arXiv subjects

Frank X. Lee

Publications and source records attributed to Frank X. Lee.

At least 55 records · Page 3Linked to original sources

arXiv2010

Magnetic Moments of Negative-Parity Baryons from Lattice QCD

We report preliminary results for the magnetic moments of negative-parity baryons extracted from mass shifts in the presence of static external magnetic fields. The calculations are done on $24^3\times 48$ quenched lattices using standard Wilson actions, with $β$=6.0 and pion mass down to about 520 MeV, and 1000 configurations. Reasonable signals for the negative-parity states are observed and the sign of their magnetic moments is established. The results are compared to model calculations.

hep-lat↗

arXiv2010

Eta-nucleon coupling constant in QCD with SU(3) symmetry breaking

We study the $η$NN coupling constant using the method of QCD sum rules starting from the vacuum-to-eta correlation function of the interpolating fields of two nucleons. The matrix element of this correlation has been taken with respect to nucleon spinors to avoid unwanted pole contribution. The SU(3)-flavor symmetry breaking effects have been accounted for via the $η$-mass, s-quark mass and eta decay constant to leading order. Out of the four sum rules obtained by taking the ratios of the two sum rules in conjunction with the two sum rules in nucleon mass, three are found to give mutually consistent results. We find the SU(3) breaking effects significant, as large as 50% of the SU(3) symmetric part.

hep-ph↗

arXiv2009

Neutron electric polarizability

We use the background field method to extract the "connected" piece of the neutron electric polarizability. We present results for quenched simulations using both clover and Wilson fermions and discuss our experience in extracting the mass shifts and the challenges we encountered when we lowered the quark mass. For the neutron we find that as the pion mass is lowered below $500\MeV$, the polarizability starts rising in agreement with predictions from chiral perturbation theory. For our lowest pion mass, $m_π=320\MeV$, we find that $α_n = 3.8(1.3)\times 10^{-4}\fm^3$, which is still only one third of the experimental value. We also present results for the neutral pion; we find that its polarizability turns negative for pion masses smaller than $500\MeV$ which is puzzling.

hep-lat↗

arXiv2009

N to $Δ$ transition amplitudes from QCD sum rules

We present a calculation of the N to $Δ$ electromagnetic transition amplitudes using the method of QCD sum rules. A complete set of QCD sum rules are derived for the entire family of transitions from the baryon octet to decuplet. They are analyzed in conjunction with the corresponding mass sum rules using a Monte-Carlo-based analysis procedure. The performance of each of the sum rules is examined using the criteria of OPE convergence and ground-state dominance, along with the role of the transitions in intermediate states. Individual contributions from the u, d and s quarks are isolated and their implications in the underlying dynamics are explored. Valid sum rules are identified and their predictions are obtained. The results are compared with experiment and other calculations.

hep-ph↗

arXiv2008

The background field method on the lattice

The background field method has been used successfully to determine hadron electromagnetic polarizabilities. Recently questions have been raised regarding the proper way to deal with the electric field on the lattice. In this paper, we show how the presence of a background electric field affects the quark hopping matrix. We use this formulation to carry out simulations on quenched configurations, and we present the results for neutron electric polarizability for pion masses as low as $500\MeV$. We find that the polarizability is roughly constant for the quark masses considered, $α\sim 1.5 \times 10^{-4} \fm^3$. While the polarizability is positive, it is significantly smaller than the experimental value due to the fact that the quark masses used are too large.

hep-lat↗

arXiv2008

Octet baryon magnetic moments from QCD sum rules

A comprehensive study is made for the magnetic moments of octet baryons in the method of QCD sum rules. A complete set of QCD sum rules is derived using the external field method and generalized interpolating fields. For each member, three sum rules are constructed from three independent tensor structures. They are analyzed in conjunction with the corresponding mass sum rules. The performance of each of the sum rules is examined using the criteria of OPE convergence and ground-state dominance, along with the role of the transitions in intermediate states. Individual contributions from the u, d and s quarks are isolated and their implications in the underlying dynamics are explored. Valid sum rules are identified and their predictions are obtained. The results are compared with experiment and previous calculations.

hep-ph↗

arXiv2007

Path Integrals in Lattice Quantum Chromodynamics

I discuss the use of path integrals to study strong-interaction physics from first principles. The underlying theory is cast into path integrals which are evaluated numerically using Monte Carlo methods on a space-time lattice. Examples are given on progress related to nuclear physics.

nucl-th↗

arXiv2007

Magnetic Moment of Vector Mesons in the Background Field Method

We report some results for the magnetic moments of vector mesons extracted from mass shifts in the presence of static external magnetic fields. The calculations are done on $24^4$ quenched lattices using standard Wilson actions, with $β$=6.0 and pion mass down to 500 MeV. The results are compared to those from the form factor method.

hep-lat↗

arXiv2006

Mass and width of strange baryon resonances in QCD sum rules

The mass spectra of strange baryons in the octet family are investigated in a finite-energy QCD sum rule approach based on the Gauss-Weierstrass transform. The phenomenological form of the spectral function is saturated by the ground state and two of the lowest excited states, considered as having opposite parities. Treating the ground state parameters as known, the masses, widths and couplings to the interpolating fields are determined and compared to experiment.

nucl-th↗

arXiv2006

On the Locality and Scaling of Overlap Fermions at Coarse Lattice Spacings

The overlap fermion offers the considerable advantage of exact chiral symmetry on the lattice, but is numerically intensive. This can be made affordable while still providing large lattice volumes, by using coarse lattice spacing, given that good scaling and localization properties are established. Here, using overlap fermions on quenched Iwasaki gauge configurations, we demonstrate directly that, with appropriate choice of negative Wilson's mass, the overlap Dirac operator's range is comfortably small in lattice units for each of the lattice spacings 0.20 fm, 0.17 fm, and 0.13 fm (and scales to zero in physical units in the continuum limit). In particular, our direct results contradict recent speculation that an inverse lattice spacing of 1 GeV is too low to have satisfactory localization. Furthermore, hadronic masses (available on the two coarser lattices) scale very well.

hep-lat↗

arXiv2006

Spin-3/2 baryons from an anisotropic lattice QCD action

The mass spectrum of baryons in the spin-3/2 sector is computed in quenched lattice QCD using a tadpole-improved anisotropic action. Both isospin 1/2 and 3/2 (the traditional decuplet) are considered, as well as members that contain strange quarks. States with positive and negative parities are isolated by parity projection, while states with spin-3/2 and spin-1/2 are separated by spin projection. The extent to which spin projection is needed is examined. The issue of optimal interpolating field is also investigated. The results are discussed in relation to previous calculations and experiment.

hep-lat↗

arXiv2006

Excited decuplet baryons from QCD sum rules

A calculation of the mass spectrum in the baryon decuplet sector is presented using the method of QCD sum rules. Sum rules are derived for states of spin-parity 3/2+- and 1/2+ using both the conventional method and a parity-projection method. The predictive ability of the sum rules is explored by a Monte-Carlo based analysis procedure in which the three phenomenological parameters (mass, coupling, threshold) are treated as free parameters and fitted simultaneously. Taken together, the results give an improved determination of the mass spectrum in this sector from the perspective of non-perturbative QCD.

nucl-th↗

arXiv2004

Electric Polarizability of Neutral Hadrons from Lattice QCD

By simulating a uniform electric field on a lattice and measuring the change in the rest mass, we calculate the electric polarizability of neutral mesons and baryons using the methods of quenched lattice QCD. Specifically, we measure the electric polarizability coefficient from the quadratic response to the electric field for 10 particles: the vector mesons $ρ^0$ and $K^{*0}$; the octet baryons n, $Σ^0$, $Λ_{o}^{0}$, $Λ_{s}^{0}$, and $Ξ^0$; and the decouplet baryons $Δ^0$, $Σ^{*0}$, and $Ξ^{*0}$. Independent calculations using two fermion actions were done for consistency and comparison purposes. One calculation uses Wilson fermions with a lattice spacing of $a=0.10 $fm. The other uses tadpole improved Lüsher-Weiss gauge fields and clover quark action with a lattice spacing $a=0.17 $fm. Our results for neutron electric polarizability are compared to experiment.

hep-lat↗

arXiv2004

The Sequential Empirical Bayes Method: An Adaptive Constrained-Curve Fitting Algorithm for Lattice QCD

We introduce the ``Sequential Empirical Bayes Method'', an adaptive constrained-curve fitting procedure for extracting reliable priors. These are then used in standard augmented-$χ^2$ fits on separate data. This better stabilizes fits to lattice QCD overlap-fermion data at very low quark mass where {\it a priori} values are not otherwise known. Lessons learned (including caveats limiting the scope of the method) from studying artificial data are presented. As an illustration, from local-local two-point correlation functions, we obtain masses and spectral weights for ground and first-excited states of the pion, give preliminary fits for the $a_0$ where ghost states (a quenched artifact) must be dealt with, and elaborate on the details of fits of the Roper resonance and $S_{11}(N^{1/2-})$ previously presented elsewhere. The data are from overlap fermions on a quenched $16^3\times 28$ lattice with spatial size $La=3.2 {\rm fm}$ and pion mass as low as $\sim 180 {\rm MeV}$.

hep-lat↗

arXiv2004

Topological Charge Evolution in the Markov-Chain of QCD

The topological charge is studied on lattices of large physical volume and fine lattice spacing. We illustrate how a parity transformation on the SU(3) link-variables of lattice gauge configurations reverses the sign of the topological charge and leaves the action invariant. Random applications of the parity transformation are proposed to traverse from one topological charge sign to the other. The transformation provides an improved unbiased estimator of the ensemble average and is essential in improving the ergodicity of the Markov chain process.

hep-lat↗

arXiv2002

Electric Polarizability of Hadrons

The electric polarizability of a hadron allows an external electric field to shift the hadron mass. We try to calculate the electric polarizability for several hadrons from their quadratic response to the field at a=0.17fm using an improved gauge field and the clover quark action. Results are compared to experiment where available.

hep-lat↗

arXiv2002

Excited Baryons and Chiral Symmetry Breaking of QCD

N* masses in the spin-1/2 and spin-3/2 sectors are computed using two non-perturbative methods: lattice QCD and QCD sum rules. States with both positive and negative parity are isolated via parity projection methods. The basic pattern of the mass splittings is consistent with experiments. The mass splitting within the same parity pair is directly linked to the chiral symmetry breaking QCD.

nucl-th↗

arXiv2002

Predictive Ability of QCD Sum Rules for Excited Baryons

The masses of octet baryons are calculated by the method of QCD sum rules. Using generalized interpolating fields, three independent sets of QCD sum rules are derived which allow the extraction of low-lying N* states with spin-parity 1/2+, 1/2- and 3/2- in both the non-strange and strange channels. The predictive ability of the sum rules is examined by a Monte-Carlo based analysis procedure in which the three phenomenological parameters (mass, coupling, threshold) are treated as free parameters simultaneously. Realistic uncertainties in these parameters are obtained by simultaneously exploring all uncertainties in the QCD input parameters. Those sum rules with good predictive power are identified and their predictions are compared with experiment where available.

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