Direct photon production at low transverse momentum in proton-proton collisions at s =2.76 and 8 TeV

Abstract

Measurements of inclusive and direct photon production at midrapidity in pp collisions at s = 2.76 and 8 TeV are presented by the ALICE experiment at the LHC. The results are reported in transverse momentum ranges of 0.4 < p T < 10 GeV/c and 0.3 < p T < 16 GeV/c, respectively. Photons are detected with the electromagnetic calorimeter (EMCal) and via reconstruction of e + e -pairs from conversions in the ALICE detector material using the central tracking system. For the final measurement of the inclusive photon spectra the results are combined in the overlapping p T interval of both methods. Direct photon spectra, or their upper limits at 90% C.L. are extracted using the direct photon excess ratio R , which quantifies the ratio of inclusive photons over decay photons generated with a decay-photon simulation. An additional hybrid method, combining photons reconstructed from conversions with those identified in the EMCal, is used for the combination of the direct photon excess ratio R , as well as the extraction of direct photon spectra or their upper limits. While no significant signal of direct photons is seen over the full p T range, R for p T > 7 GeV/c is at least one above unity and consistent with expectations from next-to-leading order pQCD calculations.