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 (\(\lesssim 5\) \({\mathrm{GeV}}/c\)).
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