Dipole Dark Photon Emission at LDMX: Theory and Simulation

Abstract

The dark photon (DP) is a hypothetical massive mediator particle introduced to explain the present cosmological abundance of dark matter (DM) candidates that are lighter than nucleons, and that can thus be effectively searched for in fixed target experiments such as the Light Dark Matter eXperiment (LDMX). This thesis studies and compares higher order electromagnetic couplings between a DP, A′ , and the Standard Model (SM) electron. These couplings are expected to have interesting experimental signatures in missing energy and momentum experiments, but have so far only been explored within semi-analytical approaches that are not able to capture the full complexity of DP production at fixed target experiments. Motivated by this, simulations of the energy, Ee, and transverse momentum, p e T , distributions of a scattered electron in the production of a DP through a bremsstrahlung process have been performed by using the software MadGraph5 (MG5). These simulations have been made for the different electromagnetic interactions and a customary kinetic mixing (KM) coupling. The DP is added through a new gauge group, U ′ (1), with mass in the range mA′ = 0.1 − 1 GeV. The results of the different interactions and masses are then compared, and the LDMX potential for differentiating them is discussed. Further, the numerical simulations have been compared with results obtained through the Weizsäcker-Williams (WW) approximation, where agreement was found for the energy but not for the transverse momentum. The reason for this is believed to be that the nucleus pT , which is neglected in this application of the WW approximation, is non-negligible. Finally, the parameter space of the different DP interactions is examined by calculating the relic abundance, Ωh 2 , that can be produced for an example where the DP couples to complex scalar DM and the SM electron. This calculation has been performed using the program micrOMEGAs and validated with semi-analytical calculations.