Search

It is expected that nucleons and their parity partners become degenerate when chiral symmetry is restored. We investigate this question in the context of the thermal transition from the hadronic phase to the quark-gluon plasma, using lattice QCD simulations with N_f=2+1 flavours. We observe a clear sign of parity doubling in the quark-gluon plasma. Besides, we find that the nucleon ground state is, within the uncertainty, largely independent of the temperature, whereas temperature effects are substantial in the negative-parity (N^*) channel, already in the confined phase., Comment: 5 pages, 5 eps figures, some more details, figure and references added; to appear in PRD

We calculate correlations between hadronic current operators as a function of their spatial separation, in a quenched lattice QCD simulation at {beta}=6.2 on a 24{sup 3}{times}48 lattice. The lattice fermion formulation used is that due to Sheikholeslami and Wohlert. The correlation functions are then compared with the corresponding quantities calculated in the infinite volume chiral limit of the noninteracting theory. The ratio of the two quantities contains information on the vacuum structure of QCD. The results obtained are consistent with previous studies, and with known hadron phenomenology, although certain features of the fermion action used make it impossible to probe the {ital r}{r_arrow}0 limit directly, thus limiting the accuracy possible.

The Thirring model, that is, a relativistic field theory of fermions with a contact interaction between vector currents, is studied for dimensionalities 2{lt}{ital d}{lt}4 using the 1/{ital N}{sub {ital f}} expansion, where {ital N}{sub {ital f}} is the number of fermion species. The model is found to have no ultraviolet divergences at leading order provided a regularization respecting current conservation is used. Explicit {ital O}(1/{ital N}{sub {ital f}}) corrections are computed, and the model is shown to be renormalizable at this order in the massless limit; renormalizability appears to hold to all orders due to a special case of Weinberg`s theorem. This implies that there is a universal amplitude for four particle scattering in the asymptotic regime. Comparisons are made with both the Gross-Neveu model and QED.

We present the first evidence from lattice simulations that the magnetic monopoles in three-dimensional compact quantum electrodynamics (${\mathrm{cQED}}_{3}$) with ${N}_{f}=2$ and ${N}_{f}=4$ four-component fermion flavors are in a plasma phase. The evidence is based mainly on the divergence of the monopole susceptibility (polarizability) with the lattice size at weak gauge couplings. A weak four-Fermi term added to the ${\mathrm{cQED}}_{3}$ action enabled simulations with massless fermions. The exact chiral symmetry of the interaction terms forbids symmetry breaking lattice discretization counterterms to appear in the theory's effective action. It is also shown that the scenario of a monopole plasma does not depend on the strength of the four-Fermi coupling. Other observables such as the densities of isolated dipoles and monopoles and the so-called specific heat show that a crossover from a dense monopole plasma to a dilute monopole gas occurs at strong couplings. The implications of our results on the stability of $U(1)$ spin liquids in two spatial dimensions are also discussed.

We investigate spectral functions extracted using the maximum entropy method from correlators measured in lattice simulations of the (2+1)-dimensional four-fermion model. This model is particularly interesting because it has both a chirally broken phase with a rich spectrum of mesonic bound states and a symmetric phase where there are only resonances. In the broken phase we study the elementary fermion, pion, sigma, and massive pseudoscalar meson; our results confirm the Goldstone nature of the π and permit an estimate of the meson binding energy. We have, however, seen no signal of σ→ππ decay as the chiral limit is approached. In the symmetric phase we observe a resonance of nonzero width in qualitative agreement with analytic expectations; in addition the ultraviolet behavior of the spectral functions is consistent with the large nonperturbative anomalous dimension for fermion composite operators expected in this model.

We present results from a numerical study of bound state wavefunctions in the (2+1)-dimensional Gross-Neveu model with staggered lattice fermions at both zero and nonzero temperature. Mesonic channels with varying quantum numbers are identified and analysed. In the strongly coupled chirally broken phase at T=0 the wavefunctions expose effects due to varying the interaction strength more effectively than straightforward spectroscopy. In the weakly coupled chirally restored phase information on fermion - antifermion scattering is recovered. In the hot chirally restored phase we find evidence for a screened interaction. The T=0 chirally symmetric phase is most readily distinguished from the symmetric phase at high T via the fermion dispersion relation., Comment: 18 pages

In this paper, we present results of numerical lattice simulations of two-flavor QED in three space-time dimensions. First, we provide evidence that chiral symmetry is spontaneously broken in the chiral and continuum limit. Next we discuss the role of an external magnetic field B on the dynamically generated fermion mass. We investigate the B dependence of the condensate through calculations with dynamical fermions using the noncompact formulation of the gauge field, and compare the results with those of a comparable study using the quenched approximation.

We present results of a Monte Carlo simulation of a three dimensional Gross-Neveu model with SU(2)xSU(2) chiral symmetry at non-zero baryon chemical potential mu, corresponding to non-zero baryon density. For mu sufficiently large there is a sharp transition between a phase where the chiral symmetry is broken by a condensate and one where the global U(1) baryon number symmetry appears to be broken by a scalar diquark condensate . There is also tentative evidence for the formation of a weaker pseudoscalar diquark condensate in the high density phase, which violoates parity.

We present results of a simulation of 2-flavor QCD on a 4x16^3 lattice using p4-improved staggered fermions with bare quark mass m/T=0.4. Derivatives of the thermodynamic grand canonical partition function Z(V,T,mu_u,mu_d) with respect to chemical potentials mu_(u,d) for different quark flavors are calculated up to sixth order, enabling estimates of the pressure and the quark number density as well as the chiral condensate and various susceptibilities as functions of mu_q = (mu_u + mu_d)/2 via Taylor series expansion. Furthermore, we analyze baryon as well as isospin fluctuations and discuss the relation between the radius of convergence of the Taylor series and the chiral critical point in the QCD phase diagram. We argue that bulk thermodynamic observables do not, at present, provide direct evidence for the existence of a chiral critical point in the QCD phase diagram. Results are compared to high temperature perturbation theory as well as a hadron resonance gas model., 38 pages, 30 encapsulated postscript figures, typo corrected, 1 footnote added