By combining the constraints of charge symmetry with new chiral extrapolation techniques and recent low mass lattice QCD simulations of the individual quark contributions to the magnetic moments of the nucleon octet, we obtain a precise determination of the strange magnetic moment of the proton. The result, namely G s M = −0.046 ± 0.019 µN , is consistent with the latest experimental measurements but an order of magnitude more precise. This poses a tremendous challenge for future experiments.

The electromagnetic properties of the baryon octet are calculated in quenched QCD on a 20 3 × 40 lattice with a lattice spacing of 0.128 fm using the fat-link irrelevant clover (FLIC) fermion action. FLIC fermions enable simulations to be performed efficiently at quark masses as low as 300 MeV. By combining FLIC fermions with an improved-conserved vector current, we ensure that discretisation errors occur only at O(a 2 ) while maintaining current conservation. Magnetic moments and electric and magnetic radii are extracted from the electric and magnetic form factors for each individual quark sector. From these, the corresponding baryon properties are constructed. Our results are compared extensively with the predictions of quenched chiral perturbation theory. We detect substantial curvature and environment sensitivity of the quark contributions to electric charge radii and magnetic moments in the low quark mass region. Furthermore, our quenched QCD simulation results are in accord with the leading non-analytic behaviour of quenched chiral perturbation theory, suggesting that the sum of higher-order terms makes only a small contribution to chiral curvature.

The electromagnetic properties of the baryon decuplet are calculated in quenched QCD on a 20 3 Â 40 lattice with a lattice spacing of 0.128 fm using the fat-link irrelevant clover fermion action with quark masses providing a pion mass as low as 300 MeV. Magnetic moments and charge radii are extracted from the electric and magnetic form factors for each individual quark sector. From these, the corresponding baryon properties are constructed. We present results for the higher-order moments of the spin-3=2 baryons, including the electric-quadrupole moment E2 and the magnetic-octupole moment M3. The world's first determination of a nonzero M3 form factor for the Á baryon is presented. With these results we provide a conclusive analysis which shows that decuplet baryons are deformed. We compare the decuplet-baryon results from a similar lattice calculation of the octet baryons. We establish that the environment sensitivity is far less pronounced for the decuplet baryons compared to the octet baryons. A surprising result is that the charge radii of the decuplet baryons are generally smaller than those of the octet baryons. The magnetic moment of the Á þ reveals a turnover in the low quark-mass region, making it smaller than the proton magnetic moment. These results are consistent with the expectations of quenched chiral perturbation theory. A similar turnover is also noticed in the magnetic moment of the AE Ã0 , but not for Ä Ã where only kaon loops can appear in quenched QCD. The electric-quadrupole moment of the À baryon is positive when the negative charge factor is included, and is equal to 0:86 AE 0:12 Â 10 À2 fm 2 , indicating an oblate shape.

By combining the constraints of charge symmetry with new chiral extrapolation techniques and recent low-mass quenched lattice QCD simulations of the individual quark contributions to the electric charge radii of the baryon octet, we obtain an accurate determination of the strange electric charge radius of the proton. While this analysis provides a value for G s E Q 2 0:1 GeV 2 in agreement with the best current data, the theoretical error is comparable with that expected from future HAPPEX results from JLab. Together with the earlier determination of G s M , this result considerably constrains the role of hidden flavor in the structure of the nucleon. DOI: 10.1103/PhysRevLett.97.022001 PACS numbers: 12.38.Gc, 12.39.Fe, 13.40.Em, 14.20.Dh One of the great challenges of modern hadron physics is to unravel the precise role of hidden flavors in the structure of the nucleon. Because of their relatively light mass, strange quarks are expected to play the biggest role, and it is with respect to strangeness that there has recently been enormous experimental progress. A new experimental investigation, in which this error is expected to be reduced by roughly a factor of 2, was conducted in late 2005. This makes it imperative to find, as we report, a way to make a theoretical estimate of comparable accuracy-or better.With respect to the strange magnetic form factor of the proton, we recently reported a calculation an order of magnitude more precise than current experiments [5,6]. This calculation exploited the advances in lattice QCD which have enabled quenched QCD (QQCD) simulations of magnetic moments at pion masses as low as 0.3-0.4 GeV [7][8][9], as well as the development of new chiral extrapolation techniques [10,11]. To minimize theoretical uncertainty, an essential input was the precise (experimental) measurements of the magnetic moments of the ground state hyperons-a luxury unfortunately not available for charge radii. Nevertheless, we show here that even the limited data on hyperon charge radii, when combined with new lattice simulations and chiral extrapolation techniques, yield a precision commensurate with the published data. Alternatively, using the best estimates for the charge radii of the valence u and d quarks, extracted from the lattice simulations, yields a determination of the strangequark contribution to the proton charge radius with an uncertainty comparable with that anticipated from the latest HAPPEX measurement.As illustrated in Fig. 1, the three point function required to extract an electromagnetic form factor in lattice QCD involves two topologically distinct processes (each incorporating an arbitrary number of gluons and quark loops). The left-hand diagram illustrates the connected insertion of the current to one of the ''valence'' quarks of the baryon. In the right-hand diagram, the external field couples to a quark loop. The latter process, where the loop involves an s quark, is entirely responsible for G s E . Charge symmetry, which is the invariance of the QCD Hamiltonian under rotation...

A hybrid molecular dynamics-Monte Carlo grand canonical simulation technique is developed for systems with constant chemical potential and temperature. The method ensures that the particle number and energy fluctuate according to the standard grand canonical probability distribution. Partial coupling and fractional particles are used to enhance the success of insertion and deletion attempts, and the method is shown to be feasible in dense liquids. The method is applied to a Lennard-Jones fluid and it gives the density as a function of chemical potential in agreement with known results. It is demonstrated that the transport coefficients can be obtained with the method by analyzing the influence of the stochastic perturbation on the diffusion constant for an isothermal system.

The chiral properties of the fat-link irrelevant clover (FLIC) fermion action are examined. The improved chiral properties of fermion actions incorporating smoothed links are realized in the FLIC action where only the irrelevant operators of the fermion action are constructed with smoothed links. In particular, the histogram of the additive mass renormalization encountered in chiral-symmetry breaking Wilson-type fermion actions is seen to narrow upon introducing fat-links in the irrelevant operators. The exceptional configuration problem of quenched QCD is reduced, enabling access to the light quark mass regime of $m_{\pi} / m_{\rho} \sim 0.35$. In particular, quenched chiral non-analytic behavior is revealed in the light quark mass dependence of the $\Delta$-baryon mass. FLIC fermions offer a promising approach to revealing the properties of full QCD at light quark masses

Abstract. Systematic uncertainties in the recent precise determination of the strangeness magnetic moment of the nucleon are identified and quantified. In summary, G s M = −0.046 ± 0.019 µN .

The Fat-Link Irrelevant Clover (FLIC) fermion action provides a new form of nonperturbative O(a) improvement and allows efficient access to the light quark-mass regime. FLIC fermions enable the construction of the nonperturbatively O(a)-improved conserved vector current without the difficulties associated with the fine tuning of the improvement coefficients. The simulations are performed with an O(a 2 ) mean-field improved plaquetteplus-rectangle gluon action on a 20 3 × 40 lattice with a lattice spacing of 0.128 fm, enabling the first simulation of baryon form factors at light quark masses on a large volume lattice. Magnetic moments, electric charge radii and magnetic radii are extracted from these form factors, and show interesting chiral nonanalytic behavior in the light quark mass regime.