Direct photon production in Pb-Pb collisions at $\sqrt{s_\rm{NN}}$ = 2.76 TeV

J. Adam , Laurent Aphecetche (1) , A. Baldisseri (2) , Guillaume Batigne (1) , I. Belikov (3) , H. Borel (2) , J. Castillo Castellanos (2) , J.L. Charvet , C. Cheshkov (4) , B. Cheynis (4) , L. Ducroux (4) , Marie Germain (1) , J.-Y. Grossiord (4) , B. Hippolyte (3) , A. Lardeux (1) , A. Maire (3) , J. Martin Blanco (1) , Gines Martinez Garcia (1) , L. Massacrier (1) , L. Molnar (3) , D.A. Moreira De Godoy (1) , A. Morreale (1) , H. Pereira Da Costa (2) , Philippe Pillot (1) , L. Ronflette (1) , Y. Schutz (1) , Alexandre Shabetai (1) , Diego Stocco (1) , B. Teyssier (4) , R. Tieulent (4) , A. Uras (4) , R. Vernet (5) , M. Wang (1) , G. Conesa Balbastre (6) , J. Faivre (6) , C. Furget (6) , R. Guernane (6) , C. Silvestre (6) , A. Vauthier (6) , V. Barret (7) , N. Bastid (7) , A. Batista Camejo (7) , P. Crochet (7) , P. Dupieux (7) , V.J.G. Feuillard (7) , S. Li (7) , X. Lopez (7) , F. Manso (7) , S. Porteboeuf-Houssais (7) , P. Rosnet (7) , B. Vulpescu (7) , Z. Conesa Del Valle (8) , I. Das (8) , B. Espagnon (8) , C. Hadjidakis (8) , I. Lakomov (8) , C. Suire (8) , J. D. Tapia Takaki (8) , M. Tarhini (8) , L. Valencia Palomo (8)

1 SUBATECH - Laboratoire SUBATECH Nantes
2 IRFU - Institut de Recherches sur les lois Fondamentales de l'Univers
3 IPHC - Institut Pluridisciplinaire Hubert Curien
4 IPNL - Institut de Physique Nucléaire de Lyon
5 CC-IN2P3 - Centre de Calcul de l'IN2P3
6 LPSC - Laboratoire de Physique Subatomique et de Cosmologie
7 LPC - Laboratoire de Physique Corpusculaire - Clermont-Ferrand
8 IPNO - Institut de Physique Nucléaire d'Orsay

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.

Domaines: Physique Nucléaire Expérimentale [nucl-ex]; Physique des Hautes Energies - Expérience [hep-ex]

J. Adam, Laurent Aphecetche, A. Baldisseri, Guillaume Batigne, I. Belikov, et al.. Direct photon production in Pb-Pb collisions at $\sqrt{s_\rm{NN}}$ = 2.76 TeV. Physics Letters B, 2016, 754, pp.235-248. ⟨10.1016/j.physletb.2016.01.020⟩. ⟨in2p3-01205166⟩