On the treatment of resonances in next-to-leading order calculations matched to a parton shower

Abstract

In this work we present a new subtraction method for next-to-leading order calculations that is particularly convenient even when narrow resonances are present. The method is particularly suitable for the implementation of next-to-leading order calculations matched to parton shower generators. It allows at the same time for the inclusion of all finite width effects, including interferences, and for a consistent treatment of resonances in the shower approach, preserving the mass of resonances near their peak. We implement our method, in a fully general and automatic way, within the POWHEG BOX framework, and illustrate it using as a test case the process of pp → µ + ν µ j b j, that is dominated by t-channel single top production.

NLO QCD predictions for off-shell t t ¯ $$ t\overline{t} $$ and t t ¯ H $$ t\overline{t}H $$ production and decay at a linear collider

“…In order to have these resonance effects under control, in Whizard we have implemented an automatized version of the resonance-aware scheme of ref. [35]. This is also a prerequisite for a future consistent matching of off-shell processes with parton showers.…”

“…Furthermore, employing the resonance-aware method of ref. [35], the process pp → bb4f has been matched consistently to parton showers, as presented recently in ref. [36].…”

“…As discussed for the first time in ref. [35], the problem is due to the fact that the momentum of the resonant particle can be different in the Born phase space and the corresponding real phase spaces with one additional gluon momentum. 2 The real-subtracted contribution to the NLO matrix element contains N + 1-particle matrix elements with corresponding kinematics, as well as Born matrix elements with factorized kinematics in the subtraction terms.…”

Scalar production and decay to top quarks including interference effects at NLO in QCD in an EFT approach

“…[78] and [79,80] resonance aware matching has been proposed within the MC@NLO and Powheg frameworks respectively, with focus on narrow coloured resonances. In our configuration the resonances are broader and the interference plays a more important role.…”

Probing the top-quark width through ratios of resonance contributions of e + e − → W + W − b b ¯ $$ {\mathrm{e}}^{+}{\mathrm{e}}^{-}\to {\mathrm{W}}^{+}{\mathrm{W}}^{-}\mathrm{b}\overline{\mathrm{b}} $$

“…However, given that at LO the squared matrix element is proportional to α 2 s and that the predicted cross section additionally carries a dependence on a factorization scale, µ F , the uncertainty due to the variation of these scales is significantly 12 This can be done within the framework of MadGraph5 aMC@NLO as well as that of the WHIZARD Event Generator, though the consistent matching to parton shower of the W + W − bb process is not totally straightforward and requires care due to the presence of intermediate coloured resonances (see the discussions in refs. [88,89]). …”