I want to calculate $$\langle x|XP|y \rangle$$ where X is the position operator and P the momentum operator, and the states are position eigenstates. But I get two different answers depending on if I insert a complete set of states.
First way:
$$\langle x|XP|y \rangle=x\langle x|P|y...
I'm having a hard time following the arguments of how the Higgs gives mass in the Standard Model. In particular, the textbook by Srednicki gives the Higgs potential as:
$$V(\phi)=\frac{\lambda}{4}(\phi^\dagger \phi-\frac{1}{2}\nu^2)^2 $$
and states that because of this, $$\langle 0 | \phi(x)...
I was following some lectures (McGreevy's notes - if anyone's interested it's page 16 of http://physics.ucsd.edu/~mcgreevy/s15/215C-2015-lectures.pdf ) and they showed the expansion so I thought it was important to understand why it could be done mathematically.
I was thinking that physically...
If you want to show that a propagator of a heavy particle reduces to a point interaction at distances large compared to the inverse mass of the particle by Taylor expanding the propagator (for simplicity take 1-dimension):
$$G(x)=\int^\infty_{-\infty} \frac{dk}{2\pi} \frac{e^{-ikx}}{k^2+M^2}=...