# Photon radiation in $e^+e^-\to$ hadrons at low energies with CARLOMAT 3.1

## TL;DR

This paper introduces a new version of the CARLOMAT Monte Carlo program that accurately simulates low-energy electron-positron annihilation into hadronic final states with photons or lepton pairs, emphasizing its enhanced features.

## Contribution

The paper presents CARLOMAT 3.1, a versatile Monte Carlo tool capable of separately or jointly modeling initial and final state radiation and incorporating the pion form factor for low-energy processes.

## Key findings

- Demonstrated CARLOMAT 3.1's capabilities in simulating various low-energy $e^+e^-$ annihilation processes.
- Showed the program's ability to handle electromagnetic gauge invariance issues.
- Provided cross section results for processes involving pions and photons.

## Abstract

We present a sample of results for the cross sections of several processes of low energetic $e^+e^-$ annihilation into final states containing pions accompanied by one or two photons, or a light lepton pair. The results, which have been obtained with a new version of a multipurpose Monte Carlo program CARLOMAT, labelled 3.1, demonstrate new capabilities of the program which, among others, include a possibility of taking into account either the initial or final state radiation separately, or both at a time, and a possibility of inclusion of the electromagnetic charged pion form factor for processes with charged pion pairs. We also discuss some problems related to the $U(1)$ electromagnetic gauge invariance.