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 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 as been more and more evidence in favor of the latter. 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' , 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 stress the importance of experimental search for doubly-heavy baryons in this context.
Highlights
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
More recently Belle collaboration confirmed this prediction, announcing [3, 4] the observation of two charged bottomonium-like resonances Zb as narrow structures in π±Υ(nS ) (n = 1, 2, 3) and π±hb(mP) (m = 1, 2) mass spectra that are produced in association with a single charged pion in Υ(5S ) decays
Since these states decay into bottomonium and a charged pion, they must contain both a bb heavy quark-antiquark pair and a du light quark-antiquark pair
Summary
In late 2007 the Belle Collaboration reported [1] anomalously large partial widths of Υ(5S ) → Υ(2S ) and Υ(5S ) → Υ(1S ), two orders of magnitude larger than the analogous decays of Υ(3S ). The proximity of the two resonances to the B∗Band B∗B∗ thresholds strongly suggests a parallel with X(3872), whose mass is almost exactly at the D∗Dthreshold This is because it is very unlikely that two “genuine" tetraquarks just happened to sit at the respective two-meson thresholds. Ref. [11] suggested that the asymptotic coupling between the two heavy mesons and a pion, gMM∗π for mQ → ∞ is approached from below, so that gBB∗π > gDD∗π
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