First Measurement Of The Underlying Event Activity In Pp And P-Pb Collisions At A Centre-of-mass Energy Of 5.02 TeV Per Nucleon Pair With ALICE

To perform precise Standard Model measurements or to search for new physics phenomena at hadron colliders,it is important to have a good understanding not only of the primary short-distance hard scattering process,but also of the accom-panying interactions of the rest of the proton-proton collision-collectively termed the underlying event(UE).As the UE is an intrinsic part of the same proton-proton collision as any signal partonic interaction,a deep understanding of the UE features and an accurate Monte Carlo description of its properties is important for the LHC physics programme.Adopting the terminology of the MC models,the UE can re-ceive contributions from Multiple Parton Interactions(MPI),i.e.additional hard scatters in the same pp collisions,from initial-and final-state radiation(ISR,FSR),from the QCD evolution of colour connections between the hard scattering and the beam-proton remnants.As it is significantly influenced by physics not currently cal-culable from first principles,the measurement of the UE properties is crucial not only for better understanding of the mechanisms involved but also to provide input for empirical tuning of the free parameters of phenomenological UE models in MC event generators.It is impossible to uniquely separate the UE from the hard scattering process on an event-by-event basis,but observables can be defined which are particularly sensitive to the properties of the UE.In order to estimate the contributions of the UE,the azimuthal plane of the event is segmented into several distinct regions with differing sensitivities to the UE-based on the azimuthal angular difference with respect to the leading(highest charged-particle transverse momentum)charged track define Transverse,Toward and Away regions.The Toward and Away regions are dominated by particle production from the hard process and are hence relatively insensitive to the UE.In contrast,the transverse region is more sensitive to the UE,and observables defined inside it are the primary focus of UE measurements.This analysis work will be essential for the study of the energy dependence of the UE,extending the ALICE measurements in pp interactions at(?)= 5.02 TeV.On top of that,for the first time,this study does focus on the study of the system dependence of the UE,extending the analysis to p-Pb interactions at(?)= 5.02 TeV.The UE is studied at reconstruction level(i.e.with no corrections applied yet),the summed pT density distribution and the azimuthal correlation of the charged particles with respect to the leading charged tracks.A significant growth of the UE activities(about a factor 2.7 in the plateau)is observed,for the same pT threshold,from pp collisions to p-Pb collisions.