Sunday, 18 January 2015

Weekend plot: spin-dependent dark matter

This weekend plot is borrowed from a nice recent review on dark matter detection:
It shows experimental limits on the spin-dependent scattering cross section of dark matter on protons. This observable is not where the most spectacular race is happening, but it is important for constraining more exotic models of dark matter. Typically, a scattering cross section in the non-relativistic limit is independent of spin or velocity of the colliding particles. However, there exist reasonable models of dark matter where the low-energy cross section is more complicated. One possibility is that the interaction strength is proportional to the scalar product of spin vectors of a dark matter particle and a nucleon (proton or neutron). This is usually referred to as the spin-dependent scattering, although other kinds of spin-dependent forces that also depend on the relative velocity are possible.

In all existing direct detection experiments, the target contains nuclei rather than single nucleons. Unlike in the spin-independent case, for spin-dependent scattering the cross section is not enhanced by coherent scattering over many nucleons. Instead, the interaction strength is proportional to the expectation values of the proton and neutron spin operators in the nucleus.  One can, very roughly, think of this process as a scattering on an odd unpaired nucleon. For this reason, xenon target experiments such as Xenon100 or LUX are less sensitive to the spin-dependent scattering on protons because xenon nuclei have an even number of protons.  In this case,  experiments that contain fluorine in their target molecules have the best sensitivity. This is the case of the COUPP, Picasso, and SIMPLE experiments, who currently set the strongest limit on the spin-dependent scattering cross section of dark matter on protons. Still, in absolute numbers, the limits are many orders of magnitude weaker than in the spin-independent case, where LUX has crossed the 10^-45 cm^2 line. The IceCube experiment can set stronger limits in some cases by measuring the high-energy neutrino flux from the Sun. But these limits depend on what dark matter annihilates into, therefore they are much more model-dependent than the direct detection limits.  


Theo Nieuwenhuizen said...

Charles Steinhardt gave a talk at Harvard CfA about the SPLASH survey with the message that at z=6 about 100 times more galaxies are observed than any theory/model predicts.
I don't understand which problem one wants to solve with all this WIMP-searching.
WIMPs are incompatible with a Galaxy full of WIMP-searchers.

Xezlec said...

Am I correct in understanding that IceCube kicks ass on this observable just by (not) directly detecting neutrinos that might be emitted from dark matter interactions? Then why do you mention the sun? Is IceCube using the sun's matter as a giant dark matter detector in some sense?

Jester said...

The Sun appears here because DM is supposed to accumulate in the Sun's core and to annihilate efficiently there. The largest neutrino flux from DM annihilation is then supposed to come from the direction of the Sun, and IceCube is searching for this. So they don't use the Sun as a detector, but rather as a storage room for large quantities of DM :)

Unknown said...

100x at z6????

That's impossible unless cosmic models r broken????

Anonymous said...

IceCube great though when in search of neutrinos you try to find them where you can. With the recent Sun events it made it a great and easy experimental lab. Believe primary focus is cosmic neutrinos.

Theo Nieuwenhuizen said...

Vik, to break cosmological models is very difficult because of sociology and psychology, but still simpler than to rearrange the cosmos. Indeed, for doing that we would have to go back in time.