arXiv subjects

Saul D. Cohen

Publications and source records attributed to Saul D. Cohen.

32 records · Page 2Linked to original sources

arXiv2010

Toward TeV Conformality

We study the chiral condensate $<\barψ ψ>$ for an SU(3) gauge theory with $N_f$ massless Dirac fermions in the fundamental representation when $N_f$ is increased from 2 to 6. For $N_f=2$, our lattice simulations of $<\barψ ψ>/F^3$, where $F$ is the Nambu-Goldstone-boson decay constant, agree with the measured QCD value. For $N_f = 6$, this ratio shows significant enhancement, presaging an even larger enhancement anticipated as $N_f$ increases further, toward the critical value for transition from confinement to infrared conformality.

hep-ph↗

arXiv2009

The role of multigrid algorithms for LQCD

We report on the first successful QCD multigrid algorithm which demonstrates constant convergence rates independent of quark mass and lattice volume for the Wilson Dirac operator. The new ingredient is the adaptive method for constructing the near null space on which the coarse grid multigrid Dirac operator acts. In addition we speculate on future prospects for extending this algorithm to the Domain Wall and Staggered discretizations, its exceptional suitability for high performance GPU code and its potential impact on simulations at the physical pion mass.

hep-lat↗

arXiv2009

Bottom-Hadron Mass Splittings from Static-Quark Action on 2+1-Flavor Lattices

We calculate bottom-hadron mass splittings with respect to $B_d$ and $Λ_b$ using full QCD with 2+1 flavors of dynamical Kogut-Susskind sea quarks and domain-wall valence quarks along with a static heavy quark. Our lattices have spatial volume of $(2.5{fm})^3$ with lattice spacing about 0.124 fm and a range of pion masses as low as 291 MeV. Our results are in agreement with experimental observations and other lattice calculations within our statistical and systematic errors. In particular, we find the mass of the $Ω_b$ to be consistent with the recent CDF measurement. We also predict the mass for the as yet unobserved $Ξ^\prime_b$ to be 5955(27) MeV.

hep-lat↗

arXiv2009

Excited-State Effective Masses in Lattice QCD

We apply black-box methods, i.e. where the performance of the method does not depend upon initial guesses, to extract excited-state energies from Euclidean-time hadron correlation functions. In particular, we extend the widely used effective-mass method to incorporate multiple correlation functions and produce effective mass estimates for multiple excited states. In general, these excited-state effective masses will be determined by finding the roots of some polynomial. We demonstrate the method using sample lattice data to determine excited-state energies of the nucleon and compare the results to other energy-level finding techniques.

hep-lat↗

arXiv2008

Light-Meson Two-Photon Decays in Full QCD

We present a study of two-photon decays of light mesons, focusing on the neutral pion decay. This important process highlights the effects of the axial anomaly in QCD but has been little studied on the lattice. By applying the Lehmann-Symanzik-Zimmermann (LSZ) reduction formula, we reconstruct the electromagnetic matrix elements from three-point vector-vector Green functions calculated on 2+1-flavor isotropic clover lattices.

hep-lat↗

arXiv2008

Challenges in Hadronic Form Factor Calculations

There is an extensive history of form factor calculations on the lattice, primarily with ground states for both initial and final states. However, there have never been any radially excited transition form factor calculations. Furthermore, the lattice faces difficulty in extracting signal from noise at large transfer momenta ($Q^2$). These measurements could give important theoretical input to experiments, such as those of JLab's 12 GeV program and studies of deformation of the nucleon. In this work, I will present a simple technique to resolve both of these difficulties and present results from anisotropic configurations showing improved signals for excited-state quantities. It should also be possible to apply this technique to isotropic lattices for calculating large-$Q^2$ form factors.

hep-lat↗

arXiv2008

First results from 2+1 dynamical quark flavors on an anisotropic lattice: light-hadron spectroscopy and setting the strange-quark mass

We present the first light-hadron spectroscopy on a set of $N_f=2+1$ dynamical, anisotropic lattices. A convenient set of coordinates that parameterize the two-dimensional plane of light and strange-quark masses is introduced. These coordinates are used to extrapolate data obtained at the simulated values of the quark masses to the physical light and strange-quark point. A measurement of the Sommer scale on these ensembles is made, and the performance of the hybrid Monte Carlo algorithm used for generating the ensembles is estimated.

hep-lat↗

arXiv2008

First Lattice Study of the $N$-$P_{11}(1440)$ Transition Form Factors

Experiments at Jefferson Laboratory, MIT-Bates, LEGS, Mainz, Bonn, GRAAL, and Spring-8 offer new opportunities to understand in detail how nucleon resonance ($N^*$) properties emerge from the nonperturbative aspects of QCD. Preliminary data from CLAS collaboration, which cover a large range of photon virtuality $Q^2$ show interesting behavior with respect to $Q^2$ dependence: in the region $Q^2 \le 1.5 {GeV}^2$, both the transverse amplitude, $A_{1/2}(Q^2)$, and the longitudinal amplitude, $S_{1/2}(Q^2)$, decrease rapidly. In this work, we attempt to use first-principles lattice QCD (for the first time) to provide a model-independent study of the Roper-nucleon transition form factor.

hep-lat↗

arXiv2007

Localization and chiral symmetry in 2+1 flavor domain wall QCD

We present results for the dependence of the residual mass of domain wall fermions (DWF) on the size of the fifth dimension and its relation to the density and localization properties of low-lying eigenvectors of the corresponding hermitian Wilson Dirac operator relevant to simulations of 2+1 flavor domain wall QCD. Using the DBW2 and Iwasaki gauge actions, we generate ensembles of configurations with a $16^3\times 32$ space-time volume and an extent of 8 in the fifth dimension for the sea quarks. We demonstrate the existence of a regime where the degree of locality, the size of chiral symmetry breaking and the rate of topology change can be acceptable for inverse lattice spacings $a^{-1} \ge 1.6$ GeV.

hep-lat↗

arXiv2007

Lattice QCD Beyond Ground States

In this work, we apply black box methods (methods not requiring input) to find excited-state energies. A variety of such methods for lattice QCD were introduced at the 3rd iteration of the numerical workshop series. We first review a selection of approaches that have been used in lattice calculations to determine multiple energy states: multiple correlator fits, the variational method and Bayesian fitting. In the second half, we will focus on a black box method, the multi-effective mass. We demonstrate the approach on anisotropic lattice data, extracting multiple states from single correlators. Without complicated operator construction or specialized fitting programs, the black box method shows good consistency with the traditional approaches.

hep-lat↗

arXiv2006

Preliminary Study of B_K on 2+1 flavor DWF lattices from QCDOC

I present some preliminary calculations of B_K on 2+1 flavor domain-wall fermion lattices from the QCDOC, including a set of 16^3x32x8 lattices with a^{-1} near 1.6 GeV. Although a final result awaits the production of a much longer run, I will compare this preliminary value to previous results.

hep-lat↗

arXiv2006

QCD, Symmetry Breaking and the Random Lattice

According to the Nielsen-Ninomiya No-Go theorem, the doubling of fermions on the lattice cannot be suppressed in a chiral theory. Whereas Wilson and staggered fermions suppress doublers with explicit breaking of chiral symmetry, the random lattice does so by spontaneous chiral symmetry breaking even in the free theory. I present results for meson masses, the chiral condensate and fermionic eigenvalues from simulations of quenched QCD on random lattices in four dimensions, focusing on chiral symmetry breaking.

hep-lat↗

arXiv2000

Terrace-Width Distributions and Step-Step Repulsions on Vicinal Surfaces: Symmetries, Scaling, Simplifications, Subtleties, and Schrödinger

For more than three decades, measurement of terrace width distributions (TWDs) of vicinal crystal surfaces have been recognized as arguably the best way to determine the dimensionless strength $\tilde{A}$ of the elastic repulsion between steps. For sufficiently strong repulsions, the TWD is expected to be Gaussian, with $\tilde{A}$ varying inversely with the squared variance. However, there has been a controversy over the proportionality constant. From another perspective the TWD can be described as a continuous generalized Wigner distribution (CGWD) essentially no more complicated than a Gaussian but a much better approximation at the few calibration points where exact solutions exist. This paper combines concisely the experimentally most useful results from several earlier papers on this subject and describes some advancements that are in progress regarding numerical tests and in using Schrödinger-equation formalism to give greater understanding of the origin of the CGWD and to give hope of extensions to more general interaction potentials between steps. There are many implications for future experiments.

cond-mat.mtrl-sci↗

arXiv2000

Extraction of Step-Repulsion Strengths from Terrace Width Distributions: Statistical and Analytic Considerations

Recently it has been recognized that the so-called generalized Wigner distribution may provide at least as good a description of terrace width distributions (TWDs) on vicinal surfaces as the standard Gaussian fit and is particularly applicable for weak repulsions between steps, where the latter fails. Subsequent applications to vicinal copper surfaces at various temperatures confirmed the serviceability of the new analysis procedure but raised some theoretical questions. Here we address these issues using analytical, numerical, and statistical methods. We propose an extension of the generalized Wigner distribution to a two-parameter fit that allows the terrace widths to be scaled by an optimal effective mean width. We discuss quantitatively the approach of a Wigner distribution to a Gaussian form for strong repulsions, how errors in normalization or mean affect the deduced interaction, and how optimally to extract the interaction from the variance and mean of the TWD. We show that correlations reduce by two orders of magnitude the number of {\em independent} measurements in a typical STM image. We also discuss the effect of the discreteness ("quantization") of terrace widths, finding that for high misorientation (small mean width) the standard continuum analysis gives faulty estimates of step interactions.

cond-mat.mtrl-sci↗