Observation of medium-induced modifications of Jet fragmentation in Pb-Pb collisions at sNN =5.02 TeV using isolated photon-tagged jets

• Main Authors: Sirunyan, A. M., Tumasyan, A. R., Adam, Wolfgang, Ambrogi, Federico, Asilar, Ece, Md. Ali, Mohd. Adli
• Format: Article
• Language: English; English
• Published: American Physical Society 2018
• Subjects: Q Science (General); QC Physics
• Online Access: http://irep.iium.edu.my/70737/1/70737_Observation%20of%20Medium-Induced%20Modifications_article.pdf; http://irep.iium.edu.my/70737/2/70737_Observation%20of%20Medium-Induced%20Modifications_scopus_new.pdf; http://irep.iium.edu.my/70737/; https://journals.aps.org/prl/pdf/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 pjet T > 30 GeV=c in events containing an isolated photon with pγ T > 60 GeV=c, using charged tracks with transverse momentum ptrk T > 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.