We present a study of semi-leptonic B̄ → Dlν̄ decays in quenched lattice QCD through a calculation of the matrix element 〈D|c̄γμb|B̄〉 on a 243 × 48 lattice at β = 6.2, using an O (a)-improved fermion action. We perform the calculation for several values of the initial and final heavy-quark masses around the charm mass, and three values of the light-(anti)quark mass around the strange mass. Because the charm quark has a bare mass which is almost 1/3 the inverse lattice spacing, we study the ensuing mass-dependent discretization errors, and propose a procedure for subtracting at least some of them non-perturbatively. ∗Present address: HLRZ, 52425 Jülich, Germany †Present address: Dept. of Physics and Astronomy, The University, Glasgow G12 8QQ, Scotland ‡Present address: FNAL, PO Box 500, Batavia IL 60510, USA §Unité Propre de Recherche 7061.

We present results of a lattice computation of the vector and axial-vector current matrix elements relevant for the semileptonic decay B0 → ϱ+−vl. The computations are performed in the quenched approximation of lattice QCD on a 243 x 48 lattice at β = 6.2, using an O(a) improved fermionic action. Our principal result is for the differential decay rate, dΓ/dq2, for the decay B0 → ϱ+l−vl in a region beyond the charm endpoint, allowing a model-independent extraction of |Vub| from experimental measurements. Heavy quark symmetry relations between radiative and semileptonic decays of B mesons into light vector mesons are also discussed.

We present a unified method for analysing form factors in B → πlνl and B → K∗γ decays. The analysis provides consistency checks on the q2 and 1M extrapolations necessary to obtain the physical decay rates. For the first time the q2 dependence of the form factors is obtained at the B scale. In the B → πlνl case, we show that pole fits to f+ may not be consistent with the q2 behaviour of f0, leading to a possible factor of two uncertainty in the decay rate and hence in the value of |Vub|2 deduced from it. For B → K∗γ, from the combined analysis of form factors T1 and T2, we find the hadronisation ratio RK∗ of the exclusive B → K∗γ to the inclusive b → sγ rates is of order 35% or 15% for constant and pole-type behaviour of T2, respectively.

We present a unified method for analysing form factors in B -> pi l nu-bar_l and B -> K* gamma decays. The analysis provides consistency checks on the q^2 and 1/M extrapolations necessary to obtain the physical decay rates. For the first time the q^2 dependence of the form factors is obtained at the B scale. In the B -> pi l nu-bar_l case, we show that pole fits to f^+ may not be consistent with the q^2 behaviour of f^0, leading to a possible factor of two uncertainty in the decay rate and hence in the value of |V_{ub}|^2 deduced from it. For B -> K* gamma, from the combined analysis of form factors T_1 and T_2, we find the hadronisation ratio R_{K^*} of the exclusive B -> K* gamma to the inclusive b -> s gamma rates is of order 35% or 15% for constant and pole-type behaviour of T_2 respectively.

We present a unified method for analysing form factors in B --> pi l ($) over bar nu(l) and B --> K*gamma decays. The analysis provides consistency checks on the q(2) and 1/M extrapolations necessary to obtain the physical decay rates. For the first time the q(2) dependence of the form factors is obtained at the B scale. In the B --> pi l ($) over bar nu(l) case, we show that pole fits to f(+) may not be consistent with the q(2) behaviour of f(0), leading to a possible factor of two uncertainty in the decay rate and hence in the value of /V-ub/(2) deduced from it. For B --> K*gamma, from the combined analysis of form factors T-1 and T-2, we find the hadronisation ratio R(K)* of the exclusive B --> K*gamma to the inclusive b --> s gamma rates is of order 35% or 15% for constant and pole-type behaviour of T-2, respectively.

We present a unified method for analysing form factors in B → πlνl and B → K∗γ decays. The analysis provides consistency checks on the q2 and 1M extrapolations necessary to obtain the physical decay rates. For the first time the q2 dependence of the form factors is obtained at the B scale. In the B → πlνl case, we show that pole fits to f+ may not be consistent with the q2 behaviour of f0, leading to a possible factor of two uncertainty in the decay rate and hence in the value of |Vub|2 deduced from it. For B → K∗γ, from the combined analysis of form factors T1 and T2, we find the hadronisation ratio RK∗ of the exclusive B → K∗γ to the inclusive b → sγ rates is of order 35% or 15% for constant and pole-type behaviour of T2, respectively.

We present results of a lattice computation of the vector and axial-vector current matrix elements relevant for the semileptonic decay B^0-bar -> rho^+ l^- nu_l-bar. The computations are performed in the quenched approximation of lattice QCD on a 24^3 x 48 lattice at beta = 6.2, using an O(a) improved fermionic action. Our principal result is for the differential decay rate, dGamma/dq^2, for the decay B^0-bar -> rho^+ l^- nu_l-bar in a region beyond the charm threshold, allowing a model-independent extraction of |V_{ub}| from experimental measurements. Heavy quark symmetry relations between radiative and semileptonic decays of B-bar mesons into light vector mesons are also discussed.

We present a study of semileptonic ($) over bar B --> Dl ($) over bar v decays in quenched lattice QCD through a calculation of the matrix element [D\($) over bar c gamma(mu)b\($) over bar B] on a 24(3) x 48 lattice at beta = 6.2, using an O(alpha)-improved fermion action. We perform the calculation for several values of the initial and final heavy-quark masses around the charm mass, and three values of the light-(anti)quark mass around the strange mass. Because the charm quark has a bare mass which is almost 1/3 the inverse lattice spacing, we study the ensuing mass-dependent discretization errors, and propose a procedure for subtracting at least some of them nonperturbatively. We extract the form factors h(+) and h(-). After radiation corrections, we find that h(+) displays no dependence on the heavy-quark mass, enabling us to identify it with an Isgur-Wise function xi. Interpolating the light-quack mass to that of the strange, we obtain an Isgur-Wise function relevant for ($) over bar B-s --> D-s*l ($) over bar v decays which has a slope -xi(s)' = 1.2(-2)(+2)(stat)(-1)(+2)(syst) at zero recoil. An extrapolation to a massless light quark enables us to obtain an Isgur-Wise function relevant for ($) over bar B --> D(*)l ($) over bar v decays. This function has a slope -xi(u,d)' = 0.9(-3)(+2)(stat)(-2)(+4)(syst) at zero recoil. We observe a slight decrease in the magnitude of the central value of the slope as the mass of the light quark is reduced; given the errors, however, the significance of this observation is limited. We then use these functions, in conjunction with heavy-quark effective theory, to extract V-cb with no free parameters from the ($) over bar B --> D*l ($) over bar v decay rate measured by the ALEPH, ARGUS, and CLEO Collaborations. Using the CLEO data, for instance, we obtain \V-cb\ = 0.037(-1-2-1)(+1+2+4)(0.99/1 + beta(A1)(1))1/1 + delta(1/mc2) where delta(1/mc2) is the power corrections inversely proportional to the square of the charm quark mass, and beta(A1)(1) is the relevant radiative correction at zero recoil. Here, the first set of errors is experimental, the second represents the statistical error, and the third represents the systematic error in our evaluation of the Isgur-Wise function. We also use our Isgur-Wise functions and heavy-quark effective theory to calculate branching ratios for ($) over bar B-(s) --> D-(s)l ($) over bar v and ($) over bar B-(s) --> D-(s)*l ($) over bar v decays.

We calculate the leading-order matrix element for the decay $B \to K^* \gamma$ in the quenched approximation of lattice QCD on a $24^3 \times 48$ lattice at $\beta=6.2$, using an O(a)-improved fermion action. Extrapolating the quark masses to their physical values we obtain an on-shell form factor of $T_1(q^2=0)=0.15+12-14$, where the errors quoted are purely statistical. We find $T_1$ is approximately independent of the spectator quark mass and extract $T_1(q^2=0)=0.15+5-4$ if this independence is assumed. We compare this with the same form factor derived (in the Standard Model) from the CLEO experimental branching ratio of $BR(B \to K^* \gamma) = (4.5 \pm 1.5 \pm 0.9) \times 10^{-5}$ and find the results to be consistent within statistical errors.

We calculate the leading-order matrix element for the decay $B \to K^* \gamma$ in the quenched approximation of lattice QCD on a $24^3 \times 48$ lattice at $\beta=6.2$, using an O(a)-improved fermion action. Extrapolating the quark masses to their physical values we obtain an on-shell form factor of $T_1(q^2=0)=0.15+12-14$, where the errors quoted are purely statistical. We find $T_1$ is approximately independent of the spectator quark mass and extract $T_1(q^2=0)=0.15+5-4$ if this independence is assumed. We compare this with the same form factor derived (in the Standard Model) from the CLEO experimental branching ratio of $BR(B \to K^* \gamma) = (4.5 \pm 1.5 \pm 0.9) \times 10^{-5}$ and find the results to be consistent within statistical errors.

We calculate the leading-order matrix element for the decay B ! Kin the quenched approximation of lattice QCD on a 243 × 48 lattice at � = 6.2, using an O(a)-improved fermion action. Extrapolating the quark masses to their physical values we obtain an on-shell form factor of T1(q2=0) = 0.15+12 14, where the errors quoted are purely statistical. We find T1 is approximately independent of the spectator quark mass and extract T1(q2=0) = 0.15+5 4 if this independence is assumed. We compare this with the same form factor derived (in the Standard Model) from the CLEO experimental branching ratio of BR(B ! K�) = (4.5 ± 1.5 ± 0.9) × 10 5 and find the results to be consistent within statistical errors.

We present a study of semi-leptonic ¯ B → Dℓ¯ ν decays in quenched lattice QCD through a calculation of the matrix element h D|¯ cγ� b| ¯ Bi on a 243 × 48 lattice at β = 6.2, using an O(a)-improved fermion action. We perform the calculation for several values of the initial and final heavy-quark masses around the charm mass, and three values of the light-(anti)quark mass around the strange mass. Because the charm quark has a bare mass which is almost 1/3 the inverse lattice spacing, we study the ensuing mass-dependent dis- cretization errors, and propose a procedure for subtracting at least some of them non-perturbatively.