Simulating hard photon production with Whizard

Abstract

One of the important goals of the proposed future $$\hbox {e}^{+}\hbox {e}^{-}$$ e + e - collider experiments is the search for 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 spectrum and angular distributions of those photons can shed light on the nature of dark matter and its interactions. Therefore, it is crucial to be able to simulate the signal and background samples in a uniform framework, to avoid possible systematic biases. The Whizard program is a flexible tool, which is widely used by $$\hbox {e}^{+}\hbox {e}^{-}$$ e + e - collaborations for simulation of many different “new physics” scenarios. We propose the procedure of merging the matrix element calculations with 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 pair-production events agree with corresponding predictions of the $$\mathcal{KK}$$ KK MC, a Monte Carlo generator providing perturbative predictions for SM and QED processes, which has been widely used in the analysis of LEP data.

Theory requirements for SM Higgs and EW precision physics at the FCC-ee

“…At the level of LO matrix elements, there is a matching procedure between explicit matrix element photons and the electron PDFs in collinear factorization in the context of WHIZARD (cf. below) which has been recently shown to work unexpectedly well [52,53].…”

Simulating hard photon production with WHIZARD

“…For proper estimate of the expected sensitivity of future e + e − colliders a realistic description of the discussed background processes is a must. Such a simulation procedure, allowing for handling the signal and background events, has been developed in [2] and is briefly described in the next section. Taking into account the expected acceptance of the experiments, 2-dimensional distributions of mono-photon events can be obtained from the discussed procedure and used to calculate upper limits on the dark matter production cross sections, as alluded to in the last section.…”

“…In the presented procedure [2] we use the following variables, calculated separately for each emitted photon, to describe kinematics of the emission:…”

“…Only photons with large values of virtualities − and + can be measured in the detector. More details about ( − , + ) plane, its correspondence to detector acceptance and consequent cuts can be found in [2].…”

“…Impact of the choice of the ISR-ME merging scale on the cross sections calculations for both background processes, for different photon mutliplicities and collision energies has been discused in details in [2]. In figure 2, example distributions of the photon transverse momenta, for radiative neutrino pair production and radiative Bhabha scattering at 3 TeV are shown, for different values of merging parameter .…”