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Silas R. Beane

Publications and source records attributed to Silas R. Beane.

At least 55 records · Page 3Linked to original sources
arXiv2009

High Statistics Analysis using Anisotropic Clover Lattices: (I) Single Hadron Correlation Functions

We present the results of high-statistics calculations of correlation functions generated with single-baryon interpolating operators on an ensemble of dynamical anisotropic gauge-field configurations generated by the Hadron Spectrum Collaboration using a tadpole-improved clover fermion action and Symanzik-improved gauge action. A total of 292,500 sets of measurements are made using 1194 gauge configurations of size 20^3 x 128 with an anisotropy parameter ξ= b_s/b_t = 3.5, a spatial lattice spacing of b_s=0.1227\pm 0.0008 fm, and pion mass of m_π~ 390 MeV. Ground state baryon masses are extracted with fully quantified uncertainties that are at or below the ~0.2%-level in lattice units. The lowest-lying negative-parity states are also extracted albeit with a somewhat lower level of precision. In the case of the nucleon, this negative-parity state is above the Nπthreshold and, therefore, the isospin-1/2 πN s-wave scattering phase-shift can be extracted using Luescher's method. The disconnected contributions to this process are included indirectly in the gauge-field configurations and do not require additional calculations. The signal-to-noise ratio in the various correlation functions is explored and is found to degrade exponentially faster than naive expectations on many time-slices. This is due to backward propagating states arising from the anti-periodic boundary conditions imposed on the quark-propagators in the time-direction. We explore how best to distribute computational resources between configuration generation and propagator measurements in order to optimize the extraction of single baryon observables.

hep-lat↗
arXiv2008

Perturbative nuclear physics

We present a new formulation of effective field theory for nucleon-nucleon (NN) interactions which treats pion interactions perturbatively, and we offer evidence that the expansion converges satisfactorily to third order in the expansion, which we have computed analytically for s and d wave NN scattering. Starting with the Kaplan-Savage-Wise (KSW) expansion about the nontrivial fixed point corresponding to infinite NN scattering length, we cure the convergence problems with that theory by summing to all orders the singular short distance part of the pion tensor interaction. This method makes possible a host of high precision analytic few-body calculations in nuclear physics.

nucl-th↗
arXiv2008

Hadronic interactions and nuclear physics

I give an overview of efforts in the last year to calculate interactions among hadrons using lattice QCD. Results discussed include the extraction of low-energy phase shifts and three-body interactions, and the study of pion and kaon condensation. A critical appraisal is offered of recent attempts to calculate nucleon-nucleon and nucleon-hyperon potentials on the lattice.

hep-lat↗
arXiv2008

Hadronic Interactions from Lattice QCD

We present an overview of recent efforts to calculate the interactions among hadrons using lattice QCD. After outlining the techniques that are used to extract scattering parameters, we detail the latest calculations of meson-meson scattering, baryon-baryon scattering and multi-meson systems obtained with domain-wall valence quarks on the staggered MILC lattices by the NPLQCD collaboration. Estimates of the computational resources required to achieve precision results in the baryon sector are presented.

hep-lat↗
arXiv2008

Multi-Pion States in Lattice QCD and the Charged-Pion Condensate

The ground-state energies of systems containing up to twelve $π^+$'s in a spatial volume V ~ (2.5 fm)^3 are computed in dynamical, mixed-action lattice QCD at a lattice spacing of ~ 0.125 fm for four different values of the light quark masses. Clean signals are seen for each ground state, allowing for a precise extraction of both the $π^+π^+$ scattering length and $π^+π^+π^+$-interaction from a correlated analysis of systems containing different numbers of $π^+$'s. This extraction of the $π^+π^+$ scattering length is consistent with than that from the $π^+π^+$-system alone. The large number of systems studied here significantly strengthens the arguments presented in our earlier work and unambiguously demonstrates the presence of a low energy $π^+π^+π^+$-interaction. The equation of state of a $π^+$ gas is investigated using our numerical results and the density dependence of the isospin chemical potential for these systems agrees well with the theoretical expectations of leading order chiral perturbation theory. The chemical potential is found to receive a substantial contribution from the $π^+π^+π^+$-interaction at the lighter pion masses. An important technical aspect of this work is the demonstration of the necessity of performing propagator contractions in greater than double precision to extract the correct results.

hep-lat↗
arXiv2007

Multi-Pion Systems in Lattice QCD and the Three-Pion Interaction

The ground-state energies of 2, 3, 4 and 5 π^+'s in a spatial volume V (2.5 fm)^3 are computed with lattice QCD. By eliminating the leading contribution from three-π^+ interactions, particular combinations of these n-π^+ ground-state energies provide precise extractions of the π^+π^+ scattering length in agreement with that obtained from calculations involving only two π^+'s. The three-π^+ interaction can be isolated by forming other combinations of the n-π^+ ground-state energies. We find a result that is consistent with a repulsive three-π^+ interaction for m_π< 352 MeV.

hep-lat↗
arXiv2007

n-Boson Energies at Finite Volume and Three-Boson Interactions

We calculate the volume dependence of the ground-state energy of n identical bosons with short-range repulsive interactions in a periodic spatial volume of side L, up to and including terms of order 1/L^6. With this result, Lattice QCD calculations of the ground-state energies of three or more pions will allow for a systematic extraction of the three-pion interaction at this order in the volume expansion.

hep-lat↗
arXiv2007

Precise Determination of the I=2 pipi Scattering Length from Mixed-Action Lattice QCD

The I=2 pipi scattering length is calculated in fully-dynamical lattice QCD with domain-wall valence quarks on the asqtad-improved coarse MILC configurations (with fourth-rooted staggered sea quarks) at four light-quark masses. Two- and three-flavor mixed-action chiral perturbation theory at next-to-leading order is used to perform the chiral and continuum extrapolations. At the physical charged pion mass, we find m_pi a_pipi(I=2) = -0.04330 +- 0.00042, where the error bar combines the statistical and systematic uncertainties in quadrature.

hep-lat↗
arXiv2006

Hyperon-Nucleon Scattering from Fully-Dynamical Lattice QCD

We present results of the first fully-dynamical lattice QCD determination of hyperon-nucleon scattering. One s-wave phase shift was determined for n Lambda scattering in both spin-channels at pion masses of 350, 490, and 590 MeV, and for n Sigma- scattering in both spin channels at pion masses of 490, and 590 MeV. The calculations were performed with domain-wall valence quarks on dynamical, staggered gauge configurations with a lattice spacing of b ~ 0.125 fm.

hep-lat↗
arXiv2006

Pi-K Scattering in Full QCD with Domain-Wall Valence Quarks

We calculate the pi+ K+ scattering length in fully-dynamical lattice QCD with domain-wall valence quarks on MILC lattices with rooted staggered sea-quarks at a lattice spacing of b=0.125 fm, lattice spatial size of L =2.5 fm and at pion masses of m_pi=290, 350, 490 and 600 MeV. The lattice data, analyzed at next-to-leading order in chiral perturbation theory, allows an extraction of the full pi K scattering amplitude at threshold. Extrapolating to the physical point gives m_pi a_3/2 = -0.0574 (+- 0.0016)(+0.0024 -0.0058) and m_pi a_1/2 = 0.1725 (+- 0.0017)(+0.0023 -0.0156) for the I=3/2 and I=1/2 scattering lengths, respectively, where the first error is statistical and the second error is an estimate of the systematic due to truncation of the chiral expansion.

hep-lat↗
arXiv2006

Strong-Isospin Violation in the Neutron-Proton Mass Difference from Fully-Dynamical Lattice QCD and PQQCD

We determine the strong-isospin violating component of the neutron-proton mass difference from fully-dynamical lattice QCD and partially-quenched QCD calculations of the nucleon mass, constrained by partially-quenched chiral perturbation theory at one-loop level. The lattice calculations were performed with domain-wall valence quarks on MILC lattices with rooted staggered sea-quarks at a lattice spacing of b=0.125 fm, lattice spatial size of L=2.5 fm and pion masses ranging from m_pi ~ 290 MeV to ~ 350 MeV. At the physical value of the pion mass, we predict M_n - M_p |(d-u) = 2.26 +- 0.57 +- 0.42 +- 0.10 MeV where the first error is statistical, the second error is due to the uncertainty in the ratio of light-quark masses, eta=m_u/m_d, determined by MILC, and the third error is an estimate of the systematic due to chiral extrapolation.

hep-lat↗
arXiv2006

I=2 pi-pi Scattering from Fully-Dynamical Mixed-Action Lattice QCD

We compute the I=2 pi-pi scattering length at pion masses of m_pi ~ 294, 348 and 484 MeV in fully-dynamical lattice QCD using Luscher's finite-volume method. The calculation is performed with domain-wall valence-quark propagators on asqtad-improved MILC configurations with staggered sea quarks at a single lattice spacing, b ~ 0.125 fm. Chiral perturbation theory is used to perform the extrapolation of the scattering length from lattice quark masses down to the physical value, and we find m_pi a_2 = -0.0426 +- 0.0006 +- 0.0003 +- 0.0018, in good agreement with experiment. The I=2 pi-pi scattering phase shift is calculated to be delta = -43 +- 10 +- 5 degrees at |p| ~ 544 MeV for m_pi ~ 484 MeV.

hep-lat↗
arXiv2004

DVCS-Dissociation of the Deuteron and the EMC Effect

The break-up of the deuteron during deeply-virtual Compton scattering, gamma* d --> gamma(*) n p, is explored. In the effective field theory describing nucleon dynamics at momenta below the pion mass, the EMC effect results from four-nucleon interactions with the twist-2 operators, appropriate for describing forward, and near-forward, matrix elements in the two-nucleon system. We point out that the break-up of the deuteron to low-energy final states during deeply-virtual Compton scattering is a process with which to explore strong-interaction physics closely related to that responsible for the EMC effect. The single-nucleon contribution to the break-up depends on the moments of the spin-dependent structure functions and contributions from local four-nucleon operators. Experimental deviations from the single-nucleon prediction would provide a probe of strong interactions complimentary to the EMC effect.

nucl-th↗
arXiv2004

Stable Pentaquarks from Strange Chiral Multiplets

The assumption of strong diquark correlations in the QCD spectrum suggests flavor multiplets of hadrons that are degenerate in the chiral limit. Generally it would be unnatural for there to be degeneracy in the hadron spectrum that is not protected by a QCD symmetry. Here we show --for pentaquarks constructed from diquarks-- that these degeneracies can be naturally protected by the full chiral symmetry of QCD. The resulting chiral multiplet structure recovers the ideally-mixed pentaquark mass spectrum of the diquark model, and interestingly, requires that the axial couplings of the pentaquarks to states outside the degenerate multiplets vanish in the chiral limit. This result suggests that if these hadrons exist, they are stable in the chiral limit and therefore have widths that scale as the fourth power of the kaon mass over the chiral symmetry breaking scale. Natural-size widths are of order a few MeV.

hep-ph↗
arXiv2004

Baryon Axial Charge in a Finite Volume

We compute finite-volume corrections to nucleon matrix elements of the axial-vector current. We show that knowledge of this finite-volume dependence --as well as that of the nucleon mass-- obtained using lattice QCD will allow a clean determination of the chiral-limit values of the nucleon and Delta-resonance axial-vector couplings.

hep-ph↗
arXiv2004

In Search of the Chiral Regime

A critical appraisal is given of a recent analysis of the quark-mass and finite-size dependence of unquenched lattice QCD data for the nucleon mass. We use this forum to estimate the boundary of the chiral regime for nucleon properties.

hep-lat↗