On the Treatment of Resonances in Next-to-Leading Order Calculations Matched to a Parton Shower

TL;DR

This work solves the challenge of performing NLO calculations matched to parton showers in the presence of intermediate resonances by introducing a resonance-aware subtraction that decomposes the cross section into definite resonance histories. Each history carries a Breit–Wigner–weighted resonance projection $P^{f_b}$, and the subtraction and real-emission terms are partitioned to preserve resonance masses in their underlying Born mappings, ensuring stable cancellations and correct showering. The method is implemented in a generalized form in POWHEG-BOX-RES and validated on a single-top $t$-channel process, showing improved convergence and discernible differences in top-mass reconstruction compared to non-resonance-aware approaches. The framework enables consistent treatment of finite-width effects and interference across production and decay, providing a practical path toward more accurate NLO+PS simulations of resonant processes.

Abstract

In this work we present a new subtraction method for next-to-leading order calculations that is particularly convenient even when narrow resonances are present. The method is particularly suitable for the implementation of next-to-leading order calculations matched to parton shower generators. It allows at the same time for the inclusion of all finite width effects, including interferences, and for a consistent treatment of resonances in the shower approach, preserving the mass of resonances near their peak. We implement our method, in a fully general and automatic way, within the POWHEG BOX framework, and illustrate it using as a test case the process of $p p \to μ^+ ν_μj_b j$, that is dominated by $t$-channel single top production.

Figures (21)

• Figure 1: Single top $t$-channel production.
• Figure 2: Feynman diagrams for $u \bar{u} \rightarrow u \bar{d} \bar{u} d$.
• Figure 3: Trees for $u \bar{u} \rightarrow u \bar{d} \bar{u} d$.
• Figure 4: Trees for $u \bar{u} \rightarrow u \bar{d} \bar{u} d g$.
• Figure 5: Insertion of a soft gluon in an internal resonance propagator.