COULD NEUTRINOS EXPLAIN THE VICTORY OF MATTER?
to pass through matter at the speed of light. In 1986 Masataka Fukugita and Tsutomu Yanagida of Tohoku University wondered if these mysterious particles might also hold the answer to the imbalance of the Universe. They came up with an extraordinary theory.
Their starting point was to propose that neutrinos actually travelled slightly slower than the speed of light and had a tiny mass. The reason for their extreme lightness, they suggested, is that neutrinos have a big brother – another, much heavier particle – that offsets their mass. The idea is called the seesaw theory because it reminded the physicists of the way a big child on a seesaw can suspend a smaller child high in the air. If you could not see the big brother on the other side, you’d wonder at the extreme lightness of the child perched high on the seesaw. So if the neutrino is extremely light, they reasoned, its brother must be superheavy.
The Japanese pair figured that these superheavy neutrinos would have been unstable. And that their decay may have been skewed toward matter. Given that they would have been created in huge numbers in the Big Bang, that might have tipped the balance towards matter over antimatter. Fukugita and Yanagida’s revolutionary idea was ignored. Their series of assumptions about neutrinos seemed tenuous, each stacked atop each other like a house of cards. Moreover there was no base to the stack because there was no evidence that the neutrino had mass.
Then, in 2001, the physics community was rocked by the discovery that neutrinos shapeshift between three possible forms as they zoom through the Universe. Think of neutrinos as a group of three close-packed riders in a bicycle peloton who take turns leading, with each leader showing a different face. This peloton behaviour could only be explained if neutrinos did carry mass ( Cosmos 66, page 25), though it must be miniscule even compared with the electron. Around the world, the physics community did a collective double-take.
Since the shapeshifting discovery, many other theories have been proposed to explain how the neutrino could be so incredibly light. But the idea of a secret neutrino big brother is the most popular.
UNFORTUNATELY, THE MASS of the proposed superheavy neutrinos is so large that researchers can’t produce them in a particle accelerator and watch them decay. But the seesaw theory does make two testable predictions about neutrinos. If they both prove true, they provide strong evidence that
neutrinos are the hero behind matter’s victory in the war of creation.
The first prediction is that neutrinos are their own antiparticle. Right now we know that as neutrinos zip through space, they always spin anticlockwise while antineutrinos spin clockwise. But a thing that spins clockwise coming toward you spins anticlockwise going away from you. This means neutrinos and antineutrinos could be two sides of the same coin. The smoking gun to prove this theory would be detecting a special radioactive process called neutrinoless double beta decay.
Regular double beta decay happens when two neutrons in the same nucleus decay simultaneously, spitting out two electrons and two neutrinos. Normally the path of the emitted electrons is unbalanced because the neutrinos carry away some of the energy – just as two struck billiard balls have unequal paths because the cue ball carries some of the energy. But if a neutrino can act as its own antiparticle, then occasionally those two neutrinos should annihilate. If this happens within the nucleus, it’s as if no neutrino was emitted at all: that’s neutrinoless double beta decay. The absence of neutrinos would leave a unique signature on the electrons that were released. Instead of being imbalanced, the paths of the electrons will be perfectly balanced. By measuring the paths of emitted electrons, physicists hope to nail the dual nature of neutrinos. “This is what the next generation of experiments are going to try to crack,” says Simon Peeters, a physicist at the University of Sussex. He is part of an international collaboration based at Canada’s Sudbury Neutrino Observatory (SNO) where some of the shapeshifting Nobel prize work was performed. When a new detector, SNO+, is operational next year, the team will watch for two tell-tale flashes of light that indicate a tellurium nucleus has decayed by emitting two electrons simultaneously ( see graphic).
The decay is incredibly rare and difficult to detect. “What you’re looking for is basically a handful of these decays in a tonne of material,” says Peeters.
Far from the ridicule that first greeted Fukugita and Yanagida, there is fierce competition to be the first to nail neutrinos as double agents. The Enriched Xenon Observatory in New Mexico is already probing 200 kilograms of liquid xenon for similar tell-tale flashes. And in Italy, the GERDA (Germanium Detector Array) uses huge crystals of the semiconductor germanium to detect the elusive decay. PROVING THAT NEUTRINOS are their own antiparticle would be the biggest coup for particle physics since the discovery of the Higgs Boson. Yet this would only be one step toward proving the seesaw theory.
The theory’s second prediction is that neutrinos, like K-mesons and B-mesons, break the rules of symmetry between matter and antimatter. We can’t directly test whether neutrinos decay in a lopsided manner. But we can check whether they break the symmetry rules. A major project called DUNE (Deep Underground Neutrino Experiment) at Fermilab, in Illinois, could soon put this idea to the test by firing the world’s most intense beam of neutrinos and antineutrinos at a detector buried 1,300kilometres away in a South Dakota mine. The long distance gives the neutrinos time to shapeshift. DUNE will probe the beam twice along the way to measure such shifts.
The laws of symmetry dictate that neutrinos and antineutrinos should shapeshift at the same rate. If they don’t, then they are part of a select club of symmetry-breakers ( see graphic) that might break all sorts of other rules. And if they behave badly, their big brother probably does too – possibly decaying in a lopsided manner, leading to our matter-dominated universe.
The US has already devoted $1 billion to the DUNE project, which should be operational by 2022, and a huge list of international collaborators, including CERN, have signed up too.
ADMITTEDLY PURSUING the weirdness of neutrinos sounds like a huge gamble to try to crack the mystery of antimatter. But they’re still the best chance we’ve got. “Most particle physicists are betting on them,” says Quinn.
What’s at stake is nothing less than our reason for being. But for now, we must wait, as two of the most sophisticated physics experiments on the planet run their course. What’s another few years to a mystery 13.8 billion years in the making?
PURSUING NEUTRINOS SOUNDS LIKE A HUGE GAMBLE TO TRY TO CRACK ANTIMATTER, BUT THEY’RE THE BEST CHANCE WE’VE GOT.