Simulating hard photon production with WHIZARD

Abstract

One of the important goals of the proposed future e‚e− collider experiments is the search for the dark matter particles using different experimental approaches. The most general search approach is based on the mono-photon signature, which is expected when production of the invisible final state is accompanied by a hard photon from initial state radiation. Analysis of the energy and angular distributions of those photons can shed light on the nature of a dark matter particle and its interactions. Therefore, it is crucial to simulate the signal and background samples in the uniform framework, to avoid possible systematic biases. The Whizard program is a flexible tool, which is widely used by e‚e− collaborations for simulation of many different "new physics" scenarios. We propose the procedure of merging the hard photon emission from matrix element with the soft photon emission parametrised as the lepton ISR structure function implemented in Whizard. It allows us to reliably simulate the mono-photon events, including the two main Standard Model background processes: radiative neutrino pair production and radiative Bhabha scattering. We demonstrate that cross sections and kinematic distributions of mono-photon in neutrino pairproduction events agree with corresponding predictions of the LEP-tuned KK MC generator. We also propose a new approach to calculating limits on dark matter production at e‚e− colliders based on the analysis of the two-dimensional distributions of the reconstructed mono-photon events.