Measurements of the production cross-section for a Z boson in association with b-jets in proton-proton collisions at $$ \sqrt{s} $$ = 13 TeV with the ATLAS detector

Abstract

This paper presents a measurement of the production cross-section of a Z boson in association with b-jets, in proton-proton collisions at √ s = 13 TeV with the ATLAS experiment at the Large Hadron Collider using data corresponding to an integrated luminosity of 35.6 fb −1. Inclusive and differential cross-sections are measured for events containing a Z boson decaying into electrons or muons and produced in association with at least one or at least two b-jets with transverse momentum p T > 20 GeV and rapidity |y| < 2.5. Predictions from several Monte Carlo generators based on leading-order (LO) or next-to-leading-order (NLO) matrix elements interfaced with a parton-shower simulation and testing different flavour schemes for the choice of initial-state partons are compared with measured cross-sections. The 5-flavour number scheme predictions at NLO accuracy agree better with data than 4-flavour number scheme ones. The 4-flavour number scheme predictions underestimate data in events with at least one b-jet.

Measurements of the inclusive and differential production cross sections of a top-quark–antiquark pair in association with a Z boson at $$\sqrt{s} = 13$$ TeV with the ATLAS detector

“…The W Z/Z Z + jets components with b-or c-jets are constrained to their MC predictions and normalisation uncertainties of 50% (W Z/Z Z + b) and 30% (W Z/Z Z +c) are assigned to them. These uncertainties are evaluated from data/MC comparisons in Z + b/c events [82], but also take into account differences in the heavyflavour jet fractions between Z + jets and W Z/Z Z + jets events. Modelling uncertainties of W Z/Z Z + jets related to the μ r and μ f scales and the PDF choice are obtained with the same prescription as for the signal.…”

Search for new phenomena in final states with b-jets and missing transverse momentum in $$ \sqrt{\mathrm{s}} $$ = 13 TeV pp collisions with the ATLAS detector

“…The matching scale between the matrix element and parton shower calculation, and the resummation scale for soft gluon emission, are also varied by factors of 0.5 and 2. As no Monte Carlo generator has been found to accurately describe Z + bb production in all observables [129], nor are these discrepancies accounted for by scale variations, an uncertainty due to the choice of generator is evaluated by comparing the nominal samples with those produced using aMC@NLO 2.3.3 + Pythia. After constraints from the control regions these variations are found to be relevant only in SRD, where modelling uncertainties dominate the systematic effect on the shape of the cos θ * bb distribution.…”

Beautiful and charming chromodipole moments

“…Inclusive and differential measurements of Z-boson production in association with b-jets have been performed at the LHC by both the ATLAS and CMS collaborations [40][41][42][43][44][45][46]. These measurements provide not only an important role in improving our quantitative understanding of perturbative QCD (see e.g.…”

“…[47]), but can also be exploited to search for BSM physics [48]. In this section we will consider the measurements performed in [45] to set bounds on the Wilson coefficient of the operator Q bG . We calculate the contribution of the operator Q bG to pp → bb Z/γ * → bb + − at LO using MadGraph5_aMCNLO together with PYTHIA 8.2 and NNPDF31_nlo_as_0118 PDFs.…”

“…Relevant Feynman diagrams are shown in figure 5. To be able compare our results to an existing analysis we employ the event selections of the recent ATLAS measurement [45] which is based on 35.6 fb −1 of 13 TeV LHC data. Electrons must satisfy the tight likelihood requirement [49], and are required to have a transverse momentum and pseudorapidity of p T (e) > 27 GeV and |η(e)| < 2.47, respectively.…”