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The total angular momentum of a bound state of quarks, such as a meson say, can be done by studying the spin and orbital angular momentum of the 2 valence quarks.

What about the sea quarks why they do not contribute? or do they?

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Delted my answer, as it was plain wrong. –  Neuneck May 14 '13 at 10:41
Prospective answers who are not not aware of the goings on in nuclear physics in the last three decades should probably take a little time to google the "proton spin crisis": it is experimentally know that that quarks do not (contrary to naive expectation) carry all the spin of the proton. This makes Revo's question rather more interesting that it may seem at first. –  dmckee May 14 '13 at 15:50
Unfortunately it is difficult to do a EMC-like experiment for mesons. The best you could hope for was a polarized meson beam on a polarized protons target, which is a lot messier at the vertex than muons on protons. Both the beam ad the analysis would be very challenging. Well, a muon--meson collider with polarized beams would do it, but getting the beam intensity would be difficult and expensive (colliders put stiffer requirements on beam quality than fixed target work if you want any rate). –  dmckee May 14 '13 at 15:54
@dmckee Very interesting, is it a crisis for proton only? how come the quark model is successful in predicting and classifying and getting the quantum numbers of different bound states right, which match the ones discovered experimentally. –  Revo May 14 '13 at 20:49
Just from isospin symmetry you expect it to hold for the neutron too. Not sure if it's been put to the test though. As for the success of the quark model...that's a very good question, if you figure it out be sure to tell someone. –  dmckee May 14 '13 at 21:00
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2 Answers

I will tackle the experimental part leaving the theoretical for the adepts of QCD on the lattice, as the link dmckee provided in the comments clarifies.

This is a picture of a gluon and a quark antiquark pair, in OPAL.

three jets

Gluon-jet studies developed into a precision technique for testing QCD at LEP. In this event from the OPAL experiment, the most energetic jet (going to the bottom of the picture) is likely to be the quark that didn't radiate a gluon. The jet moving towards the top right can be identified as a b-quark jet, because an energetic muon (red arrow) was produced in the decay of the b-hadron. This leaves the third jet as the gluon jet and permits the comparison of the properties of quark and gluon jets – an important test of QCD.

The corresponding Feynman diagram is

feynman diagram three jets

We see here that out of the blue, the energy inputed by the incoming electron positron pair becomes an offshell photon ( spin 1) and a quark antiquark pair (spin 1/2 each) , with a gluon (spin 1) emitted from one of them . The total angular momentum must add up to the spin of the photon that gave birth to them.

Note that by quantum numbers the three end products could be within a bound meson, if the energy were lower. The algebra is the same.

you say:

The total angular momentum of a bound state of quarks, such as a meson say,

We know just the total spin of the meson, because we have measured it, not how the quarks and gluons inside add up to give the measured value. For example the quarks could add up to zero spin and the spin observed be the spin of the gluon, in the picture above. In a pion which has zero spin all spins must add up to zero. The total spin does not tell us any details about the internal spin form factors unless we do specialized experiments as in the link above.

can be done by studying the spin and orbital angular momentum of the 2 valence quarks.

Do you have a link for this? We only have measurements for the spin of the bound states of quarks. The quark model does not depend on the quark spins except as a limit in conservation of angular momentum .

For example in the meson octet:


The meson octet. Particles along the same horizontal line share the same strangeness, s, while those on the same diagonals share the same charge, q.

The spin does not enter in the symmetry pattern classification, it characterizes the whole representation.

What about the sea quarks why they do not contribute? or do they?

Logic says that they must since angular momentum is not like charge and strangeness, simply additive.

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I don't know if I'm right and it's not directly my topic, but naively guessing I think that sea quarks emerge from fluctuations in the vacuum, for example as a pair of a quark and its antiquark. And as such, because of conservation of angular momentum, each of these pairs then should have zero total angular momentum.

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To repeat the comment I left on Neuneck's now deleted answer: Uhm...no. See for instance arxiv.org/abs/0904.1735 (a review article on the state of the "proton spin crisis" from a few years ago). The sea contributes significantly to the angular momentum of the nucleons. Revo's question is interesting. –  dmckee May 14 '13 at 15:48
I see.. thank you! Never heard of this problem yet, but indeed it's very interesting. Should I delete my answer? –  GriffinPeterson May 15 '13 at 12:35
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