Charged-particle production as a function of the relative transverse activity classifier 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 defined event-by-event relative to the azimuthal direction of the charged trigger particle, which is the reconstructed particle with the largest transverse momentum (pTtrig​) in the range 8<pTtrig​<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 also includes contributions from initial- and final-state radiation. The relative transverse activity classifier RT​=NchT​/⟨NchT​⟩ is used to group events according to their UE activity, where NchT​ is the charged-particle multiplicity per event in the transverse region and ⟨NchT​⟩ 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 first 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 effects in small collision systems (pp and p–Pb).