Exotic mesons, XYZ, as tetraquarks

Luciano Maiani (Rome)
DESY Auditorium, 16.45 h

In the last ten years several hidden charm or beauty mesonic resonances have been discovered at electron-positron and proton-proton colliders, which do not fit in the quarkonium scheme. In particular, the lastly discovered Z states are charged therefore, besides containing a c-c bar or b-b pair, they must include a u-d bar or d-u bar pair in their valence composition.
The structure of XYZ has been the object of several speculations, from being "molecules" made of a pair of colourless mesons, to being bound states of coloured objects such as diquark-antidiquark pairs, tetra quarks for brief, to being pure illusions, cusps in the cross section related to meson-antimeson thresholds.
In the seminar, I will discuss the diquark-antidiquark case, in the light of the most recent data and of theoretical considerations based on the N-> infinity limit, N being the number of QCD colours. Main emphasis will be on the hidden charmonium case, studied at length in Roma by our group. Consideration will also be given to hidden beauty states, studied in particular at DESY by Ali and collaborators.

application/pdf Poster (339KB)
application/pdf Slides (3.9 MB)