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I think it is just a one vertex diagram with $A$ at the initial state and $A^*$ at the final state, where $A$ and $A^*$ denote the atom in the ground and an excited state respectively. If you look at the QM calculation of such an absorption, you will see what should be ascribed to the vertex. Say, it may be a dipole interaction of a bound electron, but with ...


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The cross-section of a nucleon to a solar (few MeV) neutrino is about $10^{-46}$ m$^2$ and higher than the cross-section for leptonic reactions (see here). The mean density of the Earth is 5500 kg/m$^3$, and this is essentially made up of protons and neutrons with a number density of $3.3\times 10^{30}$ m$^{-3}$. The mean free path is $(n\sigma)^{-1} \...


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It should be noted that the differential cross section at $\theta = 0$ is not really defined - what is the difference between a particle being scattered by angle zero and not being scattered at all? However, it's completely legitimate to ask about the behavior of $\frac{d\sigma}{d\Omega}$ as $\theta$ approaches close to zero. For Rutherford scattering, the ...


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There will be as many formulae as models, because it depends what one supposes dark matter is composed of. If WIMP In particle physics and astrophysics, Weakly Interacting Massive Particles, or WIMPs, are among the last hypothetical particle physics candidates for dark matter. The term “WIMP” is given to a dark matter particle that was produced by ...


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As the general details of reheating are quite complicated, I won't say much about it, instead I'll just focus on answering the simpler part: given a Lagrangian, how to compute the decay rate (or, in the case of the first term, the cross section)? Now, the decay rate for a general reaction in which one initial particle decays into $n$ final ones is given by: ...



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