Wick's theorem Definition and 19 Threads

Let's say we want to calculate the two-point Green's function for a fermion to a given order for a two particle interaction of the form ##U(x,y)=U(y,x)##. For the first order calculation we have to do all contractions related to...

This is problem 18.3 from QFT for the gifted amateur. I must admit I'm struggling to interpret what this question is asking. Chapter 18 has applied Wick's theorem to calculate vacuum expectation values etc. But, there is nothing to suggest how it applies to a product of operators. Does the...

I'm trying to work out the Feynman diagrams for scalar-scalar scattering using the Yukawa interaction, as given in Chapter 6 of Lahiri & Pal's A First Book of Quantum Field Theory. The interaction hamiltonian is $$\mathscr{H}_{I}=h:\overline{\psi}\psi\phi:$$ where ##\psi## is a fermion field and...

Homework Statement ##T(\phi_1\Phi_2\phi_3\Phi_4)## where ## \phi_1## is ##\phi(x_1)## and ##\phi## and ##\Phi## are two different fields. By Wicks theorem ##T(\phi_1\Phi_2\phi_3\Phi_4)= : : + contracted terms.## QUESTION Are the fully contracted terms (apologies for the bad notation I'm...

Hi. My question is about nucleon-nucleon scattering. In David Tong's lecture note, he discusses Wick's theorem and nucleon scattering (page 58-60). My problem is that I don't know how to calculate the second line of eq(3.48): \begin{equation} <p'_1, p'_2|:\psi^\dagger (x_1) \psi (x_1)...

Homework Statement Question attached: Homework Equations [/B] Using the result from two fields that ## T(\phi(x) \phi(y))= : \phi(x) \phi(y) : + G(x-y)## Where ##G(x-y) = [\phi(x)^+,\phi(y)^-] ## ## : ## denotes normal ordered and ##\phi(x)^+ ## is the annihilation operator part , and...

Hi everyone, I use Wick's theorem to decompose expectation values of a string of bosonic creation and annihilation operators evaluated at the vacuum state. This can only be done when the time evolution is driven by a Hamiltonian of the form: H=\sum_{i,j}{\epsilon_{i,j} c^{\dagger}_{i}c_{j}}...

Consider the lagrangian of the real scalar field given by $$\mathcal L = \frac{1}{2} (\partial \phi)^2 - \frac{1}{2} m^2 \phi^2 - \frac{\lambda}{4!} \phi^4$$ Disregarding snail contributions, the only diagram contributing to ## \langle p_4 p_3 | T (\phi(y)^4 \phi(x)^4) | p_1 p_2 \rangle## at...

Could someone please tell me the difference between tree diagrams and loop diagrams? If I'm thinking correctly tree diagrams are before contracting? Also how do vacuum diagrams fit into the picture? Thanks!

Homework Statement [/B] Consider a real free scalar field Φ with mass m. Evaluate the following time-ordered product of field operators using Wick's theorem: ∫d^4x <0| T(Φ(x1)Φ(x2)Φ(x3)Φ(x4)(Φ(x))^4) |0> (T denotes time ordering) Homework Equations Wick's theorem: T((Φ(x1)...Φ(xn)) = ...

I'm not quite following why Wick's Theorem only applies to free fields. What part of the argument depends on a free field assumption?

So if I understood well, Normal ordering just comes due to the conmutation relation of a and a⁺? right? Is just a simple and clever simplification. Wick Theorem is analogue to normal ordering because it is related to the a and a⁺ again (so related to normal ordering, indeed). However I do not...

Hey there, first post here! I've been struggling with a detail in Second Quantization which I really need to clear out of my head. If I expand the S-matrix of a theory with an interaction Hamiltonian H_I(x) then I have S - 1= \int^{+\infty}_{-\infty} d^4 x H_I(x) +...

Hi all! I've got a question concerning Wick's theorem. I followed the proof in the book by Fetter and Walecka and it works well for particles with "normal" statistic, that means for bosons and fermons (commuting or anticommuting). But what about anyons, particles which don't commute just with a...

I'm having a bit of trouble working through the induction proof they give in the book. The step I don't understand is: (page 90 in the book, halfway down) N(\phi_2...\phi_m)\phi_1^+ + [\phi_1^+,N(\phi_2...\phi_m)] = N(\phi_1^+\phi_2...\phi_m) + N([\phi_1^+,\phi_2^-]\phi_3...\phi_m +...

Why is the Feynman diagram for the following nasty 10 point Green's function so simple: I mean it only has two external points, one vertex, and one loop: Here is the offending function: \int d^4y_1 d^4y_2 <0|T[\phi (x_1) \phi (x_2) \phi^4 (y_1) \phi^4 (y_2)]|0> which I am assuming is simply...

Hi: If we want to work out the expectation of: <0|T(φ1φ2)|0> ie. <0|<0|T(φ1φ2)|0>|0> apparently it is acceptable to pull out the <0|T(φ1φ2)|0>: So <0|<0|T(φ1φ2)|0>|0>=<0|T(φ1φ2)|0><0|I|0> I do realize this is a really stupid question, but I want to be 100% sure. Is this simply...

I have the following contour integral form of Wick's theorem (C indicating contraction): C[A(z):BC:(w)]=\frac{1}{2 \pi i} \int _w \frac{dx}{x-w} C[A(z)B(x)]C(w) + B(x)C[A(z)C(w)] Does anybody know how to evaluate contractions like C[:AB:(z)C(w)]?

Hi, I am stuck on a step Peskin & Schroeder give in their proof of Wick's Theorem (Intro to Quantum Field Theory, p 90). In the middle of the page when they consider the term with no contraction, it seems like in between the 1st and 2nd lines they somehow factor out the normal ordering...