Why does the cross section for Dark Matter decrease with increasing mass?

[kelly0303]

Hello! In most papers that present exclusion plots as cross section versus mass, the plot has a specific shape in which mostly the cross section decreases with mass. I am a bit confused why. If you assume that the density and speed of DM is constant, shouldn't a higher mass (and hence a higher energy), have a higher probability of interaction with the nucleus and hence a higher cross section?

 

[Orodruin]

Please give specific examples of references that show such plots. "I have read that" or "In papers that" is not a good starting point for a discussion.

Also, exclusion plots do not show actual cross sections, they show upper bounds on cross sections.

 

[kelly0303]

Please give specific examples of references that show such plots. "I have read that" or "In papers that" is not a good starting point for a discussion.

Also, exclusion plots do not show actual cross sections, they show upper bounds on cross sections.

I apologize, but you answered my question. However, is there any predicted relation between the cross section and energy for WIMPs?

 

[Orodruin]

That depends on which cross section you refer to and what dark matter model you are considering.

 

[mfb]

One more thing about the exclusion limits: Particles with higher mass are easier to see in the detectors as there is less background, so typically you can search for smaller cross sections there.

Often there are no clear theoretical predictions for cross sections, or the predictions differ by orders of magnitude, so we just look where ever we can. Here is a plot for WIMPs

 

[Orodruin]

Often there are no clear theoretical predictions for cross sections, or the predictions differ by orders of magnitude, so we just look where ever we can. Here is a plot for WIMPs

I think that figure (shown below for reference) is slightly misleading. The generic WIMP region should extend down in the plot as you can construct models where WIMPs essentially do not couple to nucleons at all. That you could not see such WIMPs in direct detection experiments because of the neutrino floor is a different story.

 

[mfb]

I have seen similar plots elsewhere as well. Would that region be somewhat natural (some coupling constants not too far away from 1) and the smaller ones need some very small numbers?

There are ideas how to get a bit into the neutrino floor - directional detectors, "signal over background" experiments and so on. You never reach arbitrarily small values of course, and it will get much more challenging there if nothing is found before.

 

[Vanadium 50]

Would that region be somewhat natural (some coupling constants not too far away from 1)

By that criterion, the SM electron mass is unnatural, since it's Higgs Yukawa is tiny.

One can easily cook up models where DM has a very, very tiny interaction rate with ordinary matter. That's not a problem at all.