We use a constituent model to analyze the stability of pentaquark $bar Q qqqq$ configurations with a heavy antiquark $bar c$ or $bar b$, and four light quarks $uuds$, $ddsu$ or $ssud$. The interplay between chromoelectric and chromomagnetic effects i
s not favorable, and, as a consequence, no bound state is found below the lowest dissociation threshold.
We outline the most important results regarding the stability of doubly heavy tetraquarks $QQbar qbar q$ with an adequate treatment of the four-body dynamics. We consider both color-mixing and spin-dependent effects. Our results are straightforwardly
applied to the case of all-heavy tetraquarks $QQbar Qbar Q$. We conclude that the stability is favored in the limit $M_Q/m_q gg 1$ pointing to the stability of the $bbbar ubar d$ state and the instability of all-heavy tetraquarks.
It is shown that standard constituent quark models produce $(bar c c qqq)$ hidden-charm pentaquarks, where $c$ denotes the charmed quark and $q$ a light quark, which lie below the lowest threshold for spontaneous dissociation and thus are stable in t
he limit where the internal $bar c c$ annihilation is neglected. The binding is a cooperative effect of the chromoelectric and chromomagnetic components of the interaction, and it disappears in the static limit with a pure chromoelectric potential. Their wave function contains color sextet and color octet configurations for the subsystems and can hardly be reduced to a molecular state made of two interacting hadrons. These pentaquark states could be searched for in the experiments having discovered or confirmed the hidden-charm meson and baryon resonances.
We review some recent studies on the string model of confinement inspired by the strong-coupling regime of QCD and its application to exotic multiquark configurations. This includes two quarks and two antiquarks, four quarks and one antiquark, six qu
arks, and three quarks and three antiquarks with a careful treatment of the corresponding few-body problem.
Many new states in the charmonium and bottomonium mass region were recently discovered by the BaBar, Belle and CDF Collaborations. We use the QCD Sum Rule approach to study the possible structure of some of these states. In particular we identify the
recently observed bottomonium-like resonance $Z_b^+(10610)$ with the first excitation of the tetraquark $X_b(1^{++})$, the analogue of the X(3872) state in the charm sector.
Positivity restrains the allowed domains for pairs or triples of spin observables in polarised reactions. Various domain shapes in ${1over2}+{1over2}to{1over2}+{1over2}$ reactions are displayed. Some methods to determine these domains are mentioned a
nd a new one based on the anticommutation between two observables is presented.
We explore the role of a scalar meson exchange interaction between quarks in a semirelativistic constituent quark model where the quarks are subject to a linear confinement. We search for a variational solution and show that the gap between the N=1 b
and and the ground state N=0 band increases with the strength of the scalar meson exchange interaction potential. This result has good implications on the description of the low-lying baryon resonances, especially on the Roper vs. negative parity resonances ordering.
