3
$\begingroup$

In dark matter research, one of the properties of WIMPs is that "Interactions only through the weak nuclear force and gravity, or possibly other interactions with cross-sections no higher than the weak scale;" from Wikipedia of WIMPs, so I'm curious about what does the weak scale refers to? Does it refer to the electroweak scales? And why does the cross-section no higher than the weak scale?

$\endgroup$
0

2 Answers 2

4
$\begingroup$

@Cosmas addresses the Weak scale bit. WIMPs are Thermal Relic Particles, which means that they were once upon a time in thermal equilibrium with the rest of the primordial plasma. As such, their abundance that is left over today is inversely proportional to their [thermally averaged annihilation] cross section. If that cross section were too high, their abundance today would be too low, and there would not be enough of them around to make up most of dark matter.

$\endgroup$
3
$\begingroup$

Yes, the weak scale is v ~ (1/4) TeV. Weak and EW are synonyms here, since electromagnetism has no scale at energies lower than the EW scale, where the separation occurs.

Weak cross sections σ go like $G_F^2$, where, of course, $$G_F\sim 1/v^2 \approx 10^{-5} \hbox{GeV}^{-2}. $$ In natural units, the cross section has units of inverse energy squared. For a particle of 100 GeV annihilating weakly, you might have σ ~ 1/TeV² , so, then, fractions of a picobarn.

The ambiguous phrase you read in the WP article would be better written as "... with cross section no higher than that of the weak scale".

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.