Tetraquarks are hypothetical particles made up by four quarks (two quarks and two antiquarks). Unlike mesons (quark-antiquark pairs) and baryons (three-quark or three-antiquark systems), the quarks in a tetraquark are quite loosely bound within their confinement volume by strong interactions, as can be calculated with the help of quantum chromodynamics. Their tendence to separate into two quark-antiquark systems should yield a very short lifetime, making their observation quite difficult. However, some tentative evidence for their existence exists.

A few months ago I was obliged by Marek Karliner, the chair of the Institute of Theoretical Physics of Tel Aviv in Israel, who told us about those fascinating particles in a guest post. Now a new paper by Karliner has appeared in the arXiv, and it is a quite readable one, without complex formulas and imperscrutable jargon: if you are interested in the matter I strongly advise you to give it a look.

In his new paper, Karliner explains the basics of tetraquark states; of those, the experimentally most easily accessible ones are those containing two heavy quarks bound to two light ones: and indeed, experimental evidence of charm-anticharm tetraquark candidates, and now bottom-antibottom ones, has appeared in the recent past.

The question is whether the new resonances are loosely bound molecules of two D or two B mesons (states formed by a heavy quark and a light antiquark or vice-versa) or real envelopes within which the four quarks temporarily all reside. In the former case the picture is that of two mesons that temporarily jiggle around each other by exchanging low-energy pions: QCD calculations show that the exchange provides for a tenuous attractive force, if the system has the correct quantum numbers. In the latter case, instead, we must really consider the co-existence of four quarks within the confinement volume, and this is quite interesting because it technically constitutes "exotic" physics - so far hadrons, particles made of constituents bound together by the strong force, only include the aforementioned meson and baryon systems.

That exotic hadrons exist, however, is already a fact. This is due to the fact that even a meson molecule is considered an exotic state; I however would prefer reserving the term for the tetraquark system or other real "bags" of quarks beyond the standard ones of mesonic or baryonic kind. A state with the required exotic characteristics, the Zb, has been observed by Belle two years ago in the decay of the Y(5S) bottomonium state to a charged pion and a lower-mass bottomonium.

The most interesting part of Karliner's article to me is the proposal that also six-quark states should exist, either as molecular bindings of two baryons or as real bags. The specific characteristics of heavy baryons might make the binding energy of these systems strong enough that their experimental detection could be possible (a long lifetime makes the natural width of the resonance small enough that it does not "disappear" in the large continuum background). Karliner indicates as a promising possibility the bound state of two Σb baryons, . Such a bound state would often decay to a pair of Λb baryons and two pions, a spectacular signature that could be observable despite the rarity of the state. It is possible that LHCb or CMS have sensitivity to observe it in the near future.