A Tale of Invisibility: Constraints on New Physics in $b\to s\nu\nu$

Understand

The Belle II experiment will measure the rare decays $B\to K\nu\nu$ and $B\to K^* \nu\nu$ with increased sensitivity which can hence be expected to serve as a very efficient probe of new physics.

• We calculate the relevant branching ratios in low-energy effective field theory (LEFT) including an arbitrary number of massive sterile neutrinos and discuss the expected sensitivity to the different operators.
• We also take into account the longitudinal polarisation fraction $F_L$ and the inclusive decay rate $B\to X_s\nu\nu$.
• In our investigation we consider new physics dominantly contributing to one and two operators both for massless and massive (sterile) neutrinos.