Measurement of charged-particle distributions sensitive to the underlying event in √s=13 TeV proton-proton collisions with the ATLAS detector at the LHC

We present charged-particledistributions sensitive to the underlying event, measured by the ATLAS detector in proton-proton collisions at a centre-of-mass energy of 13 TeV, in low-luminosity Large Hadron Colliderfills corresponding to an integrated luminosity of 1.6 nb−1. The distributions were constructed using charged particleswith absolute pseudorapidityless than 2.5 and with transverse momentumgreater than 500 MeV, in events with at least one such charged particlewith transverse momentumabove 1 GeV. These distributions characterise the angular distribution of energy and particle flows with respect to the charged particlewith highest transverse momentum, as a function of both that momentumand of charged-particlemultiplicity. The results have been corrected for detector effects and are compared to the predictions of various Monte Carlo event generators, experimentally establishing the level of underlying-eventactivity at LHC Run 2 energies and providing inputs for the development of event generatormodelling. The current models in use for UE modelling typically describe this data to 5% accuracy, compared with data uncertainties of less than 1%.