Somehow, you have to like a paper which contains the phrase
Now, let us assume that the effective cross section for the inelastic scattering of two cosmic rays is of the size of the Universe.
Cosmic rays are tiny things - mostly protons, or atomic nuclei - travelling through space, so the idea that they might in any sense become as big as the universe is an odd one. Actually the idea of anything apart from the universe being as big as the universe is disturbing. If your theory predicts this is happening, you might be tempted to junk it and get a new theory.
Of course, that is the point. The paper in question is another attempt at ‘reductio ad absurdum’ on the Standard Model of particle physics; part of an attempt to find out how high in energy we might have to go before the model breaks.
As we increase the energy of our accelerators, we can look at smaller and smaller structures of matter. It is a bit like being given access to more landscape to explore, extending the map of physics into terra incognita at tiny distances.
Physics is in an interesting position, now that the Higgs boson has been discovered. The “Standard Model” doesn’t predict any more new particles, no matter how tiny, and it could be considered internally complete. However, it is very far from being a theory of everything, failing to account for such major experimental facts as gravity, the different amounts of matter and antimatter in the universe, and the 85% or so of stuff that seems to be “Dark Matter”. It also struggles with neutrinos.
It would be good to have some clues to a new theory which might account for those awkward facts. Indeed, the hunt for clues is on, and the crazy assumption quoted above is part of that hunt.
In super-high-energy collisions, way above anything we can do at CERN right now, it is possible¹ to produce, or exchange, huge numbers of particles, especially Higgs bosons. The authors (Jaeckel and Khoze, from Heidelberg and Durham) do this as a thought experiment. They extrapolate the Standard Model to very high energies, and calculate particle-scattering probabilities, or cross-sections. They find that at a particular energy, the answers become stupidly large.
Since we have measured cosmic rays up to pretty high energies, Jaeckel and Khoze can use those measurements coupled with their calculations, to set a maximum energy, a limit above which the Standard Model become cosmically stupid, if you like. What this really means is that some new physics - new particles or forces or something - beyond the Standard Model, has to appear at an energy lower than that, in order to fix the problem at high energy. In turn, that might tell you how big a new particle accelerator needs to be to study that physics.
That might sound far-fetched, but similar arguments actually told us that the Large Hadron Collider (LHC) was the right size. Without the Higgs boson, the Standard Model predicted stupidly-large cross sections even at LHC energies. So we knew we would either find the Higgs, or some physics beyond the Standard Model (or, as we still hope, both).
Such arguments don’t really tell you what to expect from a new particle collider, but they can tell you how high the energy has to be for you to be guaranteed to find something. Unfortunately the current answer Jaeckel and Khoze come up with is around 1000 TeV.² Given that the highest-energy collider even being dreamed of is currently a factor ten below this, that’s bit discouraging.
Still, it’s a start. There are other arguments that indicate lower energies might be enough, and even Jaeckel and Khoze offer more hope. As well as the ‘cosmic stupid’ limit, they derive an indirect limit on the energy-scale of new physics from precision measurements of the Z boson at LEP (the previous experiment in the tunnel the LHC now occupies), again because of multiple Higgs exchanges. This also points to a very high energy. However, it implies that by improving those measurements of the Z at a future electron-positron collider we might get more information, though how much is not yet clear.
Also, the probability of producing many Higgs (or other) bosons in proton-proton collisions is predicted to rise very rapidly even before hitting the ‘cosmic stupid’ limit, and this could well show up at a 100 TeV machine; or perhaps even at the 14 TeV LHC, when it restarts in 2015. We shall have to see - and keep an eye out for lots more bosons.
¹basically because E=mc², as any fule kno.
² That’s 1000 Tera-electrovolts, or the energy an electron would get if accelerated through 1000 trillion volts.
Jon Butterworth has written a book about being involved in the discovery of the Higgs boson, Smashing Physics, available here. Some interesting events where you might be able to hear him talk about it etc are listed here. Also, Twitter.