Monday, May 19, 2014

Dark Matter News

LUX Light WIMP CDM Exclusion Confirmed By SuperCDMS

Several Earth based direct dark matter detection experiments have claimed to see hints of WIMP dark matter in the 8 GeV-20 GeV mass range: The signals from CDMSII (Silicon detector) and CoGeNT were consistent with each other. Signals from CRESTT and DAMA/LIBRA were just barely consistent with each other. The two sets of positive signals are mutually inconsistent with each other. These hints are largely ruled out by a combination of the results from the CDMSII (Germanium detector), CDMSLite, XENON10, and EDELWEISS experiments.

Finally, both the LUX experiment and the newly released results from the SuperCDMS experiment have almost identical, ultra-precise results that exclude the entire range of all four detection hints.  The dispels any concerns that the LUX result might not have been robust due to experiment specific systemic errors.

Direct detection experiments have searched for and not found WIMP dark matter at masses of up to about 200 GeV.

The bottom line is that every hint of a direct detection of dark matter (in each case by a less precise experiment) is ruled out by eight other experimental results, including two ultra-precise searches that confirm each other.  There is no light WIMP dark matter in the vicinity of Earth.

WIMP dark matter has to be less than 7 GeV in mass if it exist in the vicinity of Earth at anything approaching the relic density expected from dark halo models for the Milky Way, and must be less than 4 GeV in mass if its cross-section of interaction is in the higher end where the hints of direct dark matter detection suggested that they would be.

Other Light WIMP Model Problems

Non-Detection in Precision Electroweak Boson Decay Experiments

Naively, every weakly interacting particle with a mass of 45 GeV or less should be produced in particle-antiparticle pairs in the decays of W and Z bosons and the Higgs boson. Yet, 100% of those decays are accounted for with Standard Model particle decays.

Non-Detection In SUSY Particle Searches That Also Disfavor R-Parity Conserving SUSY

Particle accelerator searches for SUSY particles rule out their existence to the hundreds of GeVs of mass in most cases. R-parity conserving models of SUSY which might evade the naive limits of electroweak boson decays on the existence of light weakly interacting particles (because W and Z bosons might have the wrong R-parity and hence might not be produced in that manner) and are based on searches from missing transverse energy in particle accelerator events are particularly stringent.

The Case For A Hooperon

The hints of a WIMP dark matter annihilation signal from the central Milky Way by the FERMI experiment suggests a potential dark matter particle of about 25 GeV-40 GeV, sometimes called the Hooperon. A comprehensive experimental case for the Hooperon most recently made in a February 26, 2014 preprintand an April 24, 2014 preprint that attempt to explain why this has not been seen in latest LHC data.

The non-detection at the LHX, and by LUX and SuperCDMS strain this model, however, because it should be detectable just around the corner at all three of these experiments according to an April 4, 2014 preprint.  Both rule out cold dark matter in the vicinity of Earth with a Hooperon mass unless it has very, very low cross-sections of interaction and suppose a quite complex set of Hooperon properties rather than a minimal set of such properties.  Specifically, the Hooperon theorists must take the following stance to explain why it hasn't been seen elsewhere yet:
[W]e will take the DM to be a Dirac fermion and a SM gauge singlet, with couplings to right-handed down-type quarks. We take the couplings of X b;s;d with quarks to be approximately flavor diagonal, allowing us to associate each flavor in the dark sector with a corresponding flavor of quarks. In particular, we take the lightest of these new particles to be associated with the b-quark, and assume that the heavier flavors decay in this lightest state.
Ten different direct dark matter detection experiments based on Earth, many of which are extremely precise, have not seen such a particle. This radiation signal is the only indication in any experiment that hasn't been repeatedly and soundly contradicted by another experiment using a similar methodology of WIMP dark matter in the 8 GeV to 200 GeV or so range.

Needless to say, I'm skeptical of this possibility and suspect that it has some Standard Model explanation in the not terribly well understood dynamics of the central black hole of the Milky Way, or in systemic issues with the FERMI experiment itself).  It also doesn't help that no other experiment has replicated the FERMI result yet, hints of which were emerging a decade ago.

Impact, In General, Of Non-Detection Of WIMP Cold Dark Matter On SUSY and CDM Theories

In addition to disfavoring WIMP dark matter, this collection of experimental data also disfavors SUSY models generally, because one of the core features of most popular SUSY models is that dark matter is explained as a stable lighest supersymmetric particle (LSP) that is a weakly interacting massive particle in the sub-TeV, super-GeV mass range.

Now, direct dark matter detection experiments and annihilation radiation signature searches for dark matter would fail if the weak interaction cross-section of dark matter particles was basically nil. But, SUSY particles are, by assumption, weakly interacting (their weak interactions are, in general) a fundamental part of the theories of this type).

Dark matter halo distributions and dwarf galaxy level structures that are inferred from astronomy observations are inconsistent with sterile neutrino dark matter (i.e. dark matter that that interacts solely via gravity and fermion contact forces) that has a mass much in excess of 3 keV, which is a million times or more lighter than the typical cold dark matter candidate.

Hints Of A 3.5 keV Dark Matter Emission Line.  

The 3.5 keV X-Ray emission line seen from multiple galactic clusters by multiple experiments, on the other hand, is a more plausible dark matter annihilation signature, and would suggest a slowly decaying warm dark matter candidate with a mass on the order of 7 keV.  No known process creates such an emission, although, of course, there are lots of astronomy processes that are ill understood.

This possibility has problems too, but is not directly contradicted by Earth based direct dark matter detection experiments, and isn't burdened by the theoretical requirements of more traditional SUSY WIMP Cold Dark Matter models. The candidate particle would be several times heavier than the preferred mass of a warm dark matter candidate based upon other data from astronomy, but this discrepancy is far less severe than the experimental hurdles to SUSY WIMP CDM.

Super-Heavy CDM.

The IceCube experiment, as of March 2013, has seen PeV energy neutrino events which could, in principle be fit to very heavy CDM.  But, for a variety of reasons, the super-heavy cold dark matter explanation of these events has not been favored.


Anonymous said...

Thanks for the post.
How has this news not already hit the Physics Media outlet?
I haven't seen anything from New Scientist, Phys.Org, Scientific American, etc...
As you point, I think that the case for a sterile neutrino in the 2-10 keV range is getting stronger everyday. In order for neutrinos to have mass (and oscillate), we need a fourth neutrino particle. But WMAP & Planck clearly put constraints on the sum total of active neutrino mass. (or else the neutrinos would have distorted the final composition of H, He, Li, etc...)
It seems that a 'mostly-sterile' neutrino in the 2-10 keV can help explain: (a) why neutrinos have mass, (b) what is dark matter and it didn't mess up the abundance of H,He&Li, and (c) perhaps what is the cause of the 3.5 keV emission from lots of different galaxies.

SUSY is dead. Particle physics theorists need to focus on a workable "minimal neutrino Standard Model."
The questions I have about sterile neutrinos are the following: when/how were the sterile neutrinos created? (At what epoch of the Big Bang were most of them created?)Can active neutrinos of energy >10keV oscillate into sterile neutrinos? In which case, are they still being created all of the time? Or are they just being destroyed as they decay into photons? In which case, how can two sterile neutrinos of 7 keV rest mass collide and turn into 3.5 keV photons? (Are the two 7 keV neutrinos turning into 4 photons of 3.5 keV energy?)

Let me know if you have any answers to these questions.

andrew said...

Good questions. Of course, we don't even currently know of any lepton number violating processes sufficient to explain the current imbalance and I've done the numbers to know that sterile neutrinos can't solve that problem.

FWIW, I suspect that if there is a "sterile neutrino" that it will not be a higher generation or right handed variant of the SM fermions now called neutrino. More likely, it will be a gravitational sector particle and the name is just a convenient way of describing its properties. I doubt that a sterile neutrino will mix with the three fertile neutrinos of the SM.