Probing charged lepton flavor violation in an economical muon on-target experiment

This work proposes a new yet economical experiment to probe the charged lepton flavor violation (CLFV) process mediated by an extra massive neutron gauge boson $Z^{\mathrm{\prime }}$ beyond the standard model, by extending a recently proposed muon dark matter project in the Peking University Muon (PKMuon) Experiment. The devices used originally for light mass dark matter direct detection are easily adaptable to search for the ${\mu }^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}$ CLFV process leveraging the large-area, high-precision muon tracking and tomography system sandwiching a fixed target the incoming muons scatter off. The ${\mu }^{+}{\mu }^{-}$ final state signal studied in this work can be uniquely sensitive to specific CLFV parameter combinations, such as the couplings between $Z^{\mathrm{\prime }}$, electron and muon, or $Z^{\mathrm{\prime }}$ and two muons. Prospected results are obtained through detailed detector simulation for the proposal interfacing with a muon beam with energy at tens of $\mathrm{G}\mathrm{e}\mathrm{V}$ and a flux of ${10}^6{\mathrm{s}}^{-1}$. Based mainly on angular information of the incoming and outgoing particles, the expected upper limit at 95% confidence level on the coupling coefficients ${\lambda }_{e\mu }{\lambda }_{\mu \mu }$ is able to reach ${10}^{-5}$ with, for example, $Z^{\mathrm{\prime }}$ mass $0.25\mathrm{G}\mathrm{e}\mathrm{V}$, for a one year’s run.