# Search results

1. ### Basic metric diagonalization questions

I understand it is always possible to diagonalize a metric to the form diag[1,-1,\dots,-1] at any given point in spacetime because the metric is symmetric and we can always re-scale our eigenvectors. But is this achievable via a coordinate transformation? That is, would the basis...
2. ### Källen-Lehmann Representation

I have two basic questions about the full propagator (2-point function) in QFT. Am I correct that for a scalar field, it is \frac{iZ}{p^{2}-m^{2}+i \epsilon} + \int_{m^{2}}^{\infty} dX \frac{\rho[X]}{p^{2}-X+i \epsilon} ? (1) Is this form of the propagator a feature of _quantum_ field theory...
3. ### What is a bifurcation surface and a binormal vector?

I've been reading Wald's book on GR as well as his article "Thermodynamics of Black Holes" in Living Reviews in Relativity about the definitions of mass and energy in GR and the concepts of entropy and temperature of black holes. I keep coming across the words "bifurcation surface" and...
4. ### Stress-Energy Tensor from Lagrangian: Technical Question

Stress-Energy-Momentum Tensor from Lagrangian: Technical Question I've been reading about how to generate the stress-energy-momentum tensor T^{\mu \nu} from the action S = \int d^{4}x \sqrt{|g|} \mathcal{L} T^{\mu \nu} = \frac{2}{\sqrt{|g|}} \frac{\partial}{\partial g_{\mu \nu}} \left(...
5. ### Question about epsilons that occurs in calculating beta functions in QFTs w dim reg

Question about epsilons that occur in calculating beta functions in QFTs w dim reg In deriving the beta function of, say, QED using dimensional regularization we get the relation (up to 1 loop) \beta[e] = - \frac{\epsilon}{2} e - e \frac{d ln[Z_{e}]}{d ln[\mu]} \quad (1) and Z_{e} = 1...
6. ### How to compute the number of loops in a Feynman diagram?

In doing my \phi^{3} theory I didn't know exactly how to count the number of loops in a diagram given the number of vertices, internal and external lines. Is there a general algorithm in doing this? What if we have more than one interaction vertex (e.g. the Standard Model)? PS. What does it...
7. ### What is the physical mass and coupling?

I have a very basic question about exactly how to match the experimentally measured masses and coupling constants to the parameters in the lagrangian density in a given QFT. Let me specialize to a particular theory and perhaps people here can help me out. I've just computed the tadpole and...
8. ### Why Neutron Does Not Decay

Why does a neutron not decay in some nuclei? Why does it decay in other nuclei? Is there some mechanism suppressing the d quark -> u quark + W- boson channel? I'm looking for a fundamental QFT / Standard Model explanation. Thanks!
9. ### Seeking QFT exercise buddy

I'm a grad student. I've done 2 semester of QFT using Peskin but I've didn't too well due to many reasons. I now want to go back and catch up on things I should have learnt better, and then learn even more - I want to be a good field theorist. I'd like to see if there's anyone in a similar...