Pion pair production in $e^+ e^-$ annihilation at next-to-leading order matched to Parton Shower

TL;DR

This paper presents a next-to-leading order calculation of $e^+ e^- o ext{pion pair} + ext{photon}$ processes, matched with a Parton Shower, improving theoretical modeling for precision energy scan experiments and experimental data analysis.

Contribution

It introduces a novel NLO matched Parton Shower approach for pion pair production, incorporating advanced treatments of the pion form factor and composite structure.

Findings

Two methods for including the pion form factor agree remarkably well.

Radiative corrections significantly impact differential observables.

The updated Monte Carlo generator enables precise simulations for experimental analysis.

Abstract

The pion pair production in $e^+ e^-$ annihilation at flavour factories plays a crucial role in the determination of the hadronic contribution to the muon anomalous magnetic moment. The recent CMD-3 measurement of the pion form factor via energy scan displays a significant difference with the previous experimental determinations. In order to contribute to an improved theoretical description and simulation of energy scan experiments, we present a calculation of the $e^+ e^- \to \pi^+ \pi^- (\gamma)$ hadronic channel at next-to-leading order matched to a Parton Shower algorithm in QED and sQED. According to the recent advances in the literature, particular attention is paid to the treatment of the pion composite structure in loop diagrams beyond the commonly used factorised sQED approach, as well as to the modelling of multiple photon radiation through the Parton Shower algorithm. In particular, we carry out a detailed discussion on the inclusion of the pion form factor in the virtual sQED corrections according to two independent methods, inspired by the generalised vector meson dominance model and the dispersive approach, respectively. We find the two methods to be in remarkable agreement. We show phenomenological results for inclusive and differential observables which are relevant for precision energy scan measurements, focusing on the impact of the radiative corrections and the effect of the various approaches for the treatment of the pion form factor. Our calculation is implemented in an updated version of the Monte Carlo event generator BabaYaga@NLO, that can be used for fully exclusive simulations in data analysis.