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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
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.
Figures & Tables Summary References CMS Publications

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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.

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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.
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.
1 W. Busza, K. Rajagopal, and W. van der Schee Heavy ion collisions: The big picture, and the big questions 1802.04801
2 J. D. Bjorken Highly relativistic nucleus-nucleus collisions: The central rapidity region PRD 27 (1983) 140
3 CMS Collaboration Measurement of the pseudorapidity and centrality dependence of the transverse energy density in PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}= $ 2.76 TeV PRL 109 (2012) 152303 CMS-HIN-11-003
4 F. Karsch Lattice QCD at high temperature and density in Lectures on Quark Matter, p. 209 Springer, Berlin, Heidelberg, 2002 Lecture Notes in Physics, volume 583 hep-lat/0106019
5 CMS Collaboration Evidence for collective multiparticle correlations in p-Pb Collisions PRL 115 (2015) 012301 CMS-HIN-14-006
6 ATLAS Collaboration Measurement with the ATLAS detector of multi-particle azimuthal correlations in p+Pb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV PLB 725 (2013) 60 1303.2084
7 ALICE Collaboration Search for collectivity with azimuthal $\mathrm{J}/ \psi $-hadron correlations in high multiplicity p-Pb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 and 8.16 TeV PLB 780 (2018) 7 1709.06807
8 CMS Collaboration Evidence for collectivity in pp collisions at the LHC PLB 765 (2017) 193 CMS-HIN-16-010
9 ATLAS Collaboration Measurement of multi-particle azimuthal correlations in pp, p+Pb and low-multiplicity Pb+Pb collisions with the ATLAS detector EPJC 77 (2017) 428 1705.04176
10 ALICE Collaboration Multi-strange baryon production in p-Pb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}= $ 5.02 TeV PLB 758 (2016) 389 1512.07227
11 E-802 Collaboration Measurement of energy emission from O+A and p+A collisions at 14.5 GeV/$c$ per nucleon with a lead glass Array PLB 197 (1987) 285
12 E814 Collaboration Transverse energy and charged particle multiplicity in p nucleus collisions at 14.6 GeV/$c$ PRC 52 (1995) 2028
13 HELIOS Collaboration Transverse energy measurements in proton - nucleus interactions at high-energy Z. Phys. C 58 (1993) 239
14 PHENIX Collaboration Transverse energy production and charged-particle multiplicity at midrapidity in various systems from $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}= $ 7.7 to 200 GeV PRC 93 (2016) 024901 1509.06727
15 H. Sorge, R. Mattiello, H. Stocker, and W. Greiner Energy and baryon flow in nuclear collisions at 15 A GeV PRL 68 (1992) 286
16 K. Werner and P. Koch Cascading in ultrarelativistic nuclear collisions PLB 242 (1990) 251
17 B. Andersson, G. Gustafson, and B. Nilsson-Almqvist A model for low $ {p_{\mathrm{T}}} $ hadronic reactions, with generalizations to hadron-nucleus and nucleus-nucleus collisions NPB 281 (1987) 289
18 PHENIX Collaboration Transverse-energy distributions at midrapidity in p+p, d+Au, and Au+Au collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}= $ 62.4-200 GeV and implications for particle-production models PRC 89 (2014) 044905 1312.6676
19 R. Ulrich, R. Engel, and M. Unger Hadronic multiparticle production at ultra-high energies and extensive air showers PRD 83 (2011) 054026 1010.4310
20 L. Kheyn Shower center of gravity and hadronic interaction characteristics Astropart. Phys. 92 (2017) 7 1202.4989
21 X.-N. Wang and M. Gyulassy HIJING: A monte carlo model for multiple jet production in pp, pA and AA collisions PRD 44 (1991) 3501
22 T. Pierog et al. EPOS LHC: Test of collective hadronization with data measured at the CERN Large Hadron Collider PRC 92 (2015) 034906 1306.0121
23 S. Ostapchenko Monte Carlo treatment of hadronic interactions in enhanced pomeron scheme: I. QGSJET-II model PRD 83 (2011) 014018 1010.1869
24 CMS Collaboration The CMS experiment at the CERN LHC JINST 3 (2008) S08004 CMS-00-001
25 CMS Collaboration Particle-flow reconstruction and global event description with the CMS detector JINST 12 (2017) P10003 CMS-PRF-14-001
26 G. Bayatian et al. Design, performance and calibration of the CMS forward calorimeter wedges EPJC 53 (2008) 139
27 CMS Collaboration Status of zero degree calorimeter for CMS experiment AIP Conf. Proc. 867 (2006) 258 nucl-ex/0608052
28 CMS Collaboration Measurement of inclusive jet production and nuclear modifications in pPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV EPJC 76 (2016) 372 CMS-HIN-14-001
29 M. L. Miller, K. Reygers, S. J. Sanders, and P. Steinberg Glauber modeling in high energy nuclear collisions Ann. Rev. Nucl. Part. Sci. 57 (2007) 205 nucl-ex/0701025
30 CMS Collaboration Studies of dijet transverse momentum balance and pseudorapidity distributions in pPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV EPJC 74 (2014) 2951 CMS-HIN-13-001
31 H. J. Drescher et al. Parton based Gribov-Regge theory PR 350 (2001) 93 hep-ph/0007198
32 K. S. Krane Introductory nuclear physics Wiley
33 I. Angeli and K. P. Marinova Table of experimental nuclear ground state charge radii: An update Atomic Data and Nuclear Data Tables 99 (2013) 69
34 CMS Collaboration Pseudorapidity distributions of charged hadrons in proton-lead collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 and 8.16 TeV JHEP 01 (2018) 045 CMS-HIN-16-021
35 PHENIX Collaboration Systematic studies of the centrality and $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} $ dependence of the $ \text{d} E_\text{T}/\text{d}\eta $ and $ \text{d}N^\text{ch}/\text{d}\eta $ in heavy ion collisions at mid-rapidity PRC 71 (2005) 034908 nucl-ex/0409015
36 T. Abbott et al. Systematics of mid-rapidity transverse energy distributions in limited apertures from p+Be to Au+Au collisions at relativistic energies PRC 63 (2001) 064602, .[Erratum: \DOI10.1103/PhysRevC.64.029901]
37 ALICE Collaboration Measurement of transverse energy at midrapidity in Pb-Pb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 2.76 TeV PRC 94 (2016) 034903 1603.04775
38 NA49 Collaboration Hadron production in nuclear collisions from the NA49 experiment at 158 GeV/$c$/A NP A 661 (1999) 45
39 NA49 Collaboration Recent results on spectra and yields from NA49 NP A 715 (2003) 161 nucl-ex/0208014
40 NA49 Collaboration Energy dependence of pion and kaon production in central Pb+Pb collisions PRC 66 (2002) 054902 nucl-ex/0205002
41 FOPI Collaboration Central collisions of Au on Au at 150, 250 and 400 MeV/nucleon NP A 612 (1997) 493 nucl-ex/9610009
42 FOPI Collaboration Charged pion production in Au on Au collisions at 1 A GeV Z. Phys. A 357 (1997) 215
43 FOPI Collaboration Proton and pion distributions in heavy-ion collisions at SIS energies PRC 66 (2002) 034901
44 STAR Collaboration Measurements of transverse energy distributions in Au+Au collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 200 GeV PRC 70 (2004) 054907 nucl-ex/0407003
45 E802 Collaboration Simultaneous multiplicity and forward energy characterization of particle spectra in Au+Au collisions at 11.6 A GeV/$c$ PRC 59 (1999) 2173
46 F. Videbaek and O. Hansen Baryon rapidity loss and mid-rapidity stacking in high-energy nucleus-nucleus collisions PRC 52 (1995) 2684
47 W. Busza and A. S. Goldhaber Nuclear stopping power PLB 139 (1984) 235
48 BRAHMS Collaboration Nuclear stopping in Au+Au collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 200 GeV PRL 93 (2004) 102301 nucl-ex/0312023
49 BRAHMS Collaboration Nuclear stopping and rapidity loss in Au+Au collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 62.4 GeV PLB 677 (2009) 267 0901.0872
50 CMS Collaboration Studies of the nuclear stopping power in PbPb collisions at 2.76 TeV with CMS NP A 904-905 (2013) 787c
51 STAR Collaboration Bulk properties of the medium produced in relativistic heavy-ion collisions from the beam energy scan program PRC 96 (2017) 044904 1701.07065
52 W. Busza Structure and fine structure in multiparticle production data at high energies Acta Phys. Polon. B 35 (2004) 2873 nucl-ex/0410035
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