The eta ' signal from partially quenched Wilson fermions

We present new results from our ongoing study of flavor singlet pseudoscalar mesons in QCD. Our approach is based on (a) performing truncated eigenmode expansions for the hairpin diagram and (b) incorporating the ground state contribution for the connected meson propagator. First, we explain how the computations can be substantially improved by even-odd preconditioning. We extend previous results on early mass plateauing in the eta' channel of two-flavor full QCD with degenerate sea and valence quarks to the partially quenched situation. We find that early mass plateau formation persists in the partially quenched situation.Comment: Lattice2002(spectrum), 3 pages, 5 figure

Temperature dependence in random matrix models with pairing condensates

We address a number of issues raised by a manuscript of Klein, Toublan, and Verbaarschot (hep-ph/0405180) in which the authors introduce a random matrix model for QCD with two colors, two flavors, and fermions in the fundamental representation. Their inclusion of temperature terms differs from the approach adopted in previous work on this problem (Phys. Rev. D 64, 074016 (2001).) We demonstrate that the two approaches are related by a transformation that leaves the thermodynamic potential invariant and which therefore has no effect on physical observables.Comment: 8 pages, revtex4. v2: typos corrected in reference

Diffusion in the random gap model of mono- and bilayer graphene

In this paper we study the effect of a fluctuating gap in mono- and bilayer graphene, created by a random staggered potential. We identify a continuous symmetry for the two-particle Green's function which is spontaneously broken in the average two-particle Green's function and leads to a massless fermion mode. Within a loop expansion it is shown that the massless mode is dominated on large scales by small loops. This result indicates diffusion of electrons. Although the diffusion mechanism is the same in mono- and in bilayer graphene, the amount of scattering is much stronger in the latter. Physical quantities at the neutrality point, such as the density of states, the diffusion coefficient and the conductivity, are determined by the one-particle scattering rate. All these quantities vanish at a critical value of the average staggered potential, signaling a continuous transition to an insulating behavior.Comment: 16 pages, 2 figures, extended versio

Indirect forces between impurities in one-dimensional quantum liquids

We investigate the indirect interaction between two isolated impurities in a Luttinger liquid described by a microscopic lattice model. To treat the electron-electron interaction U the functional renormalization group method is used. For comparison we also study the U=0 case. We find that for a wide range of impurity parameters the impurity interaction V_{12} as a function of their separation r oscillates with decaying amplitude between being attractive and repulsive. For half-filling of the band and in a crossover regime between weak and strong impurities the interaction becomes purely attractive. For U=0 and independent of the impurity strength the amplitude of the interaction energy falls off as 1/r. For U>0 the decay for small separations and weak to intermediate impurities is governed by a U dependent exponent larger than -1, which crosses over to -1 for large r. The crossover scale depends on the impurity strength and U. We present simple pictures which explain our results in the limits of weak and strong impurities. We finally also consider attractive interactions U<0.Comment: 8 pages, 9 figures include

Mott insulator to superfluid transition in the Bose-Hubbard model: a strong-coupling approach

We present a strong-coupling expansion of the Bose-Hubbard model which describes both the superfluid and the Mott phases of ultracold bosonic atoms in an optical lattice. By performing two successive Hubbard-Stratonovich transformations of the intersite hopping term, we derive an effective action which provides a suitable starting point to study the strong-coupling limit of the Bose-Hubbard model. This action can be analyzed by taking into account Gaussian fluctuations about the mean-field approximation as in the Bogoliubov theory of the weakly interacting Bose gas. In the Mott phase, we reproduce results of previous mean-field theories and also calculate the momentum distribution function. In the superfluid phase, we find a gapless spectrum and compare our results with the Bogoliubov theory.Comment: 8 pages, 6 figures; (v2) Two references adde

Hadronic physics with domain-wall valence and improved staggered sea quarks

With the advent of chiral fermion formulations, the simulation of light valence quarks has finally become realistic for numerical simulations of lattice QCD. The simulation of light dynamical quarks, however, remains one of the major challenges and is still an obstacle to realistic simulations. We attempt to meet this challenge using a hybrid combination of Asqtad sea quarks and domain-wall valence quarks. Initial results for the proton form factor and the nucleon axial coupling are presented.Comment: Two Talks presented at Lattice2004(spectrum), LaTex, 6 pages, 6 eps figure

Magnetic Monopole Content of Hot Instantons

We study the Abelian projection of an instanton in $R^3 \times S^1$ as a function of temperature (T) and non-trivial holonomic twist ($\omega$) of the Polyakov loop at infinity. These parameters interpolate between the circular monopole loop solution at T=0 and the static 't Hooft-Polyakov monopole/anti-monopole pair at high temperature.Comment: 3 pages, LATTICE98(confine), LaTeX, PostScript figures include

Helicity Dependent and Independent Generalized Parton Distributions of the Nucleon in Lattice QCD

A complete description of the nucleon structure in terms of generalized parton distributions (GPDs) at twist 2 level requires the measurement/computation of the eight functions H, E, \tilde H, \tilde E, H_T, E_T, \tilde H_T and \tilde E_T, all depending on the three variables x, \xi and t. In this talk, we present and discuss our first steps in the framework of lattice QCD towards this enormous task. Dynamical lattice QCD results for the lowest three Mellin moments of the helicity dependent and independent GPDs are shown in terms of their corresponding generalized form factors. Implications for the transverse coordinate space structure of the nucleon as well as the orbital angular momentum (OAM) contribution of quarks to the nucleon spin are discussed in some detail.Comment: 5 pages, 5 figures, Talk presented by Ph.H. at Electron-Nucleus Scattering VIII, Elba, Italy, June 21-25, 2004; typos corrected, minor change in wording on p.4&

Quantum interface unbinding transitions

We consider interfacial phenomena accompanying bulk quantum phase transitions in presence of surface fields. On general grounds we argue that the surface contribution to the system free energy involves a line of singularities characteristic of an interfacial phase transition, occurring below the bulk transition temperature T_c down to T=0. This implies the occurrence of an interfacial quantum critical regime extending into finite temperatures and located within the portion of the phase diagram where the bulk is ordered. Even in situations, where the bulk order sets in discontinuously at T=0, the system's behavior at the boundary may be controlled by a divergent length scale if the tricritical temperature is sufficiently low. Relying on an effective interfacial model we compute the surface phase diagram in bulk spatial dimensionality $d\geq 2$ and extract the values of the exponents describing the interfacial singularities in $d\geq 3$

Quantum phases in mixtures of fermionic atoms

A mixture of spin-polarized light and heavy fermionic atoms on a finite size 2D optical lattice is considered at various temperatures and values of the coupling between the two atomic species. In the case, where the heavy atoms are immobile in comparison to the light atoms, this system can be seen as a correlated binary alloy related to the Falicov-Kimball model. The heavy atoms represent a scattering environment for the light atoms. The distributions of the binary alloy are discussed in terms of strong- and weak-coupling expansions. We further present numerical results for the intermediate interaction regime and for the density of states of the light particles. The numerical approach is based on a combination of a Monte-Carlo simulation and an exact diagonalization method. We find that the scattering by the correlated heavy atoms can open a gap in the spectrum of the light atoms, either for strong interaction or small temperatures.Comment: 15 pages, 8 figure