Abstract
During the last three years strong experimental evidence from B and charm factories has been accumulating for the existence of exotic hadronic quarkonia, narrow resonances which cannot be made from a quark and an antiquark. Their masses and decay modes show that they contain a heavy quark-antiquark pair, but their quantum numbers are such that they must also contain a light quark-antiquark pair. The main theoretical challenge has been to determine the nature of these resonances. The main possibilities are that they are either “genuine tetraquarks”, i.e. two quarks and two antiquarks within one confinement volume, or “hadronic molecules” of two heavy-light mesons. In the last few months there is more and more evidence in favor of the latter. In addition, there exist narrow resonances such as X (3872) which are not exotic but are closely related to the exotic states. Most likely such states are a mixture of hadronic molecules and excited quarkonia. I discuss the experimental data and its interpretation and provide fairly precise predictions for masses and quantum numbers of the additional exotic states which are naturally expected in the molecular picture but have yet to be observed. In addition, I provide arguments in favor of the existence of an even more exotic state – a hypothetical deuteron-like bound state of two heavy baryons. I also consider “baryon-like” states QQ′qq′ , which if found will be direct evidence not just for near-threshold binding of two heavy mesons, but for genuine tetraquarks with novel color networks. I also stress the importance of experimental search for doubly-heavy baryons which are intimately connected with doubly heavy exotics.
Highlights
In late 2007 the Belle Collaboration reported [1] anomalously large rate partial widths of Υ(5S ) → Υ(2S ) and Υ(5S ) → Υ(1S ), two orders of magnitude larger than the analogous decays of Υ(3S )
In the bottom system the attraction due to π exchange is essentially the same, but the kinetic energy is much smaller by a factor of ∼m(B)/m(D)≈2.8
The recently discovered manifestly exotic charged resonances are surprisingly narrow. This is the case in both bb systems [3, 4] – and in the exotic charmonium, namely the remarkable peak Zc(3900) at 3899.0 ± 3.6 ± 4.9 MeV with Γ = 46 ± 10 ± 20 MeV reported by BESIII [5]
Summary
In late 2007 the Belle Collaboration reported [1] anomalously large rate partial widths of Υ(5S ) → Υ(2S ) and Υ(5S ) → Υ(1S ), two orders of magnitude larger than the analogous decays of Υ(3S ). As usual in quantum mechanics, for a given potential the radius of a bound state or a resonance gets smaller when the reduced mass grows, so the D D∗ states are larger than the B B∗ states Because of this difference in size the attraction in both I=0 and I=1 charmonium channels is expected to be somewhat smaller. BESIII reported observation of another charged charmoniumlike structure Zc(4020) in e+e− → π+π−hc at (4022.9 ± 0.8 ± 2.7) MeV and width of 7.9 ± 2.7 ± 2.6 MeV At this time it is not yet clear if these are two independent resonances, or two observations of the same object at slightly different masses, possibly due to systematic effects associated with the two observation channels.
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