Theory of b --> s/d nu anti-nu

TL;DR

This paper reviews the theoretical framework and experimental prospects for detecting new physics through b to s/d neutrino-antineutrino transitions, emphasizing their sensitivity, observables, and constraints.

Contribution

It provides a comprehensive analysis of b --> s/d nu anti-nu transitions as probes of new physics, including theoretical uncertainties, observables, and implications for various models.

Findings

Inclusive B --> Xs nu anti-nu rate is theoretically clean.

Rate ratios like B --> K* nu anti-nu / B --> K* l+ l- reduce form factor uncertainties.

New physics effects can be constrained by existing B physics observables.

Abstract

The b --> s nu anti-nu transitions are sensitive probes of new physics (NP) in the form of non-standard Z penguin effects. They involve four experimentally accessible observables, among which the inclusive rate of B --> Xs nu anti-nu is the most theoretically clean, but is challenging to study experimentally. Theory errors in exclusive rates are dominated by form factors' normalization. They can be reduced in rate ratios like the K* polarization fractions in B --> K* nu anti-nu or by studying the ratio Br(B --> K nu anti-nu)/Br(B --> K l^+l^-). Measurable NP effects in b --> s/d nu anti-nu can be expressed in terms of two real parameters and are generally correlated with other flavor observables. Although in principle even in minimal flavor violating scenarios, NP can still saturate present direct bounds, in many explicit NP models new effects are already constrained by other B physics observables to be much smaller. Alternatively, b --> s/d E_miss can receive contributions from particles other than neutrinos in the final state and strong modifications of the invariant mass spectra are possible. However, interpretation of such effects in terms of bounds on interactions of such new particles is nontrivial when experimental searches employ kinematical cuts or their signal reconstruction efficiencies depend on expected signal kinematical distributions.