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Register a new userSymmetries in the angular distribution of exclusive semileptonic B decays
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We discuss a method to construct observables protected against QCD uncertainties based on the angular distribution of the exclusive Bd -> K(*0}(-> Kpi) l+ l- decay. We focus on the identification and the interpretation of all the symmetries of the distribution. They constitute a key ingredient to construct a set of so-called transverse observables. We work in the framework of QCD factorization at NLO supplemented by an estimate of power-suppressed Lambda/mb corrections. A discussion of the new physics properties of two of the transverse asymmetries, AT^{(2)} and AT^{(5)}, is presented. A comparison between the transverse asymmetry AT^{(2)} and the forward-backward asymmetry shows that AT^{(2)} emerges as an improved version of it.
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Two of the elements of the Cabibbo-Kobayashi-Maskawa quark mixing matrix, $|V_{ub}|$ and $|V_{cb}|$, are extracted from semileptonic B decays. The results of the B factories, analysed in the light of the most recent theoretical calculations, remain puzzling, because for both $|V_{ub}|$ and $|V_{cb}|$ the exclusive and inclusive determinations are in clear tension. Further, measurements in the $tau$ channels at Belle, Babar, and LHCb show discrepancies with the Standard Model predictions, pointing to a possible violation of lepton flavor universality. LHCb and Belle II have the potential to resolve these issues in the next few years. This article summarizes the discussions and results obtained at the MITP workshop held on April 9--13, 2018, in Mainz, Germany, with the goal to develop a medium-term strategy of analyses and calculations aimed at solving the puzzles. Lattice and continuum theorists working together with experimentalists have discussed how to reshape the semileptonic analyses in view of the much higher luminosity expected at Belle II, searching for ways to systematically validate the theoretical predictions in both exclusive and inclusive B decays, and to exploit the rich possibilities at LHCb.
We consider leptonic $B^-to ell^- bar u_ell$ and semileptonic $bar B to pi ell^- bar u_ell$, $bar B to rho ell^- bar u_ell$ decays and present a strategy to determine short-distance coefficients of New-Physics operators and the CKM element $|V_{ub}|$. As the leptonic channels play a central role, we illustrate this method for (pseudo)-scalar operators which may lift the helicity suppression of the corresponding transition amplitudes arising in the Standard Model. Utilising a new result by the Belle collaboration for the branching ratio of $B^-to mu^- bar u_mu$, we explore theoretically clean constraints and correlations between New Physics coefficients for leptonic final states with $mu$ and $tau$ leptons. In order to obtain stronger bounds and to extract $|V_{ub}|$, we employ semileptonic $bar B to pi ell^- bar u_ell$ and $bar B to rho ell^- bar u_ell$ decays as an additional ingredient, involving hadronic form factors which are determined through QCD sum rule and lattice calculations. In addition to a detailed analysis of the constraints on the New Physics contributions following from current data, we make predictions for yet unmeasured decay observables, compare them with experimental constraints and discuss the impact of CP-violating phases of the New-Physics coefficients.
We present a systematic global analysis of dineutrino modes $b to q , u bar u$, $q=d,s$, and charged dilepton $b to q ,ell^+ ell^-$ transitions. We derive improved or even entirely new limits on dineutrino branching ratios including decays $B^0 to (K^0 , X_s), u bar u$, $B_s to phi , u bar u$ and $B^0 to (pi^0, rho^0), u bar u$ from dineutrino modes which presently are best constrained: $B^+ to (K^+,pi^+, rho^+) , u bar u$ and $B^0 to K^{*0} , u bar u$. Using SMEFT we obtain new flavor constraints from the dineutrino modes, which are stronger than the corresponding ones from charged dilepton rare $b$-decay or Drell-Yan data, for $e tau$ and $tau tau$ final states, as well as for $mu tau$ ones in $b to s$ processes. The method also allows to put novel constraints on semileptonic four-fermion operators with top quarks. Implications for ditau modes $b to s , tau^+ tau^-$ and $b to d , tau^+ tau^-$ are worked out. Furthermore, the interplay between dineutrinos and charged dileptons allows for concrete, novel tests of lepton universality in rare $B$-decays. Performing a global fit to $b to s ,mu^+ mu^-, ,s gamma$ transitions we find that lepton universality predicts the ratio of the $B^0 to K^{*0} , u bar u$ to $B^0 to K^0 , u bar u$ ($B^+ to K^+ , u bar u$) branching fractions to be within 1.7 to 2.6 (1.6 to 2.4) at $1,sigma$, a region that includes the standard model, and that can be narrowed with improved charged dilepton data. There is sizable room outside this region where universality is broken and that can be probed with the Belle II experiment. Using results of a fit to $B^0 to mu^+ mu^-$, $B^0_sto bar{K}^{ast 0},mu^+mu^-$ and $ B^+ to pi^+, mu^+ mu^-$ data we obtain an analogous relation for $|Delta b|=|Delta d|=1$ transitions.
B meson semileptonic decays are a crucial tool in our studies of the quark mixing parameters Vcb and Vub. The interplay between experimental and theoretical challenges to achieve precision in the determination of these fundamental parameters is discussed.
We study the radiative and semileptonic B decays involving a spin-$J$ resonant $K_J^{(*)}$ with parity $(-1)^J$ for $K_J^*$ and $(-1)^{J+1}$ for $K_J$ in the final state. Using the large energy effective theory (LEET) techniques, we formulate $B to K_J^{(*)}$ transition form factors in the large recoil region in terms of two independent LEET functions $zeta_perp^{K_J^{(*)}}$ and $zeta_parallel^{K_J^{(*)}}$, the values of which at zero momentum transfer are estimated in the BSW model. According to the QCD counting rules, $zeta_{perp,parallel}^{K_J^{(*)}}$ exhibit a dipole dependence in $q^2$. We predict the decay rates for $B to K_J^{(*)} gamma$, $B to K_J^{(*)} ell^+ ell^-$ and $B to K_J^{(*)} u bar{ u}$. The branching fractions for these decays with higher $K$-resonances in the final state are suppressed due to the smaller phase spaces and the smaller values of $zeta^{K_J^{(*)}}_{perp,parallel}$. Furthermore, if the spin of $K_J^{(*)}$ becomes larger, the branching fractions will be further suppressed due to the smaller Clebsch-Gordan coefficients defined by the polarization tensors of the $K_J^{(*)}$. We also calculate the forward backward asymmetry of the $B to K_J^{(*)} ell^+ ell^-$ decay, for which the zero is highly insensitive to the $K$-resonances in the LEET parametrization.
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