Why when people/textbooks talk about strong interaction, they talk only about bound states of 2 or 3 quarks to form baryons and mesons?
Does the strong interaction allow bound states of more than 3 quarks?
If so, how is the stability of a bound state of more than 3 quarks studied?
Answer
There is no known reason that you can't have bound states like $qq\bar{q}\bar{q}$ or $qqqq\bar{q}$ or higher number excitations, but none have been observed to date.
You do have to make a color-neutral state, of course.
In the mid-2000 some folks thought that they had of pentaquark states (that the $qqqq\bar{q}$) for a while, but it was eventually concluded that they were wrong.
Added June 2013: Looks like we may have good evidence of four-quark bound states, though the detailed structure is not yet understood, and in the comments Peter Kravchuk points out that pentaquarks have come back while I wasn't paying attention (and the same state, too). Seem some egg may have moved from face to face.
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