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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(η)=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.