arXiv subjects

C. Morningstar

Publications and source records attributed to C. Morningstar.

At least 37 records · Page 2Linked to original sources

arXiv2004

Baryonic sources using irreducible representations of the double-covered octahedral group

Irreducible representations (IRs) of the double-covered octahedral group are used to construct lattice source and sink operators for three-quark baryons. The goal is to achieve a good coupling to higher spin states as well as ground states. Complete sets of local and nonlocal straight-link operators are explicitly shown for isospin 1/2 and 3/2 baryons. The orthogonality relations of the IR operators are confirmed in a quenched lattice simulation.

hep-lat↗

arXiv2004

QCD String formation and the Casimir Energy

Three distinct scales are identified in the excitation spectrum of the gluon field around a static quark-antiquark pair as the color source separation R is varied. The spectrum, with string-like excitations on the largest length scales of 2-3 fm, provides clues in its rich fine structure for developing an effective bosonic string description. New results are reported from the three-dimensional Z(2) and SU(2) gauge models, providing further insight into the mechanism of bosonic string formation. The precocious onset of string-like behavior in the Casimir energy of the static quark-antiquark ground state is observed below R=1 fm where most of the string eigenmodes do not exist and the few stable excitations above the ground state are displaced. We find no firm theoretical foundation for the widely held view of discovering string formation from high precision ground state properties below the 1 fm scale.

hep-lat↗

arXiv2003

Baryon operators and spectroscopy in lattice QCD

The construction of the operators and correlators required to determine the excited baryon spectrum is presented, with the aim of exploring the spatial and spin structure of the states while minimizing the number of propagator inversions. The method used to construct operators that transform irreducibly under the symmetries of the lattice is detailed, and the properties of example operators are studied using domain-wall fermion valence propagators computed on MILC asqtad dynamical lattices.

hep-lat↗

arXiv2003

Glueball Matrix Elements on Anisotropic Lattices

The glueball-to-vacuum matrix elements of local gluonic operators in scalar, tensor, and pseudoscalar channels are investigated numerically on several anisotropic lattices with the spatial lattice spacing in the range 0.1fm -- 0.2fm. These matrix elements are needed to predict the glueball branching ratios in $J/ψ$ radiative decays which will help to identify the glueball states in experiments. Two types of improved local gluonic operators are constructed for a self-consistent check, and the finite volume effects are also studied. The lattice spacing dependence of our results is very small and the continuum limits are reliably extrapolated.

hep-lat↗

arXiv2003

Excitations of torelon

The excitations of gluonic flux tube in a periodic lattice are examined. Monte Carlo simulations from an anisotropic lattice are presented and the comparison with effective string models is discussed.

hep-lat↗

arXiv2003

Mass spectrum of N* and source optimization

We have computed correlation functions for nucleons and extracted the masses for positive- and negative-parities. Use of group theory plays an important role in obtaining sources that have good overlap to higher spin states and minimum contamination from unwanted states. In the simulation three distinct sources and corresponding sinks that transform according to the $G_1$ irreducible representations are tested and used to form matrices of correlation functions. Diagonalizations give us clear mass splittings between low-lying states and excited states for both parities.

hep-lat↗

arXiv2003

Baryonic Operators for Lattice Simulations

The construction of baryonic operators for determining the N* excitation spectrum is discussed. The operators are designed with one eye towards maximizing overlaps with the low-lying states of interest, and the other eye towards minimizing the number of sources needed in computing the required quark propagators. Issues related to spin identification are outlined. Although we focus on tri-quark baryon operators, the construction method is applicable to both mesons and penta-quark operators.

hep-lat↗

arXiv2002

Scale setting for alpha_s beyond leading order

We present a general procedure for incorporating higher-order information into the scale-setting prescription of Brodsky, Lepage and Mackenzie. In particular, we show how to apply this prescription when the leading coefficient or coefficients in a series in the strong coupling alpha_s are anomalously small and the original prescription can give an unphysical scale. We give a general method for computing an optimum scale numerically, within dimensional regularization, and in cases when the coefficients of a series are known. We apply it to the heavy quark mass and energy renormalization in lattice NRQCD, and to a variety of known series. Among the latter, we find significant corrections to the scales for the ratio of e+e- to hadrons over muons, the ratio of the quark pole to MSbar mass, the semi-leptonic B-meson decay width, and the top decay width. Scales for the latter two decay widths, expressed in terms of MSbar masses, increase by factors of five and thirteen, respectively, substantially reducing the size of radiative corrections.

hep-ph↗

arXiv2001

Constrained Curve Fitting

We survey techniques for constrained curve fitting, based upon Bayesian statistics, that offer significant advantages over conventional techniques used by lattice field theorists.

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

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↗

arXiv2000

Scale Setting for $α_s$ Beyond Leading Order

We present a general procedure for applying the scale-setting prescription of Brodsky, Lepage and Mackenzie to higher orders in the strong coupling constant $\alphas$. In particular, we show how to apply this prescription when the leading coefficient or coefficients in a series in $\alphas$ are anomalously small. We give a general method for computing an optimum scale numerically, within dimensional regularization, and in cases when the coefficients of a series are known. We find significant corrections to the scales for $R_{e^+ e^-}$, $Γ(B \to X_u e \barν)$, $Γ(t \to b W)$, and the ratios of the quark pole to $\MSbar$ and lattice bare masses.

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

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↗

arXiv1999

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↗

arXiv1998

Spectroscopy of Hadrons with b Quarks from Lattice NRQCD

Preliminary results from an extensive lattice calculation of the B, B_c, and Υspectrum at quenched β= 6.0 are presented. The study includes radially and orbitally excited mesons, and baryons containing b quarks. The b quarks are formulated using NRQCD; for light and c quarks, a tadpole-improved clover action is used.

hep-lat↗

arXiv1998

News from the lattice

A summary of some recent results from lattice simulations is presented. These include first calculations of the strangeness magnetic moment of the nucleon, three new studies of the gluon propagator, flux tube attraction in U(1) gauge theory, and the static-quark potential and its gluonic excitations.

hep-ph↗

arXiv1998

Gluon excitations of the static-quark potential

The spectrum of gluon excitations in the presence of a static quark-antiquark pair is presented. Our results are obtained from computer simulations of gluons on anisotropic space-time lattices using an improved gauge-field action. Measurements for quark-antiquark separations r ranging from 0.1 fm to 4 fm and for various orientations on the lattice are made. Discretization errors and finite volume effects are taken into account. Surprisingly, the spectrum does not exhibit the expected onset of the universal pi/r Goldstone excitations of the effective QCD string, even for r as large as 4 fm.

hep-lat↗