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 \({p}_{T}^{\text{jet}}>30\text{}\text{}\mathrm{GeV}/c\) in events containing an isolated photon with \({p}_{T}^{\gamma }>60\text{}\text{}\mathrm{GeV}/c\), using charged tracks with transverse momentum \({p}_{T}^{\mathrm{trk}}>1\text{}\text{}\mathrm{GeV}/c\) in a cone around the jet axis. The association with an isolated photon constrains the initial \({p}_{T}\) 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) \({p}_{T}\) particles, with a transition around \(3\text{}\text{}\mathrm{GeV}/c\). 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.
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