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Register a new userSemileptonic $B to D^{**}$ decays in Lattice QCD : a feasibility study and first results
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We compute the decays ${Bto D^ast_0}$ and ${Bto D^ast_2}$ with finite masses for the $b$ and $c$ quarks. We first discuss the spectral properties of both the $B$ meson as a function of its momentum and of the $D^ast_0$ and $D^ast_2$ at rest. We compute the theoretical formulae leading to the decay amplitudes from the three-point and two-point correlators. We then compute the amplitudes at zero recoil of ${Bto D^ast_0}$ which turns out not to be vanishing contrary to what happens in the heavy quark limit. This opens a possibility to get a better agreement with experiment. To improve the continuum limit we have added a set of data with smaller lattice spacing. The ${Bto D^ast_2}$ vanishes at zero recoil and we show a convincing signal but only slightly more than 1 sigma from 0. In order to reach quantitatively significant results, we plan to fully exploit smaller lattice spacings as well as another lattice regularization.
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We present results for form factors of semileptonic decays of $D$ and $B$ mesons in 2+1 flavor lattice QCD using the MILC gauge configurations. With an improved staggered action for light quarks, we successfully reduce the systematic error from the chiral extrapolation. The results for $D$ decays are in agreement with experimental ones. The results for B decays are preliminary. Combining our results with experimental branching ratios, we then obtain the CKM matrix elements $|V_{cd}|$, $|V_{cs}|$, $|V_{cb}|$ and $|V_{ub}|$. We also check CKM unitarity, for the first time, using only lattice QCD as the theoretical input.
We present first lattice QCD results for semileptonic form factors for the decays $B_c to eta_c l u$ and $B_c to J/psi l u$ over the full $q^2$ range, using both improved non-relativistic QCD (NRQCD) and fully relativistic (HISQ) formalisms. These can be viewed as prototype calculations for pseudoscalar to pseudoscalar and pseudoscalar to vector decays involving a $b to c$ transition. In particular we can use information from the relativistic computations to fix the NRQCD current normalisations, which can then be used in improved computations of decays such as $B to D l u$ and $B to D^* l u$.
In this work we discuss in detail the non-perturbative determination of the momentum dependence of the form factors entering in semileptonic decays using unitarity and analyticity constraints. The method contains several new elements with respect to previous proposals and allows to extract, using suitable two-point functions computed non-perturbatively, the form factors at low momentum transfer $q^2$ from those computed explicitly on the lattice at large $q^2$, without any assumption about their $q^2$-dependence. The approach will be very useful for exclusive semileptonic $B$-meson decays, where the direct calculation of the form factors at low $q^2$ is particularly difficult due to large statistical fluctuations and discretisation effects. As a testing ground we apply our approach to the semileptonic $D to K ell u_ell$ decay, where we can compare the results of the unitarity approach to the explicit direct lattice calculation of the form factors in the full $q^2$-range. We show that the method is very effective and that it allows to compute the form factors with rather good precision.
The semileptonic process, B --> pi l u, is studied via full QCD Lattice simulations. We use unquenched gauge configurations generated by the MILC collaboration. These include the effect of vacuum polarization from three quark flavors: the $s$ quark and two very light flavors ($u/d$) of variable mass allowing extrapolations to the physical chiral limit. We employ Nonrelativistic QCD to simulate the $b$ quark and a highly improved staggered quark action for the light sea and valence quarks. We calculate the form factors $f_+(q^2)$ and $f_0(q^2)$ in the chiral limit for the range 16 GeV$^2 leq q^2 < q^2_{max}$ and obtain $int^{q^2_{max}}_{16 GeV^2} [dGamma/dq^2] dq^2 / |v_{ub}|^2 = 1.46(35) ps^{-1}$. Combining this with a preliminary average by the Heavy Flavor Averaging Group (HFAG05) of recent branching fraction data for exclusive B semileptonic decays from the BaBar, Belle and CLEO collaborations, leads to $|V_{ub}| = 4.22(30)(51) times 10^{-3}$. PLEASE NOTE APPENDIX B with an ERRATUM, to appear in Physical Review D, to the published version of this e-print (Phys.Rev.D 73, 074502 (2006)). Results for the form factor $f_+(q^2)$ in the chiral limit have changed significantly. The last two sentences in this abstract should now read; We calculate the form factor $f_+(q^2)$ and $f_0(q^2)$ in the chiral limit for the range 16 Gev$^2 leq q^2 < q^2_{max}$ and obtain $int^{q^2_{max}}_{16 GeV^2} [dGamma/dq^2] dq^2 / |V_{ub}|^2 = 2.07(57)ps^{-1}$. Combining this with a preliminary average by the Heavy Flavor Averagibg Group (HFAG05) of recent branching fraction data for exclusive B semileptonic decays from the BaBar, Belle and CLEO collaborations, leads to $|V_{ub}| = 3.55(25)(50) times 10^{-3}$.
We report on our study of the D meson semileptonic decays in 2+1 flavor lattice QCD. Gauge ensembles are generated at three lattice cutoffs up to 4.5 GeV and with pion masses as low as 300 MeV. We employ the Moebius domain-wall fermion action for both light and charm quarks. We report our preliminary results for the vector and scalar form factors and discuss their dependence on the momentum transfer, quark masses and lattice spacing.
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