Scientific Method / Science & Exploration

Is it now or never for dark matter WIMPs?

WIMPs, the leading candidates for dark matter, have proven elusive so far. But …

For several decades, astronomers and cosmologists have been piling up data that indicates most of the matter in the Universe is dark, interacting only via gravity. As modified theories of gravity failed to account for observation, candidates for dark mater were winnowed down until one remained in favor: the weakly interactive massive particle, or WIMP. Over the past several years, potential signals of WIMPs have appeared in space and on Earth; that, combined with the startup of the LHC, has given the research community the sense that it's close to pinning down the identity of the WIMPs. But a review in this week's Nature considers what might happen if we fail.

The behavior of dark matter pretty clearly suggests that it will come in the form of particles, instead of alternatives like large, nonradiating objects in galactic halos. There are several classes of hypothetical particles that could have the right properties to produce dark matter effects, including axions and sterile neutrinos. WIMPs, however, have become quite popular because they naturally fall out of one of the systems favored to replace the Standard Model, supersymmetry.

Supersymmetry posits an entirely new set of particles that parallel our familiar ones, with a squark for each quark and so on. In most versions of supersymmetry, only the lightest particle, the neutralino, is stable, and it is likely to have a mass that is in keeping with predictions for dark matter. Unfortunately, the minimal supersymmetric model has about 120 free parameters, and it's possible to fill these in with parameters that give the neutralino a broad range of properties. Most specific models of supersymmetry have fewer degrees of freedom, but there's still a fair bit of flexibility possible.

And that creates a bit of a problem when it comes to some of the promising astronomic data, according to Gianfranco Bertone, the review's author. WIMPs may not interact much with regular matter, but they can annihilate through collisions. The rate of annihilation had declined steadily with the Universe's expansion, but there's expected to be a continued background rate of collisions any place with a concentration of dark matter, like a galaxy. And, in fact, the PAMELA satellite has detected an unusual surplus of positrons that could be created by dark matter collisions.

Unfortunately, the most promising versions of supersymmetry don't place a WIMP in the right mass range to produce a signal of the energy that PAMELA actually sees. There are a bunch of ways to tweak supersymmetry to get something that works, but Bertone calls these solutions "ad hoc," and certainly not enough to cause anyone to favor WIMP annihilation over an unidentified astronomical source for the positrons.

Earthbound dark matter detectors face similar problems. The DAMA/LIBRA and CoGeNT experiments have both provided hints of weak dark matter interactions with regular matter, but the apparent mass of the WIMP is relatively low compared to most models. Meanwhile, a different project, XENON100, isn't seeing any of these signals. Despite the confusion over what has been seen, the detectors at least seem to agree on what hasn't. There are at least some masses and collision probabilities that have now been excluded, which provides some constraints on the versions of supersymmetry that we should be considering.

The good news is that there may be help on the way. Several new observatories and detectors are in the works, and many of the current ones are continuing to gather data, so at least some of the confusion might be resolved. The big hope on the horizon is the LHC. Bertone cites estimates that indicate most dark matter candidates should be apparent within five to 10 years of running the collider. But he also cautions that spotting something might not be as definitive as we would like. All we'd know is that the WIMP didn't decay before it left the collider, which is less than a microsecond. Dark matter has been stable for the life of the Universe.

But if the LHC fails to spot anything, the remaining supersymmetric models would require extensive fine tuning to remain compatible with both WIMPs and the LHC's results. And we'd be very unlikely to build another accelerator that could help exclude those possibilities anytime soon. That would leave it up to direct detection or astronomical results to clarify the situation, and they haven't yet. Bertone can foresee a time a decade from now where we'd simply run out of obvious options for finding a WIMP.

With no certain way of studying them, Bertone thinks that physicists would gradually start focusing on some of the alternative hypothetical particles. Which, in a lot of ways, is perfectly reasonable; the simple forms of WIMPs would have been ruled out, and in purely practical terms, it's hard to pursue a research program without an obvious way of doing an experiment. But this shift would occur despite the fact that all the reasons that WIMPs seemed appealing in the first place would still apply.

Perhaps for that reason, Bertone ends with a bit of optimism. We've got plenty of options for spotting a WIMP, and as much of a decade to go before we reach this level of disappointment.

Nature, 2010. DOI: 10.1038/nature09509  (About DOIs).

Expand full story

You must to comment.


You May Also Like