arXiv subjects
Publications and source records attributed to Ph. de Forcrand.
To elucidate the role played by instantons in chiral symmetry breaking, we explore their properties in full QCD, around the critical temperature. We study in particular spatial correlations between low-lying Dirac eigenmodes and instantons. Our measurements are compared with the predictions of instanton-based models.
We present results for mesonic propagators in temporal and spatial direction and for topological properties at T below and above the deconfining transition in quenched QCD. We use anisotropic lattices and Wilson fermions.
The short-range modes of the fermionic determinant can be absorbed in the gauge action using the loop expansion. The coefficients of this expansion and the zeroes of the polynomial approximating the remainder can be optimized by a simple, practical method. When the multiboson approach is used, this optimization results in a faster simulation with fewer auxiliary fields.
We have explored the behaviour of some improved actions based on a nonperturbative renormalization group (RG) analysis in coupling space. We calculate the RG flow in two-coupling space $(\boneone,\bonetwo)$ and examine the restoration of rotational invariance and the scaling of physical quantities $(T_c/\sqrtσ)$.
We give a brief introduction on finite temperature phase transitions in lattice QCD including a discussion on the identification of first order transitions. We present a study of the deconfinement phase transition of one-flavour QCD, using the multiboson algorithm on lattice of sizes $8^3$, $12^3$ and $16^3\times 4$. For heavy quarks our results are characteristic of a first order phase transition which gets weaker as the quark mass decreases and ends at a critical value of $κ\sim 0.1$ or in physical units at about 1.6 GeV.
We study truncation effects in the SU(3) gauge actions obtained by the Monte Carlo renormalization group method. By measuring the heavy quark potential we find that the truncation effects in the actions coarsen the lattice by 40-50 % from the original blocked lattice. On the other hand, we find that rotational symmetry of the heavy quark potentials is well recovered on such coarse lattices, which may indicate that rotational symmetry breaking terms are easily cancelled out by adding a short distance operator. We also discuss the possibility of reducing truncation effects.
1. We compare Monte Carlo results with analytic predictions for the fermion condensate, in the massive one-flavour Schwinger model. 2. We illustrate on the Schwinger model how to facilitate transitions between topological sectors by a simple reweighting method. 3. We discuss exact, non-perturbative improvement of the gauge sector.
We present results, for heavy to moderate quark masses, of a study of thermodynamic properties of 1-flavour QCD, using the multiboson algorithm. Finite-size scaling behaviour is studied on lattices of size $8^3\times 4$, $12^3\times 4$ and $16^3\times 4$. It is shown that, for heavy quarks, the peak of the Polyakov loop susceptibility grows linearly with the spatial volume, indicating a first order phase transition. The deconfinement ratio and the distribution of the norm of the Polyakov loop corroborate this result. For moderately heavy quarks the first-order transition weakens and becomes a crossover. We estimate the end point of the first-order phase transition to occur at a quark mass of about 1.6 GeV.
We study topological properties of SU(3) gauge theory using improved cooling. In the absence of fermions, we measure a topological susceptibility of $(182(8) MeV)^4$ and an instanton size $\sim 0.6$ fm. In the presence of light staggered fermions and across the chiral transition, the susceptibility drops in a manner consistent with the quenched case, and the instanton size changes little. No significant formation of bound instanton-antiinstanton pairs is observed, in particular not along the Euclidean time direction for $T > T_c$.
We present a new method of gauge fixing to standard lattice Landau gauge, Max Re Tr $\sum_{μ,x}U_{μ,x}$, in which the link configuration is recursively smeared; these smeared links are then gauge fixed by standard extremization. The resulting gauge transformation is simultaneously applied to the original links. Following this preconditioning, the links are gauge fixed again as usual. This method is free of Gribov copies, and we find that for physical parameters ($β\geq 2$ in SU(2)), it generally results in the gauge fixed configuration with the globally maximal trace. This method is a general technique for finding a unique minimum to global optimization problems.
We present first results, for heavy to moderate quark masses, of a study of thermodynamic properties of 1-flavor QCD, using the multiboson algorithm.
The finite temperature transition of QCD can be seen as a change in the structure of the hadrons and as a symmetry breaking transition -- a change in the structure of the vacuum. These phenomena are observed differently and carry complementary information. We aim at a correlated analysis involving hadronic correlators and the vacuum structure including field and density correlations, both non-trivial questions.
The correlator of heavy-quark currents is calculated in quenched Lattice QCD on a $16^3 \times 32$-lattice for $β= { 6, 6.3 }$. The renormalized charmed quark mass is extracted from the short-distance part of that correlator: $m_c^{\overline{MS}}(m_c) = 1.22(5) GeV$. We study the sensitivity of our data to the strong coupling constant.
We present an exact local bosonic algorithm for the simulation of dynamical fermions in lattice QCD. It is based on a non-hermitian polynomial approximation of the inverse of the quark matrix and a global Metropolis accept/reject correction of the systematic errors. We show that this algorithm is a real alternative to the Hybrid Monte Carlo algorithm.
We present an exact version of the local bosonic algorithm for the simulation of dynamical quarks in lattice QCD. This version is based on a non-hermitian polynomial approximation of the inverse of the quark matrix. A Metropolis test corrects the systematic errors. Two variants of this test are presented. For both of them, a formal proof is given that this Monte Carlo algorithm converges to the right distribution. Simulation data are presented for different lattice parameters. The dynamics of the algorithm and its scaling in lattice volume and quark mass are investigated.
We study on the lattice the correlator of heavy-quark currents in the vicinity of vanishing momentum. The renormalized charmed quark mass, the renormalized strong coupling constant and gluon condensate can be defined in terms of the derivatives of that correlator at zero momentum. We analyze quenched Monte-Carlo data on a small lattice $8^3*16$ for $β=6$. We generalize dispersion relations to the lattice theory in a simple way and use them successfully to fit the correlator at both small and large distances. We fit the short-distance part of the correlator with the relevant expressions of perturbative QCD on the lattice and obtain the value of the renormalized quark mass $m_c^{\bar{MS}}(m_c)\,=\,1.20(4)\,GeV$.
We investigate Lüscher's method of including dynamical Wilson fermions in a lattice simulation of QCD with two quark flavours. We measure the accuracy of the approximation by comparing it with Hybrid Monte Carlo results for gauge plaquette and Wilson loops. We also introduce an additional global Metropolis step in the update. We show that the complexity of Lüscher's algorithm compares favourably with that of the Hybrid Monte Carlo.
We study the Chern-Simons number diffusion rate in the (1+1)-dimensional latticeAbelian Higgs model at temperatures much higher than, as well as comparable to, the sphaleron energy. It is found that in the high-temperature limit the rate is likely to grow as power 2/3 of the temperature. In the intermediate-temperature regime, our numerical simulations show that very weak temperature dependence of the rate, found in previous work, persists at smaller lattice spacings. We discuss possibilities of relating the observed behavior of the rate to static finite-temperature properties of the model.