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CMS-PAS-HIN-17-006

Pseudorapidity distributions of charged hadrons in XeXe collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} =$ 5.44 TeV

CMS Collaboration

May 2018

Abstract: Measurements of the pseudorapidity distributions of charged hadrons produced in xenon-xenon collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} =$ 5.44 TeV are presented. The measurements are based on data collected by the CMS experiment at the CERN LHC. The yield of primary charged hadrons produced in non-single-diffractive xenon-xenon collisions in the pseudorapidity range $|\eta | <$ 3.2 is determined using the silicon pixel detector in the CMS tracking system. For events with the largest overlap between the colliding nuclei, the charged-hadron pseudorapidity density at midrapidity is found to be 1187 $\pm$ 36 (syst.). The rapidity distribution of charged hadrons is also presented and is found to be independent of rapidity around midrapidity, a feature that is not predicted by Monte Carlo event generators. Comparisons of per-participant charged-particle multiplicities between xenon-xenon and lead-lead collisions at similar energies show that particle production is driven by the collision geometry instead of the system size.

Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 799 (2019) 135049. The superseded preliminary plots can be found here.

Figures
png pdf	Figure 1: ${\Delta r}$ distributions for tracklets reconstructed from the two layers of the BPIX closest to the beam pipe. The distributions are normalised by the number of tracklets. The spectrum in collision data (black squares) is compared to the spectra obtained from fully simulated events generated by the EPOS [7] (LHC tune [8]), HYDJET 1.9 [9], and AMPT 1.26t5 [10] event generators. The statistical uncertainties are smaller than the marker sizes for all distributions shown.
png pdf	Figure 2: (Left) Averaged and symmetrised ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ results (grey squares) in XeXe collisions at ${\sqrt {s_{_{\mathrm {NN}}}}} =$ 5.44 TeV, for events in the 0-80% centrality interval. Predictions from the EPOS [7] (LHC tune [8]), HYDJET 1.9 [9], and AMPT 1.26t5 [10] event generators are also shown in comparison. (Right) ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ in XeXe collisions for events with 0-5% (red squares) and 50-55% (blue squares) centrality, shown in comparison to predictions from the EPOS, HYDJET, and AMPT event generators. The ratios of the ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ distributions for events in the 0-5% centrality interval to those in the 50-55% centrality interval, normalised to unity at midrapidity, are shown in the bottom panel. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible.
png pdf	Figure 2-a: Averaged and symmetrised ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ results (grey squares) in XeXe collisions at ${\sqrt {s_{_{\mathrm {NN}}}}} =$ 5.44 TeV, for events in the 0-80% centrality interval. Predictions from the EPOS [7] (LHC tune [8]), HYDJET 1.9 [9], and AMPT 1.26t5 [10] event generators are also shown in comparison. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible.
png pdf	Figure 2-b: ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ in XeXe collisions for events with 0-5% (red squares) and 50-55% (blue squares) centrality, shown in comparison to predictions from the EPOS, HYDJET, and AMPT event generators. The ratios of the ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ distributions for events in the 0-5% centrality interval to those in the 50-55% centrality interval, normalised to unity at midrapidity, are shown in the bottom panel. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible.
png pdf	Figure 3: Charged-hadron ${\mathrm {d}N/\mathrm {d}y}$ in XeXe collisions at ${\sqrt {s_{_{\mathrm {NN}}}}} =$ 5.44 TeV for events with 0-80% centrality (grey squares). The band around the data points denotes the total systematic uncertainties, while the statistical uncertainties are negligible. Predictions from the EPOS [7] (LHC tune [8]), HYDJET 1.9 [9], and AMPT 1.26t5 [10] event generators are also shown in comparison.
png pdf	Figure 4: Charged hadron ${\mathrm {d}N/\mathrm {d}\eta}$ in XeXe collisions at ${\sqrt {s_{_{\mathrm {NN}}}}} =$ 5.44 TeV at midrapidity as a function of event centrality, shown in intervals of 5%. The results are compared to previous measurements in PbPb collisions at ${\sqrt {s_{_{\mathrm {NN}}}}} =$ 2.76 and 5.02 TeV by the CMS [23] and ALICE [32,33] collaborations. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible.
png pdf	Figure 5: Average ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ at midrapidity normalised by $< N_{\mathrm {part}} >$, shown as a function of $< N_{\mathrm {part}} >$ (left) and $< N_{\mathrm {part}} > /2A$ (right), where $A$ is the atomic number of the nuclei. The results are compared to previous measurements in PbPb collisions by the CMS [23] and ALICE [32,33] collaborations. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible.
png pdf	Figure 5-a: Average ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ at midrapidity normalised by $< N_{\mathrm {part}} >$, shown as a function of $< N_{\mathrm {part}} >$, where $A$ is the atomic number of the nuclei. The results are compared to previous measurements in PbPb collisions by the CMS [23] and ALICE [32,33] collaborations. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible.
png pdf	Figure 5-b: Average ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ at midrapidity normalised by $< N_{\mathrm {part}} >$, shown as a function of $< N_{\mathrm {part}} > /2A$, where $A$ is the atomic number of the nuclei. The results are compared to previous measurements in PbPb collisions by the CMS [23] and ALICE [32,33] collaborations. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible.
png pdf	Figure 6: ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}y}$ distributions for PbPb collisions at 2.76 TeV in the 0-5% centrality interval, obtained by applying a Jacobian transformation to the existing measurement of ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ by the CMS Collaboration [23]. The band around the data points denote the systematic uncertainty, while the statistical uncertainties are negligible. The ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}y}$ distribution is compared to that measured by the ALICE Collaboration [29].

Tables
png pdf	Table 1: Summary of systematic uncertainties

Summary

The pseudorapidity distributions of charged hadrons in xenon-xenon collisions at a centre-of-mass energy of 5.44 TeV per nucleon pair are reported. The charged-hadron pseudorapidity density, ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ , at midrapidity in the 0-5% centrality interval is measured to be 1187 $\pm$ 36 (syst.). The charged-hadron rapidity density is also presented, and is found to be consistent with a flat rapidity plateau in a region around midrapidity, $| y | <$ 1. The results are compared to predictions from the EPOS (LHC tune), HYDJET 1.9, and AMPT 1.26t5 event generators. None of the event generators are able to fully describe the data in terms of the magnitude, pseudorapidity dependence, and centrality dependence of the ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ distributions, although EPOS describes well the pseudorapidity dependence and its centrality dependence. The per-participant ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ at midrapidity is observed to be larger in xenon-xenon than in lead-lead collisions at a similar energy for events with a similar number of participating nucleons, but is found to be consistent when events with similar fractional overlap are compared instead. This result demonstrates that final-state charged-hadron multiplicities are determined by the collision geometry and not the system size. These results provide important constraints on particle production mechanisms and the initial conditions in heavy ion collisions at LHC energies for models and event generators to describe the production and evolution of the strongly interacting medium in such collisions.

Additional Figures
png pdf	Additional Figure 1: Charged-hadron ${\mathrm {d}N/\mathrm {d}\eta}$ in XeXe collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.44 TeV for events with 0-5% centrality (red squares). The band around the data points denotes the total systematic uncertainties, while the statistical uncertainties are negligible. Predictions from the Epos [1] (LHC tune [2]), Hydjet 1.9 [3], and Ampt 1.26t5 [4] event generators are also shown in comparison.
png pdf	Additional Figure 2: Charged-hadron ${\mathrm {d}N/\mathrm {d}\eta}$ in XeXe collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.44 TeV for events with 50-55% centrality (blue squares). The band around the data points denotes the total systematic uncertainties, while the statistical uncertainties are negligible. Predictions from the Epos [1] (LHC tune [2]), Hydjet 1.9 [3], and Ampt 1.26t5 [4] event generators are also shown in comparison.
png pdf	Additional Figure 3: Charged-hadron ${\mathrm {d}N/\mathrm {d}y}$ in XeXe collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.44 TeV for events with 0-5% centrality (red squares). The band around the data points denotes the total systematic uncertainties, while the statistical uncertainties are negligible. Predictions from the Epos [1] (LHC tune [2]), Hydjet 1.9 [3], and Ampt 1.26t5 [4] event generators are also shown in comparison.
png pdf	Additional Figure 4: Charged-hadron ${\mathrm {d}N/\mathrm {d}y}$ in XeXe collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.44 TeV for events with 50-55% centrality (blue squares). The band around the data points denotes the total systematic uncertainties, while the statistical uncertainties are negligible. Predictions from the Epos [1] (LHC tune [2]), Hydjet 1.9 [3], and Ampt 1.26t5 [4] event generators are also shown in comparison.
png pdf	Additional Figure 5: Charged-hadron ${\mathrm {d}N/\mathrm {d}y}$ in XeXe collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.44 TeV for events with 0-5% centrality (red squares). The band around the data points denotes the total systematic uncertainties, while the statistical uncertainties are negligible. The data are compared to measurements of ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}y}$ in PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 2.76 and 5.02 TeV by the ALICE Collaboration [5,6], as well as in AuAu collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 200 GeV by the BRAHMS Collaboration [7].
png pdf	Additional Figure 6: Average ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ at midrapidity normalised by $< N_{\mathrm {part}} >$, shown as a function of $< N_{\mathrm {part}} >$. The results are compared to previous measurements in PbPb collisions by the CMS [8] and ALICE [9,10] collaborations, as well as measurements in CuCu and AuAu collisions by the PHOBOS [11] and BRAHMS [12] collaborations at RHIC. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible.
png pdf	Additional Figure 7: Average ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ at midrapidity normalised by $2A$, where $A$ is the total number of nucleons in one of the colliding nuclei, shown as a function of $< N_{\mathrm {part}} >$. The results are compared to previous measurements in PbPb collisions by the CMS [8] and ALICE [9,10] collaborations. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible.
png pdf	Additional Figure 8: Average ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ at midrapidity normalised by $2A$, where $A$ is the total number of nucleons in one of the colliding nuclei, shown as a function of $< N_{\mathrm {part}} > /2A$. The results are compared to previous measurements in PbPb collisions by the CMS [8] and ALICE [9,10] collaborations. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible.
png pdf	Additional Figure 9: Average ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ at midrapidity normalised by $< N_{\mathrm {part}} >$, shown as a function of $< N_{\mathrm {part}} >$. The normalised ${\mathrm {d}N_{\mathrm {ch}}/\mathrm {d}\eta}$ values are obtained from simulations generated with the Epos [1] (LHC tune [2]) event generator and are shown for two event selections. The blue circles correspond to events selected using the experimental definition of centrality, i.e., selected based on the sum of transverse energy in the HF calorimeters. The red squares correspond to events weighted such that the ${N_{\mathrm {part}}}$ distribution is identical to those selected by the former method, at each point. The ratio of the two values, which corresponds to the bias introduced when selecting events based on the sum of transverse energy in the forward region, is shown in the bottom panel.

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