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Register a new user2+1 Flavor Lattice QCD with Lueschers Domain-Decomposed HMC Algorithm
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We report on a study of 2+1 flavor lattice QCD with the $O(a)$-improved Wilson quarks on a $16^3times 32$ lattice at the lattice spacing $1/aapprox 2$GeV employing Lueschers domain-decomposed HMC(LDDHMC) algorithm. This is dedicated to a preliminary study for the PACS-CS project which plans to complete the Wilson-clover $N_f=2+1$ program lowering the up-down quark masses close to the physical values as much as possible. We focus on three issues: (i) how light quark masses we can reach with LDDHMC, (ii) efficiency of the algorithm compared with the conventional HMC, (iii) parameter choice for the production runs on PACS-CS.
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We present results for the nucleon axial charge g_A at a fixed lattice spacing of 1/a=1.73(3) GeV using 2+1 flavors of domain wall fermions on size 16^3x32 and 24^3x64lattices (L=1.8 and 2.7 fm) with length 16 in the fifth dimension. The length of the Monte Carlo trajectory at the lightest m_pi is 7360 units, including 900 for thermalization. We find finite volume effects are larger than the pion mass dependence at m_pi= 330 MeV. We also find that g_A exhibits a scaling with the single variable m_pi L which can also be seen in previous two-flavor domain wall and Wilson fermion calculati ons. Using this scaling to eliminate the finite-volume effect, we obtain g_A = 1.20(6)(4) at the physical pion mass, m_pi = 135 MeV, where the first and second errors are statistical and systematic. The observed finite-volume scaling also appears in similar quenched simulations, but disappear when Vge (2.4 fm)^3. We argue this is a dynamical quark effect.
Polynomial approximations to the inverse of the fermion matrix are used to filter the dynamics of the upper energy scales in HMC simulations. The use of a multiple time-scale integration scheme allows the filtered pseudofermions to be evolved using a coarse step size. We introduce a novel generalisation of the nested leapfrog which allows for far greater flexibility in the choice of time scales. We observe a reduction in the computational expense of the molecular dynamics integration of between 3--5 which improves as the quark mass decreases.
We investigate the interaction between $Omega$ baryons in the $^1S_0$ channel from 2+1 flavor lattice QCD simulations. On the basis of the HAL QCD method, the $OmegaOmega$ potential is extracted from the Nambu-Bethe-Salpeter wave function calculated on the lattice by using the PACS-CS gauge configurations with the lattice spacing $asimeq 0.09$ fm, the lattice volume $Lsimeq 2.9$ fm and the quark masses corresponding to $m_pi simeq 700$ MeV and $m_Omega simeq 1970$ MeV. The $OmegaOmega$ potential has a repulsive core at short distance and an attractive well at intermediate distance. Accordingly, the phase shift obtained from the potential shows moderate attraction at low energies. Our data indicate that the $OmegaOmega$ system with the present quark masses may appear close to the unitary limit where the scattering length diverges.
Nucleon-structure calculations of isovector vector- and axialvector-current form factors, transversity and scalar charge, and quark momentum and helicity fractions are reported from two recent 2+1-flavor dynamical domain-wall fermions lattice-QCD ensembles generated jointly by the RIKEN-BNL-Columbia and UKQCD Collaborations with Iwasaki $times$ dislocation-suppressing-determinatn-ratio gauge action at inverse lattice spacing of 1.378(7) GeV and pion mass values of 249.4(3) and 172.3(3) MeV.
We present results for light meson masses and pseudoscalar decay constants from the first of a series of lattice calculations with 2+1 dynamical flavors of domain wall fermions and the Iwasaki gauge action. The work reported here was done at a fixed lattice spacing of about 0.12 fm on a 16^3times32 lattice, which amounts to a spatial volume of (2 fm)^3 in physical units. The number of sites in the fifth dimension is 16, which gives m_{res} = 0.00308(4) in these simulations. Three values of input light sea quark masses, m_l^{sea} approx 0.85 m_s, 0.59 m_s and 0.33 m_s were used to allow for extrapolations to the physical light quark limit, whilst the heavier sea quark mass was fixed to approximately the physical strange quark mass m_s. The exact rational hybrid Monte Carlo algorithm was used to evaluate the fractional powers of the fermion determinants in the ensemble generation. We have found that f_pi = 127(4) MeV, f_K = 157(5) MeV and f_K/f_pi = 1.24(2), where the errors are statistical only, which are in good agreement with the experimental values.
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