118422773
Z(2) Excitations and Disorder in SU(2) Spin Models and Gauge Theories.
120060029
Tensor force and Debye screening in quarkonium-type mesons
We use a non-relativistic quantum-mechanical model to investigate the effect of a screening plama on two quarkonium-type mesons: the charmonium and the bottonium. The stability of these mesons in the plasma is estimated in two cases: including the tensor and spin-orbit term in the potential and without these terms. It turns out that while the bottonium is somewhat stabilized by the tensor force, the charmonium becomes less stabil due to this modification of the potential. Thus the charmonium seems to be a more sensitive probe of the quark-gluon plasma formation than it was thought to be without including the tensor force.
207778110
Optimizing the Chiral Properties of Lattice Fermions
117844992
Instantons and Fermions on the Lattice
I address the question of how much of QCD in the chiral limit is reproduced by instantons. After reconstructing the instanton content of smoothed Monte Carlo lattice configurations, I compare hadron spectroscopy on this instanton ensemble to the spectroscopy on the original “physical” smoothed configurations using a chirally optimised clover fermion action. Instantons by themselves are shown to break chiral symmetry but hadron correlators on the instanton backgrounds are strongly influenced by free quark propagation. This results in unnaturally light hadrons and a small splitting between the vector and the pseudoscalar meson channels. Superimposing a perturbative ensemble of zero momentum gauge field fluctuations on the instantons eliminates the free quarks and restores the physical hadron correlators. I argue that the torons that are present only in finite volumes, are probably needed to compensate the unnaturally large finite size effects due to the lack of confinement in the instanton ensemble.
9446326
Some remarks on Abelian dominance
Abstract We used a renormalisation group based smoothing to address two questions related to abelian dominance. smoothing enable us to extract the abelian heavy-quark potential from time-like Wilson loops on Polyakov gauge projected configurations. We obtained a very small string tension which is inconsistent with the string tension extracted from Polyakov loop correlators. This shows that the Polyakov gauge projected abelian configurations do not have a consistent physical meaning. We also applied the smoothing on SU(2) configurations to test how sensitive abelian dominance in the maximal abelian gauge is to the short distance fluctuations. We found that on smoothed SU(2) configurations the abelian string tension was about 30% smaller than the SU(2) string tension which was unaffected by smoothing. This suggests that the approximate abelian dominance found with the Wilson action is probably an accident and it has no fundamental physical relevance.
15028871
Pentaquark search with spatially non-trivial operators (II)
We study spin 1/2 isoscalar and isovector, even and odd parity candidates for the Θ + (1540) pentaquark particle using large scale lattice QCD simulations. Two complications extracting the lowest state in a given parity channel are studied. Firstly, we summarize how the energy of the lowest state can be extracted separately in the two parity channels. Secondly, after summarizing the relevant group theoretical principles we discuss how spin 1/2 pentaquark operators can be constructed.
15393945
Gapless Dirac spectrum at high temperature
Using the overlap Dirac operator I show that, contrary to some expectations, even well above the critical temperature there is not necessarily a gap in the Dirac spectrum in pure SU(2) gauge theory. This happens when the Polyakov loop and the fermion boundary condition combine to give close to periodic boundary condition for the fermions in the time direction. In this Polyakov loop sector there is a non-vanishing density of Dirac eigenvalues around zero which implies that chiral symmetry is spontaneously broken. I demonstrate this both directly and also by finding good agreement with the random matrix theory prediction for the distribution of the lowest Dirac eigenvalue. I show that the chiral condensate increases with the tempereture therefore it is very unlikely to be explained by topological fluctuations that become rapidly smaller above Tc. Finally I show that it is only a small fraction of the lowest Dirac eigenvalues that decide which Polyakov loop sector is favored by the fermion determinant if dynamical fermions are turned on. This provides a qualitative understanding of how the loss of confinement above Tc implies the restoration of chiral symmetry.
16594164
Low-lying fermion modes: Dynamical versus quenched
Abstract We compare the low-lying eigenmodes of the O(a) improved Wilson-Dirac operator on quenched and dynamical configurations and investigate methods of probing the topological properties of gauge configurations.
17614040
Unquenching the topological susceptibility with an overlap action
Abstract We estimate the quark-mass dependence of the topological susceptibility with dynamical overlap and clover fermions. Unquenching effects on the susceptibility turn out to be well approximated by a reweighting of a quenched ensemble with a low-eigenmode truncation of the fermionic determinant. We find that it is most likely due to the explicit chiral symmetry breaking of the fermion action that present day dynamical simulations do not show the expected suppression of the topological susceptibility.
119244858
Anderson localization through Polyakov loops: Lattice evidence and random matrix model
We investigate low-lying fermion modes in $SU(2)$ gauge theory at temperatures above the phase transition. Both staggered and overlap spectra reveal transitions from chaotic (random matrix) to integrable (Poissonian) behavior accompanied by an increasing localization of the eigenmodes. We show that the latter are trapped by local Polyakov loop fluctuations. Islands of such ``wrong'' Polyakov loops can therefore be viewed as defects leading to Anderson localization in gauge theories. We find strong similarities in the spatial profile of these localized staggered and overlap eigenmodes. We discuss possible interpretations of this finding and present a sparse random matrix model that reproduces these features.
125829293
A comprehensive search for the theta+ pentaquark on the lattice
We study spin 1/2 isoscalar and isovector, even and odd parity candidates for the Θ+(1540) pentaquark particle using large scale lattice QCD simulations. Previous lattice works led to inconclusive results because so far it has not been possible to unambiguously identify the known scattering spectrum and tell whether additionally a genuine pentaquark state also exists. Here we carry out this analysis using several possible wave functions (operators), including spatially non-trivial ones with unit orbital angular momentum. The cross correlator matrix we compute is 14×14 with 60 non-vanishing elements. We can clearly distinguish the lowest scattering state(s) in both parity channels up to above the expected location of the pentaquark, but we find no trace of the latter. We conclude that there are most probably no pentaquark bound states at our quark masses, corresponding to mπ =400–630 MeV. However, we cannot rule out the existence of a pentaquark state at the physical quark masses or pentaquarks with a more exotic wave function.