# Measurement of charged-particle distributions sensitive to the underlying event in $\sqrt{s} = 13$ TeV proton-proton collisions with the ATLAS detector at the LHC

## TL;DR

This paper measures charged-particle distributions in 13 TeV proton-proton collisions to understand the underlying event activity, providing data to improve Monte Carlo models with high precision at LHC energies.

## Contribution

It presents the first detailed measurement of underlying event-sensitive charged-particle distributions at 13 TeV, aiding the development of more accurate event generators.

## Key findings

- Models describe data to 5% accuracy
- Data uncertainties are less than 1%
- Provides new insights into UE activity at 13 TeV

## Abstract

We present charged-particle distributions 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 Collider fills corresponding to an integrated luminosity of 1.6 nb$^{-1}$. The distributions were constructed using charged particles with absolute pseudorapidity less than 2.5 and with transverse momentum greater than 500 MeV, in events with at least one such charged particle with transverse momentum above 1 GeV. These distributions characterise the angular distribution of energy and particle flows with respect to the charged particle with highest transverse momentum, as a function of both that momentum and of charged-particle multiplicity. 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-event activity at LHC Run 2 energies and providing inputs for the development of event generator modelling. The current models in use for UE modelling typically describe this data to 5% accuracy, compared with data uncertainties of less than 1%.