Treatment of Photon Radiation in Kinematic Fits at Future e+ e- Colliders

TL;DR

This paper presents a method to incorporate photon radiation parallel to the beam into kinematic fits at future e+ e- colliders, improving fit convergence and resolution in high-energy physics experiments.

Contribution

A novel parametrization of photon longitudinal momentum in kinematic fits that maintains fit quality even with energetic photon radiation.

Findings

Improved fit convergence in the presence of energetic photons.

Retention of fit resolution without photon radiation.

Validated method using simulated e+ e- collision data at 500 GeV.

Abstract

Kinematic fitting, where constraints such as energy and momentum conservation are imposed on measured four-vectors of jets and leptons, is an important tool to improve the resolution in high-energy physics experiments. At future e+ e- colliders, photon radiation parallel to the beam carrying away large amounts of energy and momentum will become a challenge for kinematic fitting. A photon with longitudinal momentum p_z^\gamma (\eta) is introduced, which is parametrized such that \eta ~follows a normal distribution. In the fit, \eta ~is treated as having a measured value of zero, which corresponds to p_z^\gamma (\eta)=0. As a result, fits with constraints on energy and momentum conservation converge well even in the presence of a highly energetic photon, while the resolution of fits without such a photon is retained. A fully simulated and reconstructed e+ e- -> qqqq event sample at \sqrt{s}=500 GeV is used to investigate the performance of this method under realistic conditions, as expected at the International Linear Collider.