arXiv subjects

Colin Morningstar

Publications and source records attributed to Colin Morningstar.

At least 55 records · Page 3Linked to original sources

arXiv2018

The elastic $I=3/2$ $p$-wave nucleon-pion scattering amplitude and the $Δ(1232)$ resonance from $N_{\mathrm{f}}=2+1$ lattice QCD

We present the first direct determination of meson-baryon resonance parameters from a scattering amplitude calculated using lattice QCD. In particular, we calculate the elastic $I=3/2$, $p$-wave nucleon-pion amplitude on a single ensemble of $N_{\mathrm{f}}=2+1$ Wilson-clover fermions with $m_π=280\mathrm{MeV}$ and $m_{K}=460\mathrm{MeV}$. At these quark masses, the $Δ(1232)$ resonance pole is found close to the $N-π$ threshold and a Breit-Wigner fit to the amplitude gives $g^{\mathrm{BW}}_{ΔNπ}=19.0(4.7)$ in agreement with phenomenological determinations.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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arXiv2014

Excited isovector mesons using the stochastic LapH method

The spectrum of excited isovector mesons is studied using a 32^3 x 256 anisotropic lattice with u,d quark masses set to give a pion mass near 240 MeV. Results in the bosonic isovector nonstrange symmetry channels of zero total momentum are presented using correlation matrices of unprecedented size. In addition to spatially-extended single-meson operators, large numbers of two-meson operators are used, involving a wide variety of light isovector, isoscalar, and strange meson operators of varying relative momenta. 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 is discussed.

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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.

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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.

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arXiv2013

Spectrum of excited states using the stochastic LapH method

Progress in computing the spectrum of excited baryons and mesons in lattice QCD is described. Our first results in the zero-momentum bosonic I=1, S=0, T1u+ symmetry sector of QCD using a correlation matrix of 56 operators are presented. In addition to a dozen spatially-extended meson operators, 44 two-meson operators are used, involving a wide variety of light isovector, isoscalar, and strange meson operators of varying relative momenta. 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 is discussed.

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arXiv2012

Group-theoretical construction of finite-momentum and multi-particle operators for lattice hadron spectroscopy

Determining the spectrum of hadronic excitations from Monte Carlo simulations requires the use of interpolating operators that couple to multi-particle states. Recent algorithmic advances have made the inclusion of multi-hadron operators in spectroscopy calculations a practical reality. In this talk, a procedure for constructing a set of multi-hadron interpolators that project onto the states of interest is described. To aid in the interpretation of simulation data, operators are designed to transform irreducibly under the lattice symmetry group. The identification of a set of optimal single-hadron interpolators for states with non-zero momenta is an essential intermediate step in this analysis.

hep-lat↗

arXiv2011

Excited-state hadron masses using the stochastic LapH method

Progress in computing the spectrum of excited baryons and mesons in lattice QCD is described. Large sets of spatially-extended hadron operators are used. The need for multi-hadron operators in addition to single-hadron operators is emphasized, necessitating the use of a new stochastic method of treating the low-lying modes of quark propagation which exploits Laplacian Heaviside quark-field smearing. A new glueball operator is tested and computing the mixing of this glueball operator with a quark-antiquark operator and multiple two-pion operators is shown to be feasible. Some of our initial results show warning signs about extracting high-lying resonance energies using only single-hadron operators.

hep-lat↗

arXiv2011

Excited-State Hadron Masses from Lattice QCD

Progress in computing the spectrum of excited baryons and mesons in lattice QCD is described. Large sets of spatially-extended hadron operators are used. The need for multi-hadron operators in addition to single-hadron operators is emphasized, necessitating the use of a new stochastic method of treating the low-lying modes of quark propagation which exploits Laplacian Heaviside quark-field smearing. A new glueball operator is tested, and computing the mixing of this glueball operator with a quark-antiquark operator and multiple two-pion operators is shown to be feasible.

hep-lat↗

arXiv2011

Improved stochastic estimation of quark propagation with Laplacian Heaviside smearing in lattice QCD

A new method of stochastically estimating the low-lying effects of quark propagation is proposed which allows accurate determinations of temporal correlations of single-hadron and multi-hadron operators in lattice QCD. The method is well suited for calculations in large volumes. Contributions involving quark propagation connecting hadron sink operators at the same final time can be handled in a straightforward manner, even for a large number of final time slices. The method exploits Laplacian Heaviside (LapH) smearing. ZN noise is introduced in a novel way, and variance reduction is achieved using judiciously-chosen noise dilution projectors. The method is tested using isoscalar mesons in the scalar, pseudoscalar, and vector channels, and using the two-pion system of total isospin I=0,1,2 on large anisotropic 24^3 x 128 lattices with spatial spacing a_s~0.12 fm and temporal spacing a_t~0.034 fm for pion masses mpi~390 and 240 MeV.

hep-lat↗

arXiv2010

The excited hadron spectrum in lattice QCD using a new variance reduction method

Progress in calculating the spectrum of excited baryons and mesons in lattice QCD is described. Correlation matrices of sets of spatially-extended hadron operators have been studied and their effectiveness in facilitating the extraction of excited-state energies is demonstrated. First applications of the stochastic LapH method, a new method of estimating the low-lying effects of quark propagation, are presented.

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arXiv2010

Nucleon, Delta and Omega excited state spectra at three pion mass values

The energies of the excited states of the Nucleon, Delta and Omega are computed in lattice QCD, using two light quarks and one strange quark on anisotropic lattices. The calculations are performed at three values of the pion mass: 392(4), 438(3) and 521(3) MeV. We employ the variational method with a basis of about ten interpolating operators enabling six energies to be distinguished clearly in each irreducible representation of the octahedral group. We compare our calculations of nucleon excited states with the low-lying experimental spectrum. There is reasonable agreement for the pattern of states.

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