Studies of mu-pair and pi-pair production at the electron-positron low energy colliders

TL;DR

This paper compares Monte Carlo predictions for muon and pion pair production at low-energy electron-positron colliders, focusing on the accuracy of radiative corrections and proposing improvements for pion final states.

Contribution

It introduces an extension of the KKMC Monte Carlo program to better simulate pion pair production with higher precision.

Findings

Muon pair predictions agree well across different calculations.

Current FSR models need further testing for precision below 1%.

A new method to extend KKMC for pion pairs is described.

Abstract

Predictions for the radiative return with the muon pair and pion pair final state from KKMC and PHOKHARA Monte Carlo programs are compared and discussed. The case of muon pairs is well understood, especially of the initial state radiation (ISR), where three different second order calculations agree very well. The case of the final state radiation (FSR) requires more tests. Matrix element in KKMC of the EEX type with the incomplete second order NLL corrections is not good enough for the radiative return at $Q^2<1$GeV with the precision requirement better than 1%. A method of extending the superior CEEX-type matrix element in KKMC to the pion pair final state is described.