Modern Physics Letters A

We review the theoretical and experimental evidence for the candidacy of the and the for charmed tetraquarks. We briefly review the 't Hooft interaction's influence on the single-charm (C = 1) tetraquark mass spectrum: its consequences for other observed mesons such as the , as well as its predictions of the (thus far) unobserved exotic tetraquark states. In particular we show that the small mass difference is naturally predicted by the tetraquark model, within the present experimental error bars, and thus may constitute the first "smoking gun" evidence for tetraquark existence. A short outlook of the possible future research directions is given.

We investigate the effective field theory (EFT) which gives the approximate description of the scattering of two hard small dipoles in the small x processes in QCD near the black disc limit (BDL). We argue that the perturbative QCD approaches predict the existence of tachyon and visualize it in the approximation where α′_P=0. We demonstrate that the high energy behavior of the cross-section depends strongly on the diffusion law in the impact parameter plane. On the other hand, almost threshold behavior of the cross section of the hard processes and multiplicities, i.e. fast increase of cross sections (color inflation), melting of ladders into color network and softening of the longitudinal distributions of hadrons are qualitatively insensitive to the value of diffusion in the impact parameter space. We evaluate α′_P near the black disk limit and find significant α′_P as the consequence of the probability conservation.

We construct the chiral perturbation theory for two-color QCD with two quark flavors as an effective theory on the SO(6)/SO(5) coset space. This formulation turns out to be particularly useful for extracting the physical content of the theory when finite baryon and isospin chemical potentials are introduced, and Bose–Einstein condensation sets on.

We use Wetterich's parametrization equation of state (EOS) of dark energy to a 5D Ricci-flat cosmological solution and we assume that the universe contains three major components: matter, radiation and dark energy. By using the relation between the scale factor and the redshift z, we show that the two arbitrary functions contained in the 5D solution could be solved out analytically in terms of the variable z. Thus the whole 5D solution could be constructed uniquely if the current values of the three density parameters Ω_m0, Ω_r0, Ω_x0, the EOS w₀, and the bending parameter b contained in the EOS are all known. Furthermore, we find that all the evolutions of the mass density Ω_m, the radiation density Ω_r, the dark energy density Ω_x, and the deceleration parameter q depend on the bending parameter b sensitively. Therefore it is worthwhile to study observational constraints on the bending parameter b.

We introduce coupling to three-vector potential in the (3+1)-dimensional Dirac equation. The potential is noncentral (angular-dependent) such that the Dirac equation separates completely in spherical coordinates. The relativistic energy spectrum and spinor wave functions are obtained for the case where the radial component of the vector potential is proportional to 1/r. The coupling presented in this work is a generalization of the one which was introduced by Moshinsky and Szczepaniak for the Dirac-oscillator problem.

We study analytically the evolution of massless Dirac fields in the background of the Schwarzschild black hole. It is shown that although the quasinormal frequencies are the same for opposite chirality with the same |k|, we can differentiate neutrinos from anti-neutrinos in evolution of the massless Dirac fields provided we know both stages for the quasinormal modes and the power-law tail behavior since the decay rate of the neutrinos is described by t^-(2|k|+1) while anti-neutrinos is t^-(2|k|+3).

BRAHMS collaboration¹ had studied in the recent past some properties of the detected identified charged hadrons in gold–gold collisions at as a function of p_T and collision-centrality at two rapidity values. The reported characteristics, which are not offshoots of any specific model whatsoever, and are measured in a model-independent manner, would be dealt with in the light of a phenomenological framework named combinational approach.