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

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 (p^trig_T) in the range 8 < p^trig_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 final-state radiation. The relative transverse activity classifer, R_T = N^T_ch/⟨N^T_ch⟩, is used to group events according to their UE activity, where N^T_ch is the charged-particle multiplicity per event in the transverse region and ⟨N^T_ch⟩ is the mean value over the whole analysed sample. The energy dependence of the R_T distributions in pp collisions at √s = 2.76, 5.02, 7, and 13 TeVis reported, exploring the Koba-Nielsen-Olesen (KNO) scaling properties of the multiplicity distributions. The first measurements of charged-particle p_T spectra as a function of R_T in the three azimuthal regions in pp, p–Pb, and Pb–Pb collisions at √s_NN = 5.02 TeV are also reported. Data are compared with predictions obtained from the event generators PYTHIA 8and 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).