Heavy Quark Expansion of Λb→Λ*(1520) Form Factors beyond Leading Order

Marzia Bordone

doi:10.3390/sym13040531

Abstract

I review the parametrisation of the full set of Λb→Λ*(1520) form factors in the framework of Heavy Quark Expansion, including next-to-leading-order O(αs) and, for the first time, next-to-leading-power O(1/mb) corrections. The unknown hadronic parameters are obtained by performing a fit to recent lattice QCD calculations. I investigate the compatibility of the Heavy Quark Expansion and the current lattice data, finding tension between these two approaches in the case of tensor and pseudo-tensor form factors, whose origin could come from an underestimation of the current lattice QCD uncertainties and higher order terms in the Heavy Quark Expansion.

Highlights

The flavour changing neutral current (FCNC)-mediated b → s+ `− transition plays an important role in the search for physics beyond the Standard Model (SM)
These hints, together with all available b → s+ `− data, form a coherent pattern of discrepancies. They can be addressed by introducing New Physics (NP) effects
In the Heavy Quark Expansion (HQE) the number of independent, hadronic parameters is reduced compared to the lattice QCD case, introducing strict correlations among the form factors

Summary

Introduction

The flavour changing neutral current (FCNC)-mediated b → s+ `− transition plays an important role in the search for physics beyond the Standard Model (SM). The LHCb experiment used the decay chain Λb → Λ∗ (→ pK − )`+ `− to measure R( pK ), the universality ratio between muons and electrons, finding results consistent with both the SM expectation and the measured values of RK(∗) [16] In this analysis, the various Λ∗ resonances below a certain mass threshold are not distinguished. In the HQE the number of independent, hadronic parameters is reduced compared to the lattice QCD case, introducing strict correlations among the form factors. This paper is organised as follows: in Section 2 I present the HQE of the form factors; in Section 3 I discuss the fit to lattice data; in Section 4 I conclude. The expressions of the form factors in terms of the leading and subleading IW functions are obtained by matching the helicity amplitudes with their HQE expansion. I stick to my findings and adopt the signs in Equation (22)

Form Factors Relations and Comparison with Lattice QCD Results

Relations in the Zero-Recoil Point

Form Factors Parametrisation and Fit

Conclusions