A Positive-Weight Next-to-Leading-Order Monte Carlo for Z Pair Hadroproduction
Paolo Nason, Giovanni Ridolfi
TL;DR
This work applies a positive-weight hardest-emission framework (POWHEG) to ZZ hadroproduction, enabling NLO accuracy with seamless interfacing to standard shower Monte Carlos (HERWIG, PYTHIA) and avoiding negative-weight events found in some prior methods. It formalizes a factorized cross section in terms of Born and radiation variables, and introduces a positive-weight sampling strategy for the hardest emission via a B-like function and a Sudakov form factor with near-NLL precision. The approach reproduces NLO predictions for Born-like observables while capturing radiation effects in observables such as p_T(ZZ) and angular separations, and shows consistency with MC@NLO across Tevatron and LHC energies. Overall, the method offers a robust, positive-weight alternative for NLO+shower matching with practical advantages in convergence and LHE interfacing.
Abstract
We present a first application of a previously published method for the computation of QCD processes that is accurate at next-to-leading order, and that can be interfaced consistently to standard shower Monte Carlo programs. We have considered Z pair production in hadron-hadron collisions, a process whose complexity is sufficient to test the general applicability of the method. We have interfaced our result to the HERWIG and PYTHIA shower Monte Carlo programs. Previous work on next-to-leading order corrections in a shower Monte Carlo (the MC@NLO program) may involve the generation of events with negative weights, that are avoided with the present method. We have compared our results with those obtained with MC@NLO, and found remarkable consistency. Our method can also be used as a standalone, alternative implementation of QCD corrections, with the advantage of positivity, improved convergence, and next-to-leading logarithmic accuracy in the region of small transverse momentum of the radiated parton.