Combining states without scale hierarchies with ordered parton showers

Abstract

We present a parameter-free scheme to combine fixed-order multi-jet results with parton-shower evolution. The scheme produces jet cross sections with leading-order accuracy in the complete phase space of multiple emissions, resumming large logarithms when appropriate, while not arbitrarily enforcing ordering on momentum configurations beyond the reach of the parton-shower evolution equation. This requires the development of a matrix-element correction scheme for complex phase-spaces including ordering conditions as well as a systematic scale-setting procedure for unordered phase-space points. The resulting algorithm does not require a merging-scale parameter. We implement the new method in the Vincia framework and compare to LHC data.

Helicity antenna showers for hadron colliders

“…The iterated ME correction formalism of [7,16,18,19] has been extended to cope with helicity-dependent clusterings and splitting kernels involving initialstate legs, and in this work has been applied to strongly ordered showers in a direct extension of the formalism presented in [19]. We further reported on new, user-specifiable uncertainty variations in Vincia, including renormalisationscale and splitting-kernel variations.…”

“…Vincia [17,18], again first in the context of e + e − → jets [16] and subsequently for hadron collisions [18,19]. Importantly, the most recent study in [19] extended the formalism to strongly-ordered and non-Markovian shower algorithms, expanding its applicability to essentially any shower algorithm in modern MC generators.…”

“…Importantly, the most recent study in [19] extended the formalism to strongly-ordered and non-Markovian shower algorithms, expanding its applicability to essentially any shower algorithm in modern MC generators. Although a helicitydependent (and hence computationally faster) version of the iterated-MEC algorithm was developed for final-state radiation [7], a fully-fledged helicity-dependent version for hadron collisions (and for strongly-ordered non-Markovian showers) has so far been missing.…”

“…However, general arguments indicate that the effective Sudakov factors for the nonordered histories, are probably not correct [18,24,25]. Recent efforts [19,25] have therefore shifted focus back to filling the phase space for multiple hard emissions while remaining within the paradigm of strong ordering. In particular, we take the strongly-ordered iterated-MEC formalism presented in [19] as our starting point, and adapt it to include explicit helicities.…”

“…Within the formalism presented in [19], the splitting kernels are redefined by multiplying them with the correction factor…”