Matrix elements of heavy-light mesons from a fine lattice
Results on semileptonic decay matrix elements of heavy-light mesons and charmonium spectrum and decay constant using a fine quenched lattice are presented.
hep-lat↗
arXiv subjects
S. Collins
Publications and source records attributed to S. Collins.
Unquenched determination of the kaon parameter B_K from improved staggered fermions
The use of improved staggered actions (HYP, Asqtad) has been proved to reduce the scaling corrections that affected previous calculations of B_K with unimproved (standard) staggered fermions in the quenched approximation. This improved behaviour allows us to perform a reliable calculation of B_K including quark vacuum polarization effects, using the MILC configurations with n_f=2+1 flavours of sea fermions. We perform such a calculation for a single lattice spacing, a=0.125 fm, and with kaons made up of degenerate quarks with m_s/2. The valence strange quark mass m_s is fixed to its physical value and we use two different values of the light sea quark masses. After a chiral extrapolation of the results to the physical value of the sea quark masses, we find \hat B_K = 0.83+-0.18, where the error is dominated by the uncertainty in the lattice to continuum matching at O(α_s^2). The matching will need to be improved to get the precision needed to make full use of the experimental data on ε_K to constrain the unitarity triangle.
hep-lat↗
Dynamical determination of B_K from improved staggered quarks
The scaling corrections that affected previous staggered calculations of B_K have been proved to be reduced by using improved actions (HYP, Asqtad) in the quenched approximation. This improved behaviour allows us to perform a reliable dynamical calculation of B_K including quark vacuum polarization effects using the MILC (2+1) flavour dynamical configurations. We report here on the results from such dynamical calculation. We also discuss the renormalization effects with the Asqtad action.
hep-lat↗
Electroabsorption spectroscopy of single walled nanotubes
We have measured the electric field modulated absorption of a sample of single-walled nanotubes (SWNT) suspended in a solid polyvinyl alcohol matrix. The electroabsorption (EA) spectrum roughly follows the first derivative of the absorption with respect to photon energy, scales quadratically with the electric field strength, and shows a pronounced anisotropy of light polarization with respect to the applied electric field direction. These findings indicate a quadratic Stark effect caused by a change in the polarizability of the excited states, which is common to quasi-one dimensional (1D) excitons in organic semiconductors. The EA spectrum is well described by calculations involving electron-electron interaction in the model Hamiltonian of both zigzag and chiral nanotubes. We have calculated the EA spectra for both zigzag and chiral nanotubes within a model Hamiltonian that includes electron-electron interactions. The calculations reproduce the observed quadratic Stark shift of the lowest optical exciton, as well as the more complicated behavior of the EA spectrum in the energy region that corresponds to the next higher exciton. Our findings show that the low-lying absorption bands in semiconducting SWNT are excitonic in origin, in agreement with transient optical measurements that identify the primary photoexcitations in SWNT as quasi-1D excitons with a substantial binding energy.
cond-mat.mtrl-sci↗
B_K from improved staggered quarks
We compare calculations of B_K with improved staggered quarks (HYP, Asqtad) and demonstrate the improved scaling behaviour that this gives rise to over previous calculations with unimproved staggered quarks. This enables us to perform the calculation of B_K on the MILC dynamical configurations (n_f=2+1), for which we give preliminary results.
hep-lat↗
Semileptonic B Decays from an NRQCD/D234 Action
Semileptonic B decays are studied on quenched anisotropic lattices using Symanzik improved glue, NRQCD heavy quark and D234 light quark actions. We employ constrained fits to extract ground state contributions to two- and three-point correlators. Results for the B --> pi, l nubar decay form factors are compared with previous lattice results. We find that our systematic errors (excluding quenching errors) are dominated by chiral extrapolation uncertainties.
hep-lat↗
Semileptonic B Decays from an NRQCD/D234 Action
Semileptonic, B --> pi, l nubar, decays are studied on quenched anisotropic lattices using tree-level tadpole improved Symanzik glue, NRQCD heavy quark and D234 light quark actions. Constrained fitting methods are applied to extract groundstate contributions to two-point and three-point correlators. We agree with previous lattice determinations of the form factors. The major source of systematic error here, as in previous work, comes from the chiral extrapolation to the physical pion mass. Future calculations must work at lighter quark masses to resolve this.
hep-lat↗
A nonperturbative determination of the O(a) improvement coefficient c_A and the scaling of f_pi and m^{MSbar}
We report on an investigation of the LANL method for determining the O(a) improvement coefficient c_A nonperturbatively. We find we are able to extract reliable estimates for the coefficient using this method. However, our study of systematic errors shows that for very accurate determinations of c_A, the smearing function must be tuned and the volume fixed to keep the O(a) ambiguity in c_A fixed as beta varies. Consistency was found with previous results from the LANL group and (within fairly large errors) 1-loop perturbation theory; c_A does not change significantly over the range beta=5.93-6.2. The big difference between our results and those of the ALPHA collaboration, around beta=6.0, show that the O(a) differences in c_A between the different methods can be large. We find that the lattice spacing dependence of f_pi and the renormalised quark mass is much smaller using our values of the coefficient compared to those of the ALPHA collaboration.
hep-lat↗
F_B from moving B mesons
We show results for the B meson decay constant calculated both for B mesons at rest and those with non-zero momentum and using both the temporal and spatial components of the axial vector current. It is an important check of lattice systematic errors that all these determinations of f_B should agree. We also describe how well different smearings for the B meson work at non-zero momentum - the optimal smearing has a narrow smearing for the b quark.
hep-lat↗
A nonperturbative determination of c_A and the scaling of f_pi and m^{MSbar}
We calculate the $O(a)$ improvement coefficient for the axial-vector current using the nonperturbative method proposed by the LANL group. Results for the coefficient in the range $β=5.93$ to 6.2 are presented. We find $c_A$ is close to the 1-loop tadpole-improved perturbative value. In addition, scaling of the pseudoscalar decay constant and renormalised quark mass is improved compared to that obtained using the (larger) $c_A$ values obtained by the ALPHA collaboration.
hep-lat↗
Heavy-light Mesons and Baryons with b quarks
We present lattice results for the spectrum of mesons containing one heavy quark and of baryons containing one or two heavy quarks. The calculation is done in the quenched approximation using the NRQCD formalism for the heavy quark. We analyze the dependence of the mass splittings on both the heavy and the light quark masses. Meson P-state fine structure and baryon hyperfine splittings are resolved for the first time. We fix the b quark mass using both M_B and M_{Λ_b}, and our best estimate is m_b^\MSbar(m_b^\MSbar) = 4.35(10)({}^{-3}_{+2})(10) GeV. The spectrum, obtained by interpolation to m_b, is compared with the experimental data.
hep-lat↗
Comparison Studies of Finite Momentum Correlators on Anisotropic and Isotropic Lattices
We study hadronic two- and three-point correlators relevant for heavy to light pseudoscalar meson semi-leptonic decays, using Symanzik improved glue, D234 light quark and NRQCD heavy quark actions. Detailed comparisons are made between simulations on anisotropic and isotropic lattices involving finite momentum hadrons. We find evidence that having an anisotropy helps in extracting better signals at higher momenta. Initial results for the form factors f_+(q^2) and f_0(q^2) are presented with tree-level matching of the lattice heavy-light currents.
hep-lat↗
Scaling and Further Tests of Heavy Meson Decay Constant Determinations from Nonrelativistic QCD
We present results for the B_s meson decay constant f_{B_s} from simulations at three lattice spacings in the range a^{-1}=1.1 to 2.6 GeV using NRQCD heavy quarks and clover light quarks in the quenched approximation. We study scaling of this quantity and check the consistency between mesons decaying from rest and from a state with nonzero spatial momentum. The cancellation of power law contributions that arise in the NRQCD formulation of heavy-light currents is discussed. On the coarsest lattice the D_s meson decay constant f_{D_s} is calculated. Our best values for the decay constants are given by f_{B_s} = 187(4)(4)(11)(2)(7)(6) MeV and f_{D_s} = 223(6)(31)(38)(23)(9)(^{+3}_{-1}) MeV.
hep-lat↗
A nonperturbative determination of cA
We describe a non-perturbative determination of $c_A$ using correlators containing the axial-vector and pseudoscalar currents at zero and non-zero momentum. We apply the method of Bhattacharya et al to extract $c_A$ from the requirement that the ratio of appropriate correlators for the PCAC relation becomes independent of time in the excited state region. We find that the result depends strongly on the order of the derivatives used in the PCAC relation. We also find that, using the lowest order derivatives, we cannot get a consistent value of $c_A$ between zero and non-zero momentum cases. The $c_A$ values that we obtain as we improve the derivatives are consistent and decrease in magnitude heading towards the perturbative result.
hep-lat↗
The Mass of the b Quark from Lattice NRQCD
We present results for the mass of the $b$ quark in the $\bar{MS}$ scheme obtained by calculating the binding energy of the $B$ meson in the static limit. The self energy of a static quark, $E_0^\infty$ needed for this purpose, is now known to $O(α^3)$ in the quenched approximation. We find a {\it preliminary} value of $\bar{m_b}(\bar{m_b})=4.34(7)$ GeV at $n_f=0$. The error is dominated by the remaining uncertainty in $E_0^\infty$. In addition, using $E_0^\infty$ at $O(α^2)$, we estimate that the quark mass is reduced by approximately 70 MeV when two flavours of dynamical quarks are introduced.
hep-lat↗
Scaling of the B and D meson spectrum in lattice QCD
We give results for the $B$ and the $D$ meson spectrum using NRQCD on the lattice in the quenched approximation. The masses of radially and orbitally excited states are calculated as well as $S$-wave hyperfine and $P$-wave fine structure. Radially excited $P$-states are observed for the first time. Radial and orbital excitation energies match well to experiment, as does the strange-non-strange $S$-wave splitting. We compare the light and heavy quark mass dependence of various splittings to experiment. Our $B$-results cover a range in lattice spacings of more than a factor of two. Our $D$-results are from a single lattice spacing and we compare them to numbers in the literature from finer lattices using other methods. We see no significant dependence of physical results on the lattice spacing. PACS: 11.15.Ha 12.38.Gc 14.40.Lb 14.40.Nd
hep-ph↗
Aspects of determining fBs: scaling and power-law divergences
We present preliminary results for the decay constant of the Bs meson, fBs, at three values of beta=5.7, 6.0 and 6.2 using NRQCD and clover fermions for the heavy and light quarks respectively. As a consistency check the decay constant has also been extracted from the axial-vector current at finite momentum. In addition, we discuss the cancellation of O(alpha/(aM0)) terms and the remaining uncertainty in fBs from higher order divergences.
hep-lat↗
Sea quark effects in B Spectroscopy and Decay Constants
We present comprehensive results for the spectrum and decay constants of hadrons containing a single b quark. The heavy quark is simulated using an $O(1/M)$ NRQCD action and the light quark using the $O(a)$ tadpole-improved clover action on gauge configurations containing two degenerate flavours of sea quarks at $β^{n_f=2}=5.6$ provided by the HEMCGC collaboration. We present detailed results for the lower lying $S$ and $P$ wave $B$ meson states and the $Λ_b$ baryon. We find broad agreement with experiment. In addition, we present results for the pseudoscalar and, for the first time, the vector decay constants fully consistent to $O(α/M): f_B = 186(5)(stat)(19)(pert)(9)(disc)(13)(NRQCD)(+50)(a^{-1})MeV, f_B^* = 181(6) (stat)(18)(pert)(9)(disc)(13)(NRQCD)(+55)(a^{-1})MeV$ and $f_{B_s}/f_B = 1.14(2)(stat)(-2)(κ_s)$. We present an investigation of sea quark effects in the $B$ spectrum and decay constants. We compare our results with those from similar quenched simulations at $β^{n_f=0}=6.0$. For the spectrum, the quenched results reproduce the experimental spectrum well and there is no significant difference between the quenched and $n_f=2$ results. For the decay constants, our results suggest that sea quark effects may be large. We find that $f_B$ increases by approximately 25% between $n_f=0$ and $n_f=2$.
hep-lat↗