New Searches for Muonphilic Particles at Proton Beam Dump Spectrometers

We introduce a new search strategy for visibly decaying muonphilic particles using a proton beam spectrometer modeled after the SpinQuest experiment at Fermilab. In this setup, a ${\sim}$100 GeV primary proton beam impinges on a thick fixed target and yields a secondary muon beam. As these muons traverse the target material, they scatter off nuclei and can radiatively produce hypothetical muonphilic particles as initial- and final-state radiation. If such new states decay to dimuons, their combined invariant mass can be measured with a downstream spectrometer immersed in a Tesla-scale magnetic field. For a representative setup with $3\times 10^{14}$ muons on target with typical energies of $\sim$ 20 GeV, a $15\%$ invariant mass resolution, and an effective 100 cm target length, this strategy can probe the entire parameter space for which $\sim$ 200 MeV -- GeV scalar particles resolve the muon $g-2$ anomaly. We present sensitivity to these scalar particles at the SpinQuest experiment where no additional hardware is needed and the search could be parasitically executed within the primary nuclear physics program. Future proton beam dump experiments with optimized beam and detector configurations could have even greater sensitivity.