Thursday, July 19, 2012

Heavy WIMPS Experimentally Excluded Again

The latest results from Xenon100s direct search for dark matter has ruled out heavy WIMPs (dark matter in the form of weakly interacting massive particles) in the vincinity of Earth down to something less than 55 GeV for even very slight interaction cross sections (2*10^-45 cm^2) and ever so slightly more interacting dark matter particles from about 15 GeV to 1 TeV of mass (almost the entire WIMP mass range in cold dark matter theories).  It has ruled out somewhat more strongly interacting WIMPs down to masses of about 5 GeV.  Basically, anything heavier than a bottom quark is ruled out.

A twelve megabyte file size paper on the results can be downloaded here.  The money chart, showing the exclusion range for all published direct dark matter detection experiments is on page 50 (the second to last page).

The Bottom Line In The Context Of Other Dark Matter Detection Experiments

The Xenon100 results for 2012 exclude almost the entire range of almost a dozen previous direct dark matter detection experiments, with the Xenon 10 results from 2011 providing some additional exclusions for particles of 5 GeV to 8 GeV masses with fairly high cross-sections of interaction (10^-41 cm^2 to 10^-40 cm^2). 

In particular, Xenon100's results confirms prior studies ruling out dark matter with 8 GeV to 12 GeV and contradicts prior studies (COGENT, DAMA/Na and CRESTT-II) that had suggested that there might be dark matter particles of those masses, although with not entirely consistent and fairly strong cross-sections of interaction. 

Also, the nature of these experiments is that it is easy to overlook a tiny but easily explained contribution to background noise in the detector that looks like but does not actually constitute a real dark matter detection event. This entire class of experiments is inherently biased towards false positives due to experimenter error.

Considering Xenon 100 Together With Collider Experiment Data

The Xenon100 data is a nice complement to the LHC data.

The under 15 GeV masses where some direct detection of dark matter experiments have not ruled out the presence of some kind of dark matter with a very slight cross section of interaction (which other experiments contradictorily say that they have ruled out) are well within the mass sensitivity range of current collider experiments like the LHC and prior ones like Tevatron and the LEP to direct directly through entirely different methods.  Precision electroweak decay data pretty much completely rules out the possibility of any weakly or electromagnetically interacting particle in that mass range, since otherwise more W and Z bosons would have more "missing energy" than the amount actually observed which is currently fully and very exactly explained by Standard Model neutrinos. 

It is coinceivable that collider experiments would miss, but direct detection of dark matter experiments might detect "sterile neutrinos" that interact via gravity and Fermi contact interactions, but not via the weak force or electromagnetic force or the strong force.  But, sterile neutrinos that don't interact via the weak force would not be expected to have as large of a cross-section of interaction of the one suggested by to possible by the results from COGENT, DAMA/Na, and CRESTT-II. 

To slightly overgeneralize, the heavier a dark matter particle that is a WIMP is, the easier it is for Xenon100 to see it, even if it has a very slight cross-section of interaction (and has some resolution down to the tens of GeV or even the high single digit GeV).  In contrast, LHC and prior collider experiments are good at seeing light particles, but start to lose detection resolution in the hundreds of GeVs range or higher (cutoffs vary with hypothetical particle properties).  The lighter a particle is, at least down to the hundreds of KeV mass range (and up to at least the 45 GeV or so if it interacts via the weak force at all as all known fundamental fermions and fundamental bosons with mass do, and none of the fundamental bosons without mass do), the easier it is for collider experiments to detect it.

Since their sensitivity ranges, in terms of particle masses, now overlap with each other, the two classes of experiments combined effectively rule out non-Standard Model particles in any mass range that has been considered seriously in and dark matter theories that credibly addresss the phenomenology.

Other Limitations on Dark Matter Properties.

Experimental astronomy data looking at dark matter effects in particular galaxies and galaxy clusters places bounds dark matter particle speed, mass and interaction cross-sections, as do models that try to reproduce the large scale structure of the universe on dark matter distribution and behavior. 

The data from astronomy currently seems to favor lighter, faster moving particles called "warm dark matter" in the KeV mass range (which none of the direct dark matter detection experiments are theoretically capable of observing), rather than hundreds of GeV masses favored by older "cold dark matter" theories that are increasingly at odds with the evidence from astronomy, and "hot dark matter" theories that posit that dark matters are simply ordinary neutrinos behaving in well understood ways and produced via the weak force decays of energetic particles (thereby making them move at close to the speed of light).  Cold dark matter models produce more small galaxy scale structure than we observe; warm dark matter produces less large scale structure in the universe than we observe.

Sterile Neutrinos Or Bust.

Basically, at this point, we are in a light sterile neutrinos of bust situation when it comes to non-Standard Model dark matter particles.  The theoretically well motivated alternatives to a sterile neutrino have been mostly ruled out by experimental data and astronomy observations, while beyond the Standard Model theories with sterile neutrinos (aka right handed neutrinos) that weigh more than regular neutrinos (and hence would move slower on average), are common place, since there are right and left handed versions of all fermions other than neutrinos.

FWIW, I have real doubts about whether not yet discovered, non-baryonic dark matter particles are out there at all, at least in the vicinity of the solar system.  They probably don't exist either.  But, if undiscovered kinds of dark matter particles are out there, sterile neutrinos do seem to be the best hypothetical particle candidates out there.

Not Encouraging For SUSY

In particular, this is bad news for SUSY which is independent of (1) non-detection of SUSY at the LHC, and (2) the non-detection of neutrinoless double beta decay at levels predicted for high characteristic energy scale SUSY theories which have not yet been ruled out by the LHC, because one of the most important phenomenological motivations for SUSY theories that is shared generically by almost all SUSY theories is that they provide a heavy WIMP dark matter candidate.  In general, the higher the characteristic energy scale of a SUSY theory, the heavier its lightest stable supersymmetric particle is expected to be.   So, the part of the SUSY parameter space not excluded by the LHC yet should provide the easiest for direct dark matter detection experiments to detect WIMPs.

If there is no dark matter in this mass range, then all of the SUSY theories that predict stable lightest supersymmetric particles (LSPs) in that mass range are again disfavored by the experimental data.  I have yet to see a SUSY theory that predicts a lightest supersymmetric particle of 15 GeV of mass or less, for the simple reason that collider experiments should have detected such a light LSP long ago.  Even strong SUSY proponents concede that SUSY particles that light have been experimentally excluded even in non-minimal supersymmetry theories.

Of course, this result doesn't rule out SUSY theories with no stable supersymmetric particles, but theories with no stable supersymmetric particles have one less point of phenomenology explained to recommend and empirically motivate them.  Why have a SUSY theory that can't even provide one viable dark matter candidate out of its embarassment of riches when it comes to new particles?

No comments: