Light exotics: Molecular resonances

Abstract

Highlights in the search for nonconventional (non qq) meson states are the π 1 (1400) and π 1 (1600) exotic candidates. Should they exist, mounting theoretical arguments suggest that they are tetraquark molecular resonances excitable by meson rescattering. We report a new tetraquark calculation within a model field theory approximation to Quantum Chromodynamics in the Coulomb gauge supporting this conjecture. We also strengthen this claim by consistently contrasting results with exotic state predictions for hybrid (qqg) mesons within the same theoretical framework. Our findings confirm that molecular-like configurations involving two color singlets (a resonance, not a bound state) are clearly favored over hybrid or colorexotic tetraquark meson (qqqq atoms) formation. Finally, to assist needed further experimental searches we document a useful off-plane correlator for establishing the structure of these exotic systems along with similar, but anticipated much narrower, states that should exist in the charmonium and bottomonium spectra.

Color fields computed in SU(3) lattice QCD for the static tetraquark system

“…In principle a X-shaped flux-tube as in Here we study the colour fields for the static tetraquark system, with the aim of observing the tetraquark flux tubes suggested by these static potential computations. The study of the colour fields in a tetraquark is important to discriminate between different multi-quark Hamiltonian models, quark models with two-body interactions only [22], from flip-flop models with a multi-body potential [16]. Unlike the colour fields of simpler few-body systems, say mesons, baryons and hybrids, [23][24][25][26], the tetraquark fields have not been previously studied in lattice QCD.…”

Glueballs, hybrids, multiquarks

“…Depending on the choice of free parameters of the model, various resonant-like amplitudes could be generated in different channels. General Wang et al [963] use an Hamiltonian approach to arrive at the conclusion that molecular-like configurations involving two color singlets are clearly favoured compared to hybrid (tetraquark) configurations in which aqq pair (or two pairs) carry colour.…”

Bottom-charmed meson spectrum from a QCD approach based on the Tamm-Dancoff approximation

“…The vacuum is represented as a coherent BCS ground state with quark and gluon Cooper pairs (condensates), and the hadrons interpreted as quasiparticle excitations. This approach has been successfully applied to the description of properties of some types of light and heavy mesons, glueballs, gluelumps, hybrids, and tetraquarks [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47]. These reports demonstrate that this model is efficient in retrieving the essential aspects of QCD with a minimal number of free parameters (current quark masses and dynamical constants) and yielding reasonable predictions.…”

“…That being so, this paper intends to contribute to the discussion and characterization of the B c -meson spectrum by employing a different formalism with respect to the analyses mentioned in the previous paragraph. The framework to be utilized is also known as the Coulomb gauge QCD model [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47]. This formulation is based on the exact QCD Hamiltonian in the Coulomb gauge, which is replaced by an effective Hamiltonian where the original nonperturbative confining and hyperfine interactions can be rearranged into calculable effective potentials between color densities and currents.…”