Abstract
The measurement of charged-particle event shape variables is presented in inclusive inelastic pp collisions at a center-of-mass energy of 7 TeV using the ATLAS detector at the LHC. The observables studied are the transverse thrust, thrust minor and transverse sphericity, each defined using the final-state charged particles' momentum components perpendicular to the beam direction. Events with at least six charged particles are selected by a minimum-bias trigger. In addition to the differential distributions, the evolution of each event shape variable as a function of the leading charged particle transverse momentum, charged particle multiplicity and summed transverse momentum is presented. Predictions from several Monte Carlo models show significant deviations from data.
Highlights
Event shape variables describe the structure of hadronic events and the properties of their energy flow
The following distributions are measured: (i) normalized distributions: ð1=NevÞdNev=dc?h, ð1=NevÞdNev=dTMch, ð1=NevÞdNev=dSc?h; (ii) average vPalues: hc?hi, hTMchi and hSc?hi as functions of Nch and pT; where Nev is the number of events with six or more charged particles within the selected kinematic range; PNch is the number of charged particles in an event; pT is the scalar sum of the transverse momenta of the charged particles in the event
The total correction applied to account for events lost due to the trigger and vertex requirements is less than 1% and it varies very weakly with the number of tracks associated with the primary vertex
Summary
Event shape variables describe the structure of hadronic events and the properties of their energy flow. The study of event shape observables in inclusive inelastic collisions plays an important role in understanding soft-QCD processes at LHC center-of-mass energies [18], where ‘‘soft’’ refers to interactions with. Soft interactions cannot be reliably calculated from theory and are generally described by phenomenological models, usually implemented in Monte Carlo (MC) event generators These models contain many parameters whose values are a priori unknown and need to be constrained by measurements. The measurements presented in this paper can further constrain the event generator models, which encapsulate our understanding of these soft processes In this analysis, the event shape observables are constructed from six or more primary charged particles in the pseudorapidity range jj < 2:5 and with transverse momentum pT > 0:5 GeV [26].
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