Abstract
This article reports measurements characterizing the Underlying Event (UE) associated with hard scatterings at midrapidity (|η| < 0.8) in pp collisions at sqrt{s} = 13 TeV. The hard scatterings are identified by the leading particle, the charged particle with the highest transverse momentum ( {p}_{mathrm{T}}^{mathrm{leading}} ) in the event. Charged-particle number-densities and summed transverse-momentum densities are measured in different azimuthal regions defined with respect to the leading particle direction: Toward, Transverse, and Away. The Toward and Away regions contain the fragmentation products of the hard scatterings in addition to the UE contribution, whereas particles in the Transverse region are expected to originate predominantly from the UE. The study is performed as a function of {p}_{mathrm{T}}^{mathrm{leading}} with three different pT thresholds for the associated particles, {p}_{mathrm{T}}^{mathrm{track}} > 0.15, 0.5, and 1.0 GeV/c. The charged-particle density in the Transverse region rises steeply for low values of {p}_{mathrm{T}}^{mathrm{leading}} and reaches a plateau. The results confirm the trend that the charged-particle density in the Transverse region shows a stronger increase with sqrt{s} than the inclusive charged-particle density at midrapidity. The UE activity is increased by approximately 20% when going from 7 TeV to 13 TeV pp collisions. The plateau in the Transverse region (5 < {p}_{mathrm{T}}^{mathrm{leading}} < 40 GeV/c) is further characterized by the probability distribution of its charged-particle multiplicity normalized to its average value (relative transverse activity, RT) and the mean transverse momentum as a function of RT. Experimental results are compared to model calculations using PYTHIA 8 and EPOS LHC. The overall agreement between models and data is within 30%. These measurements provide new insights on the interplay between hard scatterings and the associated UE in pp collisions.
Highlights
Toward and Away regions contain the fragmentation products of the hard scatterings in addition to the Underlying Event (UE) contribution, whereas particles in the Transverse region are expected to originate predominantly from the UE
This paper reports measurements characterizing the UE a√ssociated with hard scatterings performed at midrapidity (|η| < 0.8) in pp collisions at s = 13 TeV based on the CDF method [5], which utilizes the leading-charged particles
The present paper reports the first measurement of the RT probability distribution and the mean transverse momentum pT in the Transverse region as a function of RT
Summary
The UE observables considered in this study are based on primary charged particles reconstructed in the pseudorapidity range |η| < 0.8 with three different thresholds of the transverse momentum: ptTrack > 0.15, 0.5, and 1.0 GeV/c, for both the leading particle and the associated particles used in the correlation studies. For plTeading above the onset of the jet pedestal plateau, the UE depends only weakly on this quantity It has been proposed in [29] to study the UE properties in events that contain one leading object with plTeading in the range of the plateau, as functions of a. Where Ninc is the inclusive number of charged particles in an event and Ninc is the eventaveraged number-density, both evaluated in the Transverse region. Using this observable as an event classifier one can, as proposed in [29], test whether events with very small UE activity are compatible with equivalent measurements in e+e− collisions (jet universality) or whether the scaling behaviour towards high UE activity exhibits properties of nontrivial soft-QCD dynamics.
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