The Decay B --> K l^+ l^- at Low Hadronic Recoil and Model-Independent Delta B = 1 Constraints

TL;DR

This paper analyzes the decay B --> K l^+ l^- at low recoil, providing systematic predictions, model-independent constraints on new physics, and improved bounds on the zero of the forward-backward asymmetry based on recent experimental data.

Contribution

It offers a comprehensive theoretical framework for B --> K l^+ l^- decays at high q^2, including model-independent constraints and refined bounds on asymmetry zero points.

Findings

Decay rates depend on a common short-distance coefficient combination.

CP asymmetries are predicted to be identical for B --> K^* l^+ l^- and B --> K l^+ l^-.

New physics scales are constrained to be above 44 TeV for constructive interference.

Abstract

We study the decay B --> K l^+ l^- for l = e,mu,tau with a softly recoiling kaon, that is, for high dilepton invariant masses sqrt{q^2} of the order of the b-quark mass. This kinematic region can be treated within an operator product expansion and simplified using heavy quark symmetry, leading to systematic predictions for heavy-to-light processes such as B --> K^(*) l^+ l^-. We show that the decay rates of both B --> K^* l^+ l^- and B --> K l^+ l^- decays into light leptons depend on a common combination of short-distance coefficients. The corresponding CP-asymmetries are hence identical. Furthermore we present low recoil predictions for B --> K l^+ l^- observables, including the flat term in the angular distribution which becomes sizable for taus. We work out model-independently the constraints on Delta B = 1 operators using the most recent data from the experiments BaBar, Belle, CDF and LHCb. For constructive interference with the standard model, generic new physics is pushed up to scales above 44 TeV at 95% CL. Assuming none or small CP-violation we obtain a lower bound on the position of the zero of the forward-backward asymmetry of B^0 --> K^*0 l^+ l^- decays as q_0^2 > 1.7 GeV^2, which improves to q_0^2 > 2.6 GeV^2 for a standard model-like sign b --> s gamma amplitude.