Observation of Medium-Induced Modifications of Jet Fragmentation in Pb-Pb Collisions at root s(NN)=5.02 TeV Using Isolated Photon-Tagged Jets

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American Physical Society

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.

RADIATIVE ENERGY-LOSS, Physics, Multidisciplinary, Physics
Sirunyan, A. M., Tumasyan, A., Adam, W., Ambrogi, F., Asilar, E., Bergauer, T., Brandstetter, J., Brondolin, E., Dragicevic, M., Erö, J., Escalante Del Valle, A., Flechl, M., Friedl, M., Frühwirth, R., Ghete, V. M., Grossmann, J., Hrubec, J., Jeitler, M., … König, A. (2018). Observation of Medium-Induced Modifications of Jet Fragmentation in Pb-Pb Collisions at sNN=5.02  TeV Using Isolated Photon-Tagged Jets. In Physical Review Letters (Vol. 121, Issue 24). American Physical Society (APS). https://doi.org/10.1103/physrevlett.121.242301