Production of pions, kaons, (anti-)protons and $\phi$ mesons in Xe–Xe collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.44 TeV

Abstract

The first measurement of the production of pions, kaons, (anti-)protons and $\phi$ mesons at midrapidity in Xe–Xe collisions at $\sqrt{s_{\mathrm{NN}}} = 5.44~\text {TeV}$ is presented. Transverse momentum ($p_{\mathrm{T}}$) spectra and $p_{\mathrm{T}}$-integrated yields are extracted in several centrality intervals bridging from p–Pb to mid-central Pb–Pb collisions in terms of final-state multiplicity. The study of Xe–Xe and Pb–Pb collisions allows systems at similar charged-particle multiplicities but with different initial geometrical eccentricities to be investigated. A detailed comparison of the spectral shapes in the two systems reveals an opposite behaviour for radial and elliptic flow. In particular, this study shows that the radial flow does not depend on the colliding system when compared at similar charged-particle multiplicity. In terms of hadron chemistry, the previously observed smooth evolution of particle ratios with multiplicity from small to large collision systems is also found to hold in Xe–Xe. In addition, our results confirm that two remarkable features of particle production at LHC energies are also valid in the collision of medium-sized nuclei: the lower proton-to-pion ratio with respect to the thermal model expectations and the increase of the $\phi$-to-pion ratio with increasing final-state multiplicity.

Introduction

In recent years, the production of hadrons consisting of light flavour quarks (u, d, and s) has been extensively studied in pp, p–Pb and Pb–Pb collisions at LHC energies [1,2,3,4,5,6,7,8,9,10,11] with the aim to explore the strongly interacting Quark-Gluon Plasma (QGP) produced in heavy-ion collisions. After the formation, the QGP expands hydrodynamically reaching first a chemical freeze-out, where hadron abundances are fixed [12, 13], and then a kinetic freeze-out, where the hadron momenta are fixed.

Remarkably, a smooth evolution of the hadron chemistry, i.e. of the relative abundance of hadron species, was observed across different collision systems as a function of the final-state multiplicity [9]. This behaviour was also found to be independent of collision energy [10]. In particular, the relative abundance of strange particles with respect to the non-strange ones increases continuously from small to large multiplicities until a saturation is observed for systems in which about 100 charged particles are produced per unit of pseudorapidity [8]. This observation suggests a gradual approach to a chemical equilibrium that is assumed to originate from the same underlying physical mechanisms across different collision systems [14,15,16]. The study of the pion, kaon, (anti-)proton, and $\phi$ production in the collisions of medium-sized nuclei such as Xe provides the ultimate test for validating this picture by bridging the gap between p–Pb and Pb–Pb multiplicities.

In this context, two remarkable features of particle production are of particular interest to be verified in Xe–Xe collisions: (i) the low value of the ${\mathrm{p}}/\pi$ ratio with respect to statistical-thermal model estimates [17] and (ii) the rising trend of the $\phi /\pi$ ratio from low to high multiplicities [9]. The first observation has led to several speculations ranging from the incomplete treatment of resonance feed-down to a potential difference in chemical freeze-out temperatures among different quark flavours [18,19,20] but found its most likely explanation in the inclusion of pion-nucleon phase shifts within the statistical-thermal model framework [21]. The second effect provides strict constraints for both the canonical statistical-thermal approach in which no rise is predicted [9, 22, 23] as well as for models with only partial strangeness equilibration in which a steeper rise is expected similarly to the $\Xi$ baryon [22].

Moreover, the detailed comparison of spectral shapes in Xe–Xe and Pb–Pb collisions at similar multiplicities provides the unique opportunity to investigate the hydrodynamic expansion in systems of similar final state charged particle multiplicity and different geometrical eccentricity. Already existing data on the elliptic flow coefficient $v_{2}$ [24] show a large difference in central collisions between the two systems, as expected from the Glauber and hydrodynamical models. In contrast, the radial flow and consequently the mean transverse momenta are expected to be comparable between Xe–Xe and Pb–Pb at similar multiplicities [25]. The test of this hypothesis is one of the subjects of this manuscript. In addition, the data used in this article were collected with a lower magnetic field, thus allowing us to extend the measurement of pions to lower transverse momenta with respect to previous studies [26]. For this reason, these data may also be of great relevance for future studies of potential condensation phenomena at low transverse momenta [27].

This article is organised as follows. Section 2 describes the experimental setup and data analysis as well as the systematic uncertainties. Results and comparisons with model calculations are discussed in Sect. 3. The summary and conclusions are given in Sect. 4.

Experimental apparatus, data sample and analysis

The measurements reported in this article are obtained with the $\text {ALICE}$ central barrel which has full azimuthal coverage around midrapidity in $|\eta |$ < 0.8 [28]. A detailed description of the full $\text {ALICE}$ apparatus can be found in [29]. In October 2017, for the first time at the LHC, Xe–Xe collisions at $\sqrt{s_{\mathrm{NN}}} = 5.44~\text {TeV}$ were recorded by the ALICE experiment at an average instantaneous luminosity of about $2 \times 10^{-25}$ ${\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ and a hadronic interaction rate of 80–150 ${\mathrm{Hz}}$. In total, the Xe–Xe data sample consists of about $1.1 \times 10^6$ minimum bias (MB) events passing the event selection described below. The MB interaction trigger is provided by two arrays of forward scintillators, named V0 detectors, with a pseudorapidity coverage of $2.8< \eta < 5.1$ (V0A) and $- 3.7< \eta < -1.7$ (V0C) [30]. The V0 signal is proportional to the charged-particle multiplicity and is used to divide the Xe–Xe sample in centrality classes defined in percentiles of the hadronic cross section [31,32,33]. The analysis is carried out in the centrality classes $0{-}5 \%$, $5{-}10 \%$, $10{-}20 \%$, $20{-}30 \%$, $30{-}40 \%$, $40{-}50 \%$, $50{-}60 \%$, $60{-}70 \%$, $70{-}90 \%$. In order to reduce the statistical uncertainty, the $\phi$ measurements are obtained in wider centrality classes $0{-}10 \%$, $10{-}20 \%$, $20{-}30 \%$, $30{-}40 \%$, $40{-}50 \%$, $50{-}70 \%$, $70{-}90 \%$. The most central (peripheral) collisions are considered in the $0{-}5 \%$ ($70{-}90 \%$) class. The $90{-}100 \%$ centrality bin is not included in the analysis since it is affected by the contamination of electromagnetic processes ($\approx$ 20%). In addition, as described in [26, 34], an offline selection of the events is applied to remove the beam-background events. It combines the V0 timing information and the correlation between the sum and the difference of times measured in each of the Zero Degree Calorimeters (ZDCs) positioned at ± 112.5 m from the interaction point [29]. Due to the low instantaneous luminosity (with an average collision probability per bunch crossing of $\mu ~\approx ~0.0005$), the probability of having more than two events per collision trigger was sufficiently low that the so-called event pileup is considered negligible.

The central barrel detectors are located inside a solenoidal magnet providing a maximum magnetic field (B) of 0.5 T. A magnetic field of 0.2 T can be set when operating the magnet in its low B field configuration. The central barrel detectors are used to reconstruct tracks and measure their momenta, as well as to perform particle identification (PID). Those exploited in this analysis are (from the interaction point outwards) the Inner Tracking System (ITS) [28], the Time Projection Chamber (TPC) [35] and the Time Of Flight (TOF) detector [36]. With respect to previous analyses [26], the low amount of collected data makes it impracticable to perform PID with the High Momentum Particle IDentification detector (HMPID) [37].

The ITS is equipped with six layers of silicon detectors made of three different technologies: Silicon Pixel Detectors (SPD, first two layers from the interaction point), Silicon Drift Detectors (SDD, two middle layers) and Silicon Strip Detectors (SSD, two outermost layers). It allows the reconstruction of the collision vertex, the reconstruction of tracks and the identification of particles at low momentum ($p$  < 1 ${\mathrm{GeV}}/c$) via the measurement of their specific energy loss (${\mathrm{d}}E/{\mathrm{d}}x$). An ITS-only analysis can be performed by using a dedicated algorithm to treat the ITS as a standalone tracker, enabling the reconstruction and identification of low-momentum particles that do not reach the TPC. The TPC, a cylindrical gas detector equipped with Multi-Wire Proportional Chambers (MWPC), constitutes the main central-barrel tracking detector and is also used for PID through the ${\mathrm{d}}E/{\mathrm{d}}x$ measurements in the gas. The ${\mathrm{d}}E/{\mathrm{d}}x$ measurements obtained with the ITS and TPC detectors are shown in Fig. 1. The $\text {time-of-flight}$ measured with the TOF, a large area cylindrical detector based on Multigap Resistive Plate Chamber (MRPC) technology, combined with the momentum information measured in the TPC, is employed to identify particles at low and intermediate momenta ($\lesssim 5$ ${\mathrm{GeV}}/c$).

Fig. 1

Distribution of the ${\mathrm{d}}E/{\mathrm{d}}x$ measured in the ITS (left) and TPC (right) detectors as a function of the reconstructed track momentum in Xe–Xe collisions at $\sqrt{s_{\mathrm{NN}}} = 5.44~\text {TeV}$. The bands corresponding to the signals of $\pi ^{\pm }$, ${\mathrm{K}}^{\pm }$, ${\mathrm{p}}$ and $\overline{\mathrm{p}}$ are well separated in the relevant momentum ranges. The good separation power obtained at low momentum is one of the key features for the measurements reported in this article

The events analysed in this article are chosen according to the selection criteria described in [26]. The primary vertex is determined from tracks, including the track segments reconstructed in the SPD. The position along the beam axis (z) of the vertex reconstructed with the SPD segments and of the one reconstructed from tracks are required to be compatible within 0.5 cm with a resolution of the SPD one better than 0.25 cm. The position of the primary vertex along z is required to be within 10 cm from the nominal interaction point. These criteria ensure a uniform acceptance in the pseudorapidity region $|\eta | < 0.8$.

The results presented in this work refer to primary particles, defined as particles with a mean proper lifetime of $\tau > 1$ $\text {cm}/c$ that are either produced directly in the interaction or from decays of particles with $\tau < 1$ $\text {cm}/c$, restricted to decay chains leading to the interaction point [38]. To reduce the contamination from secondary particles from weak decays and interactions in the detector material, as well as tracks with wrongly associated hits, similar selection criteria as described in [26, 34] are used and are summarised below. Tracks reconstructed with both the TPC and the ITS are required to cross at least 70 TPC readout rows out of a maximum of 159 with a $\chi ^{2}$ normalised to the number of TPC space points (“clusters”), $\chi ^{2}/{\mathrm{cluster}}$, lower than 4. The ratio between the number of clusters and the number of crossed rows in the TPC has to be larger than 0.8. An additional cut on the track geometrical length in the TPC fiducial volume is used as in [34]. Tracks are also required to have at least two hits in the ITS detector out of which at least one has to be in the SPD. In addition, for the ITS-only analysis, the tracks must have at least three hits in the SDD + SSD layers. The $\chi ^{2}/{\mathrm{cluster}}$ is also recalculated constraining the track to pass by the primary vertex and it is required to be lower than 36. The same selection is also applied on the ITS points of the track: $\chi ^{2}_{\mathrm{ITS}}/N^{\mathrm{hits}}_{\mathrm{ITS}} < 36$. For the ITS-only analysis, this selection is restricted to $\chi ^{2}_{\mathrm{ITS}}/N^{\mathrm{hits}}_{\mathrm{ITS}} < 2.5$. Finally, the tracks are required to have a distance of closest approach (${\mathrm{DCA}}$) to the primary vertex along the beam axis lower than 2 cm. A $p_{\mathrm{T}}$-dependent selection is then applied to the ${\mathrm{DCA}}$ in the transverse plane (${\mathrm{DCA}} _{{ xy}}$): $|{\mathrm{DCA}} _{{ xy}} | < 7 \sigma _{{\mathrm{DCA}} _{{ xy}}}$ where $\sigma _{{\mathrm{DCA}} _{{ xy}}}$ is the resolution on the ${\mathrm{DCA}} _{{ xy}}$ in each $p_{\mathrm{T}}$ interval. Furthermore, the tracks associated with decay products of weakly decaying kaons (“kinks”) are rejected. This selection is not applied for kaons studied via their kink decay topology. The track selection criteria for kaons and pions from kinks will be described in the next paragraph.

The Xe–Xe data were collected by operating the detector in its low B field configuration (${\mathrm{B}} =0.2$ T). The lower magnetic field increases the probability of low momentum particles to cross the full detector thus extending the overall acceptance and reach of the analyses to lower $p_{\mathrm{T}}$. This allowed for the measurement of pions down to 50 ${\mathrm{MeV}}/c$ for the first time at the LHC with respect to past publications [