X(6200) as a compact tetraquark in the QCD string model

Recently the LHCb Collaboration announced the first observation of nontrivial structures in the double-$$J/\psi $$ J / ψ mass spectrum in the mass range 6.2–7.2 GeV, and a theoretical coupled-channel analysis of these data performed in Dong et al. (Phys Rev Lett 126:132001, 2021) evidenced the existence of a new state X(6200) close to the double-$$J/\psi $$ J / ψ threshold. Although its molecular interpretation seems the most plausible assumption, the present data do not exclude an admixture of a compact component in its wave function, for which a fully-charmed compact tetraquark is the most natural candidate. It is argued in this work that the QCD string model is compatible with the existence of a compact $$cc{\bar{c}}{\bar{c}}$$ c c c ¯ c ¯ state bound by QCD forces just below the double-$$J/\psi $$ J / ψ threshold. A nontrivial interplay of the quark dynamics associated with this compact state and the molecular dynamics provided by soft gluon exchanges between $$J/\psi $$ J / ψ mesons is discussed and the physical X(6200) is argued to be a shallow bound state, in agreement with the results of the aforementioned coupled-channel analysis of the LHCb data.

Confinement of Fermions in Tachyon Matter

In this paper, we develop a phenomenological model inspired by QCD that mimics the QCD theory. We use the gauge theory in color dielectric medium (Gϕ) coupled with fermion fields to produce scalar and vector confinements in the chromoelectric flux tube scenario. The Abelian theory will be used to approximate the non-Abelian QCD theory in a consistent manner. We will calculate vector and scalar glueballs and compare the result to the existing simulation and experimental results and projections. The QCD-like vacuum associated with the model will be calculated and its behavior studied relative to changing quark masses. We will also comment on the relationship between tachyon condensation, dual Higgs mechanism, QCD monopole condensation, and their association with confinement. The behavior of the QCD string tension obtained from the vector potential of the model will be studied to establish vector dominance in confinement theories.

The (De)confinement Transition in Tachyonic Matter at Finite Temperature

We present a QCD motivated model that mimics QCD theory. We examine the characteristics of the gauge field coupled with the color dielectric function (G) in the presence of temperature (T). The aim is to achieve confinement at low temperatures T<Tc, (Tc is the critical temperature), similar to what occurs among quarks and gluons in hadrons at low energies. Also, we investigate scalar glueballs and QCD string tension and effect of temperature on them. To achieve this, we use the phenomenon of color dielectric function in gauge fields in a slowly varying tachyon medium. This method is suitable for analytically computing the resulting potential, glueball masses, and the string tension associated with the confinement at a finite temperature. We demonstrate that the color dielectric function changes Maxwell’s equation as a function of the tachyon fields and induces the electric field in a way that brings about confinement during the tachyon condensation below the critical temperature.

Stiff self-interacting strings at high temperature QCD

We investigate the implications of Nambu-Goto (NG), Lüscher Weisz (LW) and Polyakov-Kleinert (PK) effective string actions for the Casimir energy and the width of the quantum delocalization of the string in 4-dim pure SU(3) Yang-Mills lattice gauge theory. At a temperature closer to the critical point T/Tc=0.9, we found that the next to leading-order (NLO) contributions from the expansion of the NG string in addition to the boundary terms in LW action to decrease the deviations from the lattice data in the intermediate distance scales for both the quark-antiquark QQ̅ potential and broadening of the color tube compared to the free string approximation. We conjecture possible stiffness of the QCD string through studying the effects of extrinsic curvature term in PK action and find a good fitting behavior for the lattice Monte-Carlo data at both long and intermediate quark separations regions.