Direct photon production at low transverse momentum in proton-proton collisions at $\sqrt{s}$ = 2.76 and 8 TeV

TL;DR

This paper reports measurements of inclusive and direct photon production in proton-proton collisions at 2.76 and 8 TeV, using the ALICE detector, and compares the results with theoretical predictions, finding no significant direct photon signal at low transverse momentum.

Contribution

First measurement of direct photon spectra at these energies in pp collisions using combined detection methods and novel analysis techniques.

Findings

No significant direct photon signal observed at low pT.

Direct photon excess ratio Rγ exceeds unity for pT > 7 GeV/c.

Results are consistent with next-to-leading order pQCD calculations.

Abstract

Measurements of inclusive and direct photon production at mid-rapidity in pp collisions at $\sqrt{s}=2.76$ 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 $R_{\gamma}$, 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 $R_{\gamma}$, 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, $R_{\gamma}$ for $p_{T}>7$ GeV/$c$ is at least one $\sigma$ above unity and consistent with expectations from next-to-leading order pQCD calculations.