arXiv subjects
Publications and source records attributed to UKQCD Collaboration.
We compare results obtained using the Sheikholeslami-Wohlert (SW) fermion action with tree-level and tadpole-improved coefficients for $5.7\leβ\le 6.2$.
We report on a lattice calculation of the form factors A_0 and A_3 for the pseudoscalar to vector meson semileptonic decay Bbar^0 to rho^+ l^- nubar_l. We find that resonant (or pole-type) contributions alone are unable to describe these two form factors simultaneously. For the quantity A_0(q^2=0), which is important phenomenologically for the determination of |Vub|, we extract a range of values, A_0(q^2=0) = (0.16--0.35) +0.09-0.06, where the range is due to systematic uncertainty and the quoted error is statistical. We have also determined A_2(q^2=0) = 0.28 +0.09-0.06 +0.04-0.05.
The results of an exploratory lattice study of heavy baryon spectroscopy are presented. We have computed the full spectrum of the eight baryons containing a single heavy quark, on a $24^3\times 48$ lattice at $β=6.2$, using an $O(a)$-improved fermion action. We discuss the lattice baryon operators and give a method for isolating the contributions of the spin doublets $(Σ,Σ^*)$, $(Ξ',Ξ^*)$ and $(Ω,Ω^*)$ to the correlation function of the relevant operator. We compare our results with the available experimental data and find good agreement in both the charm and the beauty sectors, despite the long extrapolation in the heavy quark mass needed in the latter case. We also predict the masses of several undiscovered baryons. We compute the $Λ-\mbox{pseudoscalar meson}$ and $Σ-Λ$ mass splittings. Our results, which have errors in the range $ 10-30\%$, are in good agreement with the experimental numbers. For the $Σ^*-Σ$ mass splitting, we find results considerably smaller than the experimental values for both the charm and the beauty baryons, although in the latter case the experimental results are still preliminary. This is also the case for the lattice results for the hyperfine splitting for the heavy mesons.
We present the calculation of the spectrum of baryons containing one heavy quark. Heavy baryon and meson mass splittings are computed and compared with experiment. We give preliminary results for the form factor $G_1$ for the semileptonic decay $ Λ_b \rightarrow Λ_c l \barν $ and investigate its flavour symmetry.
We present a study in the quenched approximation of the $B$ parameter $B_B$ and the decay constant $f_B$ using heavy-quark propagators implemented in the static approximation, and light-quark propagators computed using an $O(a)$-improved fermion action. We find a value of $\Bbstat$ close unity, and discuss the systematic errors entering into the calculation. $\fbstat$ is extracted using a variational fitting technique in order to obtain a reliable estimate of the ground state. In the second part of the talk, we describe an exploratory study of baryons containing a single heavy quark, computed using the $O(a)$-improved fermion action. We obtain masses generally in good agreement with experiment in both the charm and beauty sectors. We also report preliminary results for the form factor $G_1$ in the semi-leptonic $Λ_b \rightarrow Λ_c$ transition.
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 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 calculate correlations between hadronic current operators as a function of their spatial separation, in a quenched lattice QCD simulation at $β=6.2$ on a $24^3\times48$ lattice. The lattice fermion formulation used is that due to Sheikholeslami and Wohlert. The correlation functions are then compared with the corresponding quantities calculated in the infinite volume chiral limit of the non-interacting theory. The ratio of the two quantities contains information on the vacuum structure of QCD. Results obtained are consistent with previous studies, and with known hadron phenomenology, although certain features of the fermion action used make it impossible to probe the $r\to0$ limit directly, thus limiting the accuracy possible.
We use a method recently suggested for evaluating the slope of the Isgur-Wise function, at the zero-recoil point, on the lattice. The computations are performed in the quenched approximation to lattice QCD, on a $24^3 \times 48$ lattice at $β=6.2$, using an $O(a)$-improved action for the fermions. We have found unexpectedly large finite-volume effects in such a calculation. These volume corrections turned out to be purely geometrical and independent of the dynamics of the system. After the study of these effects on a smaller volume and for different quark masses, we give approximate expressions that account for them. Using these approximations we find $ξ^\prime(1)=-1.7 \pm 0.2$ and $ξ^\prime(1)=-1.4 +0.2-0.1$ for the slope of the Isgur-Wise function, for two mesons composed of a heavy quark slightly heavier and lighter, respectively, than the charm quark, and in both cases, a light antiquark whose mass is about that of the strange quark.
We present results of a lattice computation of the matrix elements of the vector and axial-vector currents which are relevant for the semi-leptonic decays $D \rightarrow K$ and $D \rightarrow K^*$. The computations are performed in the quenched approximation to lattice QCD on a $24^3 \times 48$ lattice at $β=6.2$, using an $O(a)$-improved fermionic action. In the limit of zero lepton masses the semi-leptonic decays $D \rightarrow K$ and $D \rightarrow K^*$ are described by four form factors: $f^{+}_K,V,A_1$ and $A_2$, which are functions of $q^2$, where $q^μ$ is the four-momentum transferred in the process. Our results for these form factors at $q^2=0$ are: $f^+_K(0)=0.67 \er{7}{8}$ , $V(0)=1.01 \err{30}{13}$ , $A_1(0)=0.70 \err{7}{10}$ , $A_2(0)=0.66 \err{10}{15}$ , which are consistent with the most recent experimental world average values. We have also determined the $q^2$ dependence of the form factors, which we find to be reasonably well described by a simple pole-dominance model. Results for other form factors, including those relevant to the decays \dpi and \drho, are also given.
We study a closed hadronic string using pure glue lattice simulation. We measure the energy of the flux state encircling the periodic boundary condition (the torelon). From these data we deduce the string fluctuation component and compare with models for the hadronic string. [The TEX source file is also available by anonymous ftp from suna.amtp.liv.ac.uk in directory pub/cmi ]
Form factors for pseudoscalar --> pseudoscalar decays of heavy-light mesons are found in quenched lattice QCD with heavy-quark masses in the range of approximately 1-2 GeV. The Isgur-Wise function, $ξ(ω)$, is extracted from these form factors. Results are in good agreement with $ξ(ω)$ derived from CLEO measurements for B --> D*.
We calculate the Isgur-Wise function by measuring the elastic scattering amplitude of a $D$ meson in the quenched approximation on a $24^3\times48$ lattice at $β=6.2$, using an $O(a)$-improved fermion action. We use this result, in conjunction with heavy-quark symmetry, to extract $|V_{cb}|$ from the experimentally measured $\bar B\to D^*l\barν\,$\ differential decay width.
First results are obtained for B mesons using a heavy propagator calculated using NRQCD, and a light Wilson propagator. Results from 13 quenched configurations of size 16^3 x 48 at β= 6.0 give a value for f_{B} of less than 200 Mev and a B^{*}-B splitting of 32(8) MeV. Superior signal/noise behaviour is observed over static propagators on the same configurations. No extrapolation to the b mass for the heavy quark is required.
We present decay constants from a quenched simulation using both propagating quarks and the static theory at $β=6.2$ on $24^3\times48$. The results using propagating quarks are used to test scaling laws predicted by the heavy quark symmetry.
We calculate the leading-order matrix element for the decay $B \to K^* γ$ in the quenched approximation of lattice QCD on a $24^3 \times 48$ lattice at $β=6.2$, 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 $BR(B \to K^* γ) = (4.5 \pm 1.5 \pm 0.9) \times 10^{-5}$ and find the results to be consistent within statistical errors.
Preliminary results are presented on the calculation of the relevant form factor for the radiative decay $\overline{B} \rightarrow K^* γ$ We find that the form factor is weakly dependent on the spectator quark mass. We compare the results obtained for a full chiral extrapolation and where this independence is assumed. Both results are found to be statistically consistent with the experimental data.
We present results for light hadrons composed of both degenerate and non-degenerate quarks in quenched lattice QCD. We calculate masses and decay constants using 60 gauge configurations with an $O(a)$--improved fermion action at $β= 6.2$. Using the $ρ$ mass to set the scale, we find hadron masses within two to three standard deviations of the experimental values (given in parentheses): $m_{K^*}=868\er{9}{8}$~MeV (892~MeV), $m_ϕ=970\err{20}{10}$~MeV (1020~MeV), $m_N=820\err{90}{60}$~MeV (938~MeV), $m_Δ=1300\errr{100}{100}$~MeV (1232~MeV) and $m_Ω=1650\err{70}{50}$~MeV (1672~MeV). Direct comparison with experiment for decay constants is obscured by uncertainty in current renormalisations. However, for ratios of decay constants we obtain $f_K/f_π=1.20\er{3}{2}$ (1.22) and $f_ϕ/f_ρ=1.13\er{2}{3}$ (1.22).