arXiv subjects

M. D'Elia

Publications and source records attributed to M. D'Elia.

At least 91 records · Page 5Linked to original sources

arXiv1999

Efficiency of the UV-filtered Multiboson algorithm

We study the efficiency of an improved Multiboson algorithm with two flavours of Wilson fermions in a realistic physical situation ($β= 5.60$, $κ= 0.156$ on a $16^3 \times 24$ lattice). The performance of this exact algorithm is compared with that of a state-of-the-art HMC algorithm: a considerable improvement is obtained for the plaquette auto-correlation time, while the two algorithms appear similarly efficient at decorrelating the topological charge.

hep-lat↗

arXiv1999

On the relevance of center vortices to QCD

In a numerical experiment, we remove center vortices from an ensemble of lattice SU(2) gauge configurations. This removal adds short-range disorder. Nevertheless, we observe long-range order in the modified ensemble: confinement is lost and chiral symmetry is restored (together with trivial topology), proving that center vortices are responsible for both phenomena. As for the Abelian monopoles, they survive but their percolation properties are lost.

hep-lat↗

arXiv1998

Gauge-invariant nonlocal quark condensates in QCD

We study, by numerical simulations on a lattice, the behaviour of the gauge-invariant nonlocal quark condensates in the QCD vacuum both in the quenched approximation and with four flavours of dynamical staggered fermions. The correlation length of the condensate is determined to be roughly twice as big as in the case of the gluon field strength correlators.

hep-lat↗

arXiv1998

Gauge-invariant quark-antiquark nonlocal condensates in lattice QCD

We study, by numerical simulations on a lattice, the behaviour of the gauge-invariant quark-antiquark nonlocal condensates in the QCD vacuum with dynamical fermions. A determination is also done in the quenched approximation and the results are compared with the full-QCD case. The fermionic correlation length is extracted and compared with the analogous gluonic quantity.

hep-lat↗

arXiv1998

Decorrelating Topology with HMC

The investigation of the decorrelation efficiency of the HMC algorithm with respect to vacuum topology is a prerequisite for trustworthy full QCD simulations, in particular for the computation of topology sensitive quantities. We demonstrate that for mpi/mrho ratios <= 0.69 sufficient tunneling between the topological sectors can be achieved, for two flavours of dynamical Wilson fermions close to the scaling region beta=5.6. Our results are based on time series of length 5000 trajectories.

hep-lat↗

arXiv1998

Gauge invariant Field Strength Correlators in QCD

Gauge invariant correlators in QCD are studied on the lattice.A Systematic determination of the correlation lengths for gluon field strength correlators and quark correlators is made.The measurement of the gluon and quark condensates is discussed.

hep-lat↗

arXiv1998

A critical comparison of different definitions of topological charge on the lattice

A detailed comparison is made between the field-theoretic and geometric definitions of topological charge density on the lattice. Their renormalizations with respect to continuum are analysed. The definition of the topological susceptibility, as used in chiral Ward identities, is reviewed. After performing the subtractions required by it, the different lattice methods yield results in agreement with each other. The methods based on cooling and on counting fermionic zero modes are also discussed.

hep-lat↗

arXiv1998

Topological properties of full QCD at the phase transition

We investigate the topological properties of the QCD vacuum with 4 flavours of dynamical staggered fermions at finite temperature. To calculate the topological susceptibility we use the field-theoretical method. As in the quenched case, a sharp drop is observed for the topological susceptibility across the phase transition.

hep-lat↗

arXiv1998

Scanning the Topological Sectors of the QCD Vacuum with Hybrid Monte Carlo

We address a long standing issue and determine the decorrelation efficiency of the Hybrid Monte Carlo algorithm (HMC), for full QCD with Wilson fermions, with respect to vacuum topology. On the basis of five state-of-the art QCD vacuum field ensembles (with 3000 to 5000 trajectories each and m_pi/m_rho-ratios in the regime >0.56, for two sea quark flavours) we are able to establish, for the first time, that HMC provides sufficient tunneling between the different topological sectors of QCD. This will have an important bearing on the prospect to determine, by lattice techniques, the topological susceptibility of the vacuum, and topology sensitive quantities like the spin content of the proton, or the eta' mass.

hep-lat↗

arXiv1997

Topology in QCD

Topology on the lattice is reviewed. In quenched QCD topological susceptibility chi is fully understood. The Witten-Veneziano mechanism for the eta' mass is confirmed. The topological susceptibility drops to zero at the deconfining phase transition. Preliminary results are also presented for chi and chi' in full QCD, and for the spin content of the proton. The only problem there is the difficulty of the usual Hybrid Monte Carlo algorithm to bring topology to equilibrium.

hep-lat↗

arXiv1997

The spin content of the proton in full QCD

We present preliminary results on the proton spin structure function in full QCD. The measurement has been done using 4 flavours of staggered fermions and an improved definition of the lattice topological charge density.

hep-lat↗

arXiv1997

Topology in SU(2) Yang-Mills theory

New results on the topology of the SU(2) Yang-Mills theory are presented. At zero temperature we obtain the value of the topological susceptibility by using the recently introduced smeared operators as well as a properly renormalized geometric definition. Both determinations are in agreement. At non-zero temperature we study the behaviour of the topological susceptibility across the confinement transition pointing out some qualitative differences with respect to the analogous result for the SU(3) gauge theory.

hep-lat↗

arXiv1997

Topology at zero and finite T in SU(2) Yang-Mills theory

We determine the topological susceptibility χat T=0 and its behaviour at finite T across the deconfining transition in pure SU(2) gauge theory. We use an improved topological charge density operator. χgoes to zero above T_c, but more slowly than in SU(3) gauge theory.

hep-lat↗

arXiv1996

New results on topological susceptibility in SU(3) gauge theory

We survey recent lattice results on QCD topological properties. The behaviour of the topological susceptibility at the deconfining phase transition has been determined. This advance has been made possible by an i) an improvement of the topological charge operator and ii) a non-perturbative determination of renormalizations.

hep-lat↗

arXiv1996

Renormalization group flow and fixed point of the lattice topological charge in the 2-d O(3) sigma-model

We study the renormalization group evolution up to the fixed point of the lattice topological susceptibility in the 2-d O(3) non-linear sigma-model. We start with a discretization of the continuum topological charge by a local charge density, polynomial in the lattice fields. Among the different choices we propose also a Symanzik--improved lattice topological charge. We check step by step in the renormalization group iteration the progressive dumping of quantum fluctuations, which are responsible for the additive and multiplicative renormalizations of the lattice topological susceptibility with respect to the continuum definition. We find that already after three iterations these renormalizations are negligible and an excellent approximation of the fixed point is achieved. We also check by an explicit calculation that the assumption of slowly varying fields in iterating the renormalization group does not lead to a good approximation of the fixed point charge operator.

hep-lat↗

arXiv1996

Hybrid Monte Carlo and topological modes of full QCD

We investigate the performance of the hybrid Monte Carlo algorithm, the standard algorithm used for lattice QCD simulations involving fermions, in updating non-trivial global topological structures. We find that the hybrid Monte Carlo algorithm has serious problems decorrelating the global topological charge at the values of $β$ and $m$ currently simulated, where continuum physics should be approximately realized. This represents a warning which must be seriously considered when simulating full QCD by hybrid Monte Carlo.

hep-lat↗

arXiv1996

Decorrelating the topology in full QCD

We investigate the performance of the hybrid Monte Carlo algorithm in updating non-trivial global topological structures. We find that the hybrid Monte Carlo algorithm has serious problems decorrelating the global topological charge. This represents a warning which must be seriously considered when simulating full QCD, regardless of the number and type of fermions, with this or any similar algorithm. Simulated tempering is examined as a means of accelerating the decorrelation.

hep-lat↗

arXiv1996

Topological susceptibility through the deconfining phase transition

We present a measurement of the topological susceptibility in SU(3) Yang-Mills theory through the deconfinement phase transition. An improved operator is used for the topological charge density. A drop by an order of magnitude is observed from the confined to the deconfined phase.

hep-lat↗