The recent measurement of $R_{ K^*}$ is yet another hint of new physics (NP), and supports the idea that it is present in $b\to s\mu^+\mu^-$ decays. We perform a combined model-independent and model-dependent analysis in order to deduce properties of this NP. Like others, we find that the NP must obey one of two scenarios: (I) $C_9^{ \mu\mu}({ \rm NP}) < 0$ or (II) $C_9^{ \mu\mu}({ \rm NP}) = - C_{ 10}^{ \mu\mu}({ \rm NP}) < 0$. A third scenario, (III) $C_9^{ \mu\mu}({ \rm NP}) = - C_{ 9}^{ \prime \mu\mu}({ \rm NP})$, is rejected largely because it predicts $R_K = 1$, in disagreement with experiment. The simplest NP models involve the tree-level exchange of a leptoquark (LQ) or a $Z'$ boson. We show that scenario (II) can arise in LQ or $Z'$ models, but scenario (I) is only possible with a $Z'$. Fits to $Z'$ models must take into account the additional constraints from $B^0_s$-${ \bar B}^0_s$ mixing and neutrino trident production. Although the LQs must be heavy, O(TeV), we find that the $Z'$ can be light, e.g., $M_{ Z'} = 10$ GeV or 200 MeV.
Categories: hep-ph hep-ex