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| CMS-HIN-16-021 ; CERN-EP-2017-261 | ||
| Pseudorapidity distributions of charged hadrons in proton-lead collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 and 8.16 TeV | ||
| CMS Collaboration | ||
| 26 October 2017 | ||
| JHEP 01 (2018) 045 | ||
| Abstract: The pseudorapidity distributions of charged hadrons in proton-lead collisions at nucleon-nucleon center-of-mass energies ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 and 8.16 TeV are presented. The measurements are based on data samples collected by the CMS experiment at the LHC. The number of primary charged hadrons produced in non-single-diffractive proton-lead collisions is determined in the pseudorapidity range $| \eta_{\text{lab}} | < $ 2.4. The charged-hadron multiplicity distributions are compared to the predictions from theoretical calculations and Monte Carlo event generators. In the center-of-mass pseudorapidity range $| \eta_{\text{cm}} | < $ 0.5, the average charged-hadron multiplicity densities ${<} \mathrm{d} N_{\text{ch}} / \mathrm{d} \eta_{\text{cm}} {>} | _ {| \eta_{\text{cm}} | < 0.5 }$ are 17.31 $\pm$ 0.01 (stat) $\pm$ 0.59 (syst) and 20.10 $\pm$ 0.01 (stat) $\pm$ 0.85 (syst) at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 and 8.16 TeV, respectively. The particle densities per participant nucleon are compared to similar measurements in proton-proton, proton-nucleus, and nucleus-nucleus collisions. | ||
| Links: e-print arXiv:1710.09355 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The $\Delta \eta $ (a) and $\Delta \phi $ (b, c) distributions of hit pairs for tracklets in pPb collisions at 8.16 TeV (squares) and from MC simulations with the EPOS and HIJING 1.3 generators (solid lines). The statistical uncertainties are smaller than the marker sizes for all distributions shown. |
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Figure 1-a:
The $\Delta \eta $ distribution of hit pairs for tracklets in pPb collisions at 8.16 TeV (squares) and from MC simulations with the EPOS and HIJING 1.3 generators (solid lines). The statistical uncertainties are smaller than the marker sizes for all distributions shown. |
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Figure 1-b:
The $\Delta \phi $ distribution of hit pairs for tracklets in pPb collisions at 8.16 TeV (squares) and from MC simulations with the EPOS and HIJING 1.3 generators (solid lines). The statistical uncertainties are smaller than the marker sizes for all distributions shown. |
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Figure 1-c:
The $\Delta \phi $ distribution of hit pairs for tracklets in pPb collisions at 8.16 TeV (squares) and from MC simulations with the EPOS and HIJING 1.3 generators (solid lines). The statistical uncertainties are smaller than the marker sizes for all distributions shown. |
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Figure 2:
Distributions of the pseudorapidity density of charged hadrons in the region $ | {\eta _{\text {lab}}} | < 2.4$ in NSD pPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 (open squares) and 8.16 TeV (full squares). The measurement at 5.02 TeV by the ALICE Collaboration [30] is shown as filled circles. The shaded boxes indicate the systematic uncertainties which, in the case of the CMS data, are correlated between the two beam energies. The proton beam goes in the positive ${\eta _{\text {lab}}}$ direction. |
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Figure 3:
Distributions of the pseudorapidity density of charged hadrons in the region $ | {\eta _{\text {lab}}} | < $ 2.4 for NSD pPb collisions at 8.16 TeV (squares) compared to predictions from the MC event generators EPOS LHC [27,17] (v3400), HIJING [14] (versions 1.3 [15] and 2.1 [12]), and DPMJET-III [16], as well as from the KLN model [11]. The shaded boxes around the data points indicate their systematic uncertainties. The proton beam goes in the positive ${\eta _{\text {lab}}} $ direction. |
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Figure 4:
Comparison of the measured $ {{\mathrm {d}}{}N_{\text {ch}}/ {\mathrm {d}}\eta _{\text {cm}}} $ at midrapidity, scaled by the number of participating nucleons (${N_{\text {part}}}$) in pPb [30,51], pAu [52], dAu [33,48,53] and central heavy ion collisions [37,38,39,40,41,42,43,44,45,46,47,26,48,49,50], as well as NSD [54,55,56,50,57,22,23] and inelastic [58,56,59,37,25] pp collisions. The AA data points at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 2.76 TeV have been shifted horizontally for visibility. The dashed curves, included to guide the eye, correspond to a fit to the data points using the same functional form as in Refs. [59,46]. |
| Tables | |
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Table 1:
Summary of the systematic uncertainties from various sources, for pPb collisions at 5.02 and 8.16 TeV. The range of values indicates the minimum and maximum uncertainties across the ${\eta _{\text {lab}}}$ range. |
| Summary |
| The pseudorapidity distributions of primary charged hadrons have been measured by the CMS experiment at the LHC in proton-lead collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 and 8.16 TeV. Based on pairs of pixel clusters from two different layers of the barrel region of the CMS pixel detector, the distributions have been obtained for NSD pPb events at both collision energies. The measured $\mathrm{d} N / \mathrm{d} \eta_{\text{lab}}$ distribution at 5.02 TeV is consistent with published results by the ALICE Collaboration. At 8.16 TeV, the measured $\mathrm{d} N / \mathrm{d} \eta_{\text{lab}} $ distribution is higher than the predictions of EPOS LHC, but significantly lower than the predictions from the HIJING 1.3 and DPMJET-III event generators. At $\eta_{\text{lab}} \approx $ 0, the measured distributions are in good agreement with calculations from the KLN gluon saturation model and predictions from the HIJING 2.1 event generator with the effects of gluon shadowing included. The charged-hadron multiplicity densities in the nucleon-nucleon center-of-mass frame, $ {<} \mathrm{d} N_{\text{ch}} / \mathrm{d} \eta_{\text{cm}} {>} | _ {| \eta_{\text{cm}} | < 0.5 } $, are 17.31 $\pm$ 0.01 (stat) $\pm$ 0.59 (syst) and 20.10 $\pm$ 0.01 (stat) $\pm$ 0.85 (syst) at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 and 8.16 TeV, respectively. When comparing the average charged-particle density per participant nucleon for pp, pA, and NN collisions as a function of collision energy, the pA results are found to be below those in central NN collisions and NSD pp collisions, but coincide with the trend seen in inelastic pp collisions. These results represent the first measurement of hadron production at this new center-of-mass energy frontier in nuclear collisions, and provide constraints for the understanding of nonperturbative QCD effects in high-energy nuclear collisions. |
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