Measurement of the prompt photon production cross section in proton-proton collisions at $\sqrt{s}$ = 7 TeV with ALICE

The knowledge of the gluon PDFs within hadrons is obtained indirectly from the results of Deep Inelastic Scattering experiments, which have a limited range towards small values of the momentum fraction x. At low x the behaviour of the PDFs is predicted by QCD evolution, which leads to a (unphysical) divergence of the gluon PDFs. In order to have a consistent description of hadrons, non-linear effects allowing for the gluon PDFs to saturate must occur. One of the proposed descriptions of the gluon fields in the saturation regime is the Colour Glass Condensate (CGC). Among the experimental observables that can help prove the validity of the CGC theory, the production of direct photons in hadronic collisions at large rapidity is one of the cleanest, but also most challenging observables. In fact, prompt photons are produced, in hadronic collisions, mainly via the qg-Compton scattering, which, at Leading Order, allows us to directly relate the energy and momentum of the final state photon to those of the initial state gluon. Photon measurements are particularly difficult because of the huge background from photons produced in the decay of short-living neutral hadrons such as the neutral pion. In this thesis, the cross section for isolated photons in pp collisions at $\sqrt{s}$ = 7 TeV, produced at midrapity, is presented. The measurement is performed by analysing the data collected by the ALICE collaboration in 2011, with the help of a trigger algorithm that selects events with a high-energy deposit in the ALICE-EMCal. A clustering procedure allows us to reconstruct the total energy of the particle interacting with the electromagnetic calorimeter. After selecting single proton-proton collision, we restrict our analysis to clusters produced only by neutral particles. To estimate the contribution of background clusters (produced both by photons from neutral hadron decays and by photons from fragmentation processes) in our sample, a data-driven method is used. The applicability of this method has been tested on realistic simulated samples, which are used to correct also for inefficiencies due to detector effects. The systematic uncertainties of the measurement have been quantified and presented. The cross section for isolated photon production in pp collisions at $\sqrt{s}$ = 7 TeV is then computed and compared to the state-of-the-art NLO pQCD calculations available. The measured cross section is found to agree well with the theoretical prediction. The result presented calls for a measurement of the same observable at large rapidity, where the onset of saturation effects is expected. For this purpose, the Forward Calorimeter has been proposed as an upgrade of the ALICE detector.