I am stuck at a computation and I would appreciate any help. $U$ is the pion matrix in chiral perturbation theory
where $\sigma_a$ are Pauli matrices, $\phi_a$ are three real scalar fields and $f$ is just a constant with mass dimensions. It is well known that this matrix is unitary, that is
Now comes the question. I want to compute this
where $Tr$ denotes trace. I know the that answer is proportional to
(if you want to know how I know this, it is claimed in page 9 of this http://arxiv.org/pdf/hep-ph/9403202v2.pdf paper). In any case, I am very puzzled here because a trace of something gives a product of traces. So, how does this follow?
I still haven't solved it but I think that equation (D.7) of this review http://arxiv.org/pdf/hep-ph/0210398v1.pdf might might help to solve it