Trijets in k_mathrm{T}-factorisation: matrix elements vs parton shower

H Van Haevermaet,P Van Mechelen,A Van Hameren,K Kutak,P Kotko

doi:10.1140/epjc/s10052-020-8193-2

Abstract

We study 3-jet event topologies in proton-proton collisions at a centre-of-mass energy of sqrt{s} = 13 mathrm{ TeV} in a configuration, where one jet is present in the central pseudorapidity region (|eta | < 2.0) while two other jets are in a more forward (same hemisphere) area (|eta | > 2.0). We compare various parton level predictions using: collinear factorisation, k_mathrm{T}-factorisation with fully off-shell matrix elements and the hybrid framework. We study the influence of different parton distribution functions, initial state radiation, final state radiation, and hadronisation. We focus on differential cross sections as a function of azimuthal angle difference between the leading dijet system and the third jet, which is found to have excellent sensitivity to the physical effects under study.

Highlights

Multiple large scales, require different formalisms utilising various all-order resummations of potentially large logarithms
At the Large Hadron Collider (LHC), many jet observables are subject to resummation and other corrections reaching beyond collinear factorisation
Good jet reconstruction capabilities allow to measure the azimuthal angle between jets, which is sensitive to soft gluon emissions and to the transverse momentum of partons inside hadrons

Summary

Introduction

A formal theoretical framework dealing with parton transverse momenta, kT, to leading power accuracy is the Transverse Momentum Dependent (TMD) factorisation theorem [2,3] (for recent applications see [4,5,6]), which holds to all-orders only for processes with a total of at most two hadrons in the initial or final state. General purpose Monte Carlo generators, like Pythia[9,10], Herwig[11,12], and Sherpa[13] use collinear factorisation in combination with parton showers to generate partons with non-zero kT. This variety of approaches with different realisations of potentially similar mechanisms calls for a detailed comparison and validation, as well as confrontation with experimental data. In this paper we investigate the kT-factorisation approach, as well as collinear factorisation supplemented with parton showers, in the context of trijet production processes.

Theoretical framework

Kinematics and Monte Carlo event generator setup

Parton level predictions

Effects of parton showers and hadronisation

Effects of matrix element parton multiplicity

Conclusions