Abstract
Direct photon production at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 2.76$ TeV was studied in the transverse momentum range $0.9 < p_\mathrm{T} < 14$ GeV$/c$. Photons were detected with the highly segmented electromagnetic calorimeter PHOS and via conversions in the ALICE detector material with the $e^+e^-$ pair reconstructed in the central tracking system. The results of the two methods were combined and direct photon spectra were measured for the 0-20%, 20-40%, and 40-80% centrality classes. For all three classes, agreement was found with perturbative QCD calculations for $p_\mathrm{T} \gtrsim 5$ GeV$/c$. Direct photon spectra down to $p_\mathrm{T} \approx 1$ GeV$/c$ could be extracted for the 20-40% and 0-20% centrality classes. The significance of the direct photon signal for $0.9 < p_\mathrm{T} < 2.1$ GeV$/c$ is $2.6\sigma$ for the 0-20% class. The spectrum in this $p_\mathrm{T}$ range and centrality class can be described by an exponential with an inverse slope parameter of $(297 \pm 12^\mathrm{stat}\pm 41^\mathrm{syst})$ MeV. State-of-the-art models for photon production in heavy-ion collisions agree with the data within uncertainties.
Highlights
The theory of the strong interaction, Quantum ChromoDynamics (QCD), predicts a transition from ordinary nuclear matter to a state where quarks and gluons are no longer confined to hadrons [1,2]
The inclusive photon spectra and double ratios of the Photon Conversion Method (PCM) and PHOS analyses are combined as two independent measurements to obtain the error-weighted average
A direct photon excess is observed for all centrality classes for pT 4 GeV/c, and for 1 pT 4 GeV/c in the most central class
Summary
The theory of the strong interaction, Quantum ChromoDynamics (QCD), predicts a transition from ordinary nuclear matter to a state where quarks and gluons are no longer confined to hadrons [1,2]. High photon emission rates at the largest temperatures in the early stage are compensated by an expanded space–time volume and blue-shift due to radial flow in the later stage This complicates the interpretation of inverse slope parameters of direct photon spectra, but a correlation between the slope and the initial temperature still exists [23]. The absence of suppression of high pT isolated photons in A–A collisions with respect to Ncoll scaled pp collisions, in contrast to the observed suppression of hadrons, is consistent with the latter being due to energy loss of hard scattered quarks and gluons in the medium. Tion for pT 14 the first measurement of GeV/c in Pb–Pb collisions adtire√cstNpNh=ot2o.n76prToedVucispresented
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