Effects of Family Non-universal $Z^{\mathrm{\prime }}$ Model in the angular observables of $B\to (\rho ,a_1){\mu }^{+}{\mu }^{-}$ decays

We present the angular distribution of the four-fold $B\to \rho (\to \pi \pi ){\mu }^{+}{\mu }^{-}$ and $B\to a_1(\to {\rho }_{\parallel ,\perp }\pi ){\mu }^{+}{\mu }^{-}$ decays both in the Standard Model and the family non-universal $Z^{\mathrm{\prime }}$ model. At the quark level, these decays are governed by the $b\to d{\mu }^{+}{\mu }^{-}$ transition. Along with different angular observables, we also give predictions of differential branching ratios, forward-backward asymmetry, longitudinal polarization fraction of $\rho$, and $a_1$ mesons. Our analysis shows that the signatures of family non-universal $Z^{\mathrm{\prime }}$ model are more distinct in the observables associated with the $B\to \rho (\to \pi \pi ){\mu }^{+}{\mu }^{-}$ decay, compared to that of the $B\to a_1(\to {\rho }_{\parallel ,\perp }\pi ){\mu }^{+}{\mu }^{-}$ decay. Future measurements of the predicted angular observables, both at current and future high energy colliders, will add to the useful complementary data required to clarify the structure of the family non-universal $Z^{\mathrm{\prime }}$ model in $\mathrm{\mid }\mathrm{\Delta }b\mathrm{\mid }$=$\mathrm{\mid }\mathrm{\Delta }d\mathrm{\mid }=1$ processes.