Abstract
We present a first implementation of collinear electroweak radiation in the Vincia parton shower. Due to the chiral nature of the electroweak theory, explicit spin dependence in the shower algorithm is required. We thus use the spinor-helicity formalism to compute helicity-dependent branching kernels, taking special care to deal with the gauge relics that may appear in computation that involve longitudinal polarizations of the massive electroweak vector bosons. These kernels are used to construct a shower algorithm that includes all possible collinear final-state electroweak branchings, including those induced by the Yang–Mills triple vector boson coupling and all Higgs couplings, as well as vector boson emissions from the initial state. We incorporate a treatment of features particular to the electroweak theory, such as the effects of bosonic interference and recoiler effects, as well as a preliminary description of the overlap between electroweak branchings and resonance decays. Some qualifying results on electroweak branching spectra at high energies, as well as effects on LHC physics are presented. Possible future improvements are discussed, including treatment of soft and spin effects, as well as issues unique to the electroweak sector.
Highlights
Work such as [21,22] has instead focussed on the incorporation of electroweak logarithms in the resummation of inclusive observables at high energies
An implementation [27,28] is available in Pythia event generator [23] which only includes the radiation of electroweak gauge bosons and does not retain any spin information
The formalism descibed in this paper makes use of the spinor-helicity formalism to compute the large number of spin-dependent branching kernels associated with the electroweak shower
Summary
Work such as [21,22] has instead focussed on the incorporation of electroweak logarithms in the resummation of inclusive observables at high energies. The only practical solution is to instead include EW effects in a systematic way in parton shower algorithms as part of a general-purpose event generator such as Pythia [23], Sherpa [24] or Herwig [25] These produce fully differential final states, and can be set up to include specific phenomena exclusive to the EW sector such the effects of symmetry breaking and gauge boson decay. Vincia is a plugin to the Pythia event generator and already allows for QCD evolution with partons of definite helicity states [36,37] This feature is especially important in the electroweak theory due to its chiral nature.
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