Probing maximal flavor changing $Z'$ in $U(1)_{L_\mu-L_\tau}$ at $\mu$TRISTAN

Abstract

We explore the potential to detect the $U(1)_{L_\mu-L_\tau}$ model featuring triplet scalars $\Delta$ at the $\mu$TRISTAN collider. The new gauge boson $Z'$, arising from the spontaneous breaking of $U(1)_{L_\mu-L_\tau}$, can exhibit maximal flavor changing interactions under the exchange symmetry, while $\Delta$ mediates the flavor conserving interactions. The absence of muon $(g-2)_\mu$ can be explained by interference effects arising from opposite contributions of $Z'$ and $\Delta$, with similar interference patterns also manifesting in the tau decay process $\tau\to \mu\nu\bar\nu$. These counteracting effects render the model phenomenologically interesting and warrant further investigation. For the mass $m_{Z'}$ in the range of hundreds of GeV, we find that $\mu^+\mu^+$ and $\mu^+e^-$ collider at the $\mu$TRISTAN can probe many regions inaccessible to current experiments and offer greater projected sensitivity than opposite-sign muon colliders. This suggests that $\mu$TRISTAN can serve as complementary exploration to the $U(1)_{L_\mu-L_\tau}$ model, providing compelling motivation for the next generation of high-energy lepton colliders.