Exclusive semileptonic B → πℓνℓ and Bs → Kℓνℓ decays through unitarity and lattice QCD

Abstract

The Cabibbo-Kobayashi-Maskawa (CKM) matrix element ∣Vub∣ is obtained from exclusive semileptonic B → πℓνℓ and Bs→ Kℓνℓ decays adopting the unitarity-based dispersion matrix approach for the determination of the hadronic form factors (FFs) in the whole kinematical range. We use lattice computations of the relevant susceptibilities and of the FFs in the large-q2 regime in order to derive their behavior in the low-q2 region without assuming any specific momentum dependence and without constraining their shape using experimental data. Then, we address the extraction of |Vub| from the experimental data, obtaining |Vub| = (3.62 ± 0.47) · 10−3 from B → π and |Vub| = (3.77 ± 0.48) · 10−3 from Bs→ K, which after averaging yield |Vub| = (3.69 ± 0.34) · 10−3. These results are compatible with the most recent inclusive value |Vub|incl = 4.13 (26) · 10−3 at the 1σ level. We also present purely theoretical estimates of the ratio of the τ/μ decay rates {R}_{pi (K)}^{tau /mu } , the normalized forward-backward asymmetry {overline{mathcal{A}}}_{FB}^{ell, pi (K)} and the normalized lepton polarization asymmetry {overline{mathcal{A}}}_{mathrm{polar}}^{ell, pi (K)} .