Low-mass dielectron production in proton–proton collisions at √s = 13 TeV with ALICE

Low-mass electron–positron pairs are a unique experimental tool to study the properties of the Quark-Gluon Plasma created in ultra-relativistic heavy-ion collisions at the CERN Large Hadron Collider (LHC). Such pairs are produced during all stages of the collision and carry the information about the whole space-time evolution of the system to the de- tector unperturbed by strong final-state interactions. The study of dielectron production in inelastic proton–proton (pp) collisions provides crucial vacuum reference needed for the interpretation of the results in heavy-ion collisions and a test of perturbative Quantum Chromodynamics (pQCD). This work summarises the measurement of low-mass dielectron pairs in pp collisions at a centre-of-mass energy of √𝑠 = 13 TeV with the ALICE detector. The dielectron production is studied as a function of invariant mass and transverse momentum at midrapidity (|𝑦$_{e}$| < 0.8). The contributions from light-hadron decays can be calculated from their cross sections independently measured in pp collisions. Correlated electron pairs from semi-leptonic charm and beauty decays, which dominate in the intermediate mass region (1.1 < 𝑚$_{ee}$ < 2.8 GeV/c$^{2}$), are used to extract the charm and beauty cross sections integrated over all transverse momenta for the first time at this collision energy at midrapidity. The data are fit with templates from two different Monte Carlo event generators, Pythia and Powheg, leading to the following values of heavy-flavour production cross-sections: d𝜎cc/d𝑦|$_{𝑦=0}$ = 974 ± 138(stat.) ± 140 (syst.) ± 214 (B.R.) μb and d𝜎bb/d𝑦|$_{𝑦=0}$ = 79 ± 14(stat.) ± 11 (syst.) ± 5 (B.R.) μb using Pythia simulations and d𝜎cc/d𝑦|𝑦=0 = 1417 ± 184(stat.) ± 204 (syst.) ± 312 (B.R.) μb and d𝜎bb/d𝑦|$_{𝑦=0}$ = 48±14(stat.)±7 (syst.)±3 (B.R.) μb using Powheg. The difference observed between Pythia and Powheg based results points to different kinematic correlations of the heavy- quark pairs in the two generators. These values, whose uncertainties are fully correlated between the two generators, are consistent with extrapolations from lower energies based on pQCD calculations. Recent studies of pp collisions with high charged-particle multiplicities showed sur- prising results similar to previous observations in heavy-ion collisions. Measurements of low-mass dielectrons could provide additional information regarding the underlying physics processes. This work compares the dielectron spectra in inelastic pp collisions and in collisions collected with high charged-particle multiplicities in various transverse-momentum intervals. The differences are consistent with the already measured multiplicity-dependent production of light and heavy hadrons at lower √𝑠. The production of direct photons in inelastic and high-multiplicity collisions is also dis- cussed. Upper limits on the contribution of direct photons to the inclusive photon spectrum are extracted at 90% confidence level and found to be in agreement with pQCD predictions.