Signatures of a Flavor Changing $Z'$ Boson in $B_q \to \gamma Z'$

Understand

Rare $B$ meson decays offer an opportunity to probe a light hidden $Z'$ boson.

• In this work we explore a new channel $B_q \to \gamma Z'$ ($q = d, s$) followed by a cascade decay of $Z'$ into an invisible (neutrino or dark matter) or charged lepton pair $\ell^+ \ell^-$ ($\ell=e ,\mu)$.
• The study is based on a simplified effective model where the down quark sector has tiny flavor-changing neutral current couplings with $Z'$.
• For the first time, we calculate ${\rm BR}(B_q \to \gamma Z')$ at the leading power of $1/m_b$ and $1/E_\gamma$.