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[tex] \sum_{a=1}^N \left[(\partial^{\mu}\phi_{a}^{\ast})(\partial_{\mu}\phi_{a})-m^{2}\phi_{a}^{\ast}\phi_{a}\right][/tex].

Is the symmetry SO(2N), SU(N) or U(N)?

It seemed quite obvious to me and some of my friends that such theory has an SO(2N) symmetry. If we view these N copies of complex K-G fields as 2N copies of real K-G fields, the Lagrangian is invariant under any rotation in the 2N dimensional space. It also seems that there should be N(N-1)/2, which is the number of generators in the SO(2N) group, conserved currents for this theory.

However, I have faced some objections to my claim. What they say is that the actual symmetry is SU(N). The reasoning for this claim was that the real and imaginary parts of these complex K-G fields cannot be considered independent, since they are related by causality. If we allowed an arbitrary SO(2N) rotation, particles and antiparticles would mix each other and the causality would be violated.

What makes me uncomfortable about this statement is that, first, I haven't been able to see any convincing formal development, rather than some hand waving arguments, for it, second, it would mean that one-component complex K-G theory doesn't have a U(1) symmetry.

If somebody said that the symmetry is U(N) due to causality, I would be less unhappy since we can save the 1-component K-G theory from not having even a U(1) symmetry.

I would be very grateful if any of you could clarify this issue, and if this causality argument is right, show me some formal elaboration to it. (or let me know where I can find it)

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# Global symmetry of an N-component Klein-Gordon theory?

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