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A hadron is made up of valance quarks ($qqq$ in the case of baryons and $q\bar q$ in the case of mesons) and sea quarks ($q\bar q$ pairs). Since all internal quantum numbers of quarks and anti-quarks are opposite, sea quarks cannot contribute to e.g. the overall strangeness or baryon number. That been said, the spin of a quark is equal to that of it's anti-quark. Thus if the $q\bar q$ pair are in a triplet state one would expect the possibility of sea quarks contributing to the overall spin of the particle - but since (as far as I know) we get only $J=0$ and $J=1$ mesons this does not appear to happen. Thus my question is:

Can sea quarks contribute to the overall spin (or even isospin) of the hadron? if so why and if not why?

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  • $\begingroup$ Would you consider a decay like $\Delta\to N\pi$ a change in isospin? The total isospin is conserved, but it leaves the nucleon and is destroyed by the non-strong decay of the pion. $\endgroup$
    – rob ♦
    Commented Mar 29, 2017 at 17:05

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Can sea quarks contribute to the overall spin (or even isospin) of the hadron? if so why and if not why?

The hadrons have specific spin, isospin and various other quantum numbers according to the tables. The "fact" or "model" that the hadron is a bag of quarks some of which are valence and some of which are sea quarks, plus gluons, means that the sum of the specific quantum numbers should add up to the hadron quantum number.

How they are distributed within the bag is a problem for the specific model to solve, so that everything adds up to the correct spin and isospin . It is the same as with the mass. The mass of the hadrons is fixed. The four vectors of the plethora of virtual particles in the hadron bag has to add up to the four vector of the hadron.

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