The Enduring Quest for Proton Decay

While all good things come to an end, a baffling exception is the proton. Along with neutrons, their close subatomic particle cousins, protons comprise the nuclei of all the chemical elements in the universe. When left on their own outside an atomic nucleus, neutrons break down into other particles within mere minutes. Protons, however, persist—and that's putting it lightly. In fact, no proton has ever been observed decaying, and increasingly stringent experiments have pushed the particle's lifetime out past 10³⁴ years. That almost unimaginably vast number (10 decillion years, for all you Latin numeral prefix fans) is over a trillion-trillion times longer than the age of the universe.

Yet physicists are pretty convinced that the proton must inevitably crumble. It is a composite particle, meaning it is built of truly fundamental units of matter (in this case, three quarks), and every other known composite particle decays into these fundamental units.

Confirming proton decay is not just a check-the-box, physics-for-physics'-sake kind of quest. At last discovering the phenomenon would be a breakthrough into new physics, enabling theoreticians to compose a so-called Grand Unified Theory that brings together three of nature's fundamental forces—namely electromagnetism plus the strong and weak forces—as aspects of the same, single force. Doing so would upend the decades-old Standard Model that powerfully captures the behavior of matter at its smallest scales, but is ultimately incomplete. Of great import to physicists and astrophysicists alike, the Standard Model is unable to account for dark matter (thought to comprise some 85% of the universe's matter), along with properties of elementary particles called neutrinos, and it doesn't at all address the force of gravity.

Scientists eager to go beyond the Standard Model have thus pinned a great degree of hope on protons ultimately kicking the bucket.

"Proton decay is about as big as it gets," says James Stone, emeritus professor of physics at Boston University and a Visiting Senior Scientist at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) at the University of Tokyo.

Stone has pursued proton decay his entire career, starting back in the 1970s. That's when the first Grand Unification Theories emerged and which predicted a proton lifetime on the order of 10²⁹ years (a somewhat more constrained 100 octillion years). While that age might still seem like something that couldn't possibly be measured, note that it's technically a half-life being sought, meaning that for a given amount of protons, half of them would decay in X number of years. Accordingly, given that protons are so infinitesimally copious, simply gathering enough of them together within a protected, highly sensitive detector and watching and waiting should statistically deliver a proton death in a reasonable span of time, at least by human reckoning.

"Many scientists immediately realized that this number could be measured if you had enough protons to watch and low enough background," Stone says.

The "background" he refers to involves natural events such as the high-energy particles raining from space called cosmic rays. As these rays pulverize atoms in the atmosphere, they produce the same sort of particles and interactions that mimic what a theorized proton decay would produce. To reduce that background, a practical solution is to place a detector experiment deep underground, where all the packed matter overhead keeps out a good bit of the unwanted, confounding signals. Careful experimental design and monitoring can then reduce any leftover background to a bare and manageable minimum.

Stone and colleagues accordingly constructed the Irvine-Michigan-Brookhaven (IMB) detector (named after contributing institutions) some 600 meters underground in a salt mine under Lake Erie. The detector held 10,000 tonnes of ultrapure water, enough to muster 10³³ protons, Stone says. IMB ran from the early 1980s to the early 1990s but did not register a proton decay. In Japan, a parallel experiment called KamiokaNDE (Kamioka Nucleon Decay Experiment) likewise turned up nada, helping show that the proton lifetime had to be longer than anticipated.

Upping the ante, Stone and his colleagues joined with the KamiokaNDE group in the early 1990s on an even bigger experiment. Stone recalls how the collaboration emerged, thanks to a handwritten note appended by a prominent Japanese researcher, Yoji Totsuka of the University of Tokyo, to a study outlining the new experiment that Totsuka sent to Stone. The note mentioned how a necessary detector for the experiment was delayed. Stone's group recognized the opportunity to contribute to the project, helping build the detector and also forge a relationship that persists to this day in part through Kavli IPMU.

The new experiment went by the moniker of Super-Kamiokande, or Super-K. As with IMB and Kamiokande, proton decay was not its sole motivation; tremendously large volumes of ultrapure water also double as premier detection spaces for neutrinos. IMB's claim to fame, in fact, was making the first-ever observation of neutrinos spawned by Supernova 1987A, the closest such stellar explosion to Earth available to modern science. Super-K, meanwhile, ended up serving a monumental role in solving the solar neutrino mystery, a longstanding conundrum over why the Sun seemed to produce too few neutrinos. (The groundbreaking answer: neutrinos toggle between three "flavors," in defiance of the Standard Model.)

Even as these water-filled underground detectors galvanized the physics and astrophysics communities with their neutrino-driven discoveries, many researchers behind the experiments kept their eyes on the original prize. "Most of us agreed that the neutrino work was important and must be done," says Stone, "but proton decay was our ultimate quest."

That journey has continued with Super-K, which holds 50,000 tonnes of pure water whose accumulated data from between 1996 and 2015 has established the minimum proton lifetime as that aforementioned 10 followed by 34 zeroes figure.

Now Stone and his fellow proton-decay hunters have their sights set on their next goal: the Hyper-Kamiokande. Currently under construction in Japan and slated to begin taking measurements in 2027, it will be the world's biggest underground water tank, holding 260,000 tonnes of water in a container some 71 meters deep and 68 meters wide.

"Hyper-K is the best hope for proton decay," says Stone. "It will be able to overtake the current Super-K proton lifetime limit in about two years. Another 10 [years] of running will add another order of magnitude."

Stone credits The Kavli Foundation and Kavli IPMU with assisting in getting the project off — er, well, under — the ground.

"It has been a great pleasure working with my Japanese colleagues and Kavli has played a significant role," Stone says. "Kavli provided us with the opportunities through its financial support and facilities to hold meetings and discuss distant future ideas like long baseline neutrino experiments and extremely ambitious projects like Hyper-K. Kavli IPMU has provided the atmosphere for creative thinking and interactions by bringing together great scientists with very different backgrounds but who are thinking about the same physics."

Stone's fingers are crossed that Hyper-K will deliver the long-sought breakthrough of proton decay. "Let's hope for a discovery," he says. "I'm a big fan of Hyper-K."

Physics aside, there would also be a final bit of profound philosophical significance to the discovery of proton decay. "It means that all matter in the universe eventually disintegrates," Stone says. At some distant, distant time, atoms would start to decompose, and all things made of matter—stars, planets, people—would cease to be.

It's an evocative notion, especially for Stone and his colleagues who have fought the good physics fight for so long.

"My quest for proton decay now spans over 40 years," Stone says. "My hope was it would be found before my retirement. Now my hope is it is found during my lifetime."