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Today on Nature's website appeared a news about the discovery of a quark quartet (formed from two quarks and two antiquarks). They say that this particle containing four quarks is confirmed. This is the link to the news Quark Quartet

Also, they state that the quarks arrangement in this new particle could have implication for quantum chromodynamics. This is where I get stuck. What implications does a quark quartet have on QCD?

The article references:

  • Liu, Z. Q. et al. Phys. Rev. Lett. 110, 252002 (2013).
  • Ablikim, M. et al. Phys. Rev. Lett. 110, 252001 (2013).

for the discovery/confirmation as well as some older papers

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Particle physics types will remember the penta-quark kerffufle a while ago. I presume that this has been checked carefully with that history in mind. – dmckee Jun 18 '13 at 19:25
Nothing forbids 4 quarks or more. The interesting quantity is lifetime. For instance, even for mesons, you could find mesons, made with 2 quarks, with lifetime = $4.5 ~10^{-24}s$ (rho mesons). You certainly cannot find a 4-quark particle with, for instance, the lifetime of a neutron. – Trimok Jun 18 '13 at 19:27
@dmckee, do you know the current state of art on penta-quark? – Peter Kravchuk Jun 18 '13 at 19:36
@Peter I believe that all claims of having seen evidence for the $\theta (\text{some mass})$ were withdrawn with good grace when many other labs couldn't find it and the analysis was called into question. – dmckee Jun 18 '13 at 19:40

2 Answers 2

The simple answer to your question (What implications does a quark quartet have on QCD?) is that it has no fundamental implications that I can imagine. It is an open question whether or not tetraquark ($q q \bar{q}\bar{q}$) or pentaquark ($qqqq\bar{q}$) states that live long enough to be classified as particles actually exist -- at present, our ability to compute effects in QCD is not good enough to make any convincing theoretical predictions, and experimental detection will be difficult (but maybe not impossible).

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There is the claim in the link given that tetraquarks are observed . – anna v Sep 15 at 5:15

Since you asked this question there have been a couple of confirmation of exotic particles which consist of four or more quarks. For example, Z(4430) recently observed in LHCb, was already discovered by Belle long time ago. This exotic particle has a $c\bar{c}d\bar{u}$ quark structure. This would lead us to think how color confinement would be satisfied? As you know baryons made of $qqq$ and mesons $q\bar{q}$ states which make them color-neutral. But the think is quarks are color charged particles and they need to make such a structure that would balance color to reach neutral state.

In QCD, a quark's color can take one of three values or charges, red, green, and blue. An antiquark can take one of three anticolors, called anti-red, anti-green, and anti-blue (represented as cyan, magenta and yellow, respectively). Quark-gluon fields and color charges are well understood for quark model so far. Baryons and mesons feigned such a simplicity until multi-quark states have found. Now, rather than leave aside the quark-model, physicist try to expand the theory for multi-quark particles.

Tetraquarks are composed of two primary color quarks and two antiquarks. Now the debate is, how do they combined into a neutral-state. There are some contradictions on this part. There are 2 predictions for multi-quark states:

  1. Molecular state model : $q\bar{q}$-$q\bar{q}$ form such two neutral mesons to become neutral.

  2. Di quark-di antiquark model : $qq$-$\bar{q}\bar{q}$ form neutral particles.

enter image description here

Both models have their own conflicts. Too figure out the inner structure of such exotic particles QCD would be the strongest theory. Researchers have failed to build abridgments of QCD that fit the data much better than the naive quark model. But these particles obviously do not fit in standard quark model so far.

See also the link for a further reading and information.

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