A Cross section-temperature equivalence

karmion
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It's assumed that interaction rate between a species of particule m and l is expressed as:

Γm=<nlσv>,

where nl is the density of the species l, σ the cross-section of species m (=probability of interaction) and v the relative velocity between the two particles.

It's also assumed that <σv>∼G²T², where G is fermi's constant.

I need to know where comes from this last equivalence relation, is there anyone that can help me please ?
 
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I don't know where this came from, but it is not true in general, as GF assumes a weak interaction, That's not true for particles in general.
 
karmion said:
It's assumed that ...
Where is it assumed?
 

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