Determination of $|V_{ub}|$ from untagged $B^0\toπ^- \ell^+ ν_{\ell}$ decays using 2019-2021 Belle II data

Precision calculations of Bd,s → π, K decay form factors in soft-collinear effective theory

“…Inspired by the encouraging experimental progresses on measuring the differential B → π ν decay rates from the BaBar [62,63], Belle [64,65] and Belle II [66] Collaborations, on the first observation of the semileptonic B s → K ν decays at the LHCb experiment [67], and on the anticipated discovery of the flavour-changing neutral current (FCNC) B → Kν ν decay process at Belle II [68], we aim at improving further theory predictions for the semileptonic B d,s → π, K decay form factors from the LCSR technique with the HQET B-meson distribution amplitudes as previously achieved in [33,35], by incorporating a various types of newly computed subleading-power contributions into the next-to-leading-JHEP03(2023)140 logarithmic (NLL) resummation improved leading-power effects in the heavy quark expansion. More specifically, the major new ingredients of the present paper can be summarized in the following.…”

“…Having at our disposal the combined BCL z-fit results for the exclusive B → π, K form factors, we are now prepared to explore their phenomenological implications on the semileptonic B (s) → π(K) ν decay observables constructed from the corresponding full angular distributions, such as the differential branching fractions, the normalized forward-backward asymmetries, the new-physics (NP) sensitive "flat terms" (vanishing in the massless lepton limit in the SM), and the lepton-flavour universality ratios, the lepton polarization asymmetries. In particular, the ever-increasing precision measurements on the binned q 2 distributions for the golden exclusive B → π ν (with = e, µ) decay processes from the BaBar Collaboration [62,63], the Belle Collaboration [64,65] as well as the Belle II [66] Table 4. Theory predictions for the correlated z-series coefficients in the vector, scalar and tensor B s → K form factors determined by fitting the BGL parametrization simultaneously against our LCSR results including a variety of the subleading-power corrections and the available lattice QCD data points from [4,8,11] with the preferred truncation N = 3.…”

“…As emphasized previously in [16], this attractive fitting strategy combines the theoretical and experimental inputs in a more efficient manner, yielding a somewhat smaller uncertainty on |V ub | numerically. Taking advantage of the available state-of-the-art experimental data sets from the three untagged measurements by the BaBar Collaboration [63] and the Belle Collaboration [64] assuming isospin symmetry, from the two tagged measurements of B0 → π + ν and B − → π 0 ν by the Belle Collaboration [65], and from the untagged B 0 → π − ν measurements by the Belle II Collaboration [66], we display the numerical fit results for both the form-factor shape parameters with the truncation N = 3 for the BCL z-expansion and |V ub | in table 5, including their correlation matrix. The quality of the binned maximum-likelihood fit can be understood from the resulting chi-square per degree of freedom χ 2 /dof = 86.79/(73 − 6) ≈ 1.30.…”

“…Theory predictions for the differential q 2 distributions of the exclusive semileptonic B → π ν (left panel) and B s → K ν (right panel) decay processes in the entire kinematic region with the distinct BCL z-series fits of the form-factor shape parameters. The available experimental measurements on the binned q 2 distributions of the "golden" decay process B → πµν µ from the BaBar [62,63], Belle [64,65] and Belle II [66] Collaborations are further displayed for an exploratory comparison.…”

“…from the BaBar [62,63], Belle [64,65] and Belle II [66] Collaborations are further shown for a numerical comparison. In addition, we collect simultaneously the obtained numerical results from fitting the BCL z-series parameterizations with three distinct scenarios of the input data points: i) only synthetic lattice data points, II) synthetic lattice data points ⊕ LCSR results, III) synthetic lattice data points ⊕ LCSR results ⊕ experimental data.…”