Underlying-event properties in pp and p–Pb collisions at $\sqrt{{s}_{\mathrm{NN}}}$ = 5.02 TeV

We report about the properties of the underlying event measured with ALICE at the LHC in pp and p–Pb collisions at $\sqrt{{s}_{\mathrm{NN}}}$ = 5.02 TeV. The event activity, quantified by charged-particle number and summed-p T densities, is measured as a function of the leading-particle transverse momentum $\left({p}_{T}^{\text{trig}}\right)$ . These quantities are studied in three azimuthal-angle regions relative to the leading particle in the event: toward, away, and transverse. Results are presented for three different p T thresholds (0.15, 0.5 and 1 GeV/c) at mid-pseudorapidity (|η| < 0.8). The event activity in the transverse region, which is the most sensitive to the underlying event, exhibits similar behaviour in both pp and p–Pb collisions, namely, a steep increase with ${p}_{T}^{\text{trig}}$ for low ${p}_{T}^{\text{trig}}$ , followed by a saturation at ${p}_{T}^{\text{trig}}\approx 5$ GeV/c. The results from pp collisions are compared with existing measurements at other centre-of-mass energies. The quantities in the toward and away regions are also analyzed after the subtraction of the contribution measured in the transverse region. The remaining jet-like particle densities are consistent in pp and p–Pb collisions for ${p}_{T}^{\text{trig}}>10$ GeV/c, whereas for lower ${p}_{T}^{\text{trig}}$ values the event activity is slightly higher in p–Pb than in pp collisions. The measurements are compared with predictions from the PYTHIA 8 and EPOS LHC Monte Carlo event generators.