arXiv subjects

John Bulava

Publications and source records attributed to John Bulava.

At least 55 records · Page 3Linked to original sources

arXiv2017

A local graph rewiring algorithm for sampling spanning trees

We introduce a Markov Chain Monte Carlo algorithm which samples from the space of spanning trees of complete graphs using local rewiring operations only. The probability distribution of graphs of this kind is shown to depend on the symmetries of these graphs, which are reflected in the equilibrium distribution of the Markov chain. We prove that the algorithm is ergodic and proceed to estimate the probability distribution for small graph ensembles with exactly known probabilities. The autocorrelation time of the graph diameter demonstrates that the algorithm generates independent configurations efficiently as the system size increases. Finally, the mean graph diameter is estimated for spanning trees of sizes ranging over three orders of magnitude. The mean graph diameter results agree with theoretical asymptotic values.

cs.DM↗

arXiv2017

Multi-hadron spectroscopy in a large physical volume

We demonstrate the efficacy of the stochastic LapH method to treat all-to-all quark propagation on a $N_f = 2+1$ CLS ensemble with large linear spatial extent $L = 5.5$ fm, allowing us to obtain the benchmark elastic isovector p-wave pion-pion scattering amplitude to good precision already on a relatively small number of gauge configurations. These results hold promise for multi-hadron spectroscopy at close-to-physical pion mass with exponential finite-volume effects under control.

hep-lat↗

arXiv2017

Scattering from finite-volume energies including higher partial waves and multiple decay channels

A new implementation of estimating the two-to-two $K$-matrix from finite-volume energies based on the Luescher formalism is described. The method includes higher partial waves and multiple decay channels, and the fitting procedure properly includes all covariances and statistical uncertainties. The method is also simpler than previously used procedures. Formulas and software for handling total spins up to $S=2$ and orbital angular momenta up to $L=6$ are presented.

hep-lat↗

arXiv2017

Estimating the two-particle $K$-matrix for multiple partial waves and decay channels from finite-volume energies

An implementation of estimating the two-to-two $K$-matrix from finite-volume energies based on the Lüscher formalism and involving a Hermitian matrix known as the "box matrix" is described. The method includes higher partial waves and multiple decay channels. Two fitting procedures for estimating the $K$-matrix parameters, which properly incorporate all statistical covariances, are discussed. Formulas and software for handling total spins up to $S=2$ and orbital angular momenta up to $L=6$ are obtained for total momenta in several directions. First tests involving $ρ$-meson decay to two pions include the $L=3$ and $L=5$ partial waves, and the contributions from these higher waves are found to be negligible in the elastic energy range.

hep-lat↗

arXiv2016

Non-perturbative renormalization of the axial current in $N_f = 3$ lattice QCD with Wilson fermions and tree-level improved gauge action

We non-perturbatively determine the renormalization factor of the axial vector current in lattice QCD with $N_f=3$ flavors of Wilson-clover fermions and the tree-level Symanzik-improved gauge action. The (by now standard) renormalization condition is derived from the massive axial Ward identity and it is imposed among Schrödinger functional states with large overlap on the lowest lying hadronic state in the pseudoscalar channel, in order to reduce kinematically enhanced cutoff effects. We explore a range of couplings relevant for simulations at lattice spacings of $\approx 0.09$ fm and below. An interpolation formula for $Z_A(g_0^2)$, smoothly connecting the non-perturbative values to the 1-loop expression, is provided together with our final results.

hep-lat↗

arXiv2016

$I=1$ and $I=2$ $π-π$ scattering phase shifts from $N_{\mathrm{f}} = 2+1$ lattice QCD

The $I=1$ $p$-wave and $I=2$ $s$-wave elastic $π$-$π$ scattering amplitudes are calculated from a first-principles lattice QCD simulation using a single ensemble of gauge field configurations with $N_{\mathrm{f}} = 2+1$ dynamical flavors of anisotropic clover-improved Wilson fermions. This ensemble has a large spatial volume $V=(3.7\mathrm{fm})^3$, pion mass $m_π = 230\mathrm{MeV}$, and spatial lattice spacing $a_s = 0.11\mathrm{fm}$. Calculation of the necessary temporal correlation matrices is efficiently performed using the stochastic LapH method, while the large volume enables an improved energy resolution compared to previous work. For this single ensemble we obtain $m_ρ/m_π = 3.350(24)$, $g_{ρππ} = 5.99(26)$, and a clear signal for the $I=2$ $s$-wave. The success of the stochastic LapH method in this proof-of-principle large-volume calculation paves the way for quantitative study of the lattice spacing effects and quark mass dependence of scattering amplitudes using state-of-the-art ensembles.

hep-lat↗

arXiv2016

Towards string breaking with 2+1 dynamical fermions using the stochastic LapH method

We investigate the use of stochastically estimated light quark propagators in correlation functions involving a static color source. To this end we compute the static-light meson pseudoscalar correlation function in the stochastic LapH framework, using an ensemble of $N_f= 2+1$ gauge configurations generated through the CLS effort. We extract the static-light as well as the static-strange mass with good statistical precision. Together with the static potential, we obtain a preliminary estimate for the expected mixing region.

hep-lat↗

arXiv2015

Pion-pion scattering and the timelike pion form factor from $N_{\mathrm{f}} = 2+1$ lattice QCD simulations using the stochastic LapH method

We report on progress applying the stochastic LapH method to estimate all-to-all propagators required in correlation functions of multi-hadron operators relevant for pion-pion scattering. Large-volume results for $I=2$ and $I=1$ pion-pion scattering phase shifts with good statistical precision are obtained from an $N_{\rm f} = 2+1$ anisotropic Wilson clover ensemble with $m_π = 240\mathrm{MeV}$. We also present a preliminary determination of the $I=1$ pion-pion scattering phase shift and timelike pion form factor on an isotropic $N_{\rm f}=2+1$ flavour ensemble generated by the Coordinated Lattice Simulation (CLS) community effort.

hep-lat↗

arXiv2015

Excited-state energies and scattering phase shifts from lattice QCD with the stochastic LapH method

Recent results in computing excited-state energies and meson-meson scattering phase shifts in lattice QCD are presented. A stochastic method of treating the low-lying modes of quark propagation that exploits Laplacian Heaviside quark-field smearing makes such studies possible now on large 32^3 x 256 and 48^3 x 128 lattices at near physical pion masses. Levels are identified using a variety of probe interpolating operators, which include both single-hadron and a large number of two-hadron operators.

hep-lat↗

arXiv2015

B-meson spectroscopy in HQET at order 1/m

We present a study of the B spectrum performed in the framework of Heavy Quark Effective Theory expanded to next-to-leading order in 1/m and non-perturbative in the strong coupling. Our analyses have been performed on Nf=2 lattice gauge field ensembles corresponding to three different lattice spacings and a wide range of pion masses. We obtain the Bs-meson mass and hyperfine splittings of the B- and Bs-mesons that are in good agreement with the experimental values and examine the mass difference m_{Bs}-m_B as a further cross-check of our previous estimate of the b-quark mass. We also report on the mass splitting between the first excited state and the ground state in the B and Bs systems.

hep-lat↗

arXiv2015

Non-perturbative improvement of the axial current in N_f=3 lattice QCD with Wilson fermions and tree-level improved gauge action

The coefficient c_A required for O(a) improvement of the axial current in lattice QCD with N_f=3 flavors of Wilson fermions and the tree-level Symanzik-improved gauge action is determined non-perturbatively. The standard improvement condition using Schroedinger functional boundary conditions is employed at constant physics for a range of couplings relevant for simulations at lattice spacings of ~ 0.09 fm and below. We define the improvement condition projected onto the zero topological charge sector of the theory, in order to avoid the problem of possibly insufficient tunneling between topological sectors in our simulations at the smallest bare coupling. An interpolation formula for c_A(g_0^2) is provided together with our final results.

hep-lat↗

arXiv2015

Non-perturbative improvement and renormalization of the axial current in N_f=3 lattice QCD

We report on a non-perturbative computation of the renormalization factor Z_A of the axial vector current in three-flavour O(a) improved lattice QCD with Wilson quarks and tree-level Symanzik improved gauge action and also recall our recent determination of the improvement coefficient c_A. Our normalization and improvement conditions are formulated at constant physics in a Schrödinger functional setup. The normalization condition exploits the full, massive axial Ward identity to reduce finite quark mass effects in the evaluation of Z_A and correlators with boundary wave functions to suppress excited state contributions in the pseudoscalar channel.

hep-lat↗

arXiv2014

Form factors for $\mathrm B_\mathrm s \to \mathrm K \ell ν$ decays in Lattice QCD

We present the current status of the computation of the form factor $f_+ (q^2)$ for the semi-leptonic decay $\mathrm B_\mathrm s \to \mathrm K \ell ν$ by the ALPHA collaboration. We use gauge configurations which were generated as part of the Coordinated Lattice Simulations (CLS) effort. They have $N_\mathrm f=2$ non-perturbatively $O(a)$ improved Wilson fermions, and pion masses down to $\approx 250 \,\mathrm {MeV}$ with $m_πL \geq 4$. The heavy quark is treated in non-perturbative Heavy Quark Effective Theory (HQET). We discuss how to extract the form factors from the correlation functions and present first results for the form factor at $q^2 = 21.23\,\mathrm{GeV}^2$ extrapolated to the continuum. Next-to-leading order terms in HQET and the chiral extrapolation still need to be included in the analysis.

hep-lat↗

arXiv2014

Pion-pion scattering phase shifts with the stochastic LapH method

Progress in calculating scattering phase shifts on $N_f=2+1$ anisotropic clover Wilson lattices is described. The stochastic LapH method facilitates computations in large volumes and for light pion masses. Results for pion masses down to 240 MeV, keeping $m_πL > 4$, are presented.

hep-lat↗

arXiv2014

Precision lattice QCD computation of the $B^*Bπ$ coupling

The static $B^{*}Bπ$ coupling, $\hat{g}_χ$, a low energy constant in the leading order heavy meson chiral Lagrangian, is determined using $N_\mathrm{f} = 2$ lattice QCD. We use CLS ensembles with lattice spacings and pion masses down to $a = 0.05\mathrm{fm}$ and $m_π=270\mathrm{MeV}$, and perform combined continuum and chiral extrapolations of our results which have a much better accuracy than previous numbers in the literature. As a by-product, we determine the coupling between the first radial excitations in the $B$ and $B^{*}$ channels ($\hat{g}_{22}$). Accounting for all uncertainties, which are dominated by the chiral extrapolation, we obtain $\hat{g}_χ= 0.492(29)$, while $\hat{g}_{22}$ is somewhat smaller. The comparison to a precise quenched computation suggests that there is little influence by the sea quarks and $\hat{g}_χ$ will not change much when a dynamical strange quark is included.

hep-lat↗

arXiv2014

Excited states in lattice QCD with the stochastic LapH method

Progress in computing the spectrum of excited baryons and mesons in lattice QCD is described. Results in the zero-momentum bosonic I=1/2, S=1, T1u symmetry sector of QCD using a correlation matrix of 58 operators are presented. All needed Wick contractions are efficiently evaluated using a stochastic method of treating the low-lying modes of quark propagation that exploits Laplacian Heaviside quark-field smearing. Level identification using probe operators is discussed.

hep-lat↗

arXiv2014

The finite volume spectrum of excited states from lattice QCD simulations

We present results for the spectrum of excited mesons obtained from temporal correlations of spatially-extended single-hadron and multi-hadron operators computed in lattice QCD. The stochastic LapH algorithm is implemented on anisotropic, dynamical lattices for isovectors for pions of mass $390$ MeV. A large correlation matrix with single-particle and two-particle probe operators is diagonalized to identify resonances. The masses of excited states in the $I=1, S=0, T_{1u}^+$ channel as well as the mixing of single and multi-particle probe operators are presented.

hep-lat↗

arXiv2013

On the $B^{*'}\to B$ transition

We present a first $N_{\rm f}=2$ lattice estimate of the hadronic coupling $g_{12}$ which parametrizes the strong decay of a radially excited $B^*$ meson into the ground state $B$ meson at zero recoil. We work in the static limit of Heavy Quark Effective Theory (HQET) and solve a Generalised Eigenvalue Problem (GEVP), which is necessary for the extraction of excited state properties. After an extrapolation to the continuum limit and a check of the pion mass dependence, we obtain $g_{12} = -0.17(4)$.

hep-lat↗