Coherent showers in decays of colored resonances

Abstract

We present a new approach to coherent parton showers in the decays of coloured resonances, based on the notion of "resonance-final" (RF) QCD antennae. A full set of mass-and helicity-dependent 2 → 3 antenna functions are defined, with the additional requirement of positivity over the respective branching phase spaces. Their singularity structure is identical to that of initial-final (IF) antennae in 2 → N hard processes (once mass terms associated with the incoming legs are allowed for), but the phase-space factorisations are different. The consequent radiation patterns respect QCD coherence (at leading colour) and reduce to DGLAP and eikonal kernels in the respective collinear and soft limits. The main novelty in the phase-space factorisation is that branchings in RF antennae impart a collective recoil to the other partons within the same decay system. An explicit implementation of these ideas, based on the Sudakov veto algorithm, is provided in the Vincia antenna-shower plug-in to the Pythia 8 Monte Carlo event generator. We apply our formalism, matched to next-to-leading order accuracy using Powheg , to top quark production at the LHC, and investigate implications for direct measurement of the top quark mass. Finally, we make recommendations for assessing theoretical uncertainties arising from parton showers in this context.

What Is the Top Quark Mass?

“…The relation of the MMC top mass parameter to any mass scheme and the question of universality and observable independence could then be obtained from computations rather than from speculations. Developments in the direction of NLL precise PSs are already underway-for instance, concerning a more precise description of the parton splitting (163)(164)(165), the restriction of dipole-type showers for global observables (166,167), finite-lifetime effects (168), and full color coherence (169,170)-but there is still a long way to go.…”

Collinear electroweak radiation in antenna parton showers

“…We use the branching kernels computed in the previous section to implement an electroweak shower in the Vincia framework, which was set out in [33][34][35][36][37][59][60][61]. Here we first provide a brief summary before continuing with a description of some details specific to the electroweak shower.…”

“…Most generally, resonances should be allowed to decay at any scale during the shower evolution of the hard scattering. Once a decay occurs, a factorized shower in the resonance system may be performed using Vincia's resonance shower described in [61]. Afterwards, the showered decay products may be joined with the rest of the hard system and the evolution can be continued.…”

Sector showers for hadron collisions

“…Helicity-and mass-dependent sector antenna functions for initial-and final-state radiation were already presented in [45,46], although without a dedicated implementation in a shower algorithm. We here present a new implementation of a sector shower based on the VINCIA antenna shower in the Pythia 8.3 framework, 3 including helicity dependence [47,48], mass corrections [20], initial-state radiation [22,49], resonance decays [50], and interleaved coherent QED branchings [51]. We define a full set of helicity-dependent sector antenna functions for initial-and final-state radiation entirely based on crossings and sums of global final-final antenna functions and validate the new shower against leadingorder matrix elements, the global VINCIA shower, PYTHIA 8.3, and experimental data.…”