arXiv subjects

P. Bicudo

Publications and source records attributed to P. Bicudo.

At least 19 recordsLinked to original sources

arXiv2021

Spectrum of very excited $Σ_g^+$ flux tubes in SU(3) gauge theory

Spectra with full towers of levels are expected due to the quantization of the string vibrations, however different theoretical models exist for the excitation spectra. First principle computations are important to test the different models and to search for novel phenomena, but so far only a few excited states of QCD flux tubes have been studied with pure gauge SU(3) lattice QCD in 3+1 dimensions. We thus aim to study a spectrum of flux tubes with static quark and antiquark sources up to a significant number of excitations. We specialize on the spectrum of the most symmetric case, namely $Σ_g^+$, where up to two levels are already published in the literature. To achieve the highest possible excitation level, we construct a large set of operators with the correct symmetry, solve the generalized eigenvalue problem and compare the results of different lattice QCD gauge actions with different lattice spacings and anisotropies.

hep-lat↗

arXiv2020

QCD vacuum replicas are metastable

A metastable phase has important physical implications, since it may form vacuum bubbles detectable experimentally, whereas an unstable vacuum instantly explodes. It is well known, due to chiral symmetry breaking, there are at least two very different QCD vacua. At T=0 and in the true vacuum, the scalar and pseudoscalar, or the vector and axial vector are not degenerate, and in the chiral limit, the pseudoscalar ground states are Goldstone bosons. At T=0 the chiral invariant vacuum is an unstable vacuum, decaying through an infinite number of scalar and pseudoscalar tachyons. Moreover QCD vacuum replicas, an infinite tower of excited vacuum solutions, which energy density lies between the true vacuum and the chiral invariant vacuum, have been predicted in the Coulomb gauge, due to the non-linearity of the mass gap equation with a confining interaction. It remained to show whether the QCD replicas are metastable or unstable. In this paper the spectrum of quark-antiquark systems, is studied in the true vacuum and in the first excited QCD replicas. The mass gap equation for the vacua and the Salpeter-RPA equation for the mesons are solved for a simple chiral invariant and confining quark model approximating QCD in the Coulomb gauge. We find no tachyons, thus showing the QCD replicas in our approach is indeed metastable. Moreover the energy spectra of the mesonic quark-antiquark systems in the first replicas are close to the one of the true vacuum.

hep-ph↗

arXiv2018

Colour field densities of the quark-antiquark excited flux tubes in SU(3) lattice QCD

We combine techniques previously utilised to study flux tube field density profiles and to study the excited spectrum of the gluonic fields produced by a static quark-antiquark pair. Working with pure gauge SU(3) fields discretised in a lattice, we utilise Wilson loops with a large basis of gluonic spacelike Wilson lines to include different excitations of the quark-antiquark flux tube. To increase the signal over noise ratio, we use the multihit technique in the temporal Wilson lines and the APE smearing in spatial Wilson lines. The number of gluonic operators combined with the space points where we compute the flux tube densities turns out to be very large, and we resort to GPUs and to CUDA codes. Computing the effective mass plot from the diagonalized correlation matrix, we separate the excitations with different two-dimensional angular momentum, parity and radial quantum numbers. We then compute the colour field density profiles for all the components of the colour electric and colour magnetic fields. We analyse our results for the first excitations of the flux tube and search for signals of novel phenomena beyond the Nambu-Goto string model, such as a longitudinal mode or an explicit gluon.

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arXiv2017

Pure gauge QCD flux tubes and their widths at finite temperature

We study the flux tubes produced by static quark-antiquark, quark-quark and quark-gluon charges at finite temperature in pure gauge SU(3) lattice QCD. Our sources are static and our lattice correlators are composed of fundamental and adjoint Polyakov loops. To signal the flux tubes, we compute the square densities of the chromomagnetic and chromoelectric fields with plaquettes, in a gauge invariant framework. We study the existence and non-existence of flux tubes both above and below the deconfinement phase transition temperature Tc. Using the Lagrangian density as a probability distribution, we also compute the widths of the flux tubes and study their widening as a function of the intercharge distance. We determine our results with both statistical and systematic errors. Our computations are performed in NVIDIA GPUs using the CUDA language.

hep-lat↗

arXiv2017

Lattice QCD static potentials of the meson-meson and tetraquark systems computed with both quenched and full QCD

We revisit the static potential for the $Q Q \bar Q \bar Q$ system using SU(3) lattice simulations, studying both the colour singlets groundstate and first excited state. We consider geometries where the two static quarks and the two anti-quarks are at the corners of rectangles of different sizes. We analyse the transition between a tetraquark system and a two meson system with a two by two correlator matrix. We compare the potentials computed with quenched QCD and with dynamical quarks. We also compare our simulations with the results of previous studies and analyze quantitatively fits of our results with anzatse inspired in the string flip-flop model and in its possible colour excitations.

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arXiv2016

Gluons at finite temperature

The gluon propagator is investigated at finite temperature via lattice simulations. In particular, we discuss its interpretation as a massive-type bosonic propagator. Moreover, we compute the corresponding spectral density and study the violation of spectral positivity. Finally, we explore the dependence of the gluon propagator on the phase of the Polyakov loop.

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arXiv2015

Chiral symmetry restoration in static-light mesons: chiral restoration theorem, the quark running mass m(k) and first chiral restoration signals in the lattice QCD spectra

Chiral symmetry restoration high in the hadron spectra is expected but it remains to be confirmed both in lattice QCD computations and in experiments. Recently, a theorem was derived, relating chiral symmetry restoration high in the hadron spectra to the spontaneous generation of the dynamical quark mass in QCD. We refine the theorem in the case of static-light mesons. Utilizing chiral quark model computations and lattice QCD results for the spectrum of mesons composed by a static antiquark and a light quark, we explore chiral symmetry restoration in the spectrum and the quark running mass m(k).

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arXiv2015

Tetraquark bound states and resonances in the unitary and microscopic triple string flip-flop quark model, the light-light-antiheavy-antiheavy $q q \bar Q\bar Q$ case study

We address $q q \bar Q\bar Q$ exotic tetraquark bound states and resonances with a fully unitarized and microscopic quark model. We propose a triple string flip-flop potential, inspired in lattice QCD tetraquarks static potentials and fluxtubes, combining meson-meson and tetraquark potentials. Our potential goes up to the color excited potential, but neglects spin-tensor potentials. To search for bound states and resonances, we first solve the two-body mesonic problem. Then we develop fully unitary techniques to address the four-body tetraquark problem. We fold the four-body Shcrödinger equation with the mesonic wavefunctions, transforming it into a two-body meson-meson problem with non-local potentials. We find bound states for some quark masses numbers, including the one reported in lattice QCD. Moreover, we also find resonances and calculate their masses and widths, by computing the $\mbox{T}$ matrix and finding it's pole positions in the complex energy plane, for some quantum numbers. However a detailed analysis of the quantum numbers where binding exists shows a discrepancy with recent lattice QCD results for the $l l \bar b \bar b$ tetraquark bound states. We conclude that the string flip-flop models need further improvement.

hep-ph↗

arXiv2015

The lattice gluon propagator in renormalizable $ξ$ gauges

We study the SU(3) gluon propagator in renormalizable $R_ξ$ gauges implemented on a symmetric lattice with a total volume of (3.25 fm)$^4$ for values of the guage fixing parameter up to $ξ=0.5$. As expected, the longitudinal gluon dressing function stays constant at its tree-level value $ξ$. Similar to the Landau gauge, the transverse $R_ξ$ gauge gluon propagator saturates at a non-vanishing value in the deep infrared for all values of $ξ$ studied. We compare with very recent continuum studies and perform a simple analysis of the found saturation with a dynamically generated effective gluon mass.

hep-lat↗

arXiv2014

Gluon screening mass at finite temperature from Landau gauge gluon propagator in lattice QCD

We address the interpretation of the Landau gauge gluon propagator at finite temperature as a massive type bosonic propagator. Using pure gauge SU(3) lattice simulations at a fixed lattice volume $\sim(6.5fm)^3$, we compute the electric and magnetic form factors, extract a gluon mass from Yukawa-like fits, and study its temperature dependence. This is relevant both for the Debye screening at high temperature $T$ and for confinement at low $T$.

hep-lat↗

arXiv2013

Flux tube widening in compact U (1) lattice gauge theory computed at T < Tc with the multilevel method and GPUs

We utilize Polyakov loop correlations to study d=3+1 compact U (1) flux tubes and the static electron-positron potential in lattice gauge theory. With the plaquette field operator, in U(1) lattice gauge theory, we probe directly the components of the electric and magnetic fields. In order to improve the signal-to-noise ratio in the confinement phase, we apply the Lüscher-Weiss multilevel algorithm. Our code is written in CUDA, and we run it in NVIDIA FERMI generation GPUs, in order to achieve the necessary efficiency for our computations. We measure in detail the quantum widening of the flux tube, as a function of the intercharge distance and at different finite temperatures T < Tc . Our results are compatible with the Effective String Theory.

hep-lat↗

arXiv2013

Inside the SU(3) quark-antiquark QCD flux tube: screening versus quantum widening

In lattice QCD, colour confinement manifests in flux tubes. We compute in detail the quark-antiquark flux tube for pure gauge SU(3) dimension $D=3+1$ for quark-antiquark distances R ranging from 0.4 fm to 1.4 fm. To increase the signal over noise ratio, we apply the improved multihit and extended smearing techniques. We detail the gauge invariant squared components of the colour electric and colour magnetic fields both in the mediator plane between the static quark and static antiquark and in the planes of the sources. We fit the field densities with appropriate ansatze and we observe the screening of the colour fields in all studied planes together with the quantum widening of the flux tube in the mediator plane. All components squared of the colour fields are non-vanishing and are consistent with a penetration length lambda ~ 0.22 to 0.24 fm and an effective screening mass mu ~ 0.9 to 0.8 GeV. The quantum widening of the flux tube is well fitted with a logarithmic law in R.

hep-lat↗

arXiv2012

SU(3) gauge invariant lattice QCD exploration of the dual superconductor picture in flux tube fusion, in the dual gluon mass, and in the dual Ginzburg-Landau parameters

The colour fields, created by a static gluon-quark-antiquark system, are computed in quenched SU(3) lattice QCD, in a $24^3\times 48$ lattice at $β=6.2$ and $a=0.07261(85)\,fm$. We compute the hybrid Wilson Loop including the cases when the gluon and the antiquark are superposed, i. e., the quark-antiquark case and when the quark and antiquark are superposed, i. e., the gluon-gluon case. The Casimir scaling is investigated, in the two gluon glueball case the Casimir scaling is consistent with the formation of an adjoint string. Measuring the decay of the tail in the mid section of the flux tube for the two gluon glueball and for the quark-antiquark meson, we determine the penetration length and present a gauge invariant effective dual gluon mass of $0.905\pm0.163\,\text{GeV}$. We also try to determine the coherence length comparing our results with the dual Ginzburg-Landau approach. With the penetration length and the possible coherence length we determine a putative Ginzburg-Landau dimensionless parameter, which is possibly consistent with a type II superconductor picture. These results are obtained at fixed quark-antiquark distance of 0.58 fm.

hep-lat↗

arXiv2012

Variational study of the flux tube recombination in the two quarks and two quarks system in Lattice QCD

The color fields in a system composed by two static quarks and two static antiquarks are studied. In particular, we consider the four particles in the corners of a rectangle, and two possible alignment of the particles, one in which the quarks are at the same side of the rectangle, and the other where they are at opposite sides. We use a variational method, to probe not only the ground state but also the first excited state. This results permit us to observe and interpret the flux-tube recombination in the mesons to mesons and the tetraquark to mesons transitions, for both states. The results are compared with previous results for the static potential and the Casimir scaling predictions.

hep-lat↗

arXiv2012

Lattice QCD computation of the SU(3) String Tension critical curve

We investigate the critical curve of the string tension sigma(T) as a function of temperature in quenched gauge invariant SU(3) lattice gauge theory. We extract sigma(T) from the colour averaged free energy of a static quark-antiquark pair. To compute the free energy, we utilize a pair of gauge invariant Polyakov loop and antiloop correlations, and apply the multihit procedure to enhance the signal to noise ratio. We find that the string tension departs from the zero temperature sigma(0) at T close to 0.5 Tc. We cover the relevant temperature range from 0.5 Tc up to the confinement temperature Tc using 57 different sets of pure gauge lattice configurations with four temporal extensions (4,6,8,12), different beta and a spatial volume of 48^3 in lattice units.

hep-lat↗

arXiv2012

Computation of pion and kaon heavy ion multiplicities in a gluon-meson model

In high energy Heavy Ion Collisions, the onset of the quark-gluon plasma is the colour glass condensate, dominated by gluons. The final state is hadronic, and dominated by pions and kaons. Here we investigate an effective approach of QCD with these bosonic fields and which can help to describe the transition of gluons into light mesons. Formally, our approach consists in integrating out the quark fields from the QCD path integral, utilizing auxiliary fields. In this way the fermionic fields are replaced by light mesons, such as the pions and sigma field. We apply our effective action to compute the number of pions and kaons per gluon emitted by a Boltzmann gluon gas, their multiplicities as function of a gluon mass. We conclude that an effective gluon mass remains finite at T=Tc.

hep-ph↗

arXiv2011

Physics Prospects at FAIR

We review the physics potential at FAIR in the light of the existing data of the RHIC-BES program and the NA49/NA61 beam and system size scan. Special emphasize will be put on the potential of fluctuations, as well as dilepton observables.

hep-ph↗

arXiv2011

Chiral symmetry crossover with a linear confining and temperature dependent quark-antiquark potential

Recently we developed a numerical technique to compute chiral symmetry breaking at T=0 with different current quark masses m0, including the current quark masses of the six standard flavours u, d, s, c, b, t. We also fitted from Lattice QCD data the quark-antiquark string tension sigma dependence on temperature T. We now utilize sigma(T) to further upgrade the chiral invariant and confinement quark model to finite temperatures T =/= 0. We study the quark mass at finite T and obtain the corresponding chiral crossover at T=Tc. The quark mass critical curve has a shape similar, but not identical, to the string tension critical curve. In the case of the lightest quarks, the quark mass and the chiral condensate essentially vanish at T=Tc, except for the small explicit chiral symmetry breaking mu and md.

hep-ph↗