Measurement of high p_T isolated prompt photons in lead-lead collisions at sqrt(s_NN)=2.76 TeV with the ATLAS detector at the LHC

TL;DR

This paper reports the measurement of high transverse momentum isolated prompt photons in lead-lead collisions at 2.76 TeV using the ATLAS detector, providing insights into initial state parton distributions and jet calibration in heavy ion collisions.

Contribution

First measurement of isolated prompt photon spectra in lead-lead collisions at the LHC, comparing results with perturbative QCD predictions.

Findings

Photon spectra measured as a function of transverse momentum and centrality.

Good agreement with perturbative QCD calculations.

Enhanced photon isolation techniques reduce background from jet fragmentation.

Abstract

Prompt photons are a powerful tool to study heavy ion collisions. Their production rates provide access to the initial state parton distribution functions and also provide a means to calibrate the expected energy of jets that are produced in the medium. The ATLAS detector measures photons with its hermetic, longitudinally segmented calorimeter, which gives excellent spatial and energy resolutions, and detailed information about the shower shape of each measured photon. This provides significant rejection against the expected background from the decays of neutral pions in jets. Rejection against jet fragmentation products is further enhanced by requiring candidate photons to be isolated. First results on the spectra of isolated prompt photons from a dataset with an integrated luminosity of approximately 0.13 nb^-1 of lead-lead collisions at sqrt(s_NN)=2.76 TeV are shown as a function of transverse momentum and centrality. The measured spectra are compared to expectations from perturbative QCD calculations.