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

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.

Submitted to: PRC
e-Print: arXiv:1803.09857 | PDF | inSPIRE
CERN-EP-2018-045

Figures

## Figure 1

 \textit{Left}: Measured identified particle yields per inelastic event in pp collisions at $\sqrt{s}=2.76$ TeV~\ifrho\else\fi including their modified Hagedorn parametrization used as input for the cocktail simulation. Statistical uncertainties are shown with vertical lines and systematic uncertainties with boxes. See the text for references to the data \textit{Right:} Ratio of primary decay photons from different sources to all primary decay photons in the decay photon simulation for pp collisions at $\sqrt{s}=2.76$ TeV. From top to bottom at high \pT\, the different sources are $\pi^0$, $\eta$, $\omega$, $\eta'$, $\rho^0$, $\phi$, \kzl, $\rho^\pm$, \kzs, $\Delta^{0/+}$, $\Sigma^0$ and $\Lambda$.

## Figure 2

 Ratio of inclusive photon invariant cross sections obtained with the PCM and \EMC\ methods to the TCM fit, \Eq{eq:Bylinkin} of the combined inclusive photon spectrum in pp collisions at $\sqrt{s}=2.76$ TeV~(\textit{left}) and 8 TeV~(\textit{right}) Statistical uncertainties are given by vertical lines and systematic uncertainties are visualized by the height of the boxes, while the bin widths are represented by the widths of the boxes.

## Figure 3

 Direct photon excess ratios obtained with the PCM, \EMC\ and \PCMEMC\ methods in pp collisions at $\sqrt{s}=2.76$ TeV~(\textit{left}) and 8 TeV~(\textit{right}) Statistical uncertainties are given by vertical lines and systematic uncertainties are visualized by the height of the boxes, while the bin widths are represented by the widths of the boxes.

## Figure 4

 Direct photon excess ratios for the combined measurements at $\sqrt{s}=2.76$ and 8 TeV including pQCD NLO predictions with CT10~, nCTEQ15~ or CTEQ6.1M~ proton PDF and GRV~ or BFG2~ FF In addition, a JETPHOX calculation~ based on NNPDF2.3QED~ proton PDF and BFG2 FF as well as a POWHEG calculation~ based on the same PDF but with the PYTHIA 8 parton shower algorithm are provided.

## Figure 5

 Upper limits of direct photon production at 90\% C.L. together with the invariant inclusive photon cross section at 2.76 and 8 TeV including pQCD NLO predictions with CT10~, nCTEQ15~ or CTEQ6.1M~ proton PDF and GRV~ or BFG2~ FF In addition, a JETPHOX calculation~ based on NNPDF2.3QED~ proton PDF and BFG2 FF as well as a POWHEG~ calculation based on the same PDF but with the PYTHIA 8 parton shower algorithm are provided Upper limits are calculated for all spectrum points where the $\Rg$ value including its total uncertainty is consistent with unity and indicated by the horizontal bars at the end of the arrows in the figure Furthermore, direct photon cross sections are given where $\Rg$ is larger than unity considering only statistical uncertainties (round markers and vertical error bars) or taking only systematic uncertainties into account (boxes)