Rejection of the Photon Induced Background in LDMX at 8 GeV

Abstract: The Light Dark Matter eXperiment (LDMX) is a proposed fixed target experiment at SLAC's LCLS-II electron beamline that will attempt direct production of sub-GeV dark matter, measurable through a missing momentum and missing energy signature. An initial run of LDMX is planned at 4 GeV beam energy, for which a study on LDMX's capabilities to separate dark matter production events from the standard model background was previously performed. Later phases of data taking at LDMX will after an upgrade of the beamline be performed at 8 GeV beam energy, which is expected to improve both the dark matter production rate and background rejection capabilities. Using a dark photon mediator between standard model and dark matter as a benchmark model with a missing momentum and missing energy signature, a comparative study on the performance of LDMX at 8 GeV is done with the same methodology as in the 4 GeV study. A variant of the complete background rejection sequence is presented, optimized for 8 GeV beam energy. The sequence is shown to reject each of ~2e14 simulated standard model backgrounds events, focusing on photo-nuclear and muon conversion in the target or forward electromagnetic calorimeter. Improvement in signal to background discrimination is seen in parts of the dark photon mass range, compared to 4 GeV beam energy.