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

TL;DR

The study proposes an economical approach to probe Z' mediated charged lepton flavor violation via muon-on-target interactions using the PKMuon facility. It develops an efficient signal-event generation workflow that couples a MadGraph-based CLFV generator with GEANT4-based detector simulation in a sandwich-like RPC geometry. For a one-year run, the projected 95% CL upper limit on the squared CLFV coupling product is about $10^{-5}$ at mZp around 0.25 GeV, with aluminum targets providing the best background-free sensitivity. The framework offers a complementary constraint to muon-decay limits and can be adapted to cosmic muon sources and future detector upgrades.

Abstract

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 $Z^\prime$ 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 $μ^+e^- \to μ^+μ^-$ CLFV process leveraging the large-area, high-precision muon tracking and tomography system sandwiching a fixed target the incoming muons scatter off. The $μ^+μ^-$ final state signal studied in this work can be uniquely sensitive to specific CLFV parameter combinations, such as the couplings between $Z^\prime$, electron and muon, or $Z^\prime$ and two muons. Prospected results are obtained through detailed detector simulation for the proposal interfacing with a muon beam with energy at tens of $\mathrm{GeV}$ 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 $λ_{eμ}λ_{μμ}$ is able to reach $10^{-5}$ with, for example, $Z^\prime$ mass $0.25\ \mathrm{GeV}$, for a one year's run.

Figures (12)

• Figure 1: $\mu^+e^- \to \ell^+\ell^-$ production diagrams, taking $\ell = \mu$ for example.
• Figure 1: Efficient $\mu^+e^- \to \ell^+\ell^-$ event generation
• Figure 2: A proposed muon experiment to probe CLFV through muon on-target collisions, which shows only minimal sets of detectors to record muon hits and tracks, with flexibility to include more detector components to cover more physics cases in the future.
• Figure 3: The $\mu^+e^- \to \ell^+\ell^-$ process in the (a) lab and (b) COM frames.
• Figure 4: $\mu^+e^- \to \mu^+\mu^-$ production cross section in variant $(m_{Z^\prime},\ E_\mu)$ configuration, for both Z' mediated s-channel and the non-resonant t-channel cases, respectively.