Direct Dark Matter Search Results Still Contradictory

Direct searches for dark matter are still producing contradictory results that are not easy to reconcile.

Some experiments that show some dark matter signal at the low mass end of the WIMP mass range. But the experiments showing some dark matter signals differ both from the experiments showing no dark matter signals, and from each other. The experiments that do show a dark matter signal indicate the existence WIMPs in mass ranges (on the order of 5% of a top quark or 10% of a W boson or Z boson) that particle accelerator data should have revealed if these particles were fundamental, but have instead been ruled out experimentally. Moreover, all known or even seriously hypothesized composite particles (mesons, quarks and exotic hadrons) are too heavy to fit the apparent dark matter signals.

Also disappointing from leading lights of the scientific world making conference presentations on dark matter are several omissions:

1. Failure to include the results of new baryonic matter surveys from elliptical galaxies that show that past estimates of the relative proportions of dark matter and baryonic matter profoundly underestimate the amount of baryonoic matter in the universe. The old estimates put the amount of dark matter at 3-5 times the amount of ordinary matter. The new ordinary matter census suggests that the actual value is tantalizingly close to 1:1. This data is not so new that dark matter keynote speeches in the fall of 2011 should still be using the old pie charts. Yet, it profoundly influences inferrences about dark matter candidates from baryongenesis and leptogenesis processes.

2. Failure to grapple seriously with studies that have shown that models of galactic rotation curves based upon Newtonian approximations are significantly overestimating the amount of dark matter necessary to produce the observed rotation curves, by omitting graviational effects found in general relativity but not Newtonian gravity in these systems. At the very least, any serious analysis of the issue should acknowledge that these papers are out there and evaluate them on the merits. Any result that can explain some of the observed anamolies with existing, well proven physical laws that don't need to be modified at all to reach a result need to be taken seriously. This further pushes the ordinary to dark matter ratio in the universe to 1:1 or even 1:0.5 or so, although probably not to zero.

3. Failure to engage warm dark matter theory analysis that suggests that large scale galactic structure considerations require a somewhat lighter and faster dark matter candidate than the one necessary to fit a traditional cold dark matter hypothesis. Given that most of the direct detection experiments are premised on certain assumptions about dark matter particle speed and are tuned to particular mass ranges, this matters quite a bit.

The big picture dark matter presentations also systemically tend to overstate how well motivated WIMP candidates are given the various direct detection and astronomy limitations on their mass. There are lots of weakly interacting particles predicted by extensions of the Standard Model, but almost all of the lighter ones have been ruled out experimentally by exhaustive high energy experimentation and almost all of the heavier ones are too heavy to fit the experimental limitations on dark matter.

For any particle in the hundreds of GeV mass range or less, collider data place stringent limitations on the properties of any particles that can be produced, and while direct detection experiments are contradictory in some respects, they are unanimous in ruling out heavier WIMP possiblities. Any fundamental WIMP candidate that can be produced in W or Z boson decay is pretty definitely ruled out in the entire mass range from the fraction of a eV mass associated with neutrinos to half of the W/Z boson mass, and no resonnance of unstable heavier particles has been detected either. Even one stable, fundamental heavy WIMP candidate is going to produce an immense quantity of missing traverse energy if it is produced in a decay from a very high energy W or Z (e.g. from top quark decay).

To escape those limitations, a WIMP must either be a composite particle of some sort, or be in a matter sector that has no weak force interactions, in addition to having no strong force and no electromagnetic interactions.

There is at least some emerging acknowledgment that the dark matter sector must be more complicated than a single WIMP model could fit.

The issue of particle stability is also huge. All matter with quarks in it except protons and neutrons, all charged leptons except electrons in the Standard Model, and all free bosons in the Standard Model except photons are massively unstable. Neutrinos are metastable, oscillating between types, but too light. Any realistic fundamental dark matter candidate needs to have a very narrow decay width (which is proportional to the reduced Planck's constant divided by the decay time period of the particle). The decay width of any realistic fundamental dark matter candidate needs to be on the order of that of an electron or neutrino.