupled to a vertex spectrometer, and
inspired to the former NA60 experiment. In the next sections we will give
details on the foreseen experimental set-up and related detector studies, and
will briefly recall some estimates of the physics performance and finally the
planning for the preparation of the experiment.

##  2 Experimental apparatus and detector studies

A sketch of the experiment can be seen in Fig. 1. The muon spectrometer is
separated from the target region by means of a thick hadron absorber, made of
BeO and graphite that filters out the hadrons. Candidate muons are tracked by
means of six tracking stations. The two upstream stations are followed by a
large-aperture toroidal magnet and by two further tracking stations. A “muon
wall” acts as a further filter to remove hadrons escaping the absorber as well
as low-momentum muons and is followed by the two final tracking stations. In
the target region a system of five closely spaced targets is followed by a
vertex spectrometer which includes from five up to ten tracking MAPS planes.
The vertex spectrometer is embedded in the gap of a dipole magnet. By matching
muon tracks in the muon and vertex spectrometers, in the coordinate and
momentum space, a measurement of the muon kinematics not affected by multiple
scattering and energy loss in the hadron absorber becomes possible. The muon
spectrometer is mounted on a system of rails that allows keeping a constant
rapidity coverage when changing the collision energy. When moving the
spectrometer downstream along the beam line (high energy collisions) the
thickness of the absorber will be increased by adding further graphite
elements.

Figure 1: Sketch of the experimental set-up (top view). See text for details.

In Fig. 2 (left) a possible arrangement of the vertex spectrometer is shown.
Each plane contains four large MAPS sensors ($15\times 15$ cm2 each). The R&D
phase is in progress in the frame of a common development of the ALICE (ITS3
project) and NA60+ collaborations. For NA60+ each sensor is based on 25 mm
long units, replicated several times through a stitching procedure. For each
stitched long sensor, the control logic to steer the priority encoders, the
interfaces for the configuration of the chip and serial data transmitters are
all located at the periphery of the sensor, outside the detector acceptance.
The thickness of the sensor will be $<0.1$% radiation length and the spatial
resolution will be $\leq 5$ $\mu$m. From the mechanical point of view the
sensors will be mounted on a light frame of carbon fiber or foam, as shown in
Fig. 2(right).

Figure 2: Possible layout of the target system and the vertex spectrometer
(left); sketch of the mechanical support of a MAPS plane (right).

For the muon tracking, due to the relatively low rates after the hadron
absorber ($\sim 2$ kHz, at the expected Pb beam rate of $10^{6}$ s-1) GEMs or
MWPCs options are considered. For the latter, a prototype is being built and
tested in the lab, and will be exposed to an SPS test beam. The tracking
stations will be built by replicating the prototype detector in such a way to
obtain the desired geometry. The two upstream stations will include 12
trapezoid detectors each, while those downstream of the magnet and the muon
wall will have 36 and 84 detectors respectively. Figure 3 shows the design of
the MWPC unit (left) and the foreseen arrangement for the largest stations
(right). The GEM option will also be studied, based on the triple GEM detector
studied for the MOLLER experiment at JLab.

Figure 3: Design of the first prototype of MWPC unit for the NA60+ muon
spectrometer (left); layout of the largest muon tracking stations (right).

The toroidal magnet also represents a major element of the set-up. A warm
pulsed magnet is foreseen, producing 0.5 T of magnetic field magnitude over a
volume of 120 m3. The toroid is made from eight sectors, each coil has 12
turns and the conductor has a square copper section with 50 mm side length and
a circular cooling channel in the centre. The current is 190 kA, and the total
power is 3 MW. A demonstrator (scale 1:5) was constructed and tested, allowing
a cross-check of various aspects of the design. Measurements of the magnetic
field in the prototype were found to be in agreement with simulations within
3%. In Fig. 4 (left) the results of the field calculations for the toroidal
magnet are reported, while on the right panel a photo of the demonstrator can
be seen. Finally, for the dipole magnet we plan to use the MEP48 magnet,
currently stored at CERN. It can deliver a 1.5 T field, over a 40 cm gap.

Figure 4: Magnetic field calculations for the NA60+ toroidal magnet (left);
the scale 1:5 demonstrator under test (right).

After integration studies, the PPE138 hall on the H8 beam line of the SPS was
chosen as installation site for the NA60+ set-up. Radio-protection
calculations have led to the design of an iron/concrete shielding, to keep the
dose levels below 3 $\mu$Sv/h externally to the experimental hall for the
intended 106 s-1 Pb beam intensity. Specific beam optics studies have shown
that a sub-millimetric beam can be delivered to the experiment from $E=30$ to
158 AGeV.

##  3 Physics performance

Some of the main results of performance studies are briefly recalled here.
With the expected Pb beam intensity, we expect to collect 1011 minimum bias
collision events in the typical 1-month period dedicated to heavy-ion running
at CERN. The corresponding dimuon statistics for central (0–5%) events amounts
to $\sim 4\cdot 10^{6}$. For the measurement of the $T_{\rm slope}$ parameter,
uncertainties as low as $\sim 3\%$ can be reached. The result on the
measurement of the energy dependence of $T_{\rm slope}$ is shown in Fig. 5
(left) together with existing measurements from HADES and NA60 and
corresponding studies for the CBM experiment at GSI, which explores a lower,
complementary energy range. It will be possible to explore the region close to
the pseudocritical temperature and detect posssible signals of a first-order
phase transition that can be expected at these energies.

Figure 5: Energy dependence of the $T_{\rm slope}$ parameter. The blue
triangles show the expected NA60+ performance (left); invariant mass
distribution of candidates for the $D^{\rm 0}\rightarrow{\rm K}\pi$ decay at
$E=60$ AGeV (right).

Open charm measurements will be performed starting from the tracks
reconstructed in the vertex spectrometer. Charm hadron candidates are built by
combining pairs or triplets of tracks with the proper charge signs. The huge
combinatorial background can be reduced via geometrical selections on the
displaced decay-vertex topology. In Fig. 5 (right) the expected signal from
the $D^{\rm 0}\rightarrow{\rm K}\pi$ decay, for $E=60$ AGeV incident energy,
is shown. Measurements of hidden charm production will also be performed. The
expected statistics range from $10^{4}$ to $10^{5}$ J/$\psi$ from low to high
SPS energy, allowing precise study of the suppression signal. Corresponding
measurements in p-A collisions will also be carried out.

##  4 Planning and conclusions

After the submission of an EoI in 2019 [1], the Letter of Intent will be sent
to the CERN SPSC at the end of 2022. The current plan, if the experiment will
be approved and funded, is to be able to take the first data in 2029, after
the currently foreseen end of the LHC Shutdown 3. The running plan for the
first 5-6 years includes a $\sim 1$ month period with a Pb beam, with a
different incident energy for each year, complemented by a few weeks of proton
beam running for calibration purposes and physics studies with p-A collisions.

## References

  * [1] NA60+ Collaboration, T. Dahms, E. Scomparin, and G. Usai, “Expression of Interest for a new experiment at the CERN SPS: NA60+,”. http://cds.cern.ch/record/2673280.