The Benefits of B ---> K* l+ l- Decays at Low Recoil

TL;DR

This paper uses heavy quark effective theory to predict B -> K* l+ l- decay observables at low recoil, highlighting opportunities for new physics searches with minimal hadronic uncertainties.

Contribution

It provides a theoretical framework for analyzing B -> K* l+ l- decays at low recoil, including next-to-leading order corrections and new angular observables with small uncertainties.

Findings

Correlated transversity amplitudes simplify decay analysis.

Existing data constrains Standard Model and beyond solutions.

Low recoil region offers complementary information to large recoil.

Abstract

Using the heavy quark effective theory framework put forward by Grinstein and Pirjol we work out predictions for B -> K* l+ l-, l = (e, mu), decays for a softly recoiling K*, i.e., for large dilepton masses sqrt{q^2} of the order of the b-quark mass m_b. We work to lowest order in Lambda/Q, where Q = (m_b, sqrt{q^2}) and include the next-to-leading order corrections from the charm quark mass m_c and the strong coupling at O(m_c^2/Q^2, alpha_s). The leading Lambda/m_b corrections are parametrically suppressed. The improved Isgur-Wise form factor relations correlate the B -> K* l+ l- transversity amplitudes, which simplifies the description of the various decay observables and provides opportunities for the extraction of the electroweak short distance couplings. We propose new angular observables which have very small hadronic uncertainties. We exploit existing data on B -> K* l+ l- distributions and show that the low recoil region provides powerful additional information to the large recoil one. We find disjoint best-fit solutions, which include the Standard Model, but also beyond-the-Standard Model ones. This ambiguity can be accessed with future precision measurements.