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as known the photon can have an isospin of 0 and 1. However, what is the ratio of both for an virtual photon (in e+e- annihilations)?

Next to an 1^-- resonance like J/Psi or Psi' it should be 1 in my opinion (as the resonance cross section is much higher than the continuum cross section).

But what ratio should one expect for the continuum? (I plan to estimate the ratio of Delta and Nucleon resonances within the continuum...)

Thanks for your help.

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Photons are isospin singlets, so they only ever have isospin zero... –  David Z Sep 10 '12 at 18:47
    
Did you confuse proton with photon? I never heard of the isospin of photon. –  Siyuan Ren Sep 11 '12 at 3:57
    
You can give the photon an isospin of 0 or 1 (e.g. compare to the PDG). This can be explained for example with the vector dominance model. There is only one wrong thing in my post. Next to an resonance is should be 0... –  Matthias Sep 12 '12 at 12:03

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up vote 1 down vote accepted

I did find the answer in a paper of J. L. Rosner (http://arxiv.org/abs/hep-ph/0411003).

The ratio is 1:9, since (Q_u − Q_d)^2 = 9(Q_u + Q_d)^2. Thus, it can described as 1/10 I_0 + 9/10 I_1.

The reason for the different coupling is the differing coupling from the photon to charge.

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