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.
Highlights
Major experimental efforts are undertaken at the Relativistic Heavy Ion Collider (RHIC) [1,2,3,4] and the Large Hadron Collider (LHC) [5,6,7,8,9,10,11,12,13] to study the conditions for the creation and the properties of the quark-gluon plasma (QGP), a deconfined partonic state predicted by the theory of strong interaction, quantum chromodynamics (QCD) [14,15]
The double ratios are measured by combining the individual inclusive photon and neutral pion spectra from the same reconstruction methods with a cocktail simulation based on the same neutral pion spectrum
For Rγ the statistical uncertainties show partial correlation between the Photon Conversion Method (PCM) and PCM-EMC, which are determined to be ∼20–50% depending on pT, since both measurements are based on the PCM inclusive photon measurement using different subsets of the data; the statistical uncertainties of the neutral pion measurements are fully independent due to their different reconstruction methods
Summary
Major experimental efforts are undertaken at the Relativistic Heavy Ion Collider (RHIC) [1,2,3,4] and the Large Hadron Collider (LHC) [5,6,7,8,9,10,11,12,13] to study the conditions for the creation and the properties of the quark-gluon plasma (QGP), a deconfined partonic state predicted by the theory of strong interaction, quantum chromodynamics (QCD) [14,15]. Direct photons, which are defined as all photons that are produced directly in scattering processes and do not originate from hadronic decays, are a powerful tool for exploring the QGP. They are produced during all stages of the collision and are basically unaffected by final-state interactions as they only participate in electromagnetic interactions [16]. They are sensitive to the early stages of the collision’s evolution. The main challenge for direct photon measurements is to distinguish them from the large background of decay photons
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
