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%.
Highlights
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 sensitive to the properties of the UE
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
In this paper we report the measurement of UE observables with the ATLAS detector [13] at the LHC, using charged particles in 1.6 nb−1 of proton-proton collisions at a centre-of-mass energy of 13 TeV
Summary
The UE observables in this study are constructed from “primary” charged particles in the pseudorapidity range |η| < 2.5,1 whose transverse momentum (pT) is required to be greater than 500 MeV. The majority of underlying-event observables study the dependences of the averaged quantities on the transverse momentum of the leading object — here the leading charged particle The development of this from low to high pT corresponds to the smooth transition from “minimum bias” interactions to the hard-scattering regime focused on by most LHC analyses, and the correlation distributions characterise how soft QCD effects co-evolve with the hard process through this transition. This analysis studies the dependence of the observables on the azimuthal angle with respect to the leading particle and each region’s charged-particle multiplicity. For the observables studied as a function of relative azimuthal angle, the leading particle is excluded from the spectrum
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