Exotic states with ccbar
R. Faccini · 2010
In the past few years the field of hadron spectroscopy has seen renewed interest due to the pubblication, initially mostly from B-Factories, of evidences of states that do not match regular spectroscopy, but are rather candidates for bound states with additional quarks or gluons.Currently, the most likely possible states beyond the mesons and the baryons are (you can find a review in [1]):• hybrids: bound states of a quark-antiquark pair and a number of gluons.The lowest lying state is expected to have quantum numbers J PC = 0 +-.The impossibility of a quarkonium state to assume these quantum numbers (see below) makes this a unique signature for hybrids.Alternatively a good signature would be the preference to decay into a quarkonium and a state that can be produced by the excited gluons (e.g.π + π -pairs).• molecules: bound states of two mesons, usually represented as [Q q][q ′ Q], where Q is the heavy quark.The system would be stable if the binding energy would set the mass of the states below the sum of the two meson masses.None of the observed states have this characteristic, while some of them have masses slightly above the sum of the constituents.In this case the two mesons can be bound by pion exchange.This means that only states decaying strongly into pions can bind with other mesons (e.g.there could be D * D states), and that the bound state could decay into it's constituents.• tetraquarks: a quark pair bound with an antiquark pair, usually represented as [Qq][ q′ Q].A full nonet of states is predicted for each spin-parity, i.e. many states are expected.There is no need for these states to be close to any threshold.In setting after these states one must also beware of threshold effects, where amplitudes might be enhanced when new hadronic final states become possible.This paper reviews the experimental state of the art in charmonium spectroscopy with emphasis on the measurements that are most likely to cast more light on the understanding of the global picture.