Abstract

A measurement of the jet activity in tbar{t} events produced in proton–proton collisions at a centre-of-mass energy of 7 TeV is presented, using 2.05 fb−1 of integrated luminosity collected by the ATLAS detector at the Large Hadron Collider. The tbar{t} events are selected in the dilepton decay channel with two identified b-jets from the top quark decays. Events are vetoed if they contain an additional jet with transverse momentum above a threshold in a central rapidity interval. The fraction of events surviving the jet veto is presented as a function of this threshold for four different central rapidity interval definitions. An alternate measurement is also performed, in which events are vetoed if the scalar transverse momentum sum of the additional jets in each rapidity interval is above a threshold. In both measurements, the data are corrected for detector effects and compared to the theoretical models implemented in MC@NLO, Powheg, Alpgen and Sherpa. The experimental uncertainties are often smaller than the spread of theoretical predictions, allowing deviations between data and theory to be observed in some regions of phase space.

Highlights

  • In this article, a jet veto is used to quantify the jet activity that arises from quark and gluon radiation produced in association with the ttsystem

  • The difference between the Monte Carlo (MC)@next-to-leading order (NLO) and POWHEG predictions is similar to the precision achieved in the measurement and as such the measurement is probing the different approaches to NLO plus parton-shower event generation

  • This implies that, for this variable, the parton shower approximations used for the subsequent emissions in MC@NLO and POWHEG are performing as well as the leading order (LO) approximations used in ALPGEN and SHERPA

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Summary

Introduction

A jet veto is used to quantify the jet activity that arises from quark and gluon radiation produced in association with the ttsystem. Where N is the number of selected ttevents and n(Q0) is the subset of these events that do not contain an additional jet with transverse momentum, pT, above a threshold, Q0, in a central rapidity interval. The gap fraction is measured for multiple values of Q0 and for four jet rapidity intervals: |y| < 0.8, 0.8 ≤ |y| < 1.5, 1.5 ≤ |y| < 2.1 and |y| < 2.1. An alternate definition of the gap fraction is used in this case, f (Qsum) n(Qsum) N. where n(Qsum) is the number of ttevents, and σ (Qsum) is the cross section, in which the scalar transverse momentum sum of the additional jets in the rapidity interval is less than Qsum. The data are expected to constrain the modelling of quark and gluon radiation in ttevents and provide useful information about the general theoretical description of jet vetoes, which have been proposed as a tool to enhance new physics signals [12,13,14], and to study the properties of new fundamental particles [15,16,17]

ATLAS detector
Theoretical predictions
Simulation samples
Event selection
Correction for detector effects
Systematic uncertainties
Results and discussion
Conclusions
Full Text
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