A Positive-Weight Next-to-Leading-Order Monte Carlo for e+e- Annihilation to Hadrons

TL;DR

This paper implements a positive-weight NLO Monte Carlo framework (Nason) for e+e- → hadrons by first generating the hardest gluon emission using a modified Sudakov form factor, then adding a simplified truncated soft shower before it, and interfacing the result with Herwig++. The approach avoids negative weights and aims to preserve soft radiation patterns, with a veto mechanism to recover the exact NLO distribution. Results show improved agreement with LEP data across many observables, particularly in soft regions, relative to Herwig++ with matrix-element corrections. Limitations arise from approximations in the truncated shower, motivating future extensions to processes with initial-state radiation in hadron-hadron collisions (e.g., Tevatron/LHC).

Abstract

We apply the positive-weight Monte Carlo method of Nason for simulating QCD processes accurate to Next-To-Leading Order to the case of e+e- annihilation to hadrons. The method entails the generation of the hardest gluon emission first and then subsequently adding a `truncated' shower before the emission. We have interfaced our result to the Herwig++ shower Monte Carlo program and obtained better results than those obtained with Herwig++ at leading order with a matrix element correction.

Figures (10)

• Figure 1: Transverse momentum, $k_T$.
• Figure 2: Phase space and $y$ solutions for $\kappa<0.08333$ in the region $x>y$.
• Figure 3: Phase space and $y$ solutions for $\kappa>0.08333$ in the region $x>y$.
• Figure 4: Phase space and distribution of hardest emissions.
• Figure 6: The distribution of the charged particle multiplicity. Data from the OPAL experiment at LEP Acton:1991aa.