- #1
Hao
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Homework Statement
(This is not homework)
This refers to question 3.8 in Peskin's QFT
Using the fact that the electromagnetic interaction term in the Dirac + EM lagrangian is invariant under Parity (P) and Charge conjugation (C), and that spin 0 and spin 1 states are odd and even under exchange of spins,
show that the
1) spin 0 positronium ground state (S wavefunction) decays into 2 photons, and that the
2) spin 1 positronium ground state (S wavefunction) must decay into 3 photons
3) The above for P, D states.
Homework Equations
EM coupling
[tex]\Delta H=\int A_{\mu}j^{\mu}d^{3}x[/tex]
We know that under parity, [tex]j^{\mu}\rightarrow (j^{0},-j^{1},-j^{2},-j^{3})[/tex]
We know that under parity, [tex]j^{\mu}\rightarrow -j^{\mu}[/tex]
The Attempt at a Solution
By handwaving, we can say that these transitions occur due to conservation of angular momentum as a photon has a spin of 1.
However, how would these transitions be derived on the basis of C and P symmetries alone?
One could probably consider the interaction matrix term:
[tex]\left\langle photons\right|\Delta H\left|positronium\right\rangle[/tex]
And determine how it transforms under C and P
The problem I have is in evaluating the P and C eigenvalues of states that contain only photons.
For a state involving a fermion and antifermion (eg. positronium), and with orbital angular momentum L, P|state> = (-1)L+1|state>. The extra factor of +1 is due to the anticommutativity of spin 1/2 creation operators.
Thanks