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

• Published in: Physical review letters 2018-12, Vol.121 (24), p.242301
• Main Author: Sirunyan, A M
• Format: Article
• Language: English
• Subjects: Collisions; Depletion; Fragmentation; Jets; Kinematics; Large Hadron Collider; NUCLEAR PHYSICS AND RADIATION PHYSICS; Partons; PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; Quark-gluon plasma; Quarks; Solenoids; Transverse momentum
• ISSN: 0031-9007; 1079-7114
• DOI: 10.1103/PhysRevLett.121.242301

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>30 GeV/c in events containing an isolated photon with pTγ>60 GeV/c, using charged tracks with transverse momentum pTtrk>1 GeV/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 3 GeV/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.