VINCIA for hadron colliders

Abstract

We present the first public implementation of antenna-based QCD initial- and final-state showers. The shower kernels are antenna functions, which capture not only the collinear dynamics but also the leading soft (coherent) singularities of QCD matrix elements. We define the evolution measure to be inversely proportional to the leading poles, hence gluon emissions are evolved in a measure inversely proportional to the eikonal, while processes that only contain a single pole (e.g., ) are evolved in virtuality. Non-ordered emissions are allowed, suppressed by an additional power of . Recoils and kinematics are governed by exact on-shell phase-space factorisations. This first implementation is limited to massless QCD partons and colourless resonances. Tree-level matrix-element corrections are included for QCD up to (4 jets), and for Drell–Yan and Higgs production up to (V / H + 3 jets). The resulting algorithm has been made publicly available in Vincia 2.0.

Systematics of quark/gluon tagging

“…• Vincia 2.001 [44], 1 Of course, the quantum numbers of the color singlet operator are not measurable event by event. The idea here is to have a fundamental definition of "quark" and "gluon" that does not reference QCD partons directly.…”

“…The large changes seen at hadron level from turning off g → qq splittings motivates further investigations into the shower/hadronization interface. Our Vincia baseline uses the default setup for version 2.001 [44], which includes "smooth ordering" and LO matrix-element corrections [113] up to O(α 3 s ) for both e + e − → qq and e + e − → gg. The coupling α s is evaluated with 2-loop running defined by α s (M Z ) = 0.118 (reinterpreted according to the CMW scheme [114]) with µ R = 0.6p ⊥ as the renormalization scale for gluon emissions and µ R = 0.5m qq for g → qq branchings.…”

Coherent Showers for the LHC

“…The method described here is now not our preferred choice, which will be explored in Ref. [10]. As mentioned before, to apply the MECs in Z production, we start the shower from the phase-space maximum, the hadronic centre-of-mass energy squared s. To do so we use the same scale as factorization scale in the hard process and apply a reweighting procedure, resulting in the following Born exclusive cross section at the shower cutoff scale µ,…”

Parton-shower uncertainties with Herwig 7: benchmarks at leading order

“…Recent development of these simulations has seen improvements in various areas, both within perturbative calculations, through matching to fixed order [2,[7][8][9][10][11][12][13][14][15], combining higher jet multiplicities [16][17][18][19][20][21][22], as well as the all-order resummation with parton showers [23][24][25][26] and also within the non-perturbative, phenomenological models [27,28]. While there are well established prescriptions on how to quantify the theoretical uncertainty of fixed-order calculations due to missing higher-order contributions [29][30][31][32][33][34][35] 1 , there is no such consensus for general resummed calculations [36][37][38][39][40][41], and parton-shower algorithms in particular [42][43][44][45][46][47], since a number of ambiguities are present within the different schemes; however, there is progress in towards this goal. Given the perturbative improvements, and the expected precision from data-taking at Run II of the Large Hadron Col- …”