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Title: Measurement of the splitting function in pp and PbPb collisions at $$\sqrt{s_{_{\mathrm{NN}}}}=$$ 5.02 TeV

Abstract

Data from heavy ion collisions suggest that the evolution of a parton shower is modified by interactions with the color charges in the dense partonic medium created in these collisions. However, the stage in the shower evolution at which the modifications occur is not known. By removing the soft wide-angle radiation inside a jet, the two leading partons can be resolved as subjets. The momentum ratio of these subjets provides information about the corresponding partons at an early stage of the shower. This substructure observable, known as the splitting function, relates to the process of a parton splitting into two other partons. The measurement is performed for jets with transverse momentum larger than 140 GeV and smaller than 500 GeV, using pp and PbPb collision samples collected by the CMS detector at a center-of-mass energy of 5.02 GeV per nucleon pair. In central PbPb collisions, the splitting function is observed to be modified toward a more unbalanced momentum ratio compared to peripheral PbPb and pp collisions. The measurements are compared to the predictions from event generators and analytical calculations using various models for the medium modification of the jet evolution.

Authors:
;
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
OSTI Identifier:
1390188
Report Number(s):
FERMILAB-PUB-17-360-CMS; CMS-HIN-16-006; CERN-EP-2017-205; arXiv:1708.09429
1620905
DOE Contract Number:
AC02-07CH11359
Resource Type:
Journal Article
Resource Relation:
Journal Name: TBD
Country of Publication:
United States
Language:
English
Subject:
73 NUCLEAR PHYSICS AND RADIATION PHYSICS; 72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS

Citation Formats

Sirunyan, Albert M, and et al. Measurement of the splitting function in pp and PbPb collisions at $\sqrt{s_{_{\mathrm{NN}}}}=$ 5.02 TeV. United States: N. p., 2017. Web.
Sirunyan, Albert M, & et al. Measurement of the splitting function in pp and PbPb collisions at $\sqrt{s_{_{\mathrm{NN}}}}=$ 5.02 TeV. United States.
Sirunyan, Albert M, and et al. 2017. "Measurement of the splitting function in pp and PbPb collisions at $\sqrt{s_{_{\mathrm{NN}}}}=$ 5.02 TeV". United States. doi:. https://www.osti.gov/servlets/purl/1390188.
@article{osti_1390188,
title = {Measurement of the splitting function in pp and PbPb collisions at $\sqrt{s_{_{\mathrm{NN}}}}=$ 5.02 TeV},
author = {Sirunyan, Albert M and et al.},
abstractNote = {Data from heavy ion collisions suggest that the evolution of a parton shower is modified by interactions with the color charges in the dense partonic medium created in these collisions. However, the stage in the shower evolution at which the modifications occur is not known. By removing the soft wide-angle radiation inside a jet, the two leading partons can be resolved as subjets. The momentum ratio of these subjets provides information about the corresponding partons at an early stage of the shower. This substructure observable, known as the splitting function, relates to the process of a parton splitting into two other partons. The measurement is performed for jets with transverse momentum larger than 140 GeV and smaller than 500 GeV, using pp and PbPb collision samples collected by the CMS detector at a center-of-mass energy of 5.02 GeV per nucleon pair. In central PbPb collisions, the splitting function is observed to be modified toward a more unbalanced momentum ratio compared to peripheral PbPb and pp collisions. The measurements are compared to the predictions from event generators and analytical calculations using various models for the medium modification of the jet evolution.},
doi = {},
journal = {TBD},
number = ,
volume = ,
place = {United States},
year = 2017,
month = 8
}
  • This paper reports the measurement of J/ψ meson production in proton–proton (pp) and proton–lead (pPb) collisions at a center-of-mass energy per nucleon pair of 5.02TeV by the CMS experiment at the LHC. The data samples used in the analysis correspond to integrated luminosities of 28pb –1 and 35nb –1 for pp and pPb collisions, respectively. Prompt and nonprompt J/ψ mesons, the latter produced in the decay of B hadrons, are measured in their dimuon decay channels. Differential cross sections are measured in the transverse momentum range of 2 < p T<30GeV/c, and center-of-mass rapidity ranges of |y CM| < 2.4more » (pp) and –2.87 < y CM < 1.93 (pPb). The nuclear modification factor, R pPb, is measured as a function of both p T and y CM. Small modifications to the J/ψ cross sections are observed in pPb relative to pp collisions. The ratio of J/ψ production cross sections in p-going and Pb-going directions, RFB, studied as functions of p T and y CM, shows a significant decrease for increasing transverse energy deposited at large pseudorapidities. Finally, these results, which cover a wide kinematic range, provide new insight on the role of cold nuclear matter effects on prompt and nonprompt J/ψ production.« less
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  • Charge-dependent anisotropy Fourier coefficients (more » $$v_n$$) of particle azimuthal distributions are measured in pPb and PbPb collisions at $$ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $$ 5.02 TeV with the CMS detector at the LHC. The normalized difference in the second-order anisotropy coefficients ($$v_2$$) between positively and negatively charged particles is found to depend linearly on the observed event charge asymmetry with comparable slopes for both pPb and PbPb collisions over a wide range of charged particle multiplicity. In PbPb, the third-order anisotropy coefficient, $$v_3$$, shows a similar linear dependence with the same slope as seen for $$v_2$$. The observed similarities between the $$v_2$$ slopes for pPb and PbPb, as well as the similar slopes for $$v_2$$ and $$v_3$$ in PbPb, are compatible with expectations based on local charge conservation in the decay of clusters or resonances, and constitute a challenge to the hypothesis that the observed charge asymmetry dependence of $$v_2$$ in heavy ion collisions arises from a chiral magnetic wave.« less
  • Measurements of azimuthal angle and transverse momentum (more » $$p_\mathrm{T}$$) correlations of isolated photons and associated jets are reported for pp and PbPb collisions at $$\sqrt{s_{_{\mathrm{NN}}}} =$$ 5.02 TeV. The data were recorded with the CMS detector at the CERN LHC. For events containing a leading isolated photon with $$p_\mathrm{T}^\gamma >$$ 40 GeV$/c$ and an associated jet with $$p_\mathrm{T}^\text{jet} >$$ 30 GeV$/c$$, the photon+jet azimuthal correlation and \pt imbalance in PbPb collisions are studied as functions of collision centrality and $$p_\mathrm{T}^\gamma$. The results are compared to pp reference data collected at the same collision energy and to predictions from several theoretical models for parton energy loss. No evidence of broadening of the photon+jet azimuthal correlations is observed, while the ratio $$p_\mathrm{T}^\text{jet}/p_\mathrm{T}^\gamma$$ decreases significantly for PbPb data relative to the pp reference. All models considered agree within uncertainties with the data. The number of associated jets per photon with $$p_\mathrm{T}^\gamma >$$ 80 GeV$/c$ is observed to be shifted towards lower $$p_\mathrm{T}^\text{jet}$$ values in central PbPb collisions compared to pp collisions.« less
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