New Spectroscopy with Charm and Beauty Multiquark States

Exotic charmonium and bottonomium resonances recently discovered are discussed and interpreted as diquark-antidiquark states containing a pair of charm quarks and a pair of light, up and down, quarks. Successes, shortcomings and predictions of the model are illustrated.Comment: Talk given at the Erice School of Subnuclear Physics, June 30, 2013, Ettore Majorana Foundation and Center for Science and Culture, Erice, Ital

Concluding Remarks: the Scientist that Lived Three Times

The highlights of the conference: The Legacy of Bruno Pontecorvo: the Scientist and the Man, held in Roma, Universit\`a La Sapienza, 11-12 September, 2013, are summarised and illustrated

The GIM Mechanism: origin, predictions and recent uses

The GIM Mechanism was introduced by Sheldon L. Glashow, John Iliopoulos and Luciano Maiani in 1970, to explain the suppression of Delta S=1, 2 neutral current processes and is an important element of the unified theories of the weak and electromagnetic interactions. Origin, predictions and uses of the GIM Mechanism are illustrated. Flavor changing neutral current processes (FCNC) represent today an important benchmark for the Standard Theory and give strong limitations to theories that go beyond ST in the few TeV region. Ideas on the ways constraints on FCNC may be imposed are briefly described.Comment: Opening Talk, Rencontres de Moriond, EW Interactions and Unified Theories, La Thuile, Valle d'Aosta, Italia, 2-9 March, 201

Tetraquarks, Pentaquarks and Dibaryons in the large NN QCD

We study the multiquark hadrons in large $N$ QCD under the 't-Hooft limit, extending Witten's picture of the baryons. We explore the decay widths of tetraquarks, pentaquarks and dibaryons. Based on the decay behaviors, we point out in the $N\to \infty$ limit decay widths of tetraquarks stay constant, while those of pentaquarks and dibaryons above certain thresholds can diverge. In the large $N$ limit, we find that the ground states of the three spectroscopic series are stable or narrow and that the excited states of pentaquarks and dibaryons above the indicated thresholds are not observables. We compare our results with those obtained in a different large $N$ generalization of tetraquarks.Comment: 9 pages, 6 figures; v2: discussions on phase space added in the appendi

Tetraquarks in the 1/N expansion and meson-meson resonances

Diquarks are found to have the right degrees of freedom to describe the tetraquark poles in hidden-charm to open-charm meson-meson amplitudes. Compact tetraquarks result as intermediate states in non-planar diagrams of the 1/N expansion and the corresponding resonances are narrower than what estimated before. The proximity of tetraquarks to meson-thresholds has an apparent role in this analysis and, in the language of meson molecules, an halving rule in the counting of states is obtained