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Spotting a proton decay in 50,000 tonnes of water could
point the way to the elusive Theory of Everything
For the last 25 years, a team of scientists have been monitoring a huge water-filled tank deep underground at a site in central Japan. They have been waiting patiently for signs of one of the most bizarre phenomena ever predicted: that all matter decays into sub-atomic dust.
It is an effect so outlandish that when it was first proposed by theorists it was dismissed as ridiculous. But now it’s at the heart of the ultimate challenge in theoretical physics: the construction of the Theory of Everything (ToE), a single set of equations describing all the forces in the Universe and the particles they influence.
Exactly what the ToE will look like is still unclear, but most theorists believe it will
show that protons – the building blocks of every atom – do not last forever, but decay
into other particles.
Yet despite decades of effort, there is still no evidence that protons don’t last forever – and no-one really knows why. Perhaps the experiments still haven’t been running long enough, or perhaps the scientists are looking in the wrong places. Or perhaps the search for the ToE, begun by Einstein almost a century ago, is fundamentally flawed.
Whatever the explanation, the resolution of this mystery will have huge implications for the quest for this ultimate theory. “If protons were seen to decay it would demonstrate that the constituents of the atom are not stable – and the most likely explanation for all that would be the unification of all the forces of nature,” says Stuart Raby, a particle theorist and professor at Ohio State University in the US. “So if we observe the decay of the proton it would signal a major paradigm shift in our understanding of nature.”
The first hints that the proton might not be stable emerged in the 1930s, but it took another 40 years for the idea to gain any credibility. By then physicists were making impressive progress towards the ToE, having unified two of the four fundamental forces of nature: electromagnetism and the so-called weak interaction, which causes neutrons to disintegrate into other particles.
Encouraged by this, theorists next tried unifying this unified ‘electroweak’ force to one of the two remaining forces, the so-called strong interaction which glues together the nuclei of atoms. Their aim was to create what became known as the Grand Unified Theory, or GUT for short – a somewhat overblown moniker, considering it would still exclude the fourth and most important fundamental force, gravity.
How it works – and how to detect it
When charged particles tear through water at ultra-high speeds, they trigger the release of an eerie blue light known as Cerenkov radiation. Put simply, light travels relatively slowly through water – or any liquid – and if charged particles blast through the liquid faster, the resulting shock wave becomes visible, with the light being largely short in wavelength and thus blue in colour. Measurements of the spread of the shockwave can reveal the nature of the particles responsible – which makes Cerenkov radiation a handy way of both revealing the presence of high-speed particles and studying their properties. Fragments from a decaying proton are thought to generate the radiation.
The Laguna (Large Apparatus studying Grand Unification and Neutrino Astrophysics) project is developing new detectors, which are sensitive to other ways in which protons can decay and capable of spotting them – especially in argon atoms, which emit light and Cerenkov radiation when ionised – even if they last 1000 times longer than current experimental limits allow. One such proposed detector is GLACIER (Giant Liquid Argon Charge Imaging ExpeRiment) in Sieroszewice, Poland.
Among the first to try to create a workable GUT were theorists Abdus Salam and Jogesh Pati at Imperial College, London. They showed that any would-be GUT creates connections between the electroweak and strong forces, known as ‘symmetries’. These mathematical connections turned out to have startling implications for particles affected by the electroweak force – notably electrons – and those involved in the strong interaction, known as quarks (rhyming with ‘forks’).
Predicted theoretically in the early 1960s, quarks are the basic building blocks of the proton and neutron, with three quarks trapped inside each one by the strong interaction. If Salam and Pati’s basic unifying idea behind the GUT were correct, the resulting symmetries implied that quarks should be able to turn themselves into particles like electrons, and vice versa. But if that happened, it would have an astonishing consequence. It would mean the end of the neutron or proton playing host to the unstable quarks. And that would mean nothing made from atoms – not even the proverbial diamonds – would last forever.
Salam and Pati knew this prediction would cause trouble, and they were right: their ideas were rejected by Physical Review Letters, the most prestigious physics journal. Salam had to personally ring up the editor and pull strings to get the paper published, and it finally appeared in 1973. By then other theorists had joined the quest for the GUT, and had reached similar conclusions.
Within a few months, Harvard University theorist Sheldon Glashow – who had already played a key role in unifying electromagnetism and the weak force – and colleague Howard Georgi published their own version of the GUT, which made the same astonishing prediction: protons don’t last forever. Like the theorists at Imperial, Glashow and Georgi knew just how outrageous their claim would sound to others. But they took comfort from a rough estimate based on their theory – that, on average, a proton would last around 100 billion billion billion (1029) years before decaying, a timescale far longer even than the age of the Universe.
This explained why no-one had ever observed a proton in the act of disintegrating, but it raised a question: how could the prediction ever be tested? The answer lay in the fact that proton decay is a random process, like throwing dice. So just as there’s more chance of getting a six by throwing lots of dice rather than just one, there’s more chance of seeing proton decay if huge numbers of protons are observed, rather than studying just one. “Unfortunately, the theory predicts a very long lifetime for the proton,” explains particle experimentalist Professor Neil Spooner of the University of Sheffield. “So you need a big detector with lots of protons.”
Fortunately, however, a tonne of water – just a cubic metre in volume – contains almost 1030 protons. So if Glashow and Georgi’s theory was right, and each proton has only a 1 in 1029 chance of decaying in a year, a few of those contained in a tonne of water should decay over the space of a year.
Physicists in the US, Europe and Japan took up the challenge, and began designing experiments capable of detecting proton decay even if it took place much more slowly than predicted by Glashow and Georgi. Most focused on huge tanks of water which would be monitored for flashes of so-called Cerenkov radiation – a tell-tale sign of the creation of particles by the decay of the proton.
But such flashes could also be triggered by cosmic rays zipping through the atmosphere, so to guard against false alarms the experiments were designed to operate deep underground. In the US a salt mine under Lake Erie, Ohio, became the site of the Irvine-Michigan-Brookhaven (IMB) decay detector, consisting of a 7000 tonne mass of water surrounded by 2000 light-sensitive detectors. In Japan, a team from the University of Tokyo set up the Kamiokande detector, with 3000 tonnes of water surrounded by 1000 detectors.
With around half a dozen detectors around the world, physicists were confident that they would observe proton decay – if Glashow and Georgi were right. Yet by the late 1980s, not a single confirmed case of proton decay had been seen. In 1995, Japanese physicists unveiled Super-Kamiokande, an upgrade of the original detector with 50,000 tonnes of water. It is still running today, but has yet to produce any evidence for proton decay. At the very least, protons are 1000 times more stable than predicted by Glashow and Georgi’s version of the GUT.
The idea that protons may not last forever was first put forward as long ago as 1938, by a long-forgotten genius called Ernst Stueckelberg (right). Born into a minor Swiss aristocratic family in 1905, Stueckelberg studied physics at Munich University, and by the early 1930s was coming up with some astonishing ideas in fundamental physics. He proposed that atoms are held together by a force carried by sub-atomic particles flitting around the nucleus. His idea was initially dismissed as ludicrous, and only rediscovered 40 years later.
What has gone wrong? The most likely explanation is that the original theory lacked some key ingredient. And most theorists think it is an extra level of cosmic unity discovered in the 1970s, known as ‘supersymmetry’. Put simply, supersymmetry links together all the particles of matter – like protons and electrons – with the forces that act between them, such as the strong and electroweak interactions. And when it is built into a candidate for the GUT, it predicts longer proton lifetimes, out of reach of today’s detectors – and also other routes or ‘channels’ of decay into sub-atomic particles that might have been missed.
Both could explain the lack of evidence for proton decay thus far. The challenge now is to come up with a new generation of detectors able to test these new predictions, which is the aim of projects such as Laguna (see ‘Proton decay’ on p71). The potential payoff is huge, says Spooner, one of the leaders of the Laguna project: “The precise lifetime and the channel of decay will tell us about new physics – and about which variety of GUT is correct. And we have new technology available that can widen the search for proton decay.”
Not everyone is so sanguine about the chances of success, however. “I think it is becoming increasingly unlikely that proton decay occurs at an experimentally observable rate,” says theorist Dr Peter Woit of Columbia University, New York. According to Woit, while supersymmetry seems to explain the lack of evidence for proton decay, it raises a host of other questions – not least why the forces and particles it unifies are now so totally different. “The whole set up is not very plausible.”
So can supersymmetry explain the absence of proton decay so far? Or is there something more basic wrong with the whole quest for cosmic unity? A huge particle accelerator located near Geneva may soon provide answers. Known as the Large Hadron Collider (LHC), it will accelerate sub-atomic particles to energies not seen since the earliest moments of the Big Bang – a time when supersymmetry should have had a powerful effect.
Most theorists expect the LHC will find evidence for supersymmetry. But if it doesn’t, they will have to find another solution to the mystery of why protons seem to last forever.
Robert Matthews is a visiting reader in Science at Aston University
Should we worry we haven’t proved proton decay exists?
Dr Peter Woit, Columbia University, New York
“One popular idea about how to create a Theory of Everything has been to fit together quarks and electrons into a larger theoretical structure, and looking for proton decay provides a very sensitive test of this idea. But the fact that proton decay has not been seen at the level predicted by the simplest models of this kind is discouraging for this line of thinking. The main hope for this sort of unification now rests on the hope that a phenomenon known as supersymmetry will be discovered in experiments being planned for the Large Hadron Collider particle accelerator now being completed at CERN in Switzerland. This would justify the use of some of the supersymmetry-based ideas which predict proton lifetimes long enough to have evaded detection so far. But if supersymmetry is not observed at the LHC in the next few years, that will conclusively kill interest in such models.”
Professor Neil Spooner, University of Sheffield, member of the Laguna project team
“Theories have to adapt to observational fact, but for proton decay it is clear that experiment lags behind the theory. There are several different predictions for the lifetime of the proton, and the most favoured are out of the reach of existing detectors. That’s because they do not contain enough protons to decay – though only by a relatively small factor, perhaps around 10 or so. The exciting thing is that it is therefore reasonable to envisage building a new generation detector capable of seeing proton decay – it’s not too much of an extrapolation even from previously used technology. There is also new technology, for instance based on liquid argon, that can widen the search to a variety of decay channels and greatly increase the prospects of seeing events. We just need to make a proper test of these with a new generation of experiments.”