arXiv.org: High-Energy Physics: Lattice

hep-lat updates on arXiv.org

• Orbifold equivalence and the sign problem at finite baryon density. (arXiv:1009.1623v1 [hep-th])
  

  We point out that SO(2N_{c}) gauge theory with N_{f} fundamental Dirac fermions does not have a sign problem at finite baryon number chemical potential \mu_{B}. One can thus use lattice Monte Carlo simulations to study this theory at finite density. The absence of a sign problem in the SO(2N_{c}) theory is particularly interesting because a wide class of observables in the SO(2N_{c}) theory coincide with observables in QCD in the large N_{c} limit, as we show using the technique of large N_{c} orbifold equivalence. We argue that the orbifold equivalence between the two theories continues to hold at \mu_{B} \neq 0 provided one adds appropriate deformation terms to the SO(2N_{c}) theory. This opens up the prospect of learning about QCD at \mu_{B} \neq 0 using lattice studies of the SO(2N_{c}) theory.

  
  
  
• $\Lambda (1405)$ from lattice QCD. (arXiv:1009.1790v1 [hep-lat])
  

  Low-lying $\Lambda$ baryons with spin 1/2 are analyzed in two-flavor lattice QCD. In order to extract two low-lying states for each parity, we construct $2 \times 2$ cross correlators from flavor SU(3) ``octet'' and ``singlet'' baryon operators, and diagonalize them. Two-flavor CP-PACS gauge configurations are employed, which are generated with the renormalization-group improved gauge action and the ${\mathcal O}(a)$-improved quark action. Simulation are performed at three different $\beta$'s, $\beta = 1.80$, 1.95 and 2.10, whose corresponding lattice spacings are $a = 0.2150$, 0.1555 and 0.1076 fm. For each cutoff, we adopt four different hopping parameters, ($\kappa_{\rm val}, \kappa_{\rm sea}$). The corresponding pion masses range from about 500 MeV to 1.1 GeV. The results are extrapolated to the physical quark-mass point. Our results indicate that there are two negative-parity $\Lambda$ states nearly degenerate at around 1.6 GeV, whereas no state as low as $\Lambda (1405)$ is observed. By extracting the flavor components of each state, we find that the lowest (1st-excited) negative-parity state is dominated by flavor-singlet (flavor-octet) component.

  
  
  
• Properties of the ground-state baryons in chiral perturbation theory. (arXiv:1009.1860v1 [hep-ph])
  

  We review recent progress in the understanding of low-energy baryon structure by means of chiral perturbation theory. In particular, we discuss the application of this formalism to the description of various properties such as the baryon-octet magnetic moments, the electromagnetic structure of decuplet resonances and the hyperon vector coupling $f_1(0)$. Moreover, we present the results on the chiral extrapolation of recent lattice QCD results on the lowest-lying baryon masses and we predict the corresponding baryonic sigma-terms.

  
  
  
• Hadron-hadron interaction from SU(2) lattice QCD. (arXiv:0912.0691v2 [hep-lat] UPDATED)
  

  We extract interhadron potentials in two-color lattice QCD from Bethe-Salpeter amplitudes on the Euclidean lattice. The simulations are performed in quenched SU(2) QCD. In the potential between two ``nucleons'' ($u C\gamma_5 d$, the lightest baryon in SU(2) system), we observe a clear repulsion in short-range region, which is similar to that in the SU(3) nucleon-nucleon potential, even though present quark masses are not very light. We define the ``quark-exchange part'' in the potential, which is induced by adding quark-exchange diagrams, or equivalently, by introducing Pauli blocking among some of quarks. The strong repulsive force in short-distance region arises only from the ``quark-exchange part'', and disappears when quark-exchange diagrams are omitted. We also find a universal long-range attractive force, which enters in any flavor channels of two scalar diquarks and whose interaction range and strength are quark-mass independent. Because of weak quark-mass dependence of interaction ranges in each component, it does not go together with the meson-exchange picture as the origin of hadronic interactions.

  
  
  
• Egalitarian Improvement to Democracy: Quark renormalization constants in N_f=2 QCD. (arXiv:1008.2224v2 [hep-lat] UPDATED)
  

  We present our results on the non-perturbative evaluation of the renormalization constant for the quark field, $Z_q$, in Landau gauge within RI-MOM scheme. Using three lattice spacing we are able to isolate lattice artefacts of various origin, both perturbative and non-perturbative. In particular, the existence of the dimension-two gluon-condensate is discussed, and confirmed.