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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@Peter I believe that all claims of having seen evidence for the $\theta (\text{some mass})$ were withdrawn with good grace when many other labs couldn't find it and the analysis was called into question.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

1 Answer 1

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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