Speaker
Description
We argue that the search for vector dark boson through $e^+e^-\to Z^{\prime}\gamma$ can determine the Lorentz structure of $Z^{\prime}l^+l^-$ couplings with the detection of leptonic decays $Z^{\prime}\to l^+l^-$. We assume a general framework that the vector dark boson interacts with ordinary fermions through vector and axial-vector couplings. As a consequence of Ward-Takahashi identity, $Z^{\prime}$ is transversely polarized in the limit $m_{Z'}\ll \sqrt{s}$ while all polarizations of $Z^{\prime}$ are produced for heavier $Z^{\prime}$. Such polarization effects can be analyzed through the angular, or equivalently, energy distributions of final-state particles in $Z^{\prime}$ decays. Taking $l^{\pm}\equiv \mu^{\pm}$, we study the energy distributions of final-state muons that correspond to parent $Z^{\prime}s$ moving in either the forward or backward directions relative to the electron beam axis in the CM frame of $e^+e^-$ pair. Such energy distributions can be used to probe the relative strengths of vector and axial-vector couplings between $Z^{\prime}$ and muons. We discuss such measurements in Belle II detector with appropriate acceptance cuts and various stages of integrated luminosities taken into account.
| Secondary track (number) | 05. Quark and Lepton Flavor Physics |
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