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| CMS-PAS-HIN-23-007 | ||
| Pseudorapidity distributions of charged hadrons in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV | ||
| CMS Collaboration | ||
| 18 September 2023 | ||
| Abstract: The pseudorapidity ($ \eta $) distributions of charged hadrons are measured using data collected at a record nucleon-nucleon center-of-mass energy of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.36 TeV for collisions of lead-lead (PbPb) ions. The data were recorded by the CMS experiment at the LHC in 2022 and correspond to an integrated luminosity of 0.3 $ \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 pseudorapidity density is found to be 2032 $ \pm $ 91 (syst) in the range $ |\eta| < $ 0.5, with negligible statistical uncertainty. This result is consistent with an extrapolation from lower energy nucleus-nucleus collision data. Comparisons are made to various Monte Carlo event generators, and to previous measurements of PbPb and xenon-xenon collisions at similar collision energies. These observations detail the dependence of midrapidity particle production on the initial collision geometry, collision energy, and system size. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The $ \mathrm{d} N_{\mathrm{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 class (right). The results have been averaged and symmetrized around $ \eta= $ 0. Predictions from the HYDJET 1.9 [18], AMPT 1.26t5 [36], and EPOS LHC v3400 [35,17] event generators are also displayed. The ratios of the $ \mathrm{d} N_{\mathrm{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, while the statistical uncertainties are negligible. |
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Figure 1-a:
The $ \mathrm{d} N_{\mathrm{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 class (right). The results have been averaged and symmetrized around $ \eta= $ 0. Predictions from the HYDJET 1.9 [18], AMPT 1.26t5 [36], and EPOS LHC v3400 [35,17] event generators are also displayed. The ratios of the $ \mathrm{d} N_{\mathrm{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, while the statistical uncertainties are negligible. |
png pdf |
Figure 1-b:
The $ \mathrm{d} N_{\mathrm{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 class (right). The results have been averaged and symmetrized around $ \eta= $ 0. Predictions from the HYDJET 1.9 [18], AMPT 1.26t5 [36], and EPOS LHC v3400 [35,17] event generators are also displayed. The ratios of the $ \mathrm{d} N_{\mathrm{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, while the statistical uncertainties are negligible. |
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Figure 2:
Charged-hadron $ \mathrm{d} N_{\mathrm{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 [15,11] and ALICE [42,14,12] Collaborations, and to measurements in CuCu and AuAu collisions by the PHOBOS Collaboration [43]. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible. |
png pdf |
Figure 2-a:
Charged-hadron $ \mathrm{d} N_{\mathrm{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 [15,11] and ALICE [42,14,12] Collaborations, and to measurements in CuCu and AuAu collisions by the PHOBOS Collaboration [43]. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible. |
png pdf |
Figure 2-b:
Charged-hadron $ \mathrm{d} N_{\mathrm{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 [15,11] and ALICE [42,14,12] Collaborations, and to measurements in CuCu and AuAu collisions by the PHOBOS Collaboration [43]. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible. |
png pdf |
Figure 3:
Average $ \mathrm{d} N_{\mathrm{ch}}/\mathrm{d} \eta $ at midrapidity normalized by $ \langle N_{\mathrm{part}} \rangle $, shown as a function of $ \langle N_{\mathrm{part}} \rangle $ (left) and $ \langle N_{\mathrm{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 [15,11] |
png pdf |
Figure 3-a:
Average $ \mathrm{d} N_{\mathrm{ch}}/\mathrm{d} \eta $ at midrapidity normalized by $ \langle N_{\mathrm{part}} \rangle $, shown as a function of $ \langle N_{\mathrm{part}} \rangle $ (left) and $ \langle N_{\mathrm{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 [15,11] |
png pdf |
Figure 3-b:
Average $ \mathrm{d} N_{\mathrm{ch}}/\mathrm{d} \eta $ at midrapidity normalized by $ \langle N_{\mathrm{part}} \rangle $, shown as a function of $ \langle N_{\mathrm{part}} \rangle $ (left) and $ \langle N_{\mathrm{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 [15,11] |
png pdf |
Figure 4:
Comparison of average $ \mathrm{d} N_{\mathrm{ch}}/\mathrm{d} \eta $ at midrapidity, scaled by $ \langle N_{\mathrm{part}} \rangle $ in pPb [44.24], pAu [45], dAu [46.47.48] and central heavy ion collisions [15.43.49.50.51.10.52.53.54.42.55.47.56.57], as well as non-single-diffractive (NSD) [19.20.57.58.59.60] and inelastic [23.43.61.62] 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. [24], 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 and $ \langle N_{\mathrm{part}} \rangle $ in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV. |
png pdf |
Table 2:
Centrality intervals and corresponding $ \langle N_{\mathrm{part}} \rangle $ values for 5.36 TeV PbPb collisions. The uncertainties in the $ N_{\mathrm{part}} $ values are determined by propagating the uncertainties in the parameters of the Glauber model, which are listed in Section 3. |
| Summary |
| The pseudorapidity ($ \eta $) distributions of charged hadrons are measured using data collected in lead-lead collisions of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV. In the 5% most central collisions, the charged-hadron pseudorapidity $ \eta $ density is found to be 2032 $ \pm $ 91 (syst) for the range $ |\eta| < $ 0.5, with negligible statistical uncertainty. Comparisons are made to predictions from the EPOS LHC v3400, HYDJET 1.9, and AMPT 1.26t5 event generators. None of the event generators are able to fully describe the measurements in terms of the magnitude, $ \eta $ dependence, and centrality dependence of the $ \mathrm{d} N_{\mathrm{ch}}/\mathrm{d} \eta $ distributions. The results are compared to previous measurements in various collision systems, including PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 2.76 and 5.02 TeV, XeXe collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.44 TeV and CuCu and AuAu collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 200 GeV, and show that the $ \mathrm{d} N_{\mathrm{ch}}/\mathrm{d} \eta $ at midrapidity is a function of the collision geometry after normalizing by 2 $ A $, where $ A $ is the atomic number of the nuclei. These results provide important constraints on models and generators, which describe multiparticle production in relativistic heavy ion collisions at a record collision energy. |
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