Isolated photon cross section in pp collisions at $\sqrt{s}$ = 8 TeV with the ALICE at LHC

In ultra-relativistic hadronic collisions, direct photons are those photons that are directly produced in elementary processes and act as a valuable tool to test perturbative Quantum Chromodynamics (pQCD) predictions. Since these photons are produced directly from parton–parton hard scatterings, they can be used to constrain the parton distribution functions (PDFs). In leading order pQCD, direct photons are mainly produced via 2→2 processes, namely quark–gluon Compton scattering and quark–antiquark annihilation. These photons together with the fragmentation or bremsstrahlung photons are referred to as the prompt photons [1]. Photons from the 2→2 processes provide clear constraints of the underlying parton kinematics, but making a clean separation between the different types of prompt photons is difficult. Additionally, measurements of direct photons are complicated due to the presence of a large photon background from hadron decays, especially from π 0 and η mesons. A selection called ”isolation criterion”, which is based on a threshold on the contributions of transverse energy/momentum from particles inside a cone around the candidate photon, is applied to suppress the decay and fragmentation photons. This has led to the prescription of ”isolated photons”.