General Analysis of B -> K^(*) l^+ l^- Decays at Low Recoil

TL;DR

This paper provides a comprehensive, model-independent analysis of B -> K^(*) l^+ l^- decays at low recoil, introducing novel observables sensitive to new physics, with implications for testing the Standard Model and probing CP violation.

Contribution

It introduces new low-recoil observables with high sensitivity to non-standard-model operators, including CP asymmetries without flavor tagging, and demonstrates their advantages over traditional methods.

Findings

Transversity observables H_T^(1,3,4,5) are insensitive to hadronic uncertainties.

The observable J_7 can probe the operator product expansion at the few-percent level.

Low-recoil observables improve sensitivity to |C_9/C_10| and CP-violating currents.

Abstract

We analyze the angular distributions of B -> K^* (-> K pi) l^+ l^- and B -> K l^+ l^- decays in the region of low hadronic recoil in a model-independent way by taking into account the complete set of dimension-six operators [sbar Gamma b][lbar Gamma' l]. We obtain several novel low-recoil observables with high sensitivity to non-standard-model Dirac structures, including CP-asymmetries which do not require flavor tagging. The transversity observables H_T^(1,3,4,5) are found to be insensitive to hadronic matrix elements and their uncertainties even when considering the complete set of operators. In the most general scenario we show that the low recoil operator product expansion can be probed at the few-percent level using the angular observable J_7. Higher sensitivities are possible assuming no tensor contributions, specifically by testing the low-recoil relation |H_T^(1)|=1. We explicitly demonstrate the gain in reach of the low-recoil observables in accessing the ratio |C_9/C_10| compared to the forward-backward asymmetry, and probing CP-violating right-handed currents Im C_10'. We give updated Standard Model predictions for key observables in B -> K^(*) l^+ l^- decays.