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J. Hein

Publications and source records attributed to J. Hein.

At least 19 recordsLinked to original sources
arXiv2012

CENTORI: a global toroidal electromagnetic two-fluid plasma turbulence code

A new global two-fluid electromagnetic turbulence code, CENTORI, has been developed for the purpose of studying magnetically-confined fusion plasmas on energy confinement timescales. This code is used to evolve the combined system of electron and ion fluid equations and Maxwell equations in toroidal configurations with axisymmetric equilibria. Uniquely, the equilibrium is co-evolved with the turbulence, and is thus modified by it. CENTORI is applicable to tokamaks of arbitrary aspect ratio and high plasma beta. A predictor-corrector, semi-implicit finite difference scheme is used to compute the time evolution of fluid quantities and fields. Vector operations and the evaluation of flux surface averages are speeded up by choosing the Jacobian of the transformation from laboratory to plasma coordinates to be a function of the equilibrium poloidal magnetic flux. A subroutine, GRASS, is used to co-evolve the plasma equilibrium by computing the steady-state solutions of a diffusion equation with a pseudo-time derivative. The code is written in Fortran 95 and is efficiently parallelized using Message Passing Interface (MPI). Illustrative examples of output from simulations of a tearing mode in a large aspect ratio tokamak plasma and of turbulence in an elongated conventional aspect ratio tokamak plasma are provided.

physics.comp-ph↗
arXiv2004

First determination of the strange and light quark masses from full lattice QCD

We compute the strange quark mass $m_s$ and the average of the $u$ and $d$ quark masses $\hat m$ using full lattice QCD with three dynamical quarks combined with experimental values for the pion and kaon masses. The simulations have degenerate $u$ and $d$ quarks with masses $m_u=m_d\equiv \hat m$ as low as $m_s/8$, and two different values of the lattice spacing. The bare lattice quark masses obtained are converted to the $\msbar$ scheme using perturbation theory at $O(alpha_s)$. Our results are: $m_s^\msbar$(2 GeV) = 76(0)(3)(7)(0) MeV, $\hat m^\msbar$(2 GeV) = 2.8(0)(1)(3)(0) MeV and $m_s/\hat m$ = 27.4(1)(4)(0)(1), where the errors are from statistics, simulation, perturbation theory, and electromagnetic effects, respectively.

hep-lat↗
arXiv2002

The determination of alpha_s from lattice QCD with 2+1 flavors of dynamical quarks

We describe the first lattice determination of the strong coupling constant with 3 flavors of dynamical quarks. The method follows previous analyses in using a perturbative expansion for the plaquette and Upsilon spectroscopy to set the scale. Using dynamical configurations from the MILC collaboration with 2+1 flavors of dynamical quarks we are able to avoid previous problems of having to extrapolate to 3 light flavors from 0 and 2. Our results agree with our previous work: alpha_s_MSbar(M_Z) = 0.121(3).

hep-lat↗
arXiv2002

The Upsilon Spectrum from Lattice QCD with 2+1 Flavors of Dynamical Quarks

We describe the bottomonium spectrum obtained on the MILC configurations which incorporate 2+1 flavors of dynamical quarks. We compare to quenched and 2 flavor results also on MILC configurations. We show that the lattice spacing determination using different quantities shows clear signs of convergence with 2+1 flavors and give results for the leptonic width and hyperfine splitting, in the form of the ratio of the 1st excited state of the Upsilon to that of the ground state.

hep-lat↗
arXiv2002

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↗
arXiv2002

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↗
arXiv2001

Constrained fitting of three-point functions

We determine matrix elements for $B \to D$ semileptonic decay. The use of the constrained fitting method and multiple smearings for both two- and three-point correlators allows an improved calculation of the form factors.

hep-lat↗
arXiv2001

Mass renormalisation for improved staggered quarks

Improved staggered quark actions are designed to suppress flavour changing strong interactions. We discuss the perturbation theory for this type of actions and show the improvements to reduce the quark mass renormalisation compared to naive staggered quarks. The renormalisations are of similar size as for Wilson quarks.

hep-lat↗
arXiv2001

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↗
arXiv2001

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↗
arXiv2000

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↗
arXiv2000

The unquenched Upsilon spectrum

We describe the bottomonium spectrum obtained on the UKQCD dynamical ensembles and its comparison to quenched results. We include a determination of alpha_s and m_b from the dynmaical results.

hep-lat↗
arXiv2000

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↗
arXiv1999

Semi-leptonic decays heavy-light to heavy light

We present results for the QCD matrix elements involved in semi-leptonic decays of B-mesons into pseudo scalar heavy light states. The application of NRQCD heavy quarks allows for quark masses around the physical b-quark. We investigate the dependence of the form factors on the external momenta and looked at the mass dependence at zero recoil. For the first time, results for radially excited decay products are presented.

hep-lat↗
arXiv1999

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↗
arXiv1998

Heavy quarks in non-relativistic lattice QCD

I give an overview of phenomenological heavy quark results obtained in NRQCD on the lattice. In particular I discuss the bottomonium and the b-light hadron spectrum. I also review recent results on the decay constants f_B and f_B_s.

hep-ph↗
arXiv1998

Pseudoscalar decay constant in the heavy light systems

We discuss the size of the higher order terms in the NRQCD expansion of the pseudoscalar decay constant. Power law divergences in the matrix elements contributing to the pseudoscalar decay constant are also investigated.

hep-lat↗