Charged-particle production as a function of the relative transverse activity classifer in pp, p–Pb, and Pb–Pb collisions at the LHC

Abstract

Measurements of charged-particle production in pp, p–Pb, and Pb–Pb collisions in the toward, away, and transverse regions with the ALICE detector are discussed. These regions are defned event-by-event relative to the azimuthal direction of the charged trigger particle, which is the reconstructed particle with the largest transverse momentum (p trig T ) in the range 8 < ptrig T < 15 GeV/c. The toward and away regions contain the primary and recoil jets, respectively; both regions are accompanied by the underlying event (UE). In contrast, the transverse region perpendicular to the direction of the trigger particle is dominated by the so-called UE dynamics, and includes also contributions from initial- and fnal-state radiation. The relative transverse activity classifer, RT = NT ch/⟨NT ch⟩, is used to group events according to their UE activity, where NT ch is the charged-particle multiplicity per event in the transverse region and ⟨NT ch⟩ is the mean value over the whole analysed sample. The energy dependence of the RT distributions in pp collisions at √ s = 2.76, 5.02, 7, and 13 TeV is reported, exploring the Koba-Nielsen-Olesen (KNO) scaling properties of the multiplicity distributions. The frst measurements of charged-particle pT spectra as a function of RT in the three azimuthal regions in pp, p–Pb, and Pb–Pb collisions at √ sNN = 5.02 TeV are also reported. Data are compared with predictions obtained from the event generators PYTHIA 8 and EPOS LHC. This set of measurements is expected to contribute to the understanding of the origin of collective-like efects in small collision systems (pp and p–Pb).