WIMPs on Death Row

One of the main arguments given for the idea of supersymmetric extensions of the standard model has been what SUSY enthusiasts call the “WIMP Miracle” (WIMP=Weakly Interacting Massive Particle). This is the claim that such SUSY models include a stable very massive weakly interacting particle that could provide an explanation for dark matter.

According to the “WIMP Miracle”, evidence for such a particle is supposed to show up as they get produced at the LHC, and at underground detectors designed to look for ones traveling through the earth. Like all other predicted SUSY particles, no evidence for such a thing has appeared at the LHC. A sequence of more and more sensitive underground experiments has also come up empty.

One of the latest of these, LUX, announced results today, see here, press release here. These are the results from the final 20 month run of the LUX detector and they are conclusively negative: no candidate events were seen, putting four times smaller bounds on the cross-section for any such particle. New Scientist has it right I think, with a story headlined Dark matter no-show puts favoured particles on death row. Ethan Siegel has a very good article, Dark Matter May Be Completely Invisible, Concludes World’s Most Sensitive Search, which includes:

The null detection is incredible, with a fantastic slew of implications:

1. Dark matter is most likely not made up, 100%, of the most commonly thought-of WIMP candidates.
2. It is highly unlikely that whatever dark matter is, in light of the LUX results, will be produced at the LHC.
3. And it is quite likely that dark matter lies outside of the standard mass range, either much lower (as with axions or sterile neutrinos) or much higher (as with WIMPzillas).

Enthusiasts are not likely to give up so easily though, with Sean Carroll tweeting that the news is only “we’re not seeing it yet, stay tuned.” Not sure what one is supposed to stay tuned to, this is pretty much a final result from LUX. There will be a next generation experiment, LZ, but that’s for after 2020. There are other competing experiments now operating, including Xenon1T, now being commissioned, which will be somewhat more sensitive. There seems to be no serious reason though to expect WIMPs to appear at somewhat lower cross-sections if they haven’t appeared yet.

With SUSY and the “WIMP miracle” now dead ideas, perhaps that will lead to focus on more promising ones. There is still a great deal that we don’t understand about neutrinos. A few days ago I saw this intriguing news about the PTOLEMY project, which I hadn’t heard about before.

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52 Responses to WIMPs on Death Row

1. GoletaBeach says:

To Low Math, well, not quite.

SUSY has a vast parameter space of possibilities that all are, (IMO) “Natural”. Naturalness is a completely subjective judgement, and I have no doubt that should SUSY show up experimentally *anywhere*, theorists will eventually proclaim the situation “Natural”. What else can they do? Call *Nature* un-*Natural*??

The space of possible neutrino parameters was and is also vast. That *any* experiment happened on the true neutrino parameters is a minor miracle.

That is the analogy: 2 vast parameter space with really no guidance as to where nature is hiding.

The resistance to the Ray Davis Homestake result was much, much stiffer than you portray. By far the strongest resistance was: people thought his experiment was just flat wrong. It was a radiochemical experiment more akin to Rutherford & Soddy than to 1960’s particle physics. The experimental community were just utterly skeptical… thought the missing neutrinos’ were just a bad calibration. And then lots of theorists referred back to the vast parameter space, and shrugged. A few took Davis seriously…. John Bahcall… then the MSW effect showed up in the 1980’s, and Davis got more respect. Turns out the MSW effect is not the origin of the Davis effect, but, still, that MSW can stimulate neutrino transitions really turned peoples heads at the time, and helped stimulate new experiments that verified Davis.

There is an excellent, excellent argument for SUSY… stabilizing quantum field theory at very high energies. How high do you need to go to see SUSY restored? Anybody’s guess. There will never really be a serious argument that any scale is natural’ until we have empirically discovered it.

Sure, we all wish that a very clever \$10,000 experiment will discover SUSY in some incredible series of unlikely events. Believe me, the experimental community probes ideas like that all the friggin’ time. And lots have something to do with Dark Matter. No matter what, though, WIMPs ain’t dead. They don’t need SUSY to still be right.