#  Investigation of QGP-like properties via identified particles production in
oxygen-oxygen collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV from EPOS4

A. M. Khan<EMAIL_ADDRESS>University of Science & Technology of
China, Hefei 230026, People’s Republic of China
M. U. Ashraf<EMAIL_ADDRESS>(Corresponding Author) Centre for
Cosmology, Particle Physics and Phenomenology (CP3), Université Catholique de
Louvain, B-1348 Louvain-la-Neuve, Belgium
Salman Khurshid Malik<EMAIL_ADDRESS>Department of Physics,
University of Jammu, 180006, India
H. M. Alfanda<EMAIL_ADDRESS>Key Laboratory of Quark and
Lepton Physics (MOE) and Institute of Particle Physics, Central China Normal
University, Wuhan 430079, China

###### Abstract

The Large Hadron Collider (LHC) aims to inject oxygen (${}^{16}O$) ions in
next run into its experiments. This include anticipated one-day physics run
focusing on $O+O$ collisions at center-of-mass energy
$\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV. In this study, we present the production
of identified particles ($\pi^{\pm}$, $K^{\pm}$ and $p(\overline{p})$) as well
as the bulk-properties of the medium produced in $O+O$ collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV from EPOS4 simulations. We report the
transverse momentum ($p_{\rm T}$) spectra, integrated yield
($\mathrm{d}N/\mathrm{d}y$), charged particle multiplicity
($\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta$), mean transverse momentum
($\langle p_{\rm T}\rangle$), $p_{\rm T}$ differential particle ratios
($K/\pi$ and $p/\pi$) for different centrality classes. We found that EPOS4
simulations reasonably reproduce the shapes and distributions of all
observables. The freeze-out parameters have been extracted by fitting the
$p_{\rm T}$ spectra of identified particles simultaneously by the Boltzmann
Gibbs Blast Wave model. We found the presence of significant radial flow
($\langle\beta_{\rm T}\rangle$) and comparable kinetic freeze-out temperature
($T_{\rm kin}$) with $Pb+Pb$ collisions at LHC energies. Additionally, a
strong anti-correlation is found between $T_{\rm kin}$ and $\langle\beta_{\rm
T}\rangle$ from central to peripheral collisions. We also compare the freeze-
out parameters from EPOS4 with AMPT and it is observed that the flow velocity
$\langle\beta_{\rm T}\rangle$ from EPOS4 is relatively higher compared to that
of AMPT. Furthermore, the foreseen data from $O+O$ collisions at the LHC, when
available, will help to better understand the heavy-ion-like behaviour in
small systems. This is due to final state multiplicity overlap in $O+O$
collisions with $pp$, $p+Pb$ and $Pb+Pb$ collisions, and it may also help to
put possible constraints on the model parameters .

.

##  I Introduction

Ultra-relativistic heavy-ion collisions at the Large Hadron Collider (LHC) and
the Relativistic Heavy Ion Collider (RHIC) lead to the formation of a novel
state of matter known as the Quark-Gluon Plasma (QGP), wherein quarks and
gluons are no longer confined. The experiments at both the LHC and RHIC play a
crucial role in facilitating and continuously exploring the properties of this
hot and dense Quantum Chromodynamics (QCD) matter. So far, several
measurements have been performed in different colliding systems at different
center-of-mass energies. The primary focus of these experiments is to study
the properties of QGP that flows hydrodynamically as a nearly perfect fluid
[1, 2]. This is achieved by the collision of symmetric heavy-ions, such as
$Pb+Pb$ and $Au+Au$. However, small colliding systems, such as proton-proton
($pp$), serve as a baseline for comparison.

In 2017, LHC collided symmetric Xenon (${}^{129}Xe$) nuclei at a center-of-
mass energy of $\sqrt{s_{\mathrm{NN}}}$ = 5.44 TeV during a pilot run. The
objective was to study a slightly smaller system size. Unexpected results were
observed, including the measurement of a deformed ${}^{129}Xe$ nucleus in
ultra-central collisions [3, 4, 5, 6, 7]. Apart from deformation, most of the
theoretical predictions quantitatively describe the experimental data [8, 9].
Although hydrodynamical scaling is well understood in larger systems such as
$Pb+Pb$ and $Xe+Xe$, it is uncertain how well it holds up to smaller systems.

Recently, experiments at the LHC observed the indications of the existence of
QCD matter in high-multiplicity collisions, specifically in $pp$ and
asymmetric $p+Pb$ collisions, which is beyond heavy-ion collisions [10, 11].
The presence of QGP-like signatures in small systems at the LHC energies has
drawn a lot of attention from the heavy-ion physics community. Therefore, it
becomes essential to further investigate the collisions of small systems at
these energies. In this context, a short run of the Oxygen-Oxygen ($O+O$)
collision, intermediate in multiplicity between $p+Pb$ and $Pb+Pb$, is
anticipated at the LHC [12, 13]. This could provide a valuable opportunity to
investigate the transitions of these phenomena from large to small systems.

Several recent theoretical studies have been performed to study particle
production mechanisms in $O+O$ collisions [14, 15, 16, 17, 18, 19, 20, 21,
22]. These intermediate ion collisions may provide a deeper insight to
investigate the underlying mechanisms of particle production, the effects of
transverse collective flow, and the production of light nuclei within the
multiplicity range from small systems ($pp$ and $p+Pb$) and large systems
($Xe+Xe$ and $Pb+Pb$) [13]. ${}^{16}O$ is doubly magic nucleus with a distinct
feature of having similar number of participating nucleons as that of $p+Pb$
system and are distributed more sparsely in the transverse plane which lead to
different subsequent evolution. This unique feature of $O$ provides enhanced
nuclear stability against decay and has a very compact nuclear structure [23].

Bulk observables, including particle spectra, charged-particle multiplicities,
and particle ratios, provide an excellent probe for investigating the
properties of the QGP. The correlation between these observables provide a
comprehensive understanding of the interplay between soft and hard processes
in the collisions, thus shed light on the equation of state of the hot
hadronic matter [24]. The recent results from the LHC revealed that final
state multiplicity within an event plays a crucial role in driving the
observed QGP-like properties in high multiplicity $pp$ collisions [10].
Therefore, it would be interesting to compare the initial and final state
effects in $O+O$ collisions, especially considering their multiplicity overlap
with high-multiplicity $pp$ collisions. In this article, we investigate the
predictions of global parameters [25] of identified particles ($\pi^{\pm}$,
$K^{\pm}$ and $p(\overline{p})$) including transverse momentum $p_{\rm T}$
spectra, charged particle multiplicity, particle ratios, and kinetic freeze-
out properties in $O+O$ collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV
from EPOS4 simulations.

The article is organized as follows: A brief introduction to motivation is
provided in Sec. I. Section II discusses the details of EPOS4. Detailed
results are presented in Sec. III. Finally, the results are summarized in Sec.
IV.

##  II Event Generator EPOS4

In this section, a brief introduction to the EPOS4 model is presented.

EPOS, Energy conserving quantum mechanical multiple scattering approach, based
on Partons (parton ladders), Off-shell remnants, and Saturation of parton
ladders, based on Monte carlo framework, provides comprehensive simulations of
high energy $pp$ and heavy-ion collisions, including both initial- and final-
state dynamics. The model parameters of EPOS4 are discussed in detail in the
Ref. [26, 27, 28, 29]. Various observables, such as particle production,
transverse momentum $p_{\rm T}$ distributions, and flow correlations, can be
explored in both small systems ($pp$ collisions) and complex heavy-ion
interactions. A combined approach of the Gribov-Regge Theory (GRT) with
eikonalsied parton model has been employed in the EPOS to treat the first
interactions just after the collisions. In this approach, the conservation
laws are satisfied and the subsequent pomerons (interactions) are treated
equally [30]. The formalism to calculate the particle production is based on
the Feynman diagrams of the QCD-inspired effective field theory, providing the
energy conservation, and is the same as used for the cross-section
calculations. The nucleons are considered to be composed of “constituents”
carrying certain fraction of the incident momentum of the nucleon. The sum of
these fractions equals unity ensuring that the momentum is conserved during
the collisions. A nucleon termed as a “spectator” does not participate
directly in the interaction. If a nucleon is directly participating the
interaction, it can either be a “participant” or it can be a “remnamt”. The
production of particles is based on the string model approach [31].

In high multiplicity $pp$ and heavy-ion collisions, the density of the
“strings” during particle interactions can become very high and the individual
strings are unable to decay independently. EPOS addresses this issue by
introducing a dynamic process of division of the segments of the strings into
the “core” and the “corona” [32, 33, 34]. Recently, author has introduced a
new developments in the EPOS framework, referred as EPOS4 [26, 27, 28, 29]. In
EPOS4, a new understanding of strong interaction is developed between four
major concepts used in $pp$ and $AA$ collisions. These concepts include
rigorous scattering, energy conservation, fatorization, and saturation. This
formalism effectively measures high $p_{\rm T}$ particle production in
factorization mode, and simulates the collective effects in high-multiplicity
events. Importantly, the implementation of a dynamical saturation scale in
this version does not impact high $p_{\rm T}$ particle production, even if the
large number of parallel scatterings occurs [26]. Further details about the
EPOS4 model can be found in Refs. [26, 27, 28, 29].

We produce $\approx$ 1 Million minimum bias events for $O+O$ collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV from EPOS4. The centrality classes have
been determined by the reference multiplicity ($Refmult$) observable, which
measures the total number of particles produced in an event at $|\eta|<0.5$.
The centrality percentile have been assigned on the basis of $Refmult$
distribution i.e., from central to peripheral collisions.

##  III Discussion

In this section, we present the predictions for different global properties of
identified particles ($\pi^{\pm}$, $K^{\pm}$ and $p(\overline{p})$), such as
charged particle multiplicity ($\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta$),
transverse momentum $p_{\rm T}$ spectra, integrated yield
($\mathrm{d}N/\mathrm{d}y$), particle ratios ($K/\pi$ and $p/\pi$), and
kinetic freeze-out properties for different centrality classes in $O+O$
collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV from EPOS4 simulations.

###  III.1 Charged particle multiplicity distributions

Charged particle pseudorapidity density distributions,
$\langle\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta\rangle$ is an important
observable in heavy-ion collisions. The pseudorapidity, $\eta$, is defined as
$\eta=-\rm{ln}[\rm{tan}(\theta/2)]$, where $\theta$ is the emission angle
relative to the beam direction.
$\langle\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta\rangle$ is proportional to
the entropy density at freeze-out. As the entropy density of a closed system
tends to increase [35], pseudorapidity density provide insights into the
initial-state density of partons and any further entropy produced during
subsequent evolution [36]. Additionally, soft processes are sensitive to the
$\langle\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta\rangle$ in the collision,
while the mean transverse mass and momentum provides insight into the hard
processes.

Figure 1: (Color online) Pseudorapidity distributions of identified particles
in $O+O$ collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV for different
centrality classes from EPOS4 simulations.

Fig. 1 shows pseudorapidity distributions of the charged particles from
central ($0$–$5\%$) to peripheral ($80$–$100\%$) $O+O$ collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV from EPOS4. It is observed that EPOS4
model is able to describe the shape of $|\eta|$ distribution and is consistent
with the recent AMPT studies in $O+O$ collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV reported in Ref. [22]. The values of
$\langle\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta\rangle$ for different
centrality classes are listed in Tab. 1.

Table 1: Average charged-particle multiplicity at $|\eta|<0.5$ in $O+O$ collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV for different centrality classes from EPOS4 simulations. Centrality(%) | $\langle\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta\rangle$
---|---
$0$ – $5$ | $236.44\pm 0.14$
$5$ – $10$ | $189.801\pm 0.13$
$10$ – $20$ | $148.437\pm 0.08$
$20$ – $30$ | $105.863\pm 0.07$
$30$ – $40$ | $75.027\pm 0.06$
$40$ – $50$ | $53.037\pm 0.05$
$50$ – $60$ | $37.146\pm 0.06$
$60$ – $80$ | $19.889\pm 0.02$
$80$ – $100$ | $5.127\pm 0.01$

###  III.2 Transverse momentum ($p_{\rm T}$) spectra

In ultra-relativistic heavy-ion collisions, the observable such as production
yield and/or $p_{\rm T}$ spectra are important to explore the particle
production mechanisms. Additionally, the $p_{\rm T}$ spectra of identified
particles in heavy-ion collisions provide deeper insights on the transverse
expansion of the QGP and the freeze-out properties of the hadronic phase.

Figure 2: (Color online) Transverse momentum $p_{\rm T}$ spectra of identified
hadrons in $0$–$5\%$ central $O+O$ collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV from EPOS4 simulations.

The $p_{\rm T}$ spectra of identified particles ($\pi^{\pm}$, $K^{\pm}$ and
$p(\overline{p})$) in central ($0$–$5\%$), mid-central ($30$–$40\%$) and
$60$–$80\%$ $O+O$ collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV from
EPOS4 is shown in Fig. 2. Solid symbols are for positive particles whereas
open symbols are for negative particles. It is evident that EOPS4 simulations
successfully reproduce the shape of $p_{\rm T}$ spectra for identified
hadrons. It has recently been reported in Ref. [29, 37] that EPOS4 reasonably
describe the experimental data at RHIC and LHC energies.

###  III.3 Integrated yield ($\mathrm{d}N/\mathrm{d}y$)

The integrated yield, $\mathrm{d}N/\mathrm{d}y$ of $\pi^{\pm}$, $K^{\pm}$ and
$p(\overline{p})$ as a function of centrality in $O+O$ collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV is shown in Fig. 3. Due to the abundance
of $\pi^{\pm}$ among the identified particles, their integrated yield is
relatively higher than that of $K^{\pm}$ and $p(\overline{p})$, as expected,
and aligns with the expectations derived from the thermalized Boltzmannian
production of secondary particles in high energy nuclear collisions.

Figure 3: (Color online) Integrated yield, $\mathrm{d}N/\mathrm{d}y$ of
$\pi^{\pm}$, $K^{\pm}$ and $p(\overline{p})$ as a function of centrality in
$O+O$ collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV from EPOS4
simulations.

###  III.4 Mean Transverse momentum ($\langle p_{\rm T}\rangle$)

Fig. 4 shows the mean transverse momentum ($\langle p_{\rm T}\rangle$) as a
function of centrality in $O+O$ collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}7$
TeV from EPOS4. We find that EPOS4 simulations does not show a strong
centrality dependence in $O+O$ collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}7$
TeV. A similar trend has been observed in $Pb+Pb$ collisions at
$\sqrt{s_{\mathrm{NN}}}$= 5.02 TeV and EPOS4 simulations are in good agreement
with the experimental data [29].

Figure 4: (Color online) Mean transverse momentum ($\langle p_{\rm T}\rangle$)
as a function of centrality in $O+O$ collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV from EPOS4.

The comparison of $\langle p_{\rm T}\rangle$ of identified particles as a
function of particle mass in $O+O$ collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV with the experimental data of $pp$
collisions at $\sqrt{s_{\mathrm{NN}}}$= 900 GeV [38] and 7 TeV [39] is shown
in Fig. 5. It has been reported in Ref. [38] that there is slight increase in
the value of $\langle p_{\rm T}\rangle$ with increasing
$\sqrt{s_{\mathrm{NN}}}$, even though the spectral shape clearly indicates an
increasing contribution from hard processes. A clear mass-ordering in $\langle
p_{\rm T}\rangle$ has been observed in $O+O$ collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV from EPOS4 simulations, following the
trend established by previous published results [38, 40, 41, 42].

Figure 5: (Color online) Mean transverse momentum ($\langle p_{\rm T}\rangle$)
as a function of particle mass in $O+O$ collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV from EPOS4. The results are compared with
$pp$ collisions at $\sqrt{s_{\mathrm{NN}}}$= 900 GeV [38] and 7 TeV [39].

###  III.5 Particle ratios

Fig. 6 (left) shows the $p_{\rm T}$ differential
$(K^{+}+K^{-})/(\pi^{+}+\pi^{-})$ ratios in central ($0$–$5\%$), mid-central
($30$–$40\%$) and peripheral ($60$–$80\%$) $O+O$ collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV from EPOS4. These ratios as a function of
$p_{\rm T}$, are often referred to as the measure of strangeness enhancement.
The measured ratio shows a clear enhancement at intermediate $p_{\rm T}$
region ($1.5<$$p_{\rm T}$ $<3$) with value up to $0.5$ in $0$–$5\%$ centrality
class. However, the number decreases up to $0.35$ in the peripheral
($60$–$80\%$) centrality class. Additionally, the enhancement seems to be
similar for both $0$–$5\%$ and $60$–$80\%$ centrality classes at low $p_{\rm
T}$ region. Fig. 6 (right) shows the ratio of the lightest baryon to the
lightest meson ($p+\overline{p}/(\pi^{+}+\pi^{-})$). This ratio shows an
increasing trend up to the value of $0.45$ at intermediate $p_{\rm T}$ region
($1.5<$$p_{\rm T}$ $<2.5$) in $0$–$5\%$ centrality class. However, the trend
seems to flatten at $p_{\rm T}$ $>1.6$ for the $30$–$40\%$ and $60$–$80\%$
centrality classes similar to that observed in $pp$ collisions at the LHC
[39]. In general, EPOS4 simulations successfully reproduce the shape of both
ratios in $O+O$ collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV.

Figure 6: (Color online) $p_{\rm T}$ differential
$(K^{+}+K^{-})/(\pi^{+}+\pi^{-})$ ratio in $O+O$ collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV for central ( $0$–$5\%$), mid-central
($30$–$40\%$) and peripheral ($60$–$80\%$) centrality classes from EPOS4.

###  III.6 Freeze-out properties

The kinetic freeze-out parameters can be extracted by fitting the $p_{\rm T}$
spectra using the Blast Wave (BW) model. This model assumes that all the
produced particles are locally thermalized at kinetic freeze-out temperature
and are moving with a common collective transverse radial flow velocity field
[43, 44, 45, 46]. The common collective flow velocity field results the change
in the spectral shape of the particle with increasing mass. The $p_{\rm T}$
distribution of the particles, assuming a radially boosted thermal source with
a kinetic freeze-out temperature ($T_{\rm kin}$) and a transverse radial flow
velocity ($\beta$) is given by [43]:

$\frac{{\rm d}^{2}N}{p_{\rm T}\,{\rm d}p_{\rm T}}\propto\int_{0}^{R}r\,{\rm
d}r\,m_{\rm T}\,I_{0}\,\left(\frac{p_{\rm T}\,\sinh\rho(r)}{T_{\rm
kin}}\right)\,\times\,K_{1}\,\left(\frac{m_{\rm T}\,\cosh\rho(r)}{T_{\rm
kin}}\right)$ (1)

where $m_{\rm T}=\sqrt{p_{\rm T}^{2}+m^{2}}$ is the transverse mass of the
hadron species. $I_{0}$ and $K_{1}$ are the modified Bessel functions. The
temperature of the localized thermal sources, from which particles radiate, is
denoted by $T_{\rm kin}$. While assuming the longitudinal expansion of the
system to be boost invariant, the velocity profile of the thermal source can
be parameterized as:

$\rho=\tanh^{-1}\beta(r)=\tanh^{-1}\left(\beta_{S}\left(\frac{r}{R}\right)^{n}\right)$
(2)

Usually, the $p_{\rm T}$ spectra of $\pi^{\pm}$, $K^{\pm}$ and
$p(\overline{p})$ are fitted simultaneously with the BW model. However, more
particles such as strange and multi-strange hadrons can also be included in
the fit, which force all the particle species to freeze-out at the same time,
a scenario that might not be true. It has been reported in Ref. [47, 48] at
top RHIC energies, that the BW fit to the spectra of strange hadrons results a
larger values of $T_{\rm kin}$. For the results presented here, we only
consider the $p_{\rm T}$ spectra of $\pi^{\pm}$, $K^{\pm}$ and
$p(\overline{p})$ in the BW model fit. The BW model is sensitive to the fit
range of the $p_{\rm T}$. We use the similar fit range as reported in Ref.
[49].

Figure 7: (Color online) Variation of $T_{\rm kin}$ and $\langle\beta_{\rm T}\rangle$ extracted from the simultaneous fit of identified hadrons, $\pi^{\pm}$, $K^{\pm}$ and $p(\overline{p})$ from BW distribution in $O+O$ collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV from EPOS4. Table 2: Kinetic freeze-out temperature ($T_{\rm kin}$) and transverse radial flow velocity ($\langle\beta_{\rm T}\rangle$) parameters obtained from Boltzmann-Gibbs Blast Wave fit for different centrality classes in $O+O$ collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV from EPOS4 simulations. Centrality(%) |  $T_{\rm kin}$ ($\mathrm{MeV}$) | $\langle\beta_{\rm T}\rangle$
---|---|---
$0$ – $5$ | $154\pm 10$ | $0.69\pm 0.02$
$5$ – $10$ | $157\pm 10$ | $0.68\pm 0.01$
$10$ – $20$ | $159\pm 15$ | $0.66\pm 0.01$
$20$ – $30$ | $163\pm 13$ | $0.65\pm 0.01$
$30$ – $40$ | $166\pm 11$ | $0.64\pm 0.01$
$40$ – $50$ | $169\pm 10$ | $0.63\pm 0.01$
$50$ – $60$ | $175\pm 12$ | $0.61\pm 0.01$
$60$ – $80$ | $185\pm 15$ | $0.58\pm 0.02$
$80$ – $100$ | $210\pm 15$ | $0.54\pm 0.02$

Figure 7 shows the variation of $T_{\rm kin}$ with $\langle\beta_{\rm
T}\rangle$ for different centrality classes in $O+O$ collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV from EPOS4 simulations. We observe that
$T_{\rm kin}$ increases from central ($0$–$5\%$) to peripheral ($80$–$100\%$)
collisions indicating the shorter life of fireball in peripheral collisions
[50]. The value of $\langle\beta_{\rm T}\rangle$ shows a decreasing trend from
central to peripheral collisions due to more rapid expansion of the system in
central collisions. This behavior is expected because of the larger system
size in central collisions, which leads the hadronic phase to last longer,
causing the value of $T_{\rm kin}$ to be higher and $\langle\beta_{\rm
T}\rangle$ to be lower. Similar observations are reported for $Au+Au$
collisions at RHIC [44] and $Pb+Pb$ collisions at the LHC [49]. We also
compared the results with the predictions from the AMPT model in $O+O$
collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}7$ TeV [22]. We found that the
values of $T_{\rm kin}$ and $\langle\beta_{\rm T}\rangle$ from EPOS4
simulations is relatively higher as compared to the AMPT simulations. Due to
the large uncertainties it is difficult to make any solid conclusion. The
foreseen experimental data in $O+O$ collisions will help to put possible
constraints and/or tuning on model simulations.

##  IV Conclusions

We report the predictions of the production of identified particles
($\pi^{\pm}$, $K^{\pm}$ and $p(\overline{p})$) as well as the bulk properties
of the medium produced in $O+O$ collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}7$
TeV from EPOS4 simulations. We present the charged particle multiplicity
($\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta$), transverse momentum ($p_{\rm
T}$) spectra, integrated yield ($\mathrm{d}N/\mathrm{d}y$), mean transverse
momentum ($p_{\rm T}$) and the kinetic freeze-out properties of these
particles in different centrality classes. It is observed that the EPOS4
simulations successfully reproduce the shape of the $p_{\rm T}$ spectra for
all particle species. The integrated yield ($\mathrm{d}N/\mathrm{d}y$) of
$\pi^{\pm}$ is higher as compared to that of $K^{\pm}$ and $p(\overline{p})$
due to their higher abundance among other particle species. The $\langle
p_{\rm T}\rangle$ shows a mass ordering in the $O+O$ system similar to that
observed for other colliding systems at RHIC and LHC. The $K^{\pm}/\pi^{\pm}$
ratios are sensitive to study the strangeness enhancement. We found that
$K^{\pm}/\pi^{\pm}$ ratios from EPOS4 simulations shows enhancement at the
intermediate $p_{\rm T}$ region. The $p/\pi$ ratio shows an increasing trend
at the intermediate $p_{\rm T}$ region in central collisions while starts to
flatten from at $p_{\rm T}$ $>1.6$ for peripheral collisions. Additionally, a
simultaneous blast-wave model fit is employed to the $p_{\rm T}$ spectra of
identified particles to extract the kinetic freeze-out parameters. A strong
anti-correlation is observed between $T_{\rm kin}$ with $\langle\beta_{\rm
T}\rangle$ in $O+O$ collisions similar to $Au+Au$ and $Pb+Pb$ systems. In the
present analysis, we observe a comparable values of $T_{\rm kin}$ and
$\langle\beta_{\rm T}\rangle$ with that of $Pb+Pb$ collisions at the LHC
energies. It would be interesting to compare these observations with the
foreseen experimental data, when available, to further constrain the model
parameters.

##  V Acknowledgements

The authors would like to thank K. Werner for useful discussions about EPOS4,
and Muhammad Aamir Shahzad for discussions and invaluable comments on the
manuscript.

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