Abstract
Fixed target missing-momentum experiments such as LDMX and M$^3$ are powerful probes of light dark matter and other light, weakly coupled particles beyond the Standard Model (SM). Such experiments involve $\sim$ 10 GeV beam particles whose energy and momentum are individually measured before and after passing through a suitably thin target. If new states are radiatively produced in the target, the recoiling beam particle loses a large fraction of its initial momentum, and no SM particles are observed in a downstream veto detector. We explore how such experiments can use kinematic variables and experimental parameters, such as beam energy and polarization, to measure properties of the radiated particles and discriminate between models if a signal is discovered. In particular, the transverse momentum of recoiling particles is shown to be a powerful tool to measure the masses of new radiated states, offering significantly better discriminating ability compared to the recoil energy alone. We further illustrate how variations in beam energy, polarization, and lepton flavor (i.e., electron or muon) can be used to disentangle the possible the Lorentz structure of the new interactions.
Highlights
Over the past decade the experimental dark matter (DM) search effort has greatly expanded in scope to explore the sub-GeV mass range
If new states are radiatively produced in the target, the recoiling beam particle loses a large fraction of its initial momentum, and no Standard Model (SM) particles are observed in a downstream veto detector
A low-current Oð1–10Þ GeV lepton beam is passed through a thin target, which is surrounded on both sides by tracking material and positioned upstream of a veto detector; the energy and momentum of individual beam particles are measured on both sides of the target
Summary
Over the past decade the experimental dark matter (DM) search effort has greatly expanded in scope to explore the sub-GeV mass range This push toward lower masses has been driven by several complementary strategies, including new direct-detection techniques (e.g., electron ionization) [1,2,3,4,5,6,7,8,9] and low-energy accelerator searches [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]—see Refs.
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