Dark Words

Postcards from the Energy Frontier by Prof Jon Butterworth

There’s an article published in Nature Physics, and available on the CERN website here, about the potential of a new generation of collider experiments at CERN. It is in the form of a commentary on the newly updated European Strategy for Particle Physics, which prioritised a new, large collider at CERN.

Since the article is written by the Director General of the laboratory, Fabiola Gianotti, and the head of the theory department, Gian Giudice, it is as you might expect quite positive about the science one could do with such experiments. So much, so predictable I guess, though I urge you have a look at the strategy (disclosure, I was involved in drafting that) and the Nature article, if you are interested in such matters.

It is also not surprising that not everyone shares their enthusiasm; even though the European Strategy was agreed after discussion among a very wide range of scientists, many of whom have themselves nothing directly to do with large colliders, or indeed with CERN, no scientific project will ever command universal support, especially when it requires major resources which could arguably be deployed elsewhere in physics, science or on something completely different. Even if, as has been the case for the LHC and past machines, the return on investment is eventually very significant, the investment itself is very substantial and carries opportunity costs.

What I did find surprising was a quote I saw from the Nature article that Dark Matter would be a “guaranteed result” of such a new collider.

Dark Matter is the name given to the presumed source of gravitational attraction responsible for various astrophysical observations, most notably the fact that the rotational velocity curves (how fast the stars rotate about the centre, depending on their distance from the centre) of most galaxies only make sense if the galaxies contain a lot more mass than we can actually see.

What Dark Matter actually is remain a mystery. A favoured candidate is a kind of heavy particle that interacts only via the Weak interaction (and via gravity, of course), generically known as a “WIMP” (Weakly Interacting Massive Particle). But there are many other possibilities, and even a chance (most cosmologists would say a very small one) that General Relativity is wrong and there is no Dark Matter at all. Cosmologists enter the picture because the Dark Matter was presumably produced in the early stages of the Big Bang, when the universe was very hot and dense so “thermal” production was possible.

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Assuming that Dark Matter is a WIMP and was produced this way seems very reasonable, and covers a wide range of popular models. It also gives us a fair chance of seeing Dark Matter in a collider; for example either the LHC, its high-luminosity version, or the potential collider-that-is-to-come after.

Sadly though, for some of the other possibilities, colliders have no chance.

So there is no guaranteed discovery of Dark Matter, no matter how big your collider. Gianotti and Guidice surely know this, so what are they on about here? They do indeed say Dark Matter is a guaranteed result. However, the full quote is:

A good example of a guaranteed result is dark matter. A proton collider operating at energies around 100 TeV will conclusively probe the existence of weakly interacting dark-matter particles of thermal origin. This will lead either to a sensational discovery or to an experimental exclusion that will profoundly influence both particle physics and astrophysics.

So they are… erm. Completely correct. It’s an either/or. Either a WIMP consistent with thermally-produced Dark Matter will be discovered, or it will be ruled out and we will have to look for a better idea. This was pretty much the same situation as we had with the Higgs boson before the LHC took data (although then it was the Standard Model at stake, not just a popular class of Dark Matter models).

Oh. Well. Gianotti and Giudice were right then. That quote seems to contain a lot more words than I could actually see. Or else I guess I fell for some clickbait.

Note added after some discussion with colleagues: The FCC document which I think the Nature article relies on says/“The FCC-hh would essentially provide a comprehensive search for WIMP DM”/. The word/“essentially”/seems important here; I take it to mean there may be some, possibly rather contrived, WIMP scenarios which would still be out of reach.

Link to FFC document: https://link.springer.com/article/10.1140/epjc/s10052-019-6904-3

Professor Jon Butterworth is a physics professor at University College London and a researcher on the ATLAS experiment at CERN involved with, amongst other things, the discovery of the Higgs Boson. He is the author of two popular science books Smashing Physics and A Map of the Invisible. Postcards From the Energy Frontier is the successor to Jon’s hugely successful blog for The Guardian, Life and Physics. He is @jonmbutterworth on Twitter.

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