Hi, since Peskin and Schroeder pretty much suppresses the indices in every equation, I am now unable to tell if a lot of these quantities are matrices or numbers. I try to look back, but I still can't seem to figure all of these out. In the Yukawa interaction, for example, the fermion propagator (in momentum space) is a matrix right? While the boson propagator is simply a number?(adsbygoogle = window.adsbygoogle || []).push({});

Also, I see a lot of expressions like [itex]\overline{u}u[/itex]. Are these numbers or outer product matrices? I am really confused on these. If that's a number, then would [itex]u\overline{u}[/itex] be a matrix? u is the dirac spinor.

Is the big M a matrix or number? (In spinor space)

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# I can't figure out if these are matrices or numbers

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