Geant4 simulation of gamma conversion to muon pair for dark matter searches

There is a direct evidence from different astronomical observations and CMB spectrum that 26% of the visible part of the Universe consists of so-called dark matter. There are many models explaining the phenomenon of dark matter, however none of them has been confirmed experimentally. It justifies the further searches for the dark matter that involve more and more various experiments, such as, for instance, SHiP experiment [1] at CERN or LDMX project [2] at SLAC. LDMX (Light Dark Matter eXperiment) is an electron fixedtarget missing momentum search for light dark matter. The main process that LDMX looks for is dark bremsstrahlung (i.e. emitting a light dark matter particle) by the several GeV electrons scattered off the target. This process is contaminated by the background of ordinary bremsstrahlung, that consists of many different processes which can be accounted for and vetoed by various counters. The experiment designs are studied with the Geant4 toolkit. Recently, one of the important background processes, that is the conversion of photon into the muon-antimuon pair in the field of nucleus, was claimed to be described improperly by Geant4 in case of the high momentum transfer to the recoiling nucleus [3, 4]. Moreover, there were suggestions of revising the process of photon conversion into muon-antimuon pair by SHiP experiment, where one also should account properly for this kind of background processes. All these suggestions served as a motivation for the present study, the main goal of which was to check the underlying physics of Geant4 G4GammaConversionToMuons class and to improve it if necessary.