arXiv subjects

Guido Cossu

Publications and source records attributed to Guido Cossu.

At least 37 records · Page 2Linked to original sources

arXiv2015

On the axial $U(1)$ symmetry at finite temperature

We study the $U(1)_A$ anomaly in two-flavor lattice QCD at finite temperature with the Möbius domain-wall Dirac operator. We generate gauge configurations in the temperature range $(0.9, 1.2) T_c$ on different physical volumes, $L=$ 2--4 fm, and lattice spacings. We measure the difference of the susceptibilities of the flavor non-singlet scalar ($χ_δ$) and pseudoscalar ($χ_π$) mesons. They are related by an axial $U(1)$ transformation and the difference vanishes if the axial $U(1)$ symmetry is respected. We identify the source of axial $U(1)$ symmetry breaking at finite temperature in the lowest eigenmodes, for the observable $χ_π- χ_δ$. We then reweight the Möbius domain-wall fermion partition function to that of the overlap-Dirac operator to fully recover chiral symmetry. Our data show a significant discrepancy in the results coming from the Möbius domain-wall valence quarks, the overlap valence quarks on our DWF configurations and the reweighted ones that have full chiral symmetry. After recovering full chiral symmetry we conclude that the difference $χ_π- χ_δ$ shows a suppression in the chiral limit that is compatible with an effective restoration of $U(1)_A$ at $T \gtrsim T_c$ in the scalar meson channels.

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arXiv2015

Violation of chirality of the Möbius domain-wall Dirac operator from the eigenmodes

We investigate the effects of the violation of the Ginsparg-Wilson (GW) relation in the Möbius domain-wall fermion formulation on the lattice with finite fifth dimension. Using a decomposion in terms of the eigenmodes of its four-dimensional effective Dirac operator, we isolate the GW-violating terms for various physical quantities including the residual mass and the meson susceptibilities relevant for the effective restoration of the axial U(1) symmetry at finite temperature. Numerical result shows that the GW-violating effect is more significant, or even overwhelming, for the quantities that are dominated by the low-lying eigenmodes.

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arXiv2015

Partial restoration of chiral symmetry in the color flux tube

Using the quark eigenmodes computed on the lattice with the overlap-Dirac operator, we investigate the spatial distribution of the chiral condensate around static color sources corresponding to quark-antiquark and three-quark systems. A flux structure of chromo fields appears in the presence of such color charges. The magnitude of the chiral condensate is reduced inside the color flux, which implies partial restoration of chiral symmetry inside hadrons. Taking a static baryon source in a periodic box as a toy model of nuclear matter, we estimate the magnitude of the chiral symmetry restoration as a function of baryon matter density.

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arXiv2014

Effects of near-zero Dirac eigenmodes on axial U(1) symmetry at finite temperature

We study the axial U(1)A symmetry of Nf = 2 QCD at finite temperature using the Dirac eigenvalue spectrum. The gauge configurations are generated employing the Mobius domain-wall fermion action on 16^3x8 and 32^3x8 lattices. The physical spatial size of these lattices is around 2 fm and 4 fm, respectively, and the simulated temperature is around 200 MeV, which is slightly above the critical temperature of the chiral phase transition. Although the Mobius domain-wall Dirac operator is expected to have a good chiral symmetry and our data actually show small values of the residual mass, we observe significant violation of the Ginsparg-Wilson relation for the low- lying eigenmodes of the Mobius domain-wall Dirac operator. Using the reweighting technique, we compute the overlap-Dirac operator spectrum on the same set of configurations and find a significant difference of the spectrum between the two Dirac operators for the low-lying eigenvalues. The overlap-Dirac spectrum shows a gap from zero, which is insensitive to the spacial volume.

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arXiv2014

Axial U(1) symmetry at finite temperature with Möbius domain-wall fermions

We investigate the axial $U(1)$ symmetry restoration at finite temperature in two flavor QCD. We employ the Möbius domain-wall formalism that is designed to achieve good chiral symmetry. We show the measurements of a difference of meson susceptibilities, sensitive to the $U(1)_A$ symmetry breaking. The signal is dominated by zero and near-zero modes. By reweighting the measure to that of overlap fermions we find a suppression of the $U(1)_A$ breaking effects above the chiral transition temperature.

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arXiv2014

Partial restoration of chiral symmetry inside hadrons

By using the overlap-Dirac operator eigenmodes, we investigate spatial distribution of the chiral condensate around static color sources for both quark-antiquark and three quark systems. In the presence of color sources, a characteristic flux-tube structure appears among them, suggesting a linear confining potential. We show that the magnitude of the condensate is reduced inside the color flux, which indicates the partial restoration of chiral symmetry inside the "hadrons." Considering a periodic box containing a static baryon source, which mimics the "nuclear matter," we estimate the chiral symmetry restoration in the presence of finite baryon number density.

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arXiv2014

Polyakov loops and the Hosotani mechanism on the lattice

We explore the phase structure and symmetry breaking in four-dimensional SU(3) gauge theory with one spatial compact dimension on the lattice ($16^3 \times 4$ lattice) in the presence of fermions in the adjoint representation with periodic boundary conditions. We estimate numerically the density plots of the Polyakov loop eigenvalues phases, which reflect the location of minima of the effective potential in the Hosotani mechanism. We find strong indication that the four phases found on the lattice correspond to SU(3)-confined, SU(3)-deconfined, SU(2) x U(1), and U(1) x U(1) phases predicted by the one-loop perturbative calculation. The case with fermions in the fundamental representation with general boundary conditions, equivalent to the case of imaginary chemical potentials, is also found to support the $Z_3$ symmetry breaking in the effective potential analysis.

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arXiv2014

Lattice QCD study of partial restoration of chiral symmetry in the flux-tube

Using the overlap-Dirac eigenmodes, we study the spatial distribution of the chiral condensate around static color sources in lattice QCD. Between the color sources, there appears a color-flux tube, which leads a linear confining potential. By measuring a local value of the chiral condensate, we show that the magnitude of the condensate is reduced inside the flux-tube for both quark-antiquark and three-quark systems. These results suggest that chiral symmetry is partially restored in the flux-tube. The reduction of the condensate is estimated to be about 20 $\sim$ 30% at the center of the flux.

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arXiv2013

Effective $θ$ term by CP-odd electromagnetic background fields

We discuss our study of QCD in the presence of CP-odd electromagnetic (e.m.) background fields. We investigate the propagation of the CP-odd term from the e.m. sector to the strong sector, inducing an effective $θ$ term. We discuss the method we have used in our lattice QCD simulations, and the results of our analysis, which are relevant to the determination of the effective pseudoscalar QED-QCD interactions. We also explore how these CP-odd e.m. background fields influence the number of the Dirac zero modes in our configurations.

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arXiv2013

Hosotani mechanism on the lattice

We explore the phase structure and symmetry breaking in four-dimensional SU(3) gauge theory with one spatial compact dimension on the lattice in the presence of fermions in the adjoint and fundamental representations with general boundary conditions. The eigenvalue phases of Polyakov loops and the associated susceptibility are measured on 16^3 x 4 lattice. We establish a correspondence between the phases found on the lattice and the gauge symmetry breaking by the Hosotani mechanism.

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arXiv2013

JLQCD IroIro++ lattice code on BG/Q

We describe our experience on the multipurpose C++ code IroIro++ designed for JLQCD to run on the BG/Q installation at KEK. We discuss some details on the performance improvements specific for the IBM Blue Gene Q.

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arXiv2013

Analysis of topological structure of the QCD vacuum with overlap-Dirac operator eigenmode

Using the eigenmodes of the overlap-Dirac operator, we study the topological structure of the QCD vacuum. We investigate the space-time profile of the low-lying eigenmodes and their contribution to the vacuum action density and chiral condensate under the existence of static color sources. We demonstrate that the low-lying Dirac eigenmode shows the flux-tube structure, which suggests the relevance to confinement. We also analyze the chiral condensate in the flux-tube. Chiral symmetry is partially restored inside the flux, and the reduction of the condensate is about 20% at the center of the tube.

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arXiv2013

Finite temperature study of the axial U(1) symmetry on the lattice with overlap fermion formulation

We examine the axial U(1) symmetry near and above the finite temperature phase transition in two-flavor QCD using lattice QCD simulations. Although the axial U(1) symmetry is always violated by quantization, (i.e.) the chiral anomaly, the correlation functions may manifest effective restoration of the symmetry in the high temperature phase. We explicitly study this possibility by calculating the meson correlators as well as the Dirac operator spectral density near the critical point. Our numerical simulations are performed on a $16^3\times 8$ lattice with two flavors of dynamical quarks represented by the overlap fermion formalism. Chiral symmetry and its violation due to the axial anomaly is manifestly realized with this formulation, which is a prerequisite for the study of the effective restoration of the axial U(1) symmetry. In order to avoid discontinuity in the gauge configuration space, which occurs for the exactly chiral lattice fermions, the simulation is confined in a fixed topological sector. It induces finite volume effect, which is well described by a formula based on the Fourier transform from the $θ$-vacua. We confirm this formula at finite temperature by calculating the topological susceptibility in the quenched theory. Our two flavor simulations show degeneracy of the meson correlators and a gap in the Dirac operator spectral density, which implies that the axial U(1) symmetry is effectively restored in the chirally symmetric phase.

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arXiv2012

Topological susceptibility and axial symmetry at finite temperature

We consider the simulation of finite temperature QCD with two flavors of dynamical overlap fermions in order to study the suppression of the axial U(1) symmetry breaking at the chiral phase transition point. As a preliminary study, pure gauge simulations are performed to investigate how fixing the topology affects physical quantities like the topological susceptibility, $χ_t$, at finite temperature, showing that it is possible to reconstruct known results from the fixed topology sector. First results on the degeneracy of meson correlators in the high temperature QGP sector are shown.

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arXiv2012

The disorder parameter of dual superconductivity in QCD revisited

We discover the origin of the pathologies of the disorder parameter used in previous papers to detect dual superconductivity of QCD vacuum, and we remove them by defining an improved disorder parameter. A check of the approach is made by numerical simulations of SU(2) gauge theory, which demonstrate that the approach is consistent and with it that deconfinement is a transition from dual superconductor to normal.

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arXiv2011

QCD simulations with staggered fermions on GPUs

We report on our implementation of the RHMC algorithm for the simulation of lattice QCD with two staggered flavors on Graphics Processing Units, using the NVIDIA CUDA programming language. The main feature of our code is that the GPU is not used just as an accelerator, but instead the whole Molecular Dynamics trajectory is performed on it. After pointing out the main bottlenecks and how to circumvent them, we discuss the obtained performances. We present some preliminary results regarding OpenCL and multiGPU extensions of our code and discuss future perspectives.

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arXiv2011

The Roberge-Weiss endpoint in N_f = 2 QCD

We present the results of extensive simulations regarding the critical behavior at the endpoint of the Roberge-Weiss transition for N_f = 2 QCD. We confirm early evidence, presented in arXiv:0909.0254, according to which the Roberge-Weiss endpoint is first order in the limit of large or small quark masses, and second order for intermediate masses. A systematic study of the transition strength as a function of the quark mass in the first order regions, permits us to estimate the tricritical values of the quark mass separating the second order region from the first order ones.

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arXiv2011

Toward a well defined monopole creation operator

The lattice implementation of monopole creation operator proves to have problems related to bulk transitions that can possibly affect the interpretation of its mean value as an order parameter for monopole condensation. Preliminary evidence is presented that these unexpected behaviours are in fact only due to lattice artefacts and do not spoil the physical interpretation of the monopole operator.

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