# Tag Info

1 vote

### What is fundamental object we are looking for in QFT?

Quantum field theory is a formalism ideally suited to study fundamental dynamics. So what it calculates is a probability (amplitude) for certain events to take place. These events are often quantified ...
• 15.1k

• 91

### Wave packet as a field configuration acting like a particle's wave function?

There is a mathematical equivalence between two different physical situations. The first situation is a wavepacket in a classical field. This describes some localized packet of energy in an ordinary ...
• 51.2k
Accepted

### Where does the energy in a fundamental interaction come from?

In your electron example, there are two problems: Although independently both electrons gain energy, you would never find two electrons close to one another to begin with, because such repulsion ...
• 698

### Wave packet as a field configuration acting like a particle's wave function?

Before the sentence you quoted, Caroll writes: Think of a state in which just one mode is involved, the mode with $k = 0$. Since $k = 2\pi/\lambda$, that’s a mode with “infinite wavelength”—basically ...
• 8,787

### Confusion about solution of the Callan-Symanzik Equation

I think it is just because we integrate backward in time, $t'$ is the time backward, and the "initial" concentration is $D(t, x)$, and the final concentration is $D_i(x)$ , and as time goes ...
• 56

• 2,474

• 4,452
1 vote
Accepted

### 2PI Effective Action from Double Legendre Transformation

It is difficult to find an explicit reference that really shows this property, but one can convince oneself from eq. (2.17) in PhysRevD.10.2428. Note here that the constant is the log of the free ...
• 436
1 vote

### Classical Dirac equation

The distinction between quantum fields and classical trajectories arises from the different scales at which we observe and describe physical phenomena. In quantum field theory (QFT), both ...
• 21
Accepted

### Derivation of two-body Coulomb interaction in momentum space

This is not possible to answer without knowing all of your source's relevant conventions, including the Fourier transform. One way you can check the book is look for a place where they use this ...
• 51.2k
1 vote
Accepted

### 2+1-dimensional $SU(N)$ Yang-Mills Theory

From the point of view of perturbation theory, QCD in lower dimensions is essentially identical to QCD in $d=3+1$. Namely, the beta function is negative, which means that the theory is UV complete, ...

### Checks of anomaly cancellation

A lot of the information you gave me is beyond my knowledge, not being a science professional myself or even a student. But I like reading physics textbooks from time to time and the chapter on ...
• 51
1 vote

### Deriving the equal time anti-commutator of the Dirac fields

The relation you are trying to show is wrong. The equal-time anti-commutation relations for the Dirac field are (see for example Peskin & Schroeder, (3.108) ) \begin{align} \big\lbrace\psi_a(\...
• 438
### Why is the $\theta$ term of QCD violating charge and parity (CP) symmetries?
Your reasoning is correct* and indeed $$E_q=0 \,.$$ Note however that only the external legs of the diagram correspond to the asymptotic states that can be measured, while ...