Abstract
We present a complete set of helicity-dependent 2rightarrow 3 antenna functions for QCD initial- and final-state radiation. The functions are implemented in the Vincia shower Monte Carlo framework and are used to generate showers for hadron-collider processes in which helicities are explicitly sampled (and conserved) at each step of the evolution. Although not capturing the full effects of spin correlations, the explicit helicity sampling does permit a significantly faster evaluation of fixed-order matrix-element corrections. A further speed increase is achieved via the implementation of a new fast library of analytical MHV amplitudes, while matrix elements from Madgraph are used for non-MHV configurations. A few examples of applications to QCD 2rightarrow 2 processes are given, comparing the newly released Vincia 2.200 to Pythia 8.226.
Highlights
The description of bremsstrahlung processes in partonshower event generators typically starts from the probability density for unpolarised partons to emit unpolarised radiation, i.e., DGLAP kernels or dipole/antenna functions summed over outgoing and averaged over incoming polarisations/helicities
Helicity-dependent radiation functions, as used for final-state radiation in Vincia for a few years [7], do have some advantages: helicity conservation can be made explicit, allowing to trace helicities through the shower; unphysical helicity configurations are prevented from contributing to sums and averages; and the explicit helicity assignments allow faster evaluations of matrix-element correction (MEC) factors, since only a single helicity amplitudes need to be evaluated for each ME-corrected parton state [7]
For well-resolved radiation, we expect these observables to be sensitive to low-order ME corrections, and the uncertainty associated with nonsingular-term variations should be reduced when Vincia’s ME corrections are switched on. (Note: Pythia does not incorporate ME corrections for QCD 2 → 2 processes.) Parton-level results for showered gg → gg events are presented in Fig. 2 with uncertainty bands
Summary
The description of bremsstrahlung processes in partonshower event generators typically starts from the probability density for unpolarised partons to emit unpolarised radiation, i.e., DGLAP kernels or dipole/antenna functions summed over outgoing and averaged over incoming polarisations/helicities. The concept of ME corrections was first developed to improve the description of radiation in Pythia ( called Jetset) outside the collinear region to agree with first-order matrix elements for e+e− → 3 jets [8,9], and was since extended to correct the first emission in a wide range of resonance-decay processes and some (colour-singlet) production processes [10,11] It was used as a component of the first ME correction strategies in Herwig [12,13], and it forms the basis of the treatment of real corrections within the Powheg formalism [14,15]. (Note that the use of Madgraph 4 puts some limitations on the configurations for which the relevant information for MEC factors can be extracted from the matrix elements This is the case for amplitudes with multiple quark pairs. Appendix B summarises a few changes in the Vincia code which we deem relevant to ensure that results obtained with the new implementation may be interpreted correctly, in particular in comparison with results obtained with earlier versions
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