A Century of Cosmic Rays

IT IS PROBABLY FAIR TO SAY that the 4th of July 2012 will go down in history as the day we discovered the Higgs boson (or, at least, something like it). The last jigsaw piece of the Standard Model of particle physics has been found. While we may not be sure if it’s an edge, a corner, or one of those really important middle bits, at least we know now that it was indeed down the back of the Sofa of Time, and not lodged somewhere in the bowels of some naughty Cosmological Canine*.

But did you know that 2012 also marks one hundred years of cosmic rays? In 1912, Victor Hess got into the basket of a rickety hydrogen balloon, ascended to an altitude of about five kilometres and found, through a series of careful measurements, that the level of ionising radiation increased the further up he went. The results of his experiments suggested that some previously unknown extraterrestrial source was responsible – that the Earth was, in fact, being bombarded by particles from outer space. Hess shared the 1936 Nobel Prize for this discovery.

Now, while this was interesting in its own right, for particle physicists cosmic rays were also (quite literally) gifts from the heavens. Until then, they were mainly playing about with cathode rays or naturally radioactive materials to probe the nature of matter (electrons were discovered with the former, the nucleus/protons/neutrons with the latter). Cosmic rays blasted open a new frontier for physicists; the energies at which they were pummelling the Earth were far beyond what was achievable in the lab at that time. This made possible the discovery of the muon, the pi mesons and the positron — particles that pushed us out of our atomic comfort zone and marked the first steps towards our current understanding of matter and forces at the fundamental level – the Standard Model.

Of course, the picture of quarks, leptons, Ws, Zs, photons and gluons that we know today has largely been pieced together by accelerator-based experiments; machines that offer the impatient physicist with cosmic rays on demand (the first giga-electron volt accelerator was named “The Cosmotron“). On Wednesday, the series of ever-larger particle smashers has reached a triumphant climax with CERN’s Large Hadron Collider and the discovery of the hitherto missing mass-bringer. But here’s the twist: the LHC hasn’t found anything else yet**. Plans to increase the energy have been pushed back to allow further study of the new resonance at 125 GeV. So how might we probe energies beyond those of the LHC that might give us an insight into physics beyond the Standard Model?

One possibility (as you may have guessed) is to return to where it all began – Hess’s cosmic rays. Of course, in true 21st century “Big Science” style, we won’t be using balloons and photographic emulsion. We’ll be using the Pierre Auger Observatory, an array of networked detectors in Argentina covering an area the size of the M25, to study the highest energy Extensive Air Showers. We’ll be using the HESS array in Namibia to probe cosmic gamma rays up to energies of 100 TeV. But most excitingly (for me, at least!) are projects such as LUCID, CERN@school and CORUS that will put cosmic ray detectors in schools to allow students to be part of cutting-edge research that will help us answer some of the biggest unanswered questions in fundamental physics.

The LUCID display at the Cosmic 100 Summer Science Exhibition.

If you want to find out more — like what some of these questions are — come along to the “Cosmic 100: A 100 year-old cosmic mystery” stand at the Royal Society’s Summer Science Exhibition this weekend — the photo is of our LUCID display; if you can’t make it, do feel free to ask a question here or below in the comments.

* Amazingly, this is probably one of the least strained analogies you’ll have heard this week.

** I can’t really emphasise the yet enough, of course. Follow ICHEP 2012 for the latest results.

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