Sensitivity study of $\mu^+ \to e^+ a \gamma $ cLFV decay with the MEG~II apparatus

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We report on a study of the sensitivity of the MEG~II apparatus in the search for the charged-lepton flavor-violating decay $\mu^+ \to e^+ a \gamma$, involving an axion-like particle in a $V\!-\!A$ chirality configuration. While forbidden in the Standard Model, this decay is predicted in several Beyond Standard Model scenarios. We performed Monte Carlo simulations with \textsc{Geant4} and analyzed data using the \textsc{ROOT} framework. Our studies indicate that an optimized data taking configuration, using a low muon beam rate ($R_\mu \simeq 1\times 10^6\,\mu/\text{s}$) and a low photon energy threshold ($10~\text{MeV}$), maximizes the experimental efficiency. Analyzing only the 2021 and 2022 binded datasets, corresponding to aproximately on six days of data taking, we show that MEG~II can reach a sensitivity limit of $F_{V\!-\!A}^{95\%\rm CL} \geq 1.13 \times 10^9\,\text{GeV}$, improving upon the 2015 TWIST result of $F_{V\!-\!A}^{95\%\rm CL} \geq 1.00 \times 10^9\,\text{GeV}$. This study demonstrates the capability of MEG~II to set world-leading constraints on this cLFV decay.

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