UKQCD Collaboration Research Articles

Spectroscopy and flavor changing decays ofB,Bsmesons in a Dirac formalism

In the framework of the relativistic independent quark model, the mass spectra and decay properties of $B$ and ${B}_{s}$ mesons are obtained using a Martin-like potential for the quark confinement. The predicted excited states are in good agreement with the experimental results as well as with the lattice QCD and other theoretical predictions. For instance, the ${B}_{2}(5747)$ as $1{^{3}P}_{2}$, ${B}_{1}(5721)$ as $1{^{3}P}_{1}$, and ${B}_{0}(5732)$ as $1{^{3}P}_{0}$ are identified. The spectroscopic parameters are used to calculate the electromagnetic transitions, pseudoscalar decay constants, hadronic decay widths, and leptonic decay widths. The present result for the decay constant, ${f}_{B}(1S)=188.56\text{ }\text{ }\mathrm{MeV}$, is in good agreement with recent lattice results (UKQCD Collaboration, Fermilab) and comparable with the experimental value of $(206.7\ifmmode\pm\else\textpm\fi{}8.9)$. The pseudoscalar decay constant for the ${B}_{s}$ meson obtained here, ${f}_{{B}_{s}}(1S)=240.21\text{ }\text{ }\mathrm{MeV}$, is in very good agreement with recent lattice QCD and QCD sum rule predictions. The predicted branching ratio for ${B}^{+}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{+}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ ($1.354\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$) is in accordance with the value, $(1.65\ifmmode\pm\else\textpm\fi{}0.34)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ reported by the Particle Data Group (PDG). The branching ratios of the rare decays ${B}_{s}^{0}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ ($(3.1\ifmmode\pm\else\textpm\fi{}0.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$) and ${B}^{0}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ ($l6.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$) as observed by the CMS and LHCb Collaborations very recently are in accordance with our predictions of $3.602\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ and $1.018\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$, respectively. The Cabibbo-favored hadronic branching ratios of ${B}^{0}\ensuremath{\rightarrow}{D}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ ($3.724\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$), ${B}^{0}\ensuremath{\rightarrow}{D}^{*\ensuremath{-}}{\ensuremath{\pi}}^{+}$ ($3.475\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$), and ${B}_{s}\ensuremath{\rightarrow}{D}_{s}^{\ensuremath{-}}{\ensuremath{\rho}}^{+}$ ($3.800\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$) are in good agreement with the respective PDG values. The mixing parameters ${x}_{q}$, ${\ensuremath{\chi}}_{q}$ for ${B}^{0}\ensuremath{-}{\overline{B}}^{0}$ (${x}_{d}=0.769$, ${\ensuremath{\chi}}_{d}=0.1859$) and ${B}_{s}\ensuremath{-}{\overline{B}}_{s}$ (${x}_{s}=26.41$, ${\ensuremath{\chi}}_{s}=0.49929$) are also found to be in excellent agreement with the PDG values of ($0.770\ifmmode\pm\else\textpm\fi{}0.008$, $0.1862\ifmmode\pm\else\textpm\fi{}0.0023$) and ($26.49\ifmmode\pm\else\textpm\fi{}0.29$, $0.499292\ifmmode\pm\else\textpm\fi{}0.000016$), respectively.

Vacuum Insertion Approximation and the [formula omitted] rule: A lattice QCD test of the naïve factorization hypothesis for K, D, B and static mesons

Motivated by a recent paper by the RBC–UKQCD Collaboration, which observes large violations of the naïve factorization hypothesis in K→ππ decays, we study in this paper the accuracy of the Vacuum Insertion Approximation (VIA) for the matrix elements of the complete basis of four-fermion ΔF=2 operators. We perform a comparison between the matrix elements in QCD, evaluated on the lattice, and the VIA predictions. We also investigate the dependence on the external meson masses by computing matrix elements for K, Ds, Bs and static mesons. In commonly used renormalization schemes, we find large violations of the VIA in particular for one of the two relevant Wick contractions in the kaon sector. These deviations, however, decrease significantly as the meson mass increases and the VIA predictions turn out to be rather well verified for B-meson matrix elements and, even better, in the infinite mass limit.

Excited B mesons from the lattice

The energies of different angular momentum states of a heavy-light meson were measured on a lattice in [C. Michael, J. Peisa for UKQCD Collaboration, Phys. Rev. D 58, 34506 (1998)]. We have now repeated this study using several different lattices, quenched and unquenched, that have different physical lattice sizes, clover coefficients and quark–gluon couplings. The heavy quark is taken to be infinitely heavy, whereas the light quark mass is approximately that of the strange quark. By interpolating and extrapolating in the light quark mass we can thus compare the lattice results with B and Bs meson experiments. Most interesting is the lowest P-wave Bs state, since it is possible that it lies below the BK threshold and hence is very narrow. Unfortunately, there are no experimental results on P-wave B or Bs mesons available at present. In addition to the energy spectrum, we measured earlier also vector (charge) and scalar (matter) radial distributions of the light quark in the S-wave states of a heavy-light meson on a lattice [A.M. Green, J. Koponen, C. Michael, P. Pennanen for UKQCD Collaboration, Phys. Rev. D 65, 014512 (2002) and Eur. Phys. J. C 28, 79 (2003), hep-lat/0206015 ]. Now we are extending the study of radial distributions to P-wave states.

Effects of dynamical quarks in UKQCD simulations

Recent results from the UKQCD Collaboration's dynamical simulations are presented. The main feature of these ensembles is that they have a fixed lattice spacing and volume, but varying sea quark mass from infinite (corresponding to the quenched simulation) down to roughly that of the strange quark mass. The main aim of this work is to uncover dynamical quark effects from these “matched” ensembles. We obtain some evidence of dynamical quark effects in the static quark potential with less effects in the hadronic spectrum.

Lattice QCD with dynamical quarks from the UKQCD collaboration

A brief overview of the lattice technique of studying QCD is presented. Recent results from the UKQCD Collaboration's simulations with dynamical quarks are then presented. In this work, the calculations are all at a fixed lattice spacing and volume, but varying sea quark mass from infinite (corresponding to the quenched simulation) down to roughly that of the strange quark mass. The main aim of this work is to uncover dynamical quark effects from these ``matched'' ensembles.

Searching for dynamical fermion effects in UKQCD simulations

We present recent results from the UKQCD collaboration's dynamical QCD simulations. This data has fixed lattice spacing but varying dynamical quark mass. We concentrate on searching for an unquenching signal in the mesonic mass spectrum where we do not find a significant effect at the quark masses considered.

Light hadron spectrum using an O(a)-improved Wilson action with two dynamical quark flavours

I present results of recent work for the UKQCD collaboration on the light hadron spectrum using a non-perturbatively O(a)-improved Wilson action with two degenerate flavours of dynamical quarks on a 16 3 × 32 lattice. Values of the bare gauge coupling and bare dynamical quark mass were chosen to keep the lattice spacing fixed at two distinct values in physical units as determined through the Sommer scale parameter, r 0. I also include results for a quenched ensemble with a lattice spacing matching one of these dynamical sets and results from an exploratory run at lighter bare quark mass.

O( a) improved Wilson quark action on anisotropic lattice

The O( a) improved Wilson quark action on the anisotropic lattice is investigated. We carry out numerical simulations in the quenched approximation at three values of lattice spacing ( a −1 σ = 1–2 GeV) with the anisotropy ξ = a σ/ a τ = 4, where a σ and a τ are the spatial and the temporal lattice spacings, respectively. Using the dispersion relation of mesons, the bare anisotropy γ F in the quark action is numerically tuned below the charm quark mass region with the statistical accuracy of 1 % level. The systematic uncertainties in the calibration are examined and found to be under control in the continuum limit. Then we compute the light hadron masses and find that they are consistent with the result of the UKQCD Collaboration on the isotropic lattice. The effect of the uncertainty in the calibration on the hadron spectrum for physical quark masses is also found to be under control.

Recent lattice QCD results from the UKQCD collaboration

The lattice technique of studying the strong interaction of matter is used to obtain predictions of the hadronic spectrum. These simulations were performed by the UKQCD collaboration using full (unquenched) QCD. Details of the results, a comparison with quenched data, and novel methods of extracting spectral properties are described.

Numerical study ofO(a)improved Wilson quark action on an anisotropic lattice

The $O(a)$ improved Wilson quark action on the anisotropic lattice is investigated. We carry out numerical simulations in the quenched approximation at three values of lattice spacing ($a_{\sigma}^{-1}=1$--2 GeV) with the anisotropy $\xi=a_{\sigma}/a_{\tau}=4$, where $a_{\sigma}$ and $a_{\tau}$ are the spatial and the temporal lattice spacings, respectively. The bare anisotropy $\gamma_F$ in the quark field action is numerically tuned by the dispersion relation of mesons so that the renormalized fermionic anisotropy coincides with that of gauge field. This calibration of bare anisotropy is performed to the level of 1 % statistical accuracy in the quark mass region below the charm quark mass. The systematic uncertainty in the calibration is estimated by comparing the results from different types of dispersion relations, which results in 3 % on our coarsest lattice and tends to vanish in the continuum limit. In the chiral limit, there is an additional systematic uncertainty of 1 % from the chiral extrapolation. Taking the central value $\gamma_F=\gamma_F^*$ from the result of the calibration, we compute the light hadron spectrum. Our hadron spectrum is consistent with the result by UKQCD Collaboration on the isotropic lattice. We also study the response of the hadron spectrum to the change of anisotropic parameter, $\gamma_F \to \gamma_F^* + \delta\gamma_F$. We find that the change of $\gamma_F$ by 2 % induces a change of 1 % in the spectrum for physical quark masses. Thus the systematic uncertainty on the anisotropic lattice, as well as the statistical one, is under control.

N* spectrum using an O( a) improved fermion action

The construction of operators and calculational methods for the determination of the N* spectrum are discussed. The masses of the parity partners of the nucleon and delta are computed from the O ( a)-improved data of the UKQCD Collaboration, and a clear splitting observed between the mass of the nucleon and its parity partner.

The topological susceptibility in ‘full’ (UK)QCD

We report first calculations of the topological susceptibility measured using the field theoretic method on SU(3) gauge configurations produced by the UKQCD collaboration with two flavours of dynamical, improved, Wilson fermions. Using three ensembles with matched lattice spacing but differing sea quark mass we find that hybrid Monte Carlo simulation appears to explore the topological sectors efficiently, and a topological susceptibility consistent with increasing linearly with the quark mass.

A first taste of dynamical fermions with an O(a) improved action

We present the first results obtained by the UKQCD Collaboration using a non-perturbatively O(a) improved Wilson quark action with two degenerate dynamical flavours.

Semileptonic B → ρ and B → π decays: Lattice and dispersive constraints

I present two recent pieces of work on semileptonic B → π, ρ decays where it is shown how lattice QCD calculations can be used to test heavy-quark symmetry and determine phenomenologically relevant quantities despite the limits on these calculations' kinematical reach. The study of semileptonic B → ρ decays was performed with the UKQCD Collaboration.

Light quark physics on different lattices

The light quark results from three different simulations are summarized. We get consistent results, within our statistics, between smeared and non smeared data. A comparison is performed with similar results from the UKQCD collaboration. All runs have been performed on two 6.4 GF APE computers [1].

Non-relativistic QCD for heavy quark systems

We employ a non-relativistic version of QCD (NRQCD) to study heavy q q bound states in the lowest approximation without fine structure. We use gluon configurations on a 6 3×48 lattice at β=6.2 from the UKQCD Collaboration. For a bare quark mass near that of the b-quark ( Ma=1.6) we obtain the bound state masses for the S-, P- and both types of D-waves. We also detect signals for two types of hybriges ( q qg states). The results are sufficiently accurate to confirm that the values of the D-wave mass from both D-waves coincide thus indicating that the cubical invariance of the lattice is restored to full rotational invariance to a good approximation. We also study S- and P-wave masses for values of the range of bare quark mass Ma=1.0, 1.3, 1.6 and 1.9. The results confirm the idea that the S/P-splitting is relatively insensitive to the value of the bare quark mass.

First results from the UKQCD collaboration

Abstract We present the first physics results for quenched QCD and for pure SU(2) gauge theory from the UKQCD collaboration. We report preliminary results for static quark potentials, the string tension and the quenched hadron spectrum on a 24 3 × 48 lattice at β = 6.2 using the standard Wilson fermion action and an O ( a ) improved nearest-neighbour Wilson fermion action. Our codes run at in excess of 1.5 Gflops in the 64-node Meiko i860-based Computing Surface at Edinburgh. We have also simulated pure gauge SU(2) theory on a 48 3 × 56 lattice at β = 2.85 to explore the approach to the continuum limit.