Inclusive photon production at forward rapidities

The primary goal of high-energy heavy-ion physics is the study of a new state of nuclear matter, quark–gluon plasma, a thermalised system of quarks and gluons. The study of proton–proton (pp) and proton–nucleus (pA) collisions provides the baseline for the interpretation of results from heavy-ion collisions. The study of pA collisions also helps researchers understand the effects of cold nuclear matter on the production of final-state particles.

Global observables, such as the number of produced particles (particle multiplicity) and their distribution in pseudorapidity (η), provide key information about particle-production mechanisms in these collisions. The total multiplicity is mostly determined by soft interactions, i.e. processes with small momentum transfer, which cannot be calculated using perturbative techniques and are instead modelled using non-perturbative phenomenological descriptions. For example, the distribution of the number of produced particles can be used to disentangle relative contributions to particle production from hard and soft processes using a two-component model.

ALICE has recently completed the measurement of the multiplicity and pseudorapidity density distributions of inclusive photons at forward rapidity, spanning the range η = 2.3 to 3.9, by using the photon multiplicity detector (PMD) in pp, pPb and Pbp collisions at a centre-of-mass energy of 5.02 TeV per nucleon pair using LHC Run 1 and 2 data. Since photons mostly originate from decays of neutral pions, this result complements existing measurements of charged-particle production. A comparative study of charged particles and inclusive photons can reveal possible similarities and differences in the underlying production mechanisms for charged and neutral particles.

The PMD uses the preshower technique, where a three-radiation-length-thick lead converter is sandwiched between two planes comprising an array of 184,320 gas-filled proportional counters. Photons are distinguished from hadrons in the PMD’s preshower plane by applying suitable thresholds on the number of detector cells and the energy deposited in reconstructed clusters.

The measured distributions are corrected for instrumental effects using a Bayesian unfolding method. This is the first time that the dependence of the inclusive photon production on the number of nucleons participating in the pPb collision and its scaling behaviour has been studied at the LHC.

Figure 1 (left) compares the pseudorapidity density distribution of inclusive photons in minimum bias pp, pPb and Pbp collisions measured at forward rapidity to that of charged particles at midrapidity. The pseudorapidity distribution of inclusive photons at forward rapidity smoothly matches that of charged particles at midrapidity, indicating that the production mechanisms for charged and neutral pions are similar. Figure 1 (right) shows the pseudorapidity density distribution of inclusive photons in pPb collisions for different multiplicity classes as estimated using the energy deposited in the zero-degree calorimeter (ZNA) at beam rapidity. The multiplicity in the most central collisions reaches values twice as large as those in minimum bias events. The data and model agree within one sigma of the measurement uncertainties.

These results of inclusive photon production in pp, pPb and Pbp collisions provide valuable input for the development of theoretical models and Monte Carlo event generators, and help to establish the baseline measurements for the interpretation of PbPb collision data.