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yeah such a system may qualify for penta quark system..


2

Mesons and Baryons are the most common objects quarks make. However, they are not the only ones. There are tetraquarks (=4 quarks, in the form $qq\bar{q}{\bar{q}}$, due to color confinement), such as the Z(4430) confirmed by LHCb with a significance of 13.9$\sigma$, see this for relevant references. The treatment of these objects theoretically is more ...


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Note that the combined rest masses of the quarks (~10 MeV/$c^2$) account for about 1% of the proton and neutron mass (~938 MeV/$c^2$), the main contribution to the mass are the gluons from the Strong Force. Since the composition of the proton and neutron are different, so is the force that binds them.


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Indeed you find the Feynman rule by differentiating the term in the Lagrangean w.r.t. the fields attached to the vertex. In the current-current case, you need to be more careful with spinor indices than in the general gauge coupling case. There are four independent spinor indices on such a vertex. You get something proportional to $$ V \propto (\gamma^\mu ...


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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 ...



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