The τ polarization in semileptonic B decays provides probes of new physics complementary to decay rate distributions of the three-body final state. Prior calculations for inclusive decays used a definition for the polarization axis that is different from the choice used in calculations (and the only measurement) for exclusive channels. To compare inclusive and exclusive predictions, we calculate the τ polarization in inclusive B→Xτν¯ using the same choice as in the exclusive decays, and construct a sum rule relating the inclusive τ polarization to a weighted sum of exclusive decay polarizations. We use this relation, experimental data, and theoretical predictions for the decays to the lightest charm or up-type hadrons to make predictions for excited channels.

Normalizing the recent LHCb measurement of Λb→Λcτν¯ to the standard model (SM) prediction for the Λb→Λcμν¯ rate, instead of a LEP measurement, provides a more consistent comparison with the SM prediction for the lepton flavor universality ratio R(Λc). This modestly increases R(Λc) compared to the quoted LHCb result, such that it no longer hints at a suppression compared to the SM, which would be hard to accommodate in new physics scenarios that enhance R(D(∗)). We point out that the fraction of excited states in inclusive semileptonic Λb decay may be significantly greater than the corresponding fraction in B decays. Possible implications are speculated upon.

We revisit the experimental and theoretical status of B→ρlν¯ and B→ωlν¯ decays. We perform a combined fit of averaged spectra from Belle and BABAR measurements with prior light-cone sum rule calculations in order to obtain more precise predictions over the full q2 range. The extracted values of |Vub| from these combined fits exhibit smaller uncertainty compared to previous extractions from B→ρlν¯ and B→ωlν¯ decays, and the central values are found to be smaller than values extracted from B→πlν¯ or inclusive measurements. We use our fit results to obtain more precise predictions in and beyond the Standard Model for the lepton universality ratios R(ρ) and R(ω), as well as several angular observables that are sensitive to the full q2 distribution, such as the longitudinal polarization of the vector meson, the τ polarization, and its forward-backward asymmetry.

We explore semileptonic B decays to the four lightest excited charm mesons, D∗∗={D0∗,D1∗,D1,D2∗}, for nonzero charged lepton mass and for all b→cν four-Fermi interactions, including calculation of the O(ΛQCD/mc,b) and O(αs) corrections to the heavy quark limit for all form factors. In the heavy quark limit, some form factors are suppressed at zero recoil; therefore, the O(ΛQCD/mc,b) corrections can be very important. The D∗∗ rates exhibit sensitivities to new physics in b→cτν mediated decays complementary to the D and D∗ modes. Since they are also important backgrounds to B→D(∗)τν, the correct interpretation of future semitauonic B→D(∗) rate measurements requires consistent treatment of both the D∗∗ backgrounds and the signals. Our results allow more precise and more reliable calculations of these B→D∗∗ν decays and are systematically improvable by better data on the e and μ modes. As an example, we show that the D∗∗ rates are more sensitive to a new cσμνb tensor interaction than the D(∗) rates.

The determination of |Vcb| from exclusive semileptonic B→D∗ν decays is sensitive to the choice of form factor parametrization. Larger |Vcb| values are obtained by fitting the Boyd-Grinstein-Lebed (BGL) versus the Caprini-Lellouch-Neubert (CLN) parametrization to recent Belle measurements. For the BGL parametrization, published fits use different numbers of parameters. We propose a method based on nested hypothesis tests to determine the optimal number of BGL parameters to fit the data, and find that six parameters are optimal to fit the Belle tagged and unfolded measurement [1]. We further explore the differences between fits that use different numbers of parameters. The fits which yield |Vcb| values in better agreement with determinations from inclusive semileptonic decays tend to exhibit tensions with heavy quark symmetry expectations. These have to be resolved before the determinations of |Vcb| from exclusive and inclusive decays can be considered understood.

We propose novel methods to determine the υ (4S)→B+B- and υ (4S)→B0B¯0 decay rates. The precision to which they and their ratio are known yields at present a limiting uncertainty around 2% in measurements of absolute B decay rates, and thus in a variety of applications, such as precision determinations of elements of the Cabibbo-Kobayashi-Maskawa matrix and flavor-symmetry relations. The new methods we propose are based in one case on exploiting the υ (5S) datasets, in the other case on the different average number of charged tracks in B± and B0 decays. We estimate future sensitivities using these methods and discuss possible measurements of fd/fu at the (HL-)LHC.

We calculate the Λb→Λc?ν form factors and decay rates for all possible b→c?ν four-Fermi interactions beyond the Standard Model, including nonzero charged lepton masses and terms up to order αsΛQCD/mc,b and ΛQCD2/mc2 in the heavy quark effective theory. At this order, we obtain model independent predictions for semileptonic Λb→Λc decays in terms of only two unknown sub-subleading Isgur-Wise functions, which can be determined from fitting LHCb and lattice QCD data. We thus obtain model independent results for Λb→Λc?ν decays, including predictions for the ratio R(Λc)=B(Λb→Λcτν)/B(Λb→Λcμν) in the presence of new physics, that are more precise than prior results in the literature, and systematically improvable with better data on the decays with μ (or e) in the final state. We also explore tests of factorization in Λb→Λcπ decays, and we emphasize the importance of measuring at LHCb the double differential rate d2Γ(Λb→Λc?ν)/(dq2dcosθ), in addition to the q2 spectrum.

The study of lepton flavor universality violation (LFUV) in semitauonic b-hadron decays has become increasingly important in light of long-standing anomalies in their measured branching fractions, and the large datasets anticipated from the LHC experiments and Belle II. In this review, a comprehensive survey of the experimental environments and methodologies for semitauonic LFUV measurements at the B factories and LHCb is undertaken, along with an overview of the theoretical foundations and predictions for a wide range of semileptonic decay observables. The future prospects of controlling systematic uncertainties down to the percent level, matching the precision of standard model (SM) predictions, are examined. Furthermore, new perspectives and caveats on combinations of the LFUV data are discussed and the world averages for the R(D(∗)) ratios are revisited. Here it is demonstrated that different treatments for the correlations of uncertainties from D∗∗ excited states can vary the current 3σ tension with the SM within a 1σ range. Prior experimental overestimates of D∗∗τν contributions may further exacerbate this. The precision of future measurements is also estimated; their power to exploit full differential information, and solutions to the inherent difficulties in self-consistent new physics interpretations of LFUV observables, are explored.

Precise measurements of b→ cτν¯ decays require large resource-intensive Monte Carlo (MC) samples, which incorporate detailed simulations of detector responses and physics backgrounds. Extracted parameters may be highly sensitive to the underlying theoretical models used in the MC generation. Because new physics (NP) can alter decay distributions and acceptances, the standard practice of fitting NP Wilson coefficients to SM-based measurements of the R(D(∗)) ratios can be biased. The newly developed Hammer software tool enables efficient reweighting of MC samples to arbitrary NP scenarios or to any hadronic matrix elements. We demonstrate how Hammer allows avoidance of biases through self-consistent fits directly to the NP Wilson coefficients. We also present example analyses that demonstrate the sizeable biases that can otherwise occur from naive NP interpretations of SM-based measurements. The Hammer library is presently interfaced with several existing experimental analysis frameworks and we provide an overview of its structure.

We perform the first global fit to inclusive B→X_{s}γ measurements using a model-independent treatment of the nonperturbative b-quark distribution function, with next-to-next-to-leading logarithmic resummation and O(α_{s}^{2}) fixed-order contributions. The normalization of the B→X_{s}γ decay rate, given by |C_{7}^{incl}V_{tb}V_{ts}^{*}|^{2}, is sensitive to physics beyond the standard model (SM). We determine |C_{7}^{incl}V_{tb}V_{ts}^{*}|=(14.77±0.51_{fit}±0.59_{theory}±0.08_{param})×10^{-3}, in good agreement with the SM prediction, and the b-quark mass m_{b}^{1S}=(4.750±0.027_{fit}±0.033_{theory}±0.003_{param})  GeV. Our results suggest that the uncertainties in the extracted B→X_{s}γ rate have been underestimated by up to a factor of 2, leaving more room for beyond-SM contributions.