I came upon the concept of irreducible vector-spinors while trying to simplify an expression involving the gravitino field.
It is claimed that an irredicible vector-spinor is gamma-traceless, i.e. ΓMψαM=0.
Are there conditions on the irreducibility besides the above equation? When can I use the relation? Does it follow from more general group-theoretic relations or is it a rehash of the equations of motion?
Answer
Yes, it follows from a simple group theoretical consideration. A gravitino is basically a spinor × a gauge field. However ψαμ is not an irreducible representation so you need to take the gamma traceless part away.
To see it, just look at spinor × a gauge field decomposition which looks as 1⊗12=32⊕12, the 1/2 part is precisely γμψμ since it transforms to itself. So you need to impose the condition γμψμ=0 to get the irreducible 3/2 part as required.
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