Are Tetraquarks Real ?
    By Tommaso Dorigo | January 18th 2014 03:22 AM | 7 comments | Print | E-mail | Track Comments
    About Tommaso

    I am an experimental particle physicist working with the CMS experiment at CERN. In my spare time I play chess, abuse the piano, and aim my dobson...

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    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. 


    The existence of dimers has many analogies in physics at all dimensional scales (binary stars, helium atoms, Cooper pairs, nucleon pairs, neutrons...) - so why not the quarks?

    John Duffield
    Interesting stuff, Tommaso. But I was a little surprised that pentaquarks weren't mentioned. I have a hunch these will turn out to be genuine. And that some will be stable. Unfortunately they seem to have rather fallen by the wayside these days. This paper of Marek's is ten years old now.
    A somewhat rhetorical question: How does one even define whether a resonance is a standard meson or a tetraquark, when one has quark-antiquark pairs coming into and out of existence continuously? Fundamentally, the question is (at best) a very poorly defined one, and whether a given state is best described by a "pure" quark-antiquark pair (bound by an undefined number of gluons, of course), or is a true tetraquark, or is a hadronic molecule, is one large grey area in which the boundaries between those cases are entirely arbitrary and just dependent on whatever anyone happens to set them to be. Mass eigenstates can in principle be any combination of those cases (plus bits of hexaquark, etc), while still preserving quantum numbers. So I don't see how Karliner could be right -- or with apologies to Pauli -- could even be wrong.

    The exotic resonances observed by Belle and BESIII cannot be standard mesons which are constructed from a quark and an antiquark. This is because they decay to a heavy quark -antiquark pair and a charged pion. A heavy quark-antiquark pair in the the decay products of a narrow resonance means that the heavy pair must have been there in the first place. It could not have been produced as a result of a quantum fluctuation of a highly excited regular meson containing only light quarks, because such a meson would have been very wide, having plenty of other open decay channels. Having a positively charged pion in the final state means that the minimal quark content of the original resonance must have included a u quark and a d antiquark. This is because quantum fluctuations cannot create a quark-antiquark pair with a net electric charge. The upshot is that the minimum quark content of these exotic resonances includes a heavy quark, a heavy antiquark and a light quark-antiquark pair with a net electric charge.

    The question whether these resonances are "genuine" tetraquarks or "molecules" of two heavy mesons is much more difficult and does not have a sharply defined answer. This is because "genuine" tetraquarks and "molecules" can mix, although the mixing is expected to be small. We have several indications from the data that the currently discussed resonances are close to being molecules. The evidence for this is discussed in the talk writeup that Tommaso refers to.

    This reminds me of a question I've never seen a definitive answer to... To what extent do the nucleons in light nuclei have an identity as opposed to the nucleus just being a bag of quarks.

    Eg., there are 2 x 2 x 3 = 12 spin x isospin x colour states, so up to Helium-4, having all the constituent quarks in the same momentum/position/energy states is consistent with Pauli-exclusion.

    (Apologies if it's a silly question - I am not a physicist).

    If I remember correctly didn't Tony Thomas in the 80's have a bag model of quarks to describe nuclei?

    "Six-quark states ... as molecular bindings of two baryons". Aren't deuterons an example of this? So it's clearly possible. How often are deuterons formed in colliders?