ar X iv : h ep - p h / 06 09 13 7 v 1 1 4 Se p 20 06 The double life of the X meson

Three years have passed since the BELLE discovery of the X(3872), and there are still (at least) two competing interpretations of this particle, which resembles a charmonium but behaves in a dramatic different way from it. Is X a molecule of two D mesons or a compact four-quark state? Are these two pictures really resolvable? The quantum mechanical intuition can also lead to more refined pictures: the X could be a sort of breathing mode of a charmonium oscillating into a molecule and back. Other particles have been discovered since then: the X(3940), Y (3940), Z(3930) (amazingly the first two have the same mass and both decay to charmonium but with a different decay pattern to open charm) and Y (4260). The latter also decays into J/ψ and could be an 'hybrid' particle (two quarks and a constituent gluon), likely the most experiment-proof interpretation so far. In this talk I will not try to describe all the experimental facts and theoretical ideas, thoroughly reported and commented elsewhere in the literature. I will rather comment on the first question here raised, namely, how far are molecules, four-quarks or charmonium-molecule oscillating states distin-guishable in principle and in the experimental practice? The suspect that the competing scenarios fade into one another could dangerously leave this field in a confused and controversial situation similar to that existing for sub-GeV scalar mesons (and for their super-GeV partners). The prominent decay mode of X(3872) is X → J/ψρ. Several studies conclude that the X cannot be an ordinary c¯ c state, even though the J/ψ invokes a charmonium assignation. The next-to-easy interpretations can be: (1) X is a D ¯ D * bound object, with the correct 1 ++ quantum numbers. Such a molecule