Measurements of inclusive and direct photon production at midrapidity 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 $0.4<p_{\mathrm{T}}<10 \text{GeV}/c$ and $0.3<p_{\mathrm{T}}<16 \text{GeV}/c$, 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 $p_{\mathrm{T}}$ 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 $p_{\mathrm{T}}$ range, $R_{\gamma }$ for $p_{\mathrm{T}}>7 \text{GeV}/c$ is at least one $\sigma$ above unity and consistent with expectations from next-to-leading order pQCD calculations.

• Received 30 March 2018

DOI:https://doi.org/10.1103/PhysRevC.99.024912

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©2019 CERN, for the ALICE Collaboration