• Open Access

Observation of Medium-Induced Modifications of Jet Fragmentation in Pb-Pb Collisions at sNN=5.02TeV Using Isolated Photon-Tagged Jets

A. M. Sirunyan et al. (CMS Collaboration)
Phys. Rev. Lett. 121, 242301 – Published 14 December 2018

Abstract

Measurements of fragmentation functions for jets associated with an isolated photon are presented for the first time in pp and Pb-Pb collisions. The analysis uses data collected with the CMS detector at the CERN LHC at a nucleon-nucleon center-of-mass energy of 5.02 TeV. Fragmentation functions are obtained for jets with pTjet>30GeV/c in events containing an isolated photon with pTγ>60GeV/c, using charged tracks with transverse momentum pTtrk>1GeV/c in a cone around the jet axis. The association with an isolated photon constrains the initial pT and azimuthal angle of the parton whose shower produced the jet. For central Pb-Pb collisions, modifications of the jet fragmentation functions are observed when compared to those measured in pp collisions, while no significant differences are found in the 50% most peripheral collisions. Jets in central Pb-Pb events show an excess (depletion) of low (high) pT particles, with a transition around 3GeV/c. This measurement shows for the first time the in-medium shower modifications of partons (quark dominated) with well-defined initial kinematics. It constitutes a new well-controlled reference for testing theoretical models of the parton passage through the quark-gluon plasma.

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  • Received 15 January 2018
  • Revised 16 October 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.242301

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

© 2018 CERN, for the CMS Collaboration

Physics Subject Headings (PhySH)

Nuclear Physics

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Vol. 121, Iss. 24 — 14 December 2018

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