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| CMS-HIN-23-007 ; CERN-EP-2024-118 | ||
| Pseudorapidity distributions of charged hadrons in lead-lead collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV | ||
| CMS Collaboration | ||
| 1 September 2024 | ||
| Submitted to Phys. Lett. B | ||
| Abstract: The pseudorapidity ($ \eta $) distributions of charged hadrons are measured using data collected at the highest ever nucleon-nucleon center-of-mass energy of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV for collisions of lead-lead ions. The data were recorded by the CMS experiment at the LHC in 2022 and correspond to an integrated luminosity of 0.30 $ \pm $ 0.03 $ \mu$b$^{-1} $. Using the CMS silicon pixel detector, the yields of primary charged hadrons produced in the range $ |\eta| < $ 2.6 are reported. The evolution of the midrapidity particle density as a function of collision centrality is also reported. In the 5% most central collisions, the charged-hadron $ \eta $ density in the range $ |\eta| < $ 0.5 is found to be 2032 $ \pm $ 91 (syst), with negligible statistical uncertainty. This result is consistent with an extrapolation from nucleus-nucleus collision data at lower center-of-mass energies. Comparisons are made to various Monte Carlo event generators and to previous measurements of lead-lead and xenon-xenon collisions at similar collision energies. These new data detail the dependence of particle production on the collision energy, initial collision geometry, and the size of the colliding nuclei. | ||
| Links: e-print arXiv:2409.00838 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ distributions in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV for events in the 0$-$80% centrality class (left) and in the 0$-$5 and 50$-$55% centrality classes (right). The results have been averaged and symmetrized around $ \eta= $ 0. Predictions from the HYDJET 1.9 [16], AMPT 1.26t5 [35], and EPOS LHC v3400 event generators are also displayed. The ratios of the $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ distributions of simulation and data, normalized to unity at midrapidity, are shown in the bottom panel. The gray bands denote the total systematic uncertainties and the statistical uncertainties are negligible. In the ratio panels, the uncertainty band displayed represents the relative uncertainty of the data. |
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Figure 1-a:
The $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ distributions in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV for events in the 0$-$80% centrality class (left) and in the 0$-$5 and 50$-$55% centrality classes (right). The results have been averaged and symmetrized around $ \eta= $ 0. Predictions from the HYDJET 1.9 [16], AMPT 1.26t5 [35], and EPOS LHC v3400 event generators are also displayed. The ratios of the $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ distributions of simulation and data, normalized to unity at midrapidity, are shown in the bottom panel. The gray bands denote the total systematic uncertainties and the statistical uncertainties are negligible. In the ratio panels, the uncertainty band displayed represents the relative uncertainty of the data. |
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Figure 1-b:
The $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ distributions in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV for events in the 0$-$80% centrality class (left) and in the 0$-$5 and 50$-$55% centrality classes (right). The results have been averaged and symmetrized around $ \eta= $ 0. Predictions from the HYDJET 1.9 [16], AMPT 1.26t5 [35], and EPOS LHC v3400 event generators are also displayed. The ratios of the $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ distributions of simulation and data, normalized to unity at midrapidity, are shown in the bottom panel. The gray bands denote the total systematic uncertainties and the statistical uncertainties are negligible. In the ratio panels, the uncertainty band displayed represents the relative uncertainty of the data. |
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Figure 2:
Charged-hadron $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV at midrapidity as a function of event centrality, shown as is (left) and normalized by 2 $ A $ (right), where $ A $ is the atomic number of the nuclei. The results are compared to measurements in PbPb and XeXe collisions by the CMS [12,9] and ALICE [41,13,10] Collaborations, and to measurements in CuCu and AuAu collisions by the PHOBOS Collaboration [42]. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible. |
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Figure 2-a:
Charged-hadron $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV at midrapidity as a function of event centrality, shown as is (left) and normalized by 2 $ A $ (right), where $ A $ is the atomic number of the nuclei. The results are compared to measurements in PbPb and XeXe collisions by the CMS [12,9] and ALICE [41,13,10] Collaborations, and to measurements in CuCu and AuAu collisions by the PHOBOS Collaboration [42]. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible. |
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Figure 2-b:
Charged-hadron $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV at midrapidity as a function of event centrality, shown as is (left) and normalized by 2 $ A $ (right), where $ A $ is the atomic number of the nuclei. The results are compared to measurements in PbPb and XeXe collisions by the CMS [12,9] and ALICE [41,13,10] Collaborations, and to measurements in CuCu and AuAu collisions by the PHOBOS Collaboration [42]. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible. |
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Figure 3:
Average $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity normalized by $ \langle N_{\text{part}} \rangle $, shown as a function of $ \langle N_{\text{part}} \rangle $ (left) and $ \langle N_{\text{part}} \rangle/2A $ (right), where $ A $ is the atomic number of the nuclei. The results are compared to measurements in PbPb and XeXe collisions by the CMS [12,9], ALICE [41,13,10] Collaborations, and to measurements in CuCu and AuAu collisions by the PHOBOS Collaboration [42]. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible. |
png pdf |
Figure 3-a:
Average $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity normalized by $ \langle N_{\text{part}} \rangle $, shown as a function of $ \langle N_{\text{part}} \rangle $ (left) and $ \langle N_{\text{part}} \rangle/2A $ (right), where $ A $ is the atomic number of the nuclei. The results are compared to measurements in PbPb and XeXe collisions by the CMS [12,9], ALICE [41,13,10] Collaborations, and to measurements in CuCu and AuAu collisions by the PHOBOS Collaboration [42]. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible. |
png pdf |
Figure 3-b:
Average $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity normalized by $ \langle N_{\text{part}} \rangle $, shown as a function of $ \langle N_{\text{part}} \rangle $ (left) and $ \langle N_{\text{part}} \rangle/2A $ (right), where $ A $ is the atomic number of the nuclei. The results are compared to measurements in PbPb and XeXe collisions by the CMS [12,9], ALICE [41,13,10] Collaborations, and to measurements in CuCu and AuAu collisions by the PHOBOS Collaboration [42]. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible. |
png pdf |
Figure 4:
Comparison of average $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity, scaled by $ \langle N_{\text{part}} \rangle $ in pPb [43,22], pAu [44], dAu [45,46,47] (pA) and central heavy ion collisions [12,42,48,49,50,8,51,52,53,41,54,46,55,56], as well as non-single-diffractive (NSD) [17,18,56,57,58,59] and inelastic [21,42,60,61] pp collisions. The data points for nucleus-nucleus (AA) collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 2.76 TeV have been shifted horizontally for visibility. The dashed curves, reproduced from Ref. [22], are included to guide the eye, and correspond to a power law functional form. |
| Tables | |
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Table 1:
Sources of systematic uncertainty affecting the measurement of charged-hadron multiplicities as a function of $ \langle N_{\text{part}} \rangle $ in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV. |
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Table 2:
Centrality intervals and corresponding $ \langle N_{\text{part}} \rangle $ values for 5.36 TeV PbPb collisions. The uncertainties in the $ N_{\text{part}} $ values are determined by propagating the uncertainties in the parameters of the Glauber model. |
| Summary |
| The pseudorapidity ($ \eta $) distributions of charged hadrons are measured in the range $ |\eta| < $ 2.6 for multiple centrality intervals using data collected in lead-lead (PbPb) collisions at the highest center-of-mass energy of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV. The dependence on $ \eta $ and centrality are compared to the event generators EPOS LHC v3400, HYDJET 1.9, and AMPT 1.26t5; none of these event generators are able to fully describe the measurements in terms of the magnitude, $ \eta $ dependence, and centrality dependence of the $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ distributions. In the 5% most central collisions, the charged-hadron density $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ for the range $ |\eta| < $ 0.5 is found to be 2032 $ \pm $ 91 (syst), with negligible statistical uncertainty. The results at midrapidity are also compared to previous measurements in various collision systems, including PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 2.76 and 5.02 TeV, xenon-xenon collisions at 5.44 TeV, as well as copper-copper and gold-gold collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 200 GeV. These comparisons are new constraints on models and generators which describe multiparticle production in relativistic heavy ion collisions. |
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