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Register a new userTensor form factor of $D to pi(K) ell u$ and $D to pi(K) ell ell$ decays with $N_f=2+1+1$ twisted-mass fermions
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We present the first lattice Nf=2+1+1 determination of the tensor form factor $f_T^{D pi(K)}(q^2)$ corresponding to the semileptonic and rare $D to pi(K)$ decays as a function of the squared 4-momentum transfer $q^2$. Together with our recent determination of the vector and scalar form factors we complete the set of hadronic matrix elements regulating the semileptonic and rare $D to pi(K)$ transitions within and beyond the Standard Model, when a non-zero tensor coupling is possible. Our analysis is based on the gauge configurations produced by ETMC with Nf=2+1+1 flavors of dynamical quarks, which include in the sea, besides two light mass-degenerate quarks, also the strange and charm quarks with masses close to their physical values. We simulated at three different values of the lattice spacing and with pion masses as small as 220 MeV. The matrix elements of the tensor current are determined for plenty of kinematical conditions in which parent and child mesons are either moving or at rest. As in the case of the vector and scalar form factors, Lorentz symmetry breaking due to hypercubic effects is clearly observed also in the data for the tensor form factor and included in the decomposition of the current matrix elements in terms of additional form factors. After the extrapolations to the physical pion mass and to the continuum and infinite volume limits we determine the tensor form factor in the whole kinematical region accessible in the experiments. A set of synthetic data points, representing our results for $f_T^{D pi(K)}(q^2)$ for several selected values of $q^2$, is provided and the corresponding covariance matrix is also available. At zero four-momentum transfer we get $f_T^{D pi}(0) = 0.506 (79)$ and $f_T^{D K}(0) = 0.687 (54)$, which correspond to $f_T^{D pi}(0)/f_+^{D pi}(0) = 0.827 (114)$ and $f_T^{D K}(0)/f_+^{D K}(0)= 0.898 (50)$.
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Using the MILC 2+1 flavor asqtad quark action ensembles, we are calculating the form factors $f_0$ and $f_+$ for the semileptonic $B_s rightarrow K ell u$ decay. A total of six ensembles with lattice spacing from $approx0.12$ to 0.06 fm are being used. At the coarsest and finest lattice spacings, the light quark mass $m_l$ is one-tenth the strange quark mass $m_s$. At the intermediate lattice spacing, the ratio $m_l/m_s$ ranges from 0.05 to 0.2. The valence $b$ quark is treated using the Sheikholeslami-Wohlert Wilson-clover action with the Fermilab interpretation. The other valence quarks use the asqtad action. When combined with (future) measurements from the LHCb and Belle II experiments, these calculations will provide an alternate determination of the CKM matrix element $|V_{ub}|$.
We present lattice results for the vector and scalar form factors of the semileptonic decays D -> pi ell u_ell and D -> K ell u_ell in the physical range of values of squared four momentum transfer q^2, obtained with N_f=2 maximally twisted Wilson fermions simulated at three different lattice spacings (a ~ 0.102 fm, 0.086 fm, 0.068 fm) with pion masses as light as 270 MeV and m_pi L gtrsim 4. The form factors are extracted using a double ratios strategy, which allows a good statistical accuracy and is independent of the vector current renormalization constant. The chiral/continuum extrapolation is performed through a simultaneous fit in the three variables (m_pi, q^2, a) using HMChPT formulae with additional O(a^2) terms that parametrically account for the lattice spacing dependence. Our results are in very good agreement with the experimental data in the full q^2 range for both D -> pi ell u_ell and D -> K ell u_ell. At zero momentum transfer we obtain f^{D->pi}(0) = 0.65(6)_{stat}(6)_{syst} and f^{D->K}(0) = 0.76(5)_{stat}(5)_{syst}, where the systematic error does not include the effects of quenching the strange and the charm quarks. Our findings are in good agreement with recent lattice calculations at N_f = 2+1.
We report on our study of the B to D^(*) ell u semileptonic decays at zero and nonzero recoils in 2+1 flavor QCD. The Mobius domain-wall action is employed for light, charm and bottom quarks at lattice cutoffs 1/a = 2.5 and 3.6 GeV. We take bottom quark masses up to approx 2.4 times the physical charm mass to control discretization effects. The pion mass is as low as M_pi sim 310 MeV. We present our preliminary results for the relevant form factors and discuss the violation of heavy quark symmetry, which is a recent important isuue on the long-standing tension in the Cabibbo-Kobayashi-Maskawa matrix element |V_{cb}| between the exclusive and inclusive decays.
We calculate the $B topiell u$ and $B_s to K ell u$ form factors in dynamical lattice QCD. We use the (2+1)-flavor RBC-UKQCD gauge-field ensembles generated with the domain-wall fermion and Iwasaki gauge actions. For the $b$ quarks we use the anisotropic clover action with a relativistic heavy-quark interpretation. We analyze two lattice spacings $a approx 0.11, 0.086$ fm and unitary pion masses as light as $M_pi approx 290$ MeV. We simultaneously extrapolate our numerical results to the physical light-quark masses and to the continuum and interpolate in the pion/kaon energy using SU(2) hard-pion chiral perturbation theory. We provide complete error budgets for the form factors $f_+(q^2)$ and $f_0(q^2)$ at three momenta that span the $q^2$ range accessible in our numerical simulations. We extrapolate these results to $q^2 = 0$ using a model-independent $z$-parametrization and present our final form factors as the $z$-coefficients and the matrix of correlations between them. Our results agree with other lattice determinations using staggered light quarks and provide important independent cross-checks. Both $B topiell u$ and $B_s to K ell u$ decays enable a determination of the CKM matrix element $|V_{ub}|$. To illustrate this, we perform a combined $z$-fit of our numerical $Btopiell u$ form-factor data with the experimental branching-fraction measurements leaving the relative normalization as a free parameter; we obtain $|V_{ub}| = 3.61(32) times 10^{-3}$, where the error includes statistical and systematic uncertainties. This approach can be applied to $B_sto K ell u$ decay to determine $|V_{ub}|$ once the process has been measured experimentally. Finally, in anticipation of future measurements, we make predictions for $B to piell u$ and $B_sto K ell u$ Standard-Model differential branching fractions and forward-backward asymmetries.
Semi-leptonic $B_s to K ell u$ and $B_s to D_s ell u$ decays provide an alternative $b$-decay channel to determine the CKM matrix elements $|V_{ub}|$ and $|V_{cb}|$ or to obtain $R$-ratios to investigate lepton flavor universality violations. In addition, these decays may shed further light on the discrepancies seen in the analysis of inclusive vs. exclusive decays. Using the nonperturbative methods of lattice QCD, theoretical results are obtained with good precision and full control over systematic uncertainties. This talk will highlight ongoing efforts of the $B$-physics program by the RBC-UKQCD collaboration.
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