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Mario Gravina

Publications and source records attributed to Mario Gravina.

At least 19 recordsLinked to original sources
arXiv2016

Importance of closed quark loops for lattice QCD studies of tetraquarks

To investigate the light scalar tetraquark candidate $a_0(980)$ (quantum numbers $J^P =0^+$), a correlation matrix including a variety of two- and four-quark interpolating operators has to be computed. We discuss efficient techniques to compute the elements of this correlation matrix, in particular diagrams with closed quark loops. Furthermore, we present evidence that such diagrams are not negligible given our precision, and their contribution is essential to obtain physically meaningful results. In particular, we find indications of the existence of an "additional" state around the two-particle thresholds of $K+ \bar K$ and $η+π$, which could correspond to the $a_0(980)$ meson.

hep-lat↗
arXiv2016

Plasmon modes of graphene nanoribbons with periodic planar arrangements

Among their amazing properties, graphene and related low-dimensional materials show quantized charge-density fluctuations--known as plasmons--when exposed to photons or electrons of suitable energies. Graphene nanoribbons offer an enhanced tunability of these resonant modes, due to their geometrically controllable band gaps. The formidable effort made over recent years in developing graphene-based technologies is however weakened by a lack of predictive modeling approaches that draw upon available {\it ab initio} methods. An example of such a framework is presented here, focusing on narrow-width graphene nanoribbons organized in periodic planar arrays. Time-dependent density-functional calculations reveal unprecedented plasmon modes of different nature at visible to infrared energies. Specifically, semimetallic~(zigzag) nanoribbons display an intraband plasmon following the energy-momentum dispersion of a two-dimensional electron gas. Semiconducting~(armchair) nanoribbons are instead characterized by two distinct intraband and interband plasmons, whose fascinating interplay is extremely responsive to either injection of charge carriers or increase in electronic temperature. These oscillations share some common trends with recent nanoinfrared imaging of confined edge and surface plasmon modes detected in graphene nanoribbons of $100$-$500$~nm width.

cond-mat.str-el↗
arXiv2015

Computation of correlation matrices for tetraquark candidates with $J^P = 0^+$ and flavor structure $q_1 \bar{q_2} q_3 \bar{q}_3$

The conjecture that several recently observed mesons have a structure, which is not dominated by an ordinary quark-antiquark pair, but by a four-quark structure, is being actively investigated both theoretical and experimentally. Such a state may be characterized as a mesonic molecule or as a diquark-antidiquark pair. Lattice QCD provides a theoretically sound framework to study such states. To quantitatively investigate the internal structure of such mesons, one needs to precisely compute correlation matrices containing several interpolating operators including two and four quarks. Here we discuss certain technical aspects of such correlation matrices suited to study tetraquark candidates with $J^P = 0^+$ and flavor structure $q_1 \bar{q_2} q_3 \bar{q}_3$, e.g.\ the $a_0(980)$ meson, the $D_{s0}^\ast$ meson and some of the charged $c \bar{c}$ $X$ states. Some numerical results for the $a_0(980)$ meson are presented.

hep-lat↗
arXiv2015

Deconfinement and universality in the 3D U(1) lattice gauge theory at finite temperature: study in the dual formulation

We study analytically and numerically the three-dimensional U(1) lattice gauge theory at finite temperature in the dual formulation. For an appropriate disorder operator, we obtain the renormalization group equations describing the critical behavior of the model in the vicinity of the deconfinement phase transition. These equations are used to check the validity of the Svetitsky-Yaffe conjecture regarding the critical behavior of the lattice U(1) model. Furthermore, we perform numerical simulations of the model for $N_t = 1, 2, 4, 8$ and compute, by a cluster algorithm, the dual correlation functions and the corresponding second moment correlation length. In this way we locate the position of the critical point and calculate critical indices.

hep-lat↗
arXiv2014

Investigation of the tetraquark candidate $a_0(980)$: technical aspects and preliminary results

We discuss technical aspects and first results of a lattice QCD study of the $a_0(980)$ state. We employ various interpolating operators of quark-antiquark, mesonic molecule, diquark-antidiquark and two-meson type. Both connected and disconnected contributions including diagrams with closed fermion loops are computed. To keep statistical errors small, it is essential to optimize the computation of these diagrams by choosing that combination of techniques most appropriate for each type of diagram from the correlation matrix of interpolating operators. We illustrate, how this can be done, by discussing certain diagrams in detail. We also present preliminary results corresponding to a $4\times 4$ submatrix computed with 2+1 flavors of clover fermions.

hep-lat↗
arXiv2014

Critical behavior and continuum scaling of $3D$ $Z(N)$ lattice gauge theories

Three-dimensional $Z(N)$ lattice gauge theories are studied numerically at finite temperature for $N$ = 5, 6, 8, 12, 13, 20 and for $N_t$=2,4,8. For each model the location of phase transitions and its critical indices are determined. The scaling of critical points with $N$ is proposed. The data obtained enable us to verify the scaling near the continuum limit for the $Z(N)$ models at finite temperatures.

hep-lat↗
arXiv2014

Phase structure of 3D Z(N) lattice gauge theories at finite temperature: large-N and continuum limits

We study numerically three-dimensional Z(N) lattice gauge theories at finite temperature, for N = 5, 6, 8, 12, 13 and 20 on lattices with temporal extension $N_t$ = 2, 4, 8. For each model, we locate phase transition points and determine critical indices. We propose also the scaling of critical points with N. The data obtained enable us to verify the scaling near the continuum limit for the Z(N) models at finite temperatures.

hep-lat↗
arXiv2013

Phase transitions in the three-dimensional Z(N) models

Phase transitions in zero-temperature 3D Z(N) lattice gauge theories are studied. We use a cluster algorithm defined for the dual formulation of the models. We also attempt to explain the nature of the intermediate continuously symmetric phase, which appears for N>5. The critical indices are calculated. The results obtained are used to study the scaling of critical points with N, as well as the scaling of finite-temperature critical points with the lattice size in the time direction, $N_t$.

hep-lat↗
arXiv2013

Critical properties of 3D Z(N) lattice gauge theories at finite temperature

The phase structure of three-dimensional Z(N>4) lattice gauge theories at finite temperature is investigated. Using the dual formulation of the models and a cluster algorithm we locate the critical points of the two transitions, determine various critical indices and compute average action and specific heat. Results are consistent with two transitions of infinite order, belonging to the universality class of two-dimensional Z(N) vector spin models.

hep-lat↗
arXiv2013

Study of the a_0(980) on the lattice

We present lattice results for the a_0(980) state using a variational approach with a variety of creation operators: quark-antiquark, mesonic molecule, diquark-antidiquark as well as two-meson type. Focus is put on recent technical advances, in particular the computation of singly disconnected diagrams.

hep-lat↗
arXiv2013

Scalar mesons and tetraquarks from twisted mass lattice QCD

We study light scalar mesons with particular focus on the a_0(980) using lattice QCD with 2+1+1 dynamical quark flavors. To investigate the structure of these scalar mesons and to identify, whether a sizeable tetraquark component is present, we use a large set of operators, including diquark-antidiquark, mesonic molecule and two-meson operators. We find that the low-lying states overlap essentially exclusively with two-meson states. This indicates that in the channels investigated no tightly bound four-quark states of either molecular or diquark-antidiquark type exist.

hep-lat↗
arXiv2012

Scalar mesons and tetraquarks by means of lattice QCD

We study the light scalar mesons a_0(980) and kappa using N_f = 2+1+1 flavor lattice QCD. In order to probe the internal structure of these scalar mesons, and in particular to identify, whether a sizeable tetraquark component is present, we use a large set of operators, including diquark-antidiquark, mesonic molecule and two-meson operators. The inclusion of disconnected diagrams, which are technically rather challenging, but which would allow us to extend our work to e.g. the f_0(980) meson, is introduced and discussed.

hep-lat↗
arXiv2012

Lattice investigation of the scalar mesons a_0(980) and κ using four-quark operators

We carry out an exploratory study of the isospin one a0(980) and the isospin one-half kappa scalar mesons using Nf=2+1+1 Wilson twisted mass fermions at one lattice spacing. The valence strange quark is included as an Osterwalder-Seiler fermion with mass tuned so that the kaon mass matches the corresponding mass in the unitary Nf=2+1+1 theory. We investigate the internal structure of these mesons by using a basis of four-quark interpolating fields. We construct diquark-diquark and molecular-typecinterpolating fields and analyse the resulting correlation matrices keeping only connected contributions. For both channels, the low-lying spectrum is found to be consistent with two-particle scattering states. Therefore, our analysis shows no evidence for an additional state that can be interpreted as either a tetraquark or a tightly-bound molecular state.

hep-lat↗
arXiv2012

Lattice investigation of the tetraquark candidates a0(980) and kappa

It is a long discussed issue whether light scalar mesons have sizeable four-quark components. We present an exploratory study of this question using N_f=2+1+1 twisted mass lattice QCD. A mixed action approach ignoring disconnected contributions is used to calculate correlator matrices consisting of mesonic molecule, diquark-antidiquark and two-meson interpolating operators with quantum numbers of the scalar mesons a0(980) (1(0^++)) and kappa (1/2(0+)). The correlation matrices are analyzed by solving the generalized eigenvalue problem. The theoretically expected free two-particle scattering states are identified, while no additional low lying states are observed. We do not observe indications for bound four-quark states in the channels investigated.

hep-lat↗
arXiv2011

Critical properties of 2D Z(N) vector models for N>4

We investigate the critical properties of two-dimensional Z(N) vector models for N larger than 4. In particular, critical points of the two phase transitions are located and some critical indices are determined. We study also the behavior of the helicity modulus and the dependence of the critical points on N.

hep-lat↗
arXiv2011

Numerical study of the phase transitions in the two-dimensional Z(5) vector model

We investigate the critical properties of the two-dimensional Z(5) vector model. For this purpose, we propose a new cluster algorithm, valid for Z(N) models with odd values of N. The two-dimensional Z(5) vector model is conjectured to exhibit two phase transitions with a massless intermediate phase. We locate the position of the critical points and study the critical behavior across both phase transitions in details. In particular, we determine various critical indices and compare the results with analytical predictions.

hep-lat↗
arXiv2011

Critical properties of the 2D Z(5) vector model

The two-dimensional Z(5) vector model is investigated through the determination of critical points and one critical index. To this purpose a new cluster algorithm has been developed valid for 2D Z(N) models with odd values of N. Results are compared with analytical predictions.

hep-lat↗