This letter reports measurements which characterize the underlying event associated with hard scatterings at mid-pseudorapidity (|η| < 0.8) in pp, p–Pb and Pb–Pb collisions at centre-of-mass energy per nucleon pair, √sNN = 5.02TeV. The measurements are performed with ALI
scatterings are identified by the leading particle defined as the charged particle with the largest transverse momentum (pT) in the collision and having 8 < pT < 15GeV/c. The pT spectra of associated particles (0.5 ≤ pT < 6GeV/c) are measured in different azimuthal regions defined with respect to the
leading particle direction: toward, transverse, and away. The associated charged particle yields in the transverse region are subtracted from those of the away and toward regions. The remaining jet-like yields are reported as a function of the multiplicity measured in the transverse region. The measurements show a suppression of the jet-like yield in the away region and an enhancement of high-pT associated particles in the toward region in central Pb–Pb collisions, as compared to minimum-bias pp collisions. These observations are consistent with previous measurements that used two-particle correlations, and with an interpretation in terms of parton energy loss in a high-density quark gluon plasma. These yield modifications vanish in peripheral Pb–Pb collisions and are not observed in either high-multiplicity pp or p–Pb collisions.
