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Martin Hasenbusch

Publications and source records attributed to Martin Hasenbusch.

At least 73 records · Page 4Linked to original sources
arXiv2005

Speeding up the HMC algorithm: Some new results

We give some new performance results for the Hybrid Monte Carlo (HMC) simulation of dynamical clover-improved Wilson fermions using an improved pseudo-fermion action. The generalisation of even-odd preconditioning for the standard Wilson fermion matrix to the clover-improved case is not unique. In the literature the so called symmetric and asymmetric versions are discussed. Most of the previous simulations of dynamical clover-improved Wilson fermions were done with the asymmetric version. Only recently, the JLQCD collaboration has pointed out that the symmetric version leads to a better performance of the HMC algorithm. Here, we show that also in combination with an improved pseudo-fermion action, the symmetric version of even-odd preconditioning leads to a better performance. For our lightest quark mass, which corresponds to $m_{PS}/m_{V} \approx 0.44$, we see a gain in performance by a factor of about 1.3.

hep-lat↗
arXiv2005

Critical behavior of two-dimensional fully frustrated XY systems

We study the phase diagram of the two-dimensional fully frustrated XY model (FFXY) and of two related models, a lattice discretization of the Landau-Ginzburg-Wilson Hamiltonian for the critical modes of the FFXY model, and a coupled Ising-XY model. We present Monte Carlo simulations on square lattices $L\times L$, $L\lesssim 10^3$. We show that the low-temperature phase of these models is controlled by the same line of Gaussian fixed points as in the standard XY model. We find that, if a model undergoes a unique transition by varying temperature, then the transition is of first order. In the opposite case we observe two very close transitions: a transition associated with the spin degrees of freedom and, as temperature increases, a transition where chiral modes become critical. If they are continuous, they belong to the Kosterlitz-Thouless and to the Ising universality class, respectively. Ising and Kosterlitz-Thouless behavior is observed only after a preasymptotic regime, which is universal to some extent. In the chiral case, the approach is nonmonotonic for most observables, and there is a wide region in which finite-size scaling is controlled by an effective exponent $ν_{\rm eff} \approx 0.8$. This explains the result $ν\approx 0.8$ of many previous studies using smaller lattices.

cond-mat.stat-mech↗
arXiv2005

Transitions and crossover phenomena in fully frustrated XY systems

We study the two-dimensional fully frustrated XY (FFXY) model and two related models, a discretization of the Landau-Ginzburg-Wilson Hamiltonian for the critical modes of the FFXY model and a coupled Ising-XY model, by means of Monte Carlo simulations on square lattices L x L, L=O(10^3). We show that their phase diagram is characterized by two very close chiral and spin transitions, at T_ch > T_sp respectively, of the Ising and Kosterlitz-Thouless type. At T_ch the Ising regime sets in only after a preasymptotic regime, which appears universal to some extent. The approach is nonmonotonic for most observables, with a wide region controlled by an effective exponent nu_eff=0.8.

cond-mat.stat-mech↗
arXiv2005

Comparing the Nambu-Goto string with LGT results

We discuss a way to evaluate the full prediction for the interquark potential which is expected from the effective Nambu-Goto string model. We check the correctness of the prescription reproducing the results obtained with the zeta function regularization for the first two perturbative orders. We compare the predictions with existing Monte Carlo data for the (2+1) dimensional Z(2), SU(2) and SU(3) gauge theories: in the low temperature regime, we find good agreement for large enough interquark distances, but an increasing mismatch between theoretical predictions and numerical results is observed as shorter and shorter distances are investigated. On the contrary, at high temperatures (approaching the deconfinement transition from below) a remarkable agreement between Monte Carlo data and the expectations from the Nambu-Goto effective string is observed for a wide range of interquark distances.

hep-lat↗
arXiv2005

Instability of the O(5) multicritical behavior in the SO(5) theory of high-Tc superconductors

We study the nature of the multicritical point in the three-dimensional O(3)+O(2) symmetric Landau-Ginzburg-Wilson theory, which describes the competition of two order parameters that are O(3) and O(2) symmetric, respectively. This study is relevant for the SO(5) theory of high-Tc superconductors, which predicts the existence of a multicritical point in the temperature-doping phase diagram, where the antiferromagnetic and superconducting transition lines meet. We investigate whether the O(3)+O(2) symmetry gets effectively enlarged to O(5) approaching the multicritical point. For this purpose, we study the stability of the O(5) fixed point. By means of a Monte Carlo simulation, we show that the O(5) fixed point is unstable with respect to the spin-4 quartic perturbation with the crossover exponent $ϕ_{4,4}=0.180(15)$, in substantial agreement with recent field-theoretical results. This estimate is much larger than the one-loop $ε$-expansion estimate $ϕ_{4,4}=1/26$, which has often been used in the literature to discuss the multicritical behavior within the SO(5) theory. Therefore, no symmetry enlargement is generically expected at the multicritical transition. We also perform a five-loop field-theoretical analysis of the renormalization-group flow. It shows that bicritical systems are not in the attraction domain of the stable decoupled fixed point. Thus, in these systems--high-Tc cuprates should belong to this class--the multicritical point corresponds to a first-order transition.

cond-mat.supr-con↗
arXiv2004

Lattice artefacts in SU(3) lattice gauge theory with a mixed fundamental and adjoint plaquette action

We investigated SU(3) lattice gauge theory with a fundamental and adjoint plaquette term in the action. The purpose is to test whether the choice of a negative adjoint coupling can reduce lattice artefacts and improve the scaling b ehaviour. To this end, we have studied the finite temperature phase transition, the static potential and the mass of the 0^{++} glueball. We found that indeed the lattice artefacts in e.g. m_{0^{++}}/T_c can be reduced considerably compared with the pure Wilson (fundamental) gauge action at the same lattice spacing.

hep-lat↗
arXiv2003

Effective String Picture for Confining Gauge Theories at Finite Temperature

We study the ``effective string picture'' of confinement, deriving theoretical predictions for the interquark potential at finite temperature. At low temperatures, the leading string correction to the linear confining potential between a heavy quark-antiquark pair is the "Lüscher term''. Assuming a Nambu-Goto effective string action, subleading contributions can be worked out in an analytical way. We also discuss the contribution given by a possible ``boundary term'' in the effective action, and compare these predictions with high precision results from simulations of lattice Z(2) gauge theory in three dimensions, obtained with an algorithm that exploits the duality of the Z(2) gauge model with the Ising spin model.

hep-lat↗
arXiv2003

Full QCD Algorithms towards the Chiral Limit

I discuss the behaviour of algorithms for dynamical fermions as the sea-quark mass decreases. I focus on the Hybrid-Monte-Carlo (HMC) algorithm applied to two degenerate flavours of Wilson fermions. First, I briefly review the performance obtained in large scale HMC simulations. Then I discuss a modified pseudo-fermion action for the HMC simulation that has been introduced three years ago. I summarize recent results obtained with this pseudo-fermion action by the QCDSF and the ALPHA collaborations. I comment on alternatives to the HMC, like the Multiboson algorithm and variants of it.

hep-lat↗
arXiv2003

Effective string picture for confinement at finite temperature: theoretical predictions and high precision numerical results

The effective string picture of confinement is used to derive theoretical predictions for the interquark potential at finite temperature. At short distances, the leading string correction to the linear confining potential between a heavy quark-antiquark pair is the "Lüscher term''. We assume a Nambu--Goto effective string action, and work out subleading contributions in an analytical way. We discuss the contribution given by a possible ``boundary term'' in the effective action, comparing these predictions with results from simulations of lattice $Z_2$ gauge theory in three dimensions, obtained with an algorithm which exploits the duality of the $Z_2$ gauge model with the Ising spin model.

hep-lat↗
arXiv2001

Nonstandard Cutoff Effects in the Nonlinear Sigma Model

High precision measurements of the renormalized zero-momentum 4-point coupling g_R and of the Luscher-Weisz-Wolff running coupling gbar(L) = L*m(L) performed with two different lattice actions in the non-perturbative region confirm the earlier observations, that the cutoff effects look linear, in contrast to perturbative considerations. The use of different actions allows one to make a more reliable estimate on the continuum limit. The measurements were done for infinite volume correlation length up to 350.

hep-lat↗
arXiv2001

Speeding up the Hybrid-Monte-Carlo algorithm for dynamical fermions

We propose a modification of the Hybrid-Monte-Carlo algorithm that allows for a larger step-size of the integration scheme at constant acceptance rate. The key ingredient is that the pseudo-fermion action is split into two parts. We test our proposal at the example of the two-dimensional lattice Schwinger model with two degenerate flavours of Wilson-fermions.

hep-lat↗
arXiv2001

Irrelevant operators in the two-dimensional Ising model

By using conformal-field theory, we classify the possible irrelevant operators for the Ising model on the square and triangular lattices. We analyze the existing results for the free energy and its derivatives and for the correlation length, showing that they are in agreement with the conformal-field theory predictions. Moreover, these results imply that the nonlinear scaling field of the energy-momentum tensor vanishes at the critical point. Several other peculiar cancellations are explained in terms of a number of general conjectures. We show that all existing results on the square and triangular lattice are consistent with the assumption that only nonzero spin operators are present.

cond-mat.stat-mech↗
arXiv2001

First results on the running coupling in QCD with two massless flavours

We report on the non-perturbative computation of the running coupling of two-flavour QCD in the Schr"odinger functional scheme. The corresponding Lambda-parameter, which describes the coupling strength at high energy, is related to a low energy scale which still remains to be connected to a hadronic ``experimentally'' observable quantity. We find the non-perturbative evolution of the coupling indispensable to avoid untolerable errors in the estimated Lambda-parameter.

hep-lat↗
arXiv2000

Comparative Benchmarks of full QCD Algorithms

We report performance benchmarks for several algorithms that we have used to simulate the Schr"odinger functional with two flavors of dynamical quarks. They include hybrid and polynomial hybrid Monte Carlo with preconditioning. An appendix describes a method to deal with autocorrelations for nonlinear functions of primary observables as they are met here due to reweighting.

hep-lat↗
arXiv2000

Eliminating leading corrections to scaling in the 3-dimensional O(N)-symmetric phi^4 model: N=3 and 4

We study corrections to scaling in the O(3)- and O(4)-symmetric phi^4 model on the three-dimensional simple cubic lattice with nearest neighbour interactions. For this purpose, we use Monte Carlo simulations in connection with a finite size scaling method. We find that there exists a finite value of the coupling lambda^*, for both values of N, where leading corrections to scaling vanish. As a first application, we compute the critical exponents nu=0.710(2) and eta=0.0380(10) for N=3 and nu=0.749(2) and eta=0.0365(10) for N=4.

cond-mat↗
arXiv2000

Critical behavior of the three-dimensional XY universality class

We improve the theoretical estimates of the critical exponents for the three-dimensional XY universality class. We find alpha=-0.0146(8), gamma=1.3177(5), nu=0.67155(27), eta=0.0380(4), beta=0.3485(2), and delta=4.780(2). We observe a discrepancy with the most recent experimental estimate of alpha; this discrepancy calls for further theoretical and experimental investigations. Our results are obtained by combining Monte Carlo simulations based on finite-size scaling methods, and high-temperature expansions. Two improved models (with suppressed leading scaling corrections) are selected by Monte Carlo computation. The critical exponents are computed from high-temperature expansions specialized to these improved models. By the same technique we determine the coefficients of the small-magnetization expansion of the equation of state. This expansion is extended analytically by means of approximate parametric representations, obtaining the equation of state in the whole critical region. We also determine the specific-heat amplitude ratio.

cond-mat.stat-mech↗
arXiv2000

High-precision estimate of g4 in the 2D Ising model

We compute the renormalized four-point coupling in the 2d Ising model using transfer-matrix techniques. We greatly reduce the systematic uncertainties which usually affect this type of calculations by using the exact knowledge of several terms in the scaling function of the free energy. Our final result is g4=14.69735(3).

hep-th↗