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


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 ...


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.


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 ...


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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