Nucleon axial charge in full lattice QCD

The nucleon axial charge is calculated as a function of the pion mass in full QCD. Using domain wall valence quarks and improved staggered sea quarks, we present the first calculation with pion masses as light as 354 MeV and volumes as large as (3.5 fm)3. We show that finite volume effects are small for our volumes and that a constrained fit based on finite volume chiral perturbation theory agrees with experiment within 7% statistical errors.     

Electromagnetic properties of relativistic quarks in confining potentials

We investigate electromagnetic properties of the quark core of nucleons in a model with massless quarks in confining potentials. We find quark core rms radii of 0.6 fm or smaller to be compatible with form factor data corrected for pion cloud effects. Relations between the magnetic form factor and the axial vector or pseudoscalar form factors will also be discussed.     

Spin structure of the pion from the instanton vacuum

We investigate the spin structure of the pion within the framework of the nonlocal chiral quark model from the instanton vacuum. We first evaluate the tensor form factors of the pion for the first and second moment (n=1,2)$(n=1,2)$ and compare it with the lattice data. Combining the tensor form factor of the pion with the electromagnetic one, we determine the impact-parameter dependent probability density of transversely polarized quarks inside the pion. It turns out that the present numerical results for the tensor form factor as well as those for the probability density are in good agreement with the lattice data. We also discuss the distortion of the spatial distribution of the quarks in the transverse plane inside the pion.     

Kaon cloud and baryon magnetic moments

The kaon cloud correction to the magnetic moments of the octet baryons that arises when chiral SU(2) × SU(2) symmetry is enlarged to SU(3) × SU(3) is calculated in the chiral bag model. The effect is considerably smaller than the pion cloud effect provided the bag radius is R ? 1 fm. We discuss the implications of this result on our ongoing attempt to incorporate chiral symmetry into the bag model of the nucleon and on the scale sizes associated with (nearly) massless quarks and massive quarks.     

Extracting low-energy hadron-hadron physics from lattice QCD

Lattice QCD simulations of the hadron-hadron interactions can be very useful in situations where experimental guidance to phenomenological calculations is missing. Moreover, they can determine how nuclear processes depend upon the fundamental constants of nature. Within this framework, I will discuss recent efforts carried out by the Nuclear Physics Lattice QCD collaboration, and present some results for the low-energy scattering parameters in the strange baryon-baryon sector. These simulations have been performed with domain-wall valence quarks on dynamical staggered gauge configurations with lattice spacing of b = 0.125 fm and pion masses between m _π ≈ 300 and 500?MeV.     

Evidence for a bound H dibaryon from lattice QCD

We present evidence for the existence of a bound H dibaryon, an I=0, J=0, s=-2 state with valence quark structure uuddss, at a pion mass of m(π)～389 MeV. Using the results of lattice QCD calculations performed on four ensembles of anisotropic clover gauge-field configurations, with spatial extents of L～2.0, 2.5, 3.0, and 3.9 fm at a spatial lattice spacing of b(s)～0.123 fm, we find an H dibaryon bound by B(∞)(H)=16.6±2.1±4.6 MeV at a pion mass of m(π)～389 MeV.     

Lattice QCD Calculation of ππ Scattering Length

We study s-wave pion-pion scattering length in lattice QCD for pion masses ranging from 330 MeV to 466 MeV.In the "Asqtad" improved staggered fermion formulation,we measure full ππ four-point correlators for isospin I = 0 and 2 channels,and use chiral perturbation theory at next-to-leading order to extrapolate our simulation results.Extrapolating to the physical pion mass yields scattering lengths as mπaI=2 0 = 0.041 6(2) and m π a I =0 0 = 0.186(2) for isospin I = 2 and 0 channels,respectively.Our lattice simulation for ππ scattering length in I = 0 channel is an exploratory study,where we include the disconnected contribution,and our preliminary result is near to its experimental value.These simulations are carried out with MILC 2 + 1 flavor gauge configurations at lattice spacing a ≈ 0.15 fm.     

Phenomenological sizes of confinement regions in baryons

Standard order of magnitude estimates from QCD indicate that the radius of the quarkgluon core in the nucléon is Λ _QCD ^?1 ?1 fm. However, in work with the chiral bag model, we have found that the effective confinement size for low energy reactions can be as small as ～ 1/2 fm or smaller. This shrinking of the effective confinement size has been attributed to the pressure of the pion cloud surrounding the quark core. The concept of confinement size is evidently subtle in light-quark systems, due to the chiral vacuum structure. This is indicated by the “Cheshire Cat” phenomenon, in which physical observables tend to be insensitive to the bag radiusR. In four dimensions, no exact Cheshire Cat principle has yet been established but it is likely to involve infinitely many mesons. We suggest that when strange quarks are present, a qualitative change occurs in the Cheshire Cat picture; in particular, we propose that strangeness provides an obstruction to this picture. We present a phenomenological indication that when strange quarks are present, the bag radiusR is frozen at a value substantially larger than 0.5 fm by as much as a factor of two. Roughly speaking, the Cheshire Cat picture emerges from a near cancellation between repulsive quark kinetic and attractive pion-cloud energies in the case of the nucleon. In theΛ andΣ particles, however, replacement of one up or down quark by a strange quark removes ～ 1/N_c of the attraction from the coupling of the quarks to the pion cloud. This upsets the balance needed for the Cheshire Cat phenomenon and makes larger strange baryons more favorable energetically than the 0.5 fm ones appropriate for pureu- andd-systems. Since the above argument is crude, we appeal strongly to phenomenology. We find that magnetic moments of strange baryons favor a bag radius R?1.1 fm. We find that the excited states of theΛ-hyperons favor similarly large bag radii. Somewhat less convincingly, we argue that — due to perturbative effects — the bag radius appropriate to the Δ(1232) lies intermediate between that of the nucleon and of the strange baryons.     

Flavor symmetry breaking and scaling for improved staggered actions in quenched QCD

We present a study of the flavor symmetry breaking in the pion spectrum for the p4-improved fermion action. Three different variants of the p4 action – p4fat3, p4fat7, and p4fat7tad – are compared to the Asqtad and naive staggered actions. To study the pattern of symmetry breaking, we measure all 15 pion masses in the four-flavor staggered theory. The measurements are done on a quenched gauge background, generated using a one-loop improved Symanzik action with β=10/g²=7.40, 7.75, and 8.00, corresponding to lattice spacings of approximately a= 0.31 fm, 0.21 fm, and 0.14 fm. PACS 11.15.Ha; 11.30.Rd; 12.38.Aw; 12.38.-t; 12.38.Gc     

Nucleon-nucleon scattering from fully dynamical lattice QCD

We present results of the first fully dynamical lattice QCD determination of nucleon-nucleon scattering lengths in the 1S0 channel and 3S1 - 3D1 coupled channels. The calculations are performed with domain-wall valence quarks on the MILC staggered configurations with a lattice spacing of b = 0.125 fm in the isospin-symmetric limit, and in the absence of electromagnetic interactions.     

Baryon structure from Lattice QCD

We present recent lattice results on the baryon spectrum, nucleon electromagnetic and axial form factors, nucleon to △ transition form factors as well as the △ electromagnetic form factors. The masses of the low lying baryons and the nucleon form factors are calculated using two degenerate flavors of twisted mass fermions down to pion mass of about 270 MeV. We compare to the results of other collaborations. The nucleon to △ transition and △ form factors are calculated in a hybrid scheme, which uses staggered sea quarks and domain wall valence quarks. The dominant magnetic dipole nucleon to △ transition form factor is also evaluated using dynamical domain wall fermions. The transverse density distributions of the △ in the infinite momentum frame are extracted using the form factors determined from lattice QCD.     

Axial couplings and strong decay widths of heavy hadrons

We calculate the axial couplings of mesons and baryons containing a heavy quark in the static limit using lattice QCD. These couplings determine the leading interactions in heavy hadron chiral perturbation theory and are central quantities in heavy quark physics, as they control strong decay widths and the light quark mass dependence of heavy hadron observables. Our analysis makes use of lattice data at six different pion masses, 227 MeV相似文献   

Left-right asymmetry and orbiting valence quarks in polarized protons and antiprotons

It is shown that the asymmetry observed in the recent single-spin large-x _F inclusive pion production experiments at high-energies should be considered as a strong indication for the existence of orbiting valence quarks in polarized protons and antiprotons. The underlying physical picture in which orbiting valence quarks give rise to flavor-currents is discussed. The close connection between these currents and the measured quantities — in particular baryon's magnetic moment and the abovementioned asymmetry — is pointed out.     

Single spin asymmetry in inclusive pion production,Collins effect and the string model

We calculated the single spin asymmetry in the inclusive pion production in the fragmentation region of transversely polarized proton-proton collisions. We generated the asymmetry at the level of fragmentation function (Collins effect) by the Lund coloured string mechanism. We compared our results to the presently available experimental data. We obtained a qualitative agreement with the data after assuming that the transverse polarizations of the u and the d quarks in the proton are +1 and ? 1, respectively, at x_B=1.     

Numerical simulation of QCD with u,d, s and c quarks in the twisted-mass Wilson formulation

A first study of numerical Monte Carlo simulations with two quark doublets, a mass-degenerate one and a mass-split one, interpreted as u, d, s and c quarks, is carried out in the framework of the twisted mass Wilson lattice formulation. Tuning the bare parameters of this theory is explored on 12³·24 and 16³·32 lattices at lattice spacings a≃0.20 fm and a≃0.15 fm, respectively.     

格点QCD组态产生研究

组态是格点QCD计算的基础,本文利用开源软件Chroma产生了一组格点QCD组态,格距为0.105 fm,体积为323×64,π介子质量为220 MeV,格点上的夸克作用量采用Wilson clover作用量.这组组态可用于格点QCD中研究核子结构和强子谱等物理问题.     

Pion properties at finite isospin chemical potential with isospin symmetry breaking

Pion properties at finite temperature, finite isospin and baryon chemical potentials are investigated within the SU(2) NJL model. In the mean field approximation for quarks and random phase approximation fpr mesons, we calculate the pion mass, the decay constant and the phase diagram with different quark masses for the u quark and d quark, related to QCD corrections, for the first time. Our results show an asymmetry between μI 0 and μI 0 in the phase diagram, and different values for the charged pion mass(or decay constant) and neutral pion mass(or decay constant) at finite temperature and finite isospin chemical potential. This is caused by the effect of isospin symmetry breaking, which is from different quark masses.     

Pion Tensor Generalized Parton Distributions in a Covariant Constituent Quark Model

The pion tensor generalized parton distributions are evaluated within a covariant, analytic constituent quark model. The generalized form factors for the first two Mellin moments and the probability densities of polarized quarks in the impact parameter space are derived and compared with lattice QCD and quark model results.     

N-to-Delta axial transition form factors from lattice QCD

We evaluate the N to Delta axial transition form factors in lattice QCD with no dynamical sea quarks, with two degenerate flavors of dynamical Wilson quarks, and using domain wall valence fermions with three flavors of staggered sea quarks. We predict the ratio C(5)(A)(q(2))/C(3)(V)(q(2)) relevant for parity violating asymmetry experiments and verify the off-diagonal Goldberger-Treiman relation.