Last week, when I wasn’t watching democracy bleed, I was participating in an international virtual workshop, attended by experts from many countries. This meeting of particle experimenters and particle theorists focused on the hypothetical possibility known as “hidden valleys” or “dark sectors”. (As shorthand I’ll refer to them as “HV/DS”). The idea of an HV/DS is that the known elementary particles and forces, which collectively form the Standard Model of particle physics, might be supplemented by additional undiscovered particles that don’t interact with the known forces (other than gravity), but have forces of their own. All sorts of interesting and subtle phenomena, such as this one or this one or this one, might arise if an HV/DS exists in nature.
Of course, according to certain self-appointed guardians of truth, the Standard Model is clearly all there is to be found at the Large Hadron Collider [LHC], all activities at CERN are now just a waste of money, and there’s no point in reading this blog post. Well, I freely admit that it is possible that these individuals have a direct line to God, and are privy to cosmic knowledge that I don’t have. But as far as I know, physics is still an experimental science; our world may be going backwards in many other ways, but I don’t think we should return to Medieval modes of thought, where the opinion of a theorist such as Aristotle was often far more important than actually checking whether that opinion was correct.
According to the methods of modern science, the views of any particular scientist, no matter how vocal, have little value. It doesn’t matter how smart they are; even Nobel Prize-winning theorists have often been wrong. For instance, Murray Gell-Mann said for years that quarks were just a mathematical organizing principle, not actual particles; Martinus Veltman insisted there would be no Higgs boson; Frank Wilczek was confident that supersymmetry would be found at the LHC; and we needn’t rehash all the things that Newton and Einstein were wrong about. In general, theorists who make confident proclamations about nature have a terrible track record, and only get it right very rarely.
The central question for modern science is not about theorists at all. It is this: “What do we know from experiments?”
And when it comes to the possibility of an HV/DS, the answer is “not much… not yet anyway.”
The good news is that we do not need to build another multibillion dollar experimental facility to search for this kind of physics. The existing LHC will do just fine for now; all we need to do is take full advantage of its data. But experimenters and theorists working together must develop the right strategies to search for the relevant clues in the LHC’s vast data sets. That requires completely understanding how an HV/DS might manifest itself, a matter which is far from simple.
Last week’s workshop covered many topics related to these issues. Today I’ll just discuss one: an example of a powerful, novel search strategy used by the ATLAS experiment. (It’s over a year old, but it appeared as my book was coming out, and I was too busy to cover it then.) I’ll explain why it is a good way to look for strong forces in a hidden valley/dark sector, and why it covers ground that, in the long history of particle physics, has never previously been explored.
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