Abstract
Jets created in association with a photon can be used as a calibrated probe to study energy loss in the medium created in nuclear collisions. Measurements of the transverse momentum balance between isolated photons and inclusive jets are presented using integrated luminosities of 0.49 nb−1 of Pb + Pb collision data at sNN=5.02 TeV and 25 pb−1 of pp collision data at s=5.02 TeV recorded with the ATLAS detector at the LHC. Photons with transverse momentum 63.1<pTγ<200 GeV and |ηγ|<2.37 are paired with all jets in the event that have pTjet>31.6 GeV and pseudorapidity |ηjet|<2.8. The transverse momentum balance given by the jet-to-photon pT ratio, xJγ, is measured for pairs with azimuthal opening angle Δϕ>7π/8. Distributions of the per-photon jet yield as a function of xJγ, (1/Nγ)(dN/dxJγ), are corrected for detector effects via a two-dimensional unfolding procedure and reported at the particle level. In pp collisions, the distributions are well described by Monte Carlo event generators. In Pb + Pb collisions, the xJγ distribution is modified from that observed in pp collisions with increasing centrality, consistent with the picture of parton energy loss in the hot nuclear medium. The data are compared with a suite of energy-loss models and calculations.
Highlights
The energy loss of fast partons traversing the hot, deconfined medium created in nucleus–nucleus collisions can be studied in a controlled and systematic way through the analysis of jets produced in association with a high transverse momentum prompt photon [1,2,3,4,5,6,7]
Measurements of prompt photon production in Au + Au collisions at the Relativistic Heavy Ion Collider (RHIC) [8] and Pb + Pb collisions at the Large Hadron Collider (LHC) [9] have confirmed that, since photons do not participate in the strong interaction, their production rates are not modified by the medium [10]
For the primary simulation samples, the Pythia 8.186 [38] generator was used with the NNPDF23LO parton distribution function (PDF) set [39], and generator parameters which were tuned to reproduce a set of minimum-bias data [40]
Summary
The energy loss of fast partons traversing the hot, deconfined medium created in nucleus–nucleus collisions can be studied in a controlled and systematic way through the analysis of jets produced in association with a high transverse momentum (pT) prompt photon [1,2,3,4,5,6,7]. Are complementary to measurements such as the dijet pT balance [11,12,13] Whereas those measurements report the ratio of the transverse momenta of two final-state jets, both of which may have lost energy, photon–jet events provide an alternative system in which one high-pT object is certain to remain unaffected by the hot nuclear medium. In the LHC studies, the distribution of the photon–jet azimuthal separation, φ, was found to be consistent with that in simulated photon–jet events embedded into a heavy-ion background, and the jet-to-photon transverse momentum ratio, xJγ. The per-photon jet yield (1/Nγ )(dN/dxJγ ) distribution was shifted to significantly smaller values in Pb + Pb data Carlo event generators and analytic calculations [26,27,28,29]
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