Mario Gravina

Lattice investigation of the scalar mesons a0(980) and κ using four-quark operators

A bstractWe carry out an exploratory study of the isospin one a0(980) and the isospin one-half κ 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-type interpolating 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.

Strange and charm baryon masses with two flavors of dynamical twisted mass fermions

The masses of the low-lying strange and charm baryons are evaluated using two degenerate flavors of twisted mass sea quarks for pion masses in the range of about 260 MeV to 450 MeV. The strange and charm valence quark masses are tuned to reproduce the mass of the kaon and Dmeson at the physical point. The tree-level Symanzik improved gauge action is employed. We use three values of the lattice spacing, corresponding to β = 3.9, β = 4.05 and β = 4.2 with r0/a = 5.22(2), r0/a = 6.61(3) and r0/a = 8.31(5) respectively. We examine the dependence of the strange and charm baryons on the lattice spacing and strange and charm quark masses. The pion mass dependence is studied and physical results are obtained using heavy baryon chiral perturbation theory to extrapolate to the physical point.

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 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.

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.

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.

Investigation of light and heavy tetraquark candidates using lattice QCD

We review the status of an ongoing long-term lattice investigation of the spectrum and structure of tetraquark candidates. We focus on the light scalar meson a0(980). First steps regarding the study of a possibly existing cc tetraquark are also outlined.

BKT phase transitions in strongly coupled 3D Z(N) LGT at finite temperature

We investigate, both analytically and numerically, the phase diagram of three-dimensional Z(N) lattice gauge theories at finite temperature for N > 4. These models, in the strong coupling limit, are equivalent to a generalized version of vector Potts models in two dimension, with Polyakov loops playing the role of Z(N) spins. It is argued that the effective spin models have two phase transitions of infinite order (i.e. BKT). Using a cluster algorithm we confirm this conjecture, locate the position of the critical points and extract various critical indices.

Computation of correlation matrices for tetraquark candidates with

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^+

J^P = 0^+

and flavor structure

and flavor structure