Saturday, February 23, 2019

Rumors of the WIMP miracle's death have been greatly exaggerated

Ethan Siegel has shown us another example of the profound difference between careful scientists on one side and zealous activists on the other side (the side where he sadly belongs) when he wrote
The 'WIMP Miracle' Hope For Dark Matter Is Dead
The bold statement from his title is repeated very many times in his text:
[...] The big hope was for a WIMP miracle, a great prediction of supersymmetry. It’s 2019, and that hope is now dashed. Direct detection experiments have thoroughly ruled out the WIMPs we were hoping for. [...] Theorists can always tweak their models, and have done so many times, pushing the anticipated cross-section down and down as null result after null result rolls in. That’s the worst kind of science you can do, however: simply shifting the goalposts for no physical reason other than your experimental constraints have become more severe. There is no longer any motivation, other than preferring a conclusion that the data rules out, in doing so. [...]
And to make sure that you won't overlook them, he repeats the thesis that the "WIMP miracle is dead" at several other places.



Well, the theories incorporating the WIMP miracle are more constrained than a decade ago but they're not dead. You know, clickbait writers and activists such as Ethan Siegel love oversimplified statements. They particularly love the idea that they "totally killed" an idea. That's how they become attractive for simple-minded readers.

However, in real science, you may only kill an idea by actually falsifying it. And that hasn't been the case of the WIMP miracle.



The energy density of the Universe is composed of
  • (almost 70%) dark energy, some non-localized substance that has the negative pressure \(p=-\rho\) which makes its stress energy tensor \(T_{\mu\nu}\) proportional to the metric tensor \(g_{\mu\nu}\), a choice that preserves the local Lorentz symmetry (the cosmological constant is dark energy for which this form of the stress-energy tensor is precise and constant all over the Cosmos and its history)
  • (about 25%) dark matter, something we only see through its gravitational effects, e.g. on the galactic rotation curves, but not electromagnetically; theories that this matter doesn't exist and the curves are due to modifications of the gravitational force are known as MOND but those theories faced much bigger trouble than dark matter in recent years
  • (about 5%) of the visible, baryonic matter – most of this mass is composed of protons and neutrons and they form atoms which send us electromagnetic radiation so that we see them
If we talk about the dark matter, the second component exists. It may hypothetically be composed of some small black holes or MACHOs or RAMBOs or SIMPs etc. but the option that still looks like the most likely single choice to most cosmologists is that dark matter is composed of a new type of a particle. One of the possible types of a new particle species that makes up dark matter is an axion; however, I think it is fair to say that the most motivated one is still WIMP, the Weakly Interacting Massive Particle (the acronym shows the physicists' characteristic playfulness in inventing terminology).

A nice "coincidence" that has been known for decades – the WIMP miracle – is that if this particle has the mass around \(100\GeV\) through \(1\TeV\) or so, then the thermal evolution in cosmology leads to the prediction that the current abundance of the particle should be almost exactly high enough to account for the observed 25% of the energy density of the Universe. What's nice about this mass is that it is also the right mass for the particle to explain the lightness of the Higgs boson – the mass is close to the Higgs mass itself. So this new particle could explain both the lightness of the Higgs as well as the abundance of dark matter. When "one product" is capable of solving "two problems" at the same moment, it looks like a good product, you surely understand why. The mass of the hypothetical particle may be estimated in two different ways that approximately agree. And this nontrivial agreement is evidence supporting the hypothesis.

The single most attractive subtype of WIMP is the LSP, the lightest superpartner of a Standard Model particle in theories with supersymmetry. LSPs may be neutralinos, the superpartners of the Higgs, photon, and Z-boson (a superposition of the photino, higgsino, and zino that happens to be the lightest mass eigenstate). Because the number of superpartners is conserved modulo two (R-parity) in many supersymmetric models, the single lightest superpartner can't decay to anything lighter while preserving the R-parity, and is therefore stable. (You may generalize this clever idea and invent other new particles independent of SUSY with symmetries or charges that are only analogous to the R-parity but that also make the lightest charged particle stable.)

In the recent decade, two classes of experiments have made the picture marginally less likely than before. The LHC hasn't found any new physics which shows that the most straightforward or naive assumptions about naturalness don't work (and some naturalness needs to be assumed for the WIMP miracle to exist); and the most expensive direct search experiments for the dark matter (XENON, LUX, and others) haven't found a WIMP, either, although they already got to the sensitivity that excludes quite a fraction of the simplest vanilla models involving the WIMP miracle.

The space of possible "theories including the numbers" – the parameter spaces – have been constrained. It is impossible to tell you any precise percentage of the spaces that have been eliminated. It may be 10%, 50%, 90%, or something else, depending on your chosen measure on the spaces, assumptions about the allowed degree of fine-tuning, about the deviations from the thermal history of the Universe, and lots of other things.

But even if you said that 95% of the parameter space is excluded, it would simply not allow you to defend the statement that the WIMP miracle is "dead" – because an experimental "proof" at the 95% confidence level is still extremely weak (we also call it "two-sigma evidence") and can't be reasonably called a "proof" at all.

Now, if you're reading this blog post seriously, I want you to do some "research" in the sense of "comparative literature". Try to read excerpts from a dozen of actual technical physicists' articles (the PDF files are found after one or two clicks) that contain "WIMP" or "WIMP miracle", especially the recent ones, and decide whether Ethan Siegel's assertions agree with the literature:
You may try many other searches and combinations. Open some PDF files. You will see that they say what I say. Experiments looking for WIMPs are ongoing and new ones are being planned. Theorists keep on discussing both WIMP and WIMP miracle because it's still one of the most intriguing observations that indicate what the dark matter could be composed of – and how heavy it could be.

The theories are under pressure that most of the authors of those papers are aware of but the general concept simply isn't dead.

Let me mention some data about the "WIMP miracle" in the title. INSPIRE finds 10 papers with such titles and they were posted in 2009, 2010, 2010, 2011, 2012, 2012, 2012, 2013, 2015, and 2017. The last two papers include adjectives in front of the "WIMP miracle" – "fraternal" and "of the second kind", respectively. You might say that the frequency of the "WIMP miracle" decreased but physicists generally don't see the question about the validity of the "WIMP miracle" as a settled one simply because it is not settled.

When new experiments achieve a greater sensitivity than their predecessors, it's always possible that they will make a discovery – and some physicists are genuinely expecting the new results. Siegel's and other people's assumption that they can't ever find anything is pure faith, it has nothing whatever to do with the scientific arguments. The experiments may continue to find null results for 100 years but it just isn't known now.

Like many other laymen, Siegel just totally misunderstands what great theoretical physics is when he says:
Theorists can always tweak their models, and have done so many times, pushing the anticipated cross-section down and down as null result after null result rolls in. That’s the worst kind of science you can do, however: simply shifting the goalposts for no physical reason other than your experimental constraints have become more severe.
On the contrary, it's the most correct thing that theorists should do when experiments gradually tighten the limits. The reason is known as "Bayesian inference". When new experimental data arrive, theorists – and all scientists – must adjust their opinions about the probabilities of various theories, scenarios, and statements. This adjustment is no heresy, it is not unethical. On the contrary, it's absolutely essential because this is what underlies the statement that the truth in science ultimately boils down to empirical facts. "Shifting the goalposts" is just a silly negative synonym for "taking the empirical facts into account" – which is actually a good, essential thing. The new experimental evidence may be "just one physical reason" but it's a very important one (and Siegel's attempt to mock it is silly) because all of science is ultimately governed by the empirical facts.

But when the theory is "fuzzy", like the WIMP miracle or naturalness of the Higgs mass, and the experiments are "gradual", then the adjustments of the beliefs are and must be "gradual", too. Anything else would be irrational and unrepresentative of the actual experimental facts!



In this documentary movie, Ethan Siegel is one of the simpletons who want to "burn the WIMP" because she's made of wood etc.

It's exciting for Siegel and his friends to burn the witches and WIMPs but it's not really science. The lady should have been allowed to live – and scientists must be allowed to do research into theories they see as intriguing and/or likely. And theories with some sort of a WIMP miracle are still among the top ones. And believe me, I have absolutely no horse there. Even on this blog, you find "WIMP miracle" at roughly five places and you may see that "I don't really have a strong belief". And an axion dark matter is almost as OK for me as a WIMP. But I also know that Siegel's black-and-white statements are untrue.

Siegel may be the kind of a guy who loves simple statements by himself – and simple-minded conclusions. But not all insights in physics are simple – if it were the case, a simpleton would be the ideal person to make the discovery. Instead, many discoveries that physics has to make are really new – and the ideal person who may do them is a Newton, not Simpleton. ;-)

Unfortunately, the gap between activists pretending to be the "voices of science" or "communicators between the public and science" – such as Siegel – and the actual science is growing deeper. Many of these people have turned to nothing else than full-blown haters of the scientific research whose own methodology is much closer to the most self-evident examples of fraud than to proper research based on the scientific integrity.

Be ashamed, Siegel.

I would also like to point out that these frantic activists' efforts to "burn the witches" or "declare the WIMP miracle dead" are seen in many other contexts. The movement to ban the fossil fuels is an example. As a source of energy, fossil fuels have faced some competitors whose importance was pretty much growing in recent years (and for the same reason, the relative importance of fossil fuels was decreasing, although the statement must be interpreted very carefully, especially in the wake of the fracking revolution) but they still remain an important source of energy, almost certainly the most important one for the mankind, just like the WIMP miracle is probably remaining the most important "numerical clue" in all of the research of dark matter. Needless to say, there's a funny difference between the people who want to "burn the witch" and those who fight against fossil fuels. The haters of the fossil fuels actually don't want to "burn the fossil fuels" at all! ;-) Their hatred is manifested by their desire to ban the burning of the fossil fuels. Too bad that the witches didn't face similar haters, they could have survived. ;-)

The opponents may perhaps (and in some cases) correctly observe some "negative trend" for the single most important paradigm or technology – fossil fuels or WIMP – and they're doing "momentum trading", effectively assuming that the value of fossil fuels or WIMP (and there are many such examples) is zero. But it is not zero, the financial markets couldn't work well just with "momentum traders" (who are really just a destabilizing force), and healthy dark matter science can't do without WIMPs just like the modern industrial economy would be devastated without the fossil fuels right now.

At any rate, the main point of this blog post is that you just can't replace the scientific qualities, careful analyses, and integrity by premature conclusions or by your decision to be very radical and straightforward. Premature conclusions and radical prejudices are among the greatest enemies of science.

No comments:

Post a Comment