# Questions tagged [thermal-field-theory]

Thermal Field Theory or Finite Temperature Field Theory deals with of methods to calculate expectation values of physical observables of a Quantum Field Theory at finite temperature.

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### Symmetry restoration in accelerated frames

I am trying to understand the symmetry restoration in accelerated frames. I was reading 1 and 2. Can somebody help me to understand how they got Eq. 7.12 in 1 or Equation just after Eq.3.3 in 2? In ...
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### Interpreting the derivative of the effective potential with the tadpole

I am currently going through the Finite Temperature Field Theory notes by Mariano Quiros, and I stumbled upon a claim that I have seen often in the literature, namely that the first derivative of the ...
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Consider the equilibrium time ordered and retarded Green's function for two operators $A,B$, defined as follows $$G^T(t,t') \equiv -i\langle T A(t)B(t') \rangle=-i\theta(t-t')\langle A(t)B(t')\rangle ... 2 votes 1 answer 268 views ### Confusion about trace in the vertex term of Lagrangian I was reading through Mariano Quirós's lecture notes titled "Finite Temperature Field Theory and Phase Transitions". In Sec. 1.2, the author is calculating the one-loop effective potential at T=0. ... 0 votes 0 answers 100 views ### Understanding Matsubara summation I'm trying to understand matsubara summation. Let us say I have f(i\omega) = 1. Obviously, the matsubara summation \sum_{\omega_n} f(i\omega_n) diverges. So, I use a weighing function. Let us ... 0 votes 1 answer 87 views ### How does one modify the decay width of a particle (QFT/Thermal Field theory style) when a particle is travelling through matter I believe a particle's decay width/rate should depend on whether they are in matter or vacuum, but am unsure of where to find a prescription describing this phenomena. Please could someone point me in ... 2 votes 1 answer 56 views ### Guessing the temperature dependence of a decay rate \Gamma(A\to B+B) For a two-body decay of the form$$A\to B+B$$if the interaction strength controlling the decay is \lambda, the Feynman amplitude \mathcal{M} will contain a factor of \lambda from the vertex ... 1 vote 0 answers 52 views ### Classical chaos at finite temperature Is there any finite temperature generalization of classical chaos? In quantum chaos, at least with regards to out-of-time-order correlators, the generalization is clear - one simply takes a thermal ... 2 votes 0 answers 94 views ### Reference for Feynman diagram technique(position space) in Thermal Field Theory I am trying to study perturbative expansion of Sachdev-Ye-Kitaev model, where I know that the dominant terms are the Melonic diagrams. I am interested in seeing how perturbative corrections affect the ... 1 vote 0 answers 65 views ### Electroweak phase transition and finite temperature field theory formalism We do our calculations in standard quantum field theory at zero temperature where we can derive pole mass and renormalized mass and ... Due to my understanding, pole mass is independent of any energy ... 2 votes 0 answers 28 views ### How does thermalisation make incoherent waves into coherent ones? Thermalisation is the process by which out-of-equilibrium systems reach equilibrium. Coherence refers to the phases of waves being a constant difference apart. While reading a paper on axion stars, I ... 2 votes 0 answers 147 views ### Thermal density matrix QFT The density matrix of a system at finite temperature is give by$$\langle\psi_1|\rho|\psi_2\rangle=\frac{1}{Z}\langle\psi_1|e^{-\beta H}|\psi_2\rangle, $$where Z is a normalization constant. We ... 1 vote 0 answers 49 views ### Normal ordering in 2D thermal CFT I am trying to understand the notion of normal ordering in thermal CFT in 2D CFT, for instance I consider a two-point function of scalar primary operator with \Delta dimension at finite temperature ... 1 vote 0 answers 53 views ### How can we calculate the imaginary part of a fraction that has a term i0_+ in the denominator? I have recently started dealing with thermal field theory for fermions and I am faced with a paper that, at some point, tries to calculate the imaginary part of a fraction that looks like:$$\frac{1}{...
Assume that the spectrum of some supersymmetric theory is discrete, then the Witten index is expected to be independent of temperature given by $T = 1/\beta$. However, it is well-known (see this) that ...
### What effect does multiplying $\mathscr{L}$ by $-1$ have on the propagator?
I am following along Ashok Das' development of Thermofield dynamics in his book Finite Temperature Field Theory. Here you have two real scalar fields $\phi_1$ and $\phi_2$ with Lagrangian density  \...