Treatment of photon radiation in kinematic fits at future e+e− colliders

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 , γ ( η ) is introduced, which is parametrized such that η follows a normal distribution. In the fit, η is treated as having a measured value of zero, which corresponds to p z , γ = 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 − → q q ¯ q q ¯ event sample at s = 500 GeV is used to investigate the performance of this method under realistic conditions, as expected at the International Linear Collider.