Speaker
Description
Direct photons are produced in various processes in pp, p-A and A-A collision and are sensitive to details of the space-time evolution the medium produced in heavy-ion collisions. The low $p_{\rm T}$ part of the direct photon spectrum is expected to be dominated by thermal direct photons - thermal radiation of hot matter, i.e., of the quark-gluon plasma and the hadron gas. At RHIC energies it is a challenge for hydrodynamical models to simultaneously describe the yield and the elliptic flow of direct photons in p-A and A-A collisions, which is often referred to as the direct-photon puzzle.
With ALICE, photons can be detected with either of the two electromagnetic calorimeters, EMCal and PHOS, and via reconstruction of $e^+e^-$ pairs from conversions in the ALICE detector material using the central tracking system. An additional hybrid method, combining the conversion information with that of the calorimeters has been developed. Where ever possible the results were combined to reflect our best estimate of the inclusive photon spectra and flow, $R_{\gamma}$, as well as the extraction of direct photon spectra or their upper limits and the flow.
In this talk, the first measurements on the direct photon production in pp collisions at $\sqrt{s} = 2.76$ and 8 TeV, as well as in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV will be presented. Below 3 GeV/$c$, $R_\gamma$ was found to be consistent with unity in these collision systems. Furthermore, the direct photon spectra are in agreement with pQCD next-to-leading order calculations within the uncertainties. In addition, the final results on the direct photon elliptic flow ($v_{2}^{\gamma dir,SP}$) at $\sqrt{s_{\rm NN}} = 2.76$ TeV from the Pb-Pb run in 2010 in the $p_{\rm T}$ range of $0.9 < p_{\rm T} < 6.2$ GeV/$c$ will be presented in this talk. Comparisons to PHENIX results and to predictions of several theoretical models will be presented in order to shed light on the status of the direct photon puzzle at LHC energies.
| Content type | Experiment |
|---|---|
| Collaboration | ALICE |
| Centralised submission by Collaboration | Presenter name already specified |
