Abstract
The Belle II experiment will measure the rare decays B → Kνν and B → K∗νν 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 FL and the inclusive decay rate B → Xsνν. In our investigation we consider new physics dominantly contributing to one and two operators both for massless and massive (sterile) neutrinos. Our results show a powerful interplay of the exclusive decay rates B → Kνν and B → K∗νν, and a surprisingly large sensitivity of the inclusive decay mode to vector operators even under conservative assumptions about its uncertainty. Furthermore, the sensitivity of FL is competitive with the branching ratio of B → K∗νν in the search for new physics contributing to scalar operators and thus also complementary to B → Kνν and B → Xsνν.
Highlights
A promising avenue to probe and constrain extensions of the SM is via the investigation of rare processes
Quantum chromodynamics (QCD) involved in exclusive decays is entirely captured via an appropriate set of form factors, whereas the inclusive decay mode is at leading order given by the underlying parton-level process which is calculable in perturbation theory and receives corrections only at quadratic order in the heavy-quark effective theory (HQET) expansion
In the first three subsections, we demonstrate the reach for new physics in b → sνν processes at Belle II under the assumption of no experimental evidence of an enhancement or suppression of the SM expectation
Summary
We consider the Standard Model extended by an arbitrary number of sterile neutrinos and work entirely within LEFT [27]. The differential decay rate is symmetric under exchange of the final-state neutrinos and under exchange of the quark-flavour indices sb ↔ bs for the scalar and tensor operators. As the final-state neutrinos escape unobserved from the detector, there are two independent observables which can be parameterised in terms of the coefficients G00,0 and G20,0 of the Wigner-D functions in the differential decay rate [31]. Where we integrate over the full kinematic range in q2 As it is the case for B → Kνν there is no interference between the scalar, vector, and tensor operators because of the different chiralities and the symmetry properties of the scalar and tensor Wilson coefficients. Note that the result does not include QCD corrections and subleading HQET corrections which generally lead to a suppression of the differential decay rate. As there are currently no projected sensitivities for the inclusive decay mode at Belle II, QCD and subleading HQET corrections are left for future work
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