Abstract
We introduce the hypothesis that diquarks and antidiquarks in tetraquarks are separated by a potential barrier. We show that this notion can answer satisfactorily long standing questions challenging the diquark–antidiquark model of exotic resonances. The tetraquark description of X and Z resonances is shown to be compatible with present limits on the non-observation of charged partners X±, of the X(3872) and the absence of a hyperfine splitting between two different neutral states. In the same picture, Zc and Zb particles are expected to form complete isospin triplets plus singlets. It is also explained why the decay rate into final states including quarkonia are suppressed with respect to those having open charm/beauty states.
Highlights
The observed lowest lying X and Z states are found very close or slightly above the meson–meson thresholds with the corresponding quantum numbers
With positive and finite δ values, a reasonable alternative description is in terms of compact tetraquarks, as in the diquark– antidiquark model [8,9]
We have to underscore that the existence of exotic charged charmed resonances with decays into ψ(nS) π ±, ρ± · · · was a prediction of the diquark–antidiquark model [8] and an unwanted/unnecessary feature for molecular models
Summary
The observed lowest lying X and Z states are found very close or slightly above the meson–meson thresholds with the corresponding quantum numbers. Forces among different parts that tend to destroy the diquark, e.g. attraction between quarks and antiquarks, reduce the binding energy of the diquark These effects increase at decreasing distance and produce a repulsion among diquark and antidiquark [19], i.e. a component in the potential increasing at decreasing distance. For the large charm quark mass, the two-lengths picture leads, in addition, to iii) an exponentially suppressed amplitude for X(3872) → J /ψ π π , with respect to D 0 D∗0, qualitatively explaining the large branching fraction of the latter to the former mode, in spite of its much smaller phase space, as observed in the phenomenology [22].
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
