Fri·Sep 21, 2007Friday·September 21, 2007

1. 01*

   Nucleon form factors from quenched lattice QCD with domain wall fermions

   Shoichi Sasaki (Univ. of Tokyo)🇯🇵 · Takeshi Yamazaki (Univ. of Connecticut)🇺🇸

   We present a quenched lattice calculation of the weak nucleon form factors: vector (F_V(q^2)), induced tensor (F_T(q^2)), axial-vector (F_A(q^2)) and induced pseudo-scalar (F_P(q^2)) form factors. Our simulations are performed on three different lattice sizes L^3 x T=24^3 x 32, 16^3 x 32 and 12^3 x 32 with a lattice cutoff of 1/a = 1.3 GeV and light quark masses down to about 1/4 the strange quark mass (m_{pi} = 390 MeV) using a combination of the DBW2 gauge action and domain wall fermions. The physical volume of our largest lattice is about (3.6 fm)^3, where the finite volume effects on form factors become negligible and the lower momentum transfers (q^2 = 0.1 GeV^2) are accessible. The q^2-dependences of form factors in the low q^2 region are examined. It is found that the vector, induced tensor, axial-vector form factors are well described by the dipole form, while the induced pseudo-scalar form factor is consistent with pion-pole dominance. We obtain the ratio of axial to vector coupling g_A/g_V=F_A(0)/F_V(0)=1.219(38) and the pseudo-scalar coupling g_P=m_{mu}F_P(0.88m_{mu}^2)=8.15(54), where the errors are statistical erros only. These values agree with experimental values from neutron beta decay and muon capture on the proton. However, the root mean squared radii of the vector, induced tensor and axial-vector underestimate the known experimental values by about 20%. We also calculate the pseudo-scalar nucleon matrix element in order to verify the axial Ward-Takahashi identity in terms of the nucleon matrix elements, which may be called as the generalized Goldberger-Treiman relation.

   hep-lathep-phPRD(2008)·48 citations
2. 02*

   Large volume behavior of Yang-Mills propagators

   Christian S. Fischer🇩🇪 · Reinhard Alkofer🇦🇹 · Axel Maas🇸🇰 · Jan M. Pawlowski🇩🇪 · Lorenz von Smekal🇦🇺

   We summarize results on finite-volume effects in the propagators of Landau gauge Yang-Mills theory using Dyson-Schwinger equations on a 4-dimensional torus. We demonstrate explicitly how the solutions for the gluon and the ghost propagator tend towards their respective infinite volume forms in the corresponding limit. We discuss the relation of our solutions with results from lattice Monte-Carlo simulations.

   hep-latPoS(2007)·9 citations
3. 03*

   Disconnected contributions to hadronic structure: a new method for stochastic noise reduction

   S. Collins🇩🇪 · G. Bali🇩🇪 · A. Schäfer🇩🇪

   We present a new method for reducing the stochastic noise of all-to-all propagators based on stopping the inversion of the propagator before convergence. The method is easy to implement, unbiased and independent of the quark action. Applying this method to the calculation of disconnected loops needed for hadronic structure observables we find savings in computer time of factors of 4-12 depending on the operator inserted in the loop. When combined with a hopping parameter expansion technique we obtain combined gains of up to factors of 30 for some operators.

   hep-latPoS(2007)·48 citations
4. 04*

   Lambda-parameter of lattice QCD with Symanzik improved gluon actions

   A. Skouroupathis🇨🇾 · H. Panagopoulos (Department of Physics, University of Cyprus)🇨🇾

   We compute the ratio Lambda_L/Lambda_MS, where the scale parameter Lambda_L is associated with a lattice formulation of QCD. We consider a 3-parameter family of gluon actions, which are most frequently used for O(a) improvement a` la Symanzik. The gluon action is put togeter with standard discretizations for fermions (Wilson/clover, overlap), to provide Lambda_L for several possible combinations of fermion and gluon actions. We employ the background field technique in order to calculate the 1PI 2-point function of the background field; this leads to the coupling constant renormalization function, Z_g, at 1-loop level. Our results are obtained for an extensive range of values for the Symanzik coefficients.

   hep-latPRD(2007)·9 citations
5. 05*

   Magnetic bion condensation: A new mechanism of confinement and mass gap in four dimensions

   Mithat Unsal🇺🇸

   In recent work, we derived the long-distance confining dynamics of certain QCD-like gauge theories formulated on small $S^1\times {\mathbb{R}}^3$ based on symmetries, an index theorem, and Abelian duality. Here, we give the microscopic derivation. The solution reveals a new mechanism of confinement in QCD(adj) in the regime where we have control over both perturbative and nonperturbative aspects. In particular, consider SU(2) QCD(adj) theory with $1\le n_f\le 4$ Majorana fermions, a theory which undergoes gauge symmetry breaking at small $S^1$. If the magnetic charge of the BPS monopole is normalized to unity, we show that confinement occurs due to condensation of objects with magnetic charge 2, not 1. Because of index theorems, we know that such an object cannot be a two identical monopole configuration. Its net topological charge must vanish, and hence it must be topologically indistinguishable from the perturbative vacuum. We construct such non-self-dual topological excitations, the magnetically charged, topologically null molecules of a BPS monopole and $\overline{\mathrm{KK}}$ antimonopole, which we refer to as magnetic bions. An immediate puzzle with this proposal is the apparent Coulomb repulsion between the BPS-$\overline{\mathrm{KK}}$ pair. An attraction which overcomes the Coulomb repulsion between the two is induced by $2n_f$-fermion exchange. Bion condensation is also the mechanism of confinement in $\mathbb{N}=1$ SYM on the same four-manifold. The SU(N) generalization hints a possible hidden integrability behind nonsupersymmetric QCD of affine Toda type, and allows us to analytically compute the mass gap in the gauge sector. We currently do not know the extension to ${\mathbb{R}}^4$.

   ↳ hep-thhep-lathep-phPRD(2009)·257 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.