Updated WIMP Exclusion Diagram

This is an update to a post from a few years ago, which itself was an update to a webpage I wrote in 2008, with many updates in between. At that time, the goalposts for detecting WIMPs had already moved repeatedly. I felt some need then to write down a brief synopsis of the history of a beloved hypothesis (including by myself) that had obviously failed as the goalposts were in motion again. That was sixteen years ago.

It is important to remember where we started from, which is now ancient history lost in the myths of time to most who are now working in the field. Indeed, when I search for mention of the WIMP miracle, the theoretical argument that launched a thousand underground detection experiments, little comes up: this essential element of the field has been memory-holed after its failure. I suppose that’s to be expected, as the same thing happened with the decay of the B₀ meson: once heralded as the “golden test” for supersymmetry, it simply stopped getting mentioned after it didn’t work out.

The original expectation for WIMPs was a particle of mass around 100 GeV/c² with an interaction cross-section of about 10^-39 cm². While I remember this, it is getting rare to find this statement, so let me quote a particle physicist:

“The most appealing possibility – a weak scale dark matter particle interacting with matter via Z-boson exchange – leads to the cross section of order 10^-39 cm²“

14 April 2011 Resonaances

To translate a little bit, the Z-boson is a carrier of the weak nuclear force (as photons are for electromagnetism), so this envisions an otherwise normal interaction that involves a new particle, the WIMP. The weak force is, well, weak, so the interaction probability is small, as quantified by the tiny cross section of 10^-39 cm². That makes such interactions rare, but particle physicists are talented at detecting such phenomena. It helps to have a lot of target material in your detector in a place that is well-shielded from background interference, hence all the giant underground WIMP experiments. Consequently, to continue the quote above,

“the cross section of order 10^-39 cm² … was excluded back in the 80s by the first round of dark matter experiments.”

And so the goalposts were set in motion. There were many steps along this path, so I’ll highlight only one, circa 2008. To complete the quote from Resonaances,

“There exists another natural possibility for WIMP dark matter: a particle interacting via Higgs boson exchange. This would lead to the cross section in the 10^-42 – 10^-46 cm² ballpark (depending on the Higgs mass and on the coupling of dark matter to the Higgs).”

So the interaction via the Z-boson had been excluded, but one can have other interactions, this one via the Higgs (which had not quite yet been detected: discovery was in 2012; the Resonaances quote is from 2011. Since then, the Higgs might be said to be “too normal” to make room for any of this.) The possibility of Higgs exchange leads to the blue-green predicted region of Trotta et al. (2008) in the exclusion diagram shown below. If one looks for such plots in the literature, one finds a natural tendency for their upper limits to migrate downwards along with the limits they portray. I thought it might be instructive to update the plot to show the full range of progress:

I call out the 2008 threshold because we had a conference here at CWRU in 2009 (while I was at the University of Maryland) at which the Trotta et al. prediction was presented. I had already become skeptical of the moving goalposts, so I wondered how much of the probability density was in the tail to low cross-section. A low-likelihood tail seems a lot more probable once the head is lopped off! I made this point at the time, and asked how important the tail was. The answer was about 2% or the probability. The speaker went on to express the usual overconfidence that WIMPs would be detected in the more likely region (marked by an X in the blue region with the handy arrow pointing to it).

The experimentalists have done a fabulous job in increasing the sensitivity of their experiments so that they can see to ever lower interaction cross section. Had WIMPs existed as predicted initially, or subsequently, they would have been detected by now. These experiments have succeeded in failing quite brilliantly. I had long before shown that the astronomical data did not add up for any flavor of dark matter. Maybe WIMPs don’t live in this universe?

While we’d be happy to detect dark matter anywhere in parameter space, the WIMP does have sweet spots: first 10^-39 cm² then 10^-44 cm². Now that those are gone, what’s next? From the particle physics perspective, I’ve heard it said that the next logical expectation for the cross-section is around 10^-48 cm². This apparently follows from “two-loop corrections.” I have only a vague idea of what that means, but in my practical experience it translates to “a difficult-to-compute effect so exotic that it likely has no bearing on reality, except maybe in the sixth place of decimals.”

More generally, this continual moving of the cross section goalpost is what I meant back in 2008 by the scientific version of the express elevator to hell. It just keeps going down, and can do so forever. I keep warning my colleagues about these things, and they keep not heeding the warnings. Being a scientific Cassandra is getting old.

The problem with pushing detection limits to still lower cross-sections like 10^-48 cm² is that the universe is indeed full of weakly interacting particles with at least a little bit of mass: neutrinos. These are not as massive as WIMPs, and should not be confused with them: neutrinos are Standard Model particles that are known to exist and to have a very small mass (< 1 eV) while WIMPs are expected to be hundreds of GeV and require entirely new physics beyond the Standard Model. I shouldn’t need to say this, but WIMPs and neutrinos are very different beasts. However, they do both have mass and interact weakly, so I’ve noticed that some of the more rabid advocates of dark matter mix these two in order to claim that we know weakly interacting dark matter exists. That much is technically true, but in technical parlance it is also some bold bullshit. Hmmm, actually, I think it is worse than ordinary bullshit. It is willful scientific disinformation that intentionally sews confusion by conflating the unconfirmed existence of WIMPs with the known existence of neutrinos in order to lend an air of certainty to a failed hypothesis.

Meanwhile, experimental progress proceeds apace. The coming generation of WIMP detectors should be sensitive to the solar and atmospheric neutrino background. That is astrophysically interesting, as it can probe nuclear reactions in the sun and, in principle, those in every supernova that have ever exploded. This has bugger all to do with dark matter. However, since that’s what people are looking for, what they built these detectors to find, and they’re completely convinced dark matter exists, and a Nobel prize awaits whoever detects it first, I expect that the first neutrino detections will be misinterpreted as WIMP detections. There will be much arguing between groups, claims and counterclaims, and after a few years it will be recognized that these coming detections are neutrinos not WIMPs. First there will probably be many over-hyped claims that mislead the public into thinking dark matter has been detected.

But there I go being a scientific Cassandra again.