New Predictions for Λ_{b}→Λ_{c} Semileptonic Decays and Tests of Heavy Quark Symmetry.

Florian U Bernlochner,Dean J Robinson,Zoltan Ligeti,William L Sutcliffe

doi:10.1103/physrevlett.121.202001

Abstract

The heavy quark effective theory makes model independent predictions for semileptonic Λ_{b}→Λ_{c} decays in terms of a small set of parameters. No subleading Isgur-Wise function occurs at order Λ_{QCD}/m_{c,b}, and only two subsubleading functions enter at order Λ_{QCD}^{2}/m_{c}^{2}. These features allow us to fit the form factors and decay rates calculated up to order Λ_{QCD}^{2}/m_{c}^{2} to LHCb data and lattice QCD calculations. We derive a significantly more precise standard model prediction for the ratio B(Λ_{b}→Λ_{c}τν[over ¯])/B(Λ_{b}→Λ_{c}μν[over ¯]) than prior results, and find the expansion in Λ_{QCD}/m_{c} well behaved, addressing a long-standing question. Our results allow more precise and reliable calculations of Λ_{b}→Λ_{c}ℓν[over ¯] rates, and are systematically improvable with better data on the μ (or e) modes.

Highlights

Introduction.—Semileptonic decays mediated by b → clνtransitions give tantalizing hints of deviations from the standard model (SM) in the ratios
Precision control of hadronic matrix elements are crucial to predict the ratios of decay rates: a better understanding of the heavy quark expansion to OðΛ2QCD=m2cÞ is required, as it is largely responsible for the different uncertainty estimates of RðDÃÞ in the SM [2,3,4]
In this Letter, we examine the heavy quark effective theory (HQET) predictions at OðΛ2QCD=m2cÞ and fit them to a recent LHCb measurement of Λb → Λcμν [15] and/or lattice QCD (LQCD) results [16]

Summary

Introduction

Introduction.—Semileptonic decays mediated by b → clνtransitions give tantalizing hints of deviations from the standard model (SM) in the ratios. In this Letter, we examine the HQET predictions at OðΛ2QCD=m2cÞ and fit them to a recent LHCb measurement of Λb → Λcμν [15] and/or lattice QCD (LQCD) results [16].

Results

Conclusion