Shape of exclusion plots of WIMP

[BillKet]

Hello! What is the reason why the exclusion plots of WIMP experiments have that "U" like shape? And what sets the minimum of the curve? Thank you!

 

[mfb]

You can find this discussed in the publications. It's largely a combination of two effects:
* We have a pretty good estimate for the WIMP mass density from gravity, but detectors need the WIMP particle density, which is the mass density divided by the particle mass. Larger mass -> smaller particle density -> fewer expected interactions at the same cross section -> weaker cross section limits. That's why all these lines are parallel and following a simple linear relation for large WIMP masses.
* At low WIMP masses the energy transfer to the detector gets smaller, so the detectors quickly lose sensitivity.