On Tuesday the 2015 Nobel Prize for Physics was awarded for neutrino physics at the Sudbury Neutrino Observatory (SNO), in Canada, and the SuperKamiokamde experiment in Japan. The start of the press release reads:

and apart from the physics, there’s a really interesting and important feature of that announcement, which I will come back to shortly. Before that though, as I said on the Guardian’s live blog,



The discovery of neutrino oscillations is the only definite “Beyond the Standard Model” physics discovery to have happened during my career.

and like most great scientific breakthroughs, the results not only answered a long-standing anomaly, but also kicked off several exciting new directions of research.

The long-standing anomaly that was solved was the “solar neutrino problem”, first observed in experiments led by Raymond Davis Jr. and John Bahcall in the late 1960s and early 1970s¹.



The essence of the problem with solar neutrinos was that there weren’t enough of them.



Our understanding of the Sun and nuclear physics led to a prediction that a certain number of neutrinos should be being produced by the fusion reactions in the Sun. These would be “electron-neutrinos”, meaning they are produced in a association with electrons (as in radioactive beta decay of unstable nuclei). They interact rarely, so are very hard to detect, but one thing they can do is scatter off nuclei and produce electrons, called a “charged-current” interaction, sort of the inverse of the process by which they were created. Bahcall and Davis predicted how often this should happen - in the specific case of neutrinos from the Sun hitting chlorine nuclei and producing argon plus an electron - and designed and built experiments to measure it. They saw that it did not happen as often as expected.

I remember being taught this during my doctoral training at Oxford. Either we didn’t understand the Sun, or we didn’t understand nuclear physics, or the experiments were wrong, or the Standard Model of particle physics was wrong. As a budding particle physicist I was sure the other lot must have made a mistake somewhere.



If the problem were indeed with particle physics, the solution would be intriguing though. It would mean introducing different masses for the neutrinos. In the Standard Model, the neutrino mass was zero. Different masses would allow the neutrinos to oscillate - to change type - as they travelled from the Sun to the Earth. The neutrinos hitting Davis’ detector might not be electron-neutrinos anymore, but might have changed into muon-neutrinos or tau-neutrinos - the other two options in the Standard Model. They would therefore not produce electrons when they interacted, and the detector would be blind to them (the neutrino energy is not high enough to produce the more massive muon or tau).

The crucial innovation of the SNO was that it was designed to detect neutrinos of any flavour, by measuring the “neutral current” interaction, in which the neutrino breaks up a nucleus, but no electron (or muon or tau) is produced. This required detectors of unprecedented sensitivity; to confidently detect the tiny signal of a broken nucleus, all other extraneous radioactivity has to be reduced to the lowest possible level, and measured precisely. They were able to measure the total number of neutrinos coming from the Sun, regardless of whether they oscillated or not. They could also measure the charged current interactions of electron-neutrinos, and thus measure the oscillations directly.



SuperKamiokande measured neutrino oscillations from higher-energy muon-neutrinos, produced by cosmic rays in the upper atmosphere. This showed that neutrinos did have mass; SNO showed that this was indeed the solution to the solar neutrino problem.

Some of the DPhil students studying at the same time as me in Oxford, back in the early 1990’s, were working on SNO. Oxford was the only UK university working on SNO then. Neutrino physics was not in favour in the UK. My current university, UCL, had been heavily involved in the technology that made Kamiokande and then SuperKamiokande work, but for whatever reasons was not able to stay involved, and so missed out on that physics. Oxford managed to stay the course, and made several critical technological contributions to SNO.



They developed one of the two extraction processes for removing radioactive radium and thorium, based on technology developed at BNFL for removing nuclear waste. They also developed an assay system used to measure the remaining levels of uranium and thorium, worked on the actual assembly of the experiment, and were lead developers of the simulation and analysis software. Professor Dave Wark, one of the UK leaders on SNO, described to me a particular challenge - finding a way of building mirrors, part of the detection system, which would remain reflective for a decade in ultra-pure water. Ultra-pure water is surprisingly virulent stuff². It has a few free hydrogen ions (H⁺ - protons) and an equal number of free hydroxide ions (OH⁻), so it can behave a bit like an acid and a bleach, but most importantly it is an absolutely excellent solvent.



These were big contributions, but there were of course many others, not least from the Canadian hosts of the experiment. SuperKamiokande too was the result of the labours - intellectual, physical and political - of hundreds, probably thousands, of physicists and engineers. Several colleagues have cited Herb Chen (SNO), and Yoji Totsuka (SuperKamiokande) as founding leaders of the experiments, both sadly no longer with us. So it goes, with long-term, large-scale science.



And that brings me back to the Nobel Prize announcement at the top of the article.



Note that right underneath the names of the recipients of the prize are the names of their collaborations. I checked previous announcements, and the only other time this has happened, at least since 1980, was in 2011, when the prize was awarded

for the discovery of the accelerating expansion of the Universe through observations of distant supernovae

to Saul Perlmetter (The Supernova Cosmology Project) and to Brian Schmidt and Adam Riess (The High-z Supernova Search Team). As with this year’s prize, I don’t think anyone doubted that the recipients made massive personal contributions and the committee chose well, but similarly, few would doubt that an element of the glory is rightly shared by all those members of the collaborations who contributed.

The exciting new directions of research kicked off by the discovery of neutrino oscillations continue to be explored. How fast exactly do neutrinos oscillate? Are there only three types? What are their masses? Do neutrinos and antineutrinos behave the same or are there subtle differences? Maybe the neutrino is even its own antiparticle? SNO (now SNO+) and SuperKamiokande continue to play a role, along with many other current and planned experiments. As Wark says

These were the experiments that broke the Standard Model of particle physics, which has stood up to every other assault that we have thrown at it.

Understanding neutrinos in depth is critical to understanding how the universe works and where it came from. There is still quite a way to go; and it is the shared vision and dedication of collaborative science that will stay the distance.



¹ Davis won a share of the Nobel in 2002 for this along with Masatoshi Koshiba who worked on the SuperKamiokande experiment and its predecessors.



²According to well-established homeopathic principles³, ultra-pure water is infinitely potent, of course.

³ i.e. nonsense.



Jon Butterworth’s book Smashing Physics is available as “Most Wanted Particle” in Canada & the US and was shortlisted for the Royal Society Winton Prize for Science Books.