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| CMS-HIN-14-014 ; CERN-EP-2018-268 | ||
| Centrality and pseudorapidity dependence of the transverse energy density in pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 13 October 2018 | ||
| Phys. Rev. C 100 (2019) 024902 | ||
| Abstract: The almost hermetic coverage of the CMS detector is used to measure the distribution of transverse energy, $ {E_{\mathrm{T}}} $, over 13.2 units of pseudorapidity, $\eta$, for pPb collisions at a center-of-mass energy per nucleon pair of ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The huge angular acceptance exploits the fact that the CASTOR calorimeter at $-6.6 < \eta < -5.2$ is effectively present on both sides of the colliding system because of a switch in the proton-going and lead-going beam directions. This wide acceptance enables the study of correlations between well-separated angular regions and makes the measurement a particularly powerful test of event generators. For minimum bias pPb collisions the maximum value of ${\mathrm{d}}{E_{\mathrm{T}}}/{\mathrm{d}}\eta$ is 22 GeV, which implies an $ {E_{\mathrm{T}}} $ per participant nucleon pair comparable to that of peripheral PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 2.76 TeV. The increase of ${\mathrm{d}}{E_{\mathrm{T}}}/{\mathrm{d}}\eta$ with centrality is much stronger for the lead-going side than for the proton-going side. The $\eta$ dependence of ${\mathrm{d}}{E_{\mathrm{T}}}/{\mathrm{d}}\eta$ is sensitive to the $\eta$ range in which the centrality variable is defined. Several modern generators are compared to these results but none is able to capture all aspects of the $\eta$ and centrality dependence of the data and the correlations observed between different $\eta$ regions. | ||
| Links: e-print arXiv:1810.05745 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Transverse energy density versus $\eta $ from minimum bias pPb collisions at. at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The proton is moving towards positive $\eta $. The statistical uncertainties are smaller than the size of the data points and the total errors are dominated by the systematics. The systematic uncertainties are largely correlated point to point within the central and with the HF regions and so shown by gray bands there. The systematic uncertainties for the most forward and backward data points i.e. $\eta = {\pm}5.9$ are uncorrelated with those of central and HF regions and so are shown as vertical bars. Predictions from the EPOS-LHC (red solid), QGSJET-II (green dashed), and hijing (blue dotted) event generators are also shown. |
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Figure 2:
Transverse energy density per participating nucleon-nucleon pair evaluated at $\eta _\text {cm}$ versus $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} $ for minimum bias pAu, pU, dAu, and pPb collisions. The uncertainties are generally smaller than the size of the data points. Also shown are the corresponding results for central AuAu and PbPb collisions, as well as simulation for minimum bias pPb collisions from three event generators [13,3,37,35,36,38,39,40,41,42,43,44,45]. |
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Figure 3:
Transverse energy density versus $\eta $ and centrality from 5.02 TeV pPb collisions for the HF-Double (left), HF-Single (center), and $N_\text {track}$ (right) centrality definitions for data and for predictions from the EPOS-LHC, QGSJET-II, and hijing event generators, for 0-10% (upper), 40-50% (middle), and 70-80% (lower) central collisions. The uncertainties are dominated by the systematic components, which are largely correlated point-to-point in the central region and in HF, and which are shown by gray bands there. |
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Figure 4:
Transverse energy density per participating nucleon-nucleon pair versus $N_\text {part}$ for different $\eta $ ranges. The HF-Single method was used to define centrality. The total experimental uncertainties are shown by gray bands. |
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Figure 5:
Ratio of peripheral to central ${E_{\mathrm {T}}}$ production, $S_\text {PC}$, as a function of $\eta $ for three centrality ranges for HF-Double (left), HF-Single (middle), and $N_\text {track}$ (right) for data, and for the EPOS-LHC, QGSJET-II, and hijing event generators. The systematic uncertainties are dominant and are of comparable size to the data points. |
| Tables | |
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Table 1:
Systematic uncertainties in $ {\mathrm {d}} {E_{\mathrm {T}}} / {\mathrm {d}}\eta $ and $S_\text {PC}$ for the tracker region, the HF region, and the CASTOR region as a function of centrality defined by HF-Double. The $S_\text {PC}$ ratio is by construction unity for 0-10% centrality and is not defined for minimum bias events. |
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Table 2:
Values of exponents from fitting the energy dependence of $ {\mathrm {d}}N^\pm / {\mathrm {d}}\eta $ [34] and $ {\mathrm {d}} {E_{\mathrm {T}}} / {\mathrm {d}}\eta $ at midrapidity to a function of the form $s_{\text {NN}}^\gamma $ for minimum bias proton-nucleus and central nucleus-nucleus collisions. |
| Summary |
|
In this paper we report the centrality and pseudorapidity ($\eta$) dependence of transverse energy ($ {E_{\mathrm{T}}} $) production from pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV over 13.2 units of $\eta$. The $ {E_{\mathrm{T}}} $ per participant pair in minimum bias pPb events at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV is comparable to that of peripheral PbPb collisions at 2.76 TeV. At midrapidity the energy density at a proper time $\tau_0= $ 1 fm/$c$ is of order of 4.5 GeV/fm$^3$ for the top 10% most central pPb collisions, which is comparable to those observed in PbPb collisions. As the centrality of the collision increases, the total $ {E_{\mathrm{T}}} $ increases dramatically and the mean $\eta$ of the $ {E_{\mathrm{T}}} $ distribution moves towards the lead-going side of the collision. For central collisions, the peak of ${\mathrm{d}}{E_{\mathrm{T}}}/{\mathrm{d}}\eta$ is 1.4 units below the center-of-mass rapidity. This pseudorapidity shift is almost the same as for pU collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 20 GeV. The EPOS-LHC event generator gives a good description of the minimum bias ${\mathrm{d}}{E_{\mathrm{T}}}/{\mathrm{d}}\eta$ distribution and peaks at an $\eta$ value close to that of the data for all centralities. The centrality dependence of $ {E_{\mathrm{T}}} $ production for QGSJET-II is stronger than that of the data. This model is below the data for 70-80% peripheral events and almost a factor of two above the data for the 10% most central events. Near midrapidity the hijing generator tends to underestimate the magnitude of ${\mathrm{d}}{E_{\mathrm{T}}}/{\mathrm{d}}\eta$ and for central collisions predicts a peak that is at significantly lower $\eta$ than in the data. Similarly to what has been seen in particle production at lower energy [52], the ${\mathrm{d}}{E_{\mathrm{T}}}/{\mathrm{d}}\eta$ per participating nucleon-nucleon pair increases with the number of nucleons that participate in the collisions ($N_\text{part}$) for $\eta$ values on the lead side; it is rather independent of $N_\text{part}$ near midrapidity; and it decreases with $N_\text{part}$ for $\eta$ values on the proton side. The $\eta$ region used to define centrality has a strong impact on the nature of the events selected. There is a significant autocorrelation of the $\eta$ range used to define centrality with ${\mathrm{d}}{E_{\mathrm{T}}}/{\mathrm{d}}\eta$ both for data, and the EPOS-LHC, QGSJET-II and hijing event generators. None of the tested event generators are able to capture all aspects of the autocorrelations seen in data. It is clear that cosmic ray event generators have difficulties modeling proton-lead collisions. While the proton-lead system is significantly larger than the proton-nitrogen and proton-oxygen collisions occurring in air showers, these data illustrate the need for a better understanding of nuclear effects. Ultimately, protons colliding with light nuclei would be most valuable for this purpose. |
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