CMS-PAS-HIN-22-005

CMS-PAS-HIN-22-005
Net-charge fluctuations in PbPb collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 5.02 TeV
CMS Collaboration
31 July 2023

Abstract: Net-charge fluctuation measurements for lead-lead (PbPb) and proton-proton (pp) collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} $ = 5.02 TeV are presented. The PbPb and pp data were obtained with the CMS detector at the CERN LHC and correspond to integrated luminosities of 0.607 and 252 nb$^{-1}$, respectively. A measure of the relative strength of the correlated pairs is determined for a wide range of relative pseudorapidity (with $ \Delta \eta < $ 4.8) and in different centrality intervals using reconstructed tracks with transverse momenta in the range 0.5-3.0$ \,\text{GeV} $. An excess of opposite sign pairs is observed that increases with the charged-particle multiplicity.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Summary of the measured (left panel) $ \nu_\mathrm{(+-,dyn)} $ as a function of centrality and (right panel) $ \langle N_{\text{ch}}\rangle \nu_\mathrm{(+-,dyn)} $ as a function of the average number of participating nucleons ($ \langle N_\mathrm{part} \rangle $) for PbPb and pp collisions at 5.02 TeV. The horizontal line corresponds to zero net-charge fluctuations. The rectangular open boxes represent the systematic uncertainty and the vertical bars indicate the statistical uncertainty. On the right panel, the predictions from HYDJET [24] and PYTHIA8 [26] event generators in PbPb and pp collisions, respectively, are also shown.
png pdf	Figure 1-a: Summary of the measured (left panel) $ \nu_\mathrm{(+-,dyn)} $ as a function of centrality and (right panel) $ \langle N_{\text{ch}}\rangle \nu_\mathrm{(+-,dyn)} $ as a function of the average number of participating nucleons ($ \langle N_\mathrm{part} \rangle $) for PbPb and pp collisions at 5.02 TeV. The horizontal line corresponds to zero net-charge fluctuations. The rectangular open boxes represent the systematic uncertainty and the vertical bars indicate the statistical uncertainty. On the right panel, the predictions from HYDJET [24] and PYTHIA8 [26] event generators in PbPb and pp collisions, respectively, are also shown.
png pdf	Figure 1-b: Summary of the measured (left panel) $ \nu_\mathrm{(+-,dyn)} $ as a function of centrality and (right panel) $ \langle N_{\text{ch}}\rangle \nu_\mathrm{(+-,dyn)} $ as a function of the average number of participating nucleons ($ \langle N_\mathrm{part} \rangle $) for PbPb and pp collisions at 5.02 TeV. The horizontal line corresponds to zero net-charge fluctuations. The rectangular open boxes represent the systematic uncertainty and the vertical bars indicate the statistical uncertainty. On the right panel, the predictions from HYDJET [24] and PYTHIA8 [26] event generators in PbPb and pp collisions, respectively, are also shown.
png pdf	Figure 2: The measured $ \langle N_{\text{ch}}\rangle \nu_\mathrm{(+-,dyn)} $ as a function of $ \Delta \eta $ for different centrality ranges. The results are presented both in PbPb and pp collisions. The rectangular open boxes represent the systematic uncertainties, while the vertical bars indicate the statistical uncertainties. The dotted and dashed lines indicate the results from HIJING [25] and HYDJET [24] event generators, respectively.
png pdf	Figure 3: $ \langle N_{\text{ch}}\rangle \nu_\mathrm{(+-,dyn)} $ (left $ y $-axis) and $ D $-measure (right $ y $-axis) values as a function of $ \Delta \eta $ for the 0-5% centrality class. The rectangular open boxes represent the systematic uncertainties, while the vertical bars indicate the statistical uncertainties. The theoretical predictions for the HRG [12,13,17,13,16] and QGP [12,13,17,13,16] limits are indicated with the dotted line and band, respectively.

Summary

In this note, the dynamical net-charge fluctuations in terms of $ \nu_\mathrm{(+-,dyn)} $ in lead-lead (PbPb) and proton-proton (pp) collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} $ = 5.02 TeV are reported. The measurement takes advantage of the large pseudorapidity coverage of the CMS detector to, for the first time, determine the dynamical net-charge fluctuations in pseudorapidity windows where the influence of the QGP state is expected to be significant. The $ \nu_\mathrm{(+-,dyn)} $ values are found to become increasingly more negative going from central to peripheral collisions. The $ \nu_\mathrm{(+-,dyn)} $ values are found to be negative in all cases, implying the dominance of the correlation of opposite sign charged particles. We also determine the charged particle multiplicity and $ \Delta\eta $ window dependence of the product $ \nu_\mathrm{(+-,dyn)} $, where this product is related to the charge particle fluctuations per entropy. We find the $ \langle N_\mathrm{ch} \rangle \nu_\mathrm{(+-,dyn)} $ values decrease monotonically with increasing relative pseudorapidity window ($ \Delta \eta $) for all centrality ranges. Firm conclusions based on the current experimental results require a better understanding of how global charge conservation affects the fluctuation observables.

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