Every second, thousands of billions of high-energy particles called neutrinos fly through your body at close to the speed of light. Fortunately, they’re harmless – although distinctly strange. The 2015 Nobel prize in physics was awarded to two scientists who discovered neutrinos continually shape-shift as they move through space.
“Neutrinos are real oddballs,” says Paul Langacker, a particle physicist at Princeton University in New Jersey. Even within the weird world of quantum physics, neutrinos stand out as eccentrics. Each time physicists solve one riddle of neutrino behaviour, they unveil a new set of baffling questions.
Take the discovery for which Takaaki Kajita of the University of Tokyo and Arthur McDonald of Queen’s University in Kingston, Canada, shared this year’s Nobel Prize in physics. In experiments performed in 1998 and 2001, their teams showed why neutrino detectors often pick up far fewer of the particles than expected, including only detecting a third of the neutrinos produced by the Sun.
They demonstrated that this is because neutrinos continually flip between three possible forms, essentially disguising their identities as they go. That is remarkable enough. But the discovery also indicates that the standard model of physics, our best theory of particle physics, is incomplete – something uncharted must lie beyond it.
“I have done something very bad today by proposing a particle that cannot be detected.”
Neutrinos were almost disowned by the physicist who first proposed them. In 1930, Austrian quantum physicist Wolfgang Pauli was trying to understand a form of radioactivity known as beta decay. Observing the decay that occurred when radioactive atoms spit out electrons, he was puzzled as to why some of the energy seemed to be going missing. Pauli proposed that new particles – neutrinos – were carrying the energy away, escaping detection because they have no mass and do not interact with other matter. Neutrinos were a “desperate remedy” to the problem, Pauli admitted. He wrote in his journal: “I have done something very bad today by proposing a particle that cannot be detected.”
In fact, these tiny, unreactive, uncharged particles can be detected – with great difficulty. Neutrinos don’t ordinarily interact with other matter, but very occasionally a neutrino will smash into an atom, sparking a shower of photons. In 1956, American physicists Frederick Reines and Clyde Cowan finally proved neutrinos exist when they detected flashes from neutrinos coming from a nuclear reactor in South Carolina. This earned Reines the 1995 physics Nobel (Cowan passed away in 1974).
Since the 1950s we’ve known that neutrinos should also be created in the thermonuclear reactions in the heart of the Sun. But when physicists first began detecting solar neutrinos in 1967, they found only one third the number of particles they expected. Some physicists thought this meant our theory of how the Sun shines was wrong. Others thought the problem must lie in the way neutrinos were counted.
By the mid-1970s, physicists had catalogued all known subatomic particles into a table called the standard model. The table included slots for three types, or “flavours”, of neutrinos, labelled electron neutrinos, muon neutrinos and tau neutrinos.
According to thermonuclear theory, the Sun only produces electron neutrinos – so early detectors were designed to look for them. Italian physicist Bruno Pontecorvo suggested the answer to the solar neutrino problem was that the electron neutrinos created in the Sun were switching between their three possible identities as they travelled through space – forming an equal mix before reaching Earth. Hence a neutrino detector calibrated to pick up only electron neutrinos would detect only a third as many as expected.
To explain this odd behaviour, Pontecorvo invoked quantum theory, which tells us tiny particles can sometimes behave like waves. Neutrinos are made up of several subunits; effectively they are three waves superimposed on each other. A neutrino’s identity depends on how these different waves are mixed together, a bit like how you can form different colours by mixing red, green and blue.
Pontecorvo speculated that if these subunits were to differ slightly in mass, their waves would oscillate at slightly different rates as they travel through space. This shifting combination of waves would manifest as the neutrino switching identity.
Other physicists were reluctant to accept this idea, as it would mean conceding that neutrinos must have mass – and that the standard model is wrong.
Pontecorvo’s idea could only be tested by making more accurate neutrino counts – so huge, highly sensitive neutrino detectors were constructed for the purpose.
The Super-Kamiokande Neutrino Detection experiment was performed in a bath filled with 50,000 tonnes of ultra-pure water in an old mine 1,000 metres beneath Mount Kamioka, near the city of Hida in Japan. Tens of thousands of electronic eyes – each one capable of detecting a single photon – were placed around the tub, to detect any direct strikes between a neutrino and a water molecule.
The hundred-strong team, led by Kajita, looked not at neutrinos produced by the Sun, but at a neutrino source closer to home: the muon neutrinos created in the Earth’s atmosphere when cosmic rays crash into air molecules. Their experiments asked the question, do muon neutrinos change identity as they travel from the upper atmosphere to the Earth?
The team compared the number of muon neutrinos arriving from directly overhead with muon neutrinos formed on the other side of Earth (and entering the detector from below after slipping straight through the middle of the planet). They reasoned the neutrinos created directly above them should be more plentiful because they had the least amount of time to flip identity. Sure enough, the team found that the further the muon neutrinos travelled, the fewer muon neutrinos there were. This was the first direct evidence that neutrinos can change identity. Kajita and his team presented these results in 1998.
Meanwhile in Canada, McDonald’s team was gearing up for another crucial experiment, finally attempting to reconcile the longstanding mystery of the missing solar neutrinos. Their detector liquid was heavy water, in which one of the hydrogen atoms in water is replaced with the slightly heavier deuterium.
Some kinds of neutrino collisions, such as when an electron neutrino hits a neutron, have a signature unique to that neutrino type. But when a deuterium nucleus is hit by a neutrino, it gives the same signature no matter what type of neutrino is involved. So using heavy water, McDonald could measure both types of collisions at the same time – and calculate the total number of neutrinos coming from the Sun, as well as what slice of that pie were electron neutrinos.
The experiment to measure the total number of neutrinos did, indeed, count three times as many neutrinos as the experiment to detect electron neutrinos. This was the clincher – electron neutrinos emitted by the Sun must be changing identity on their journey to the Earth.
Kajita and McDonald’s experiments independently showed neutrinos can flip identity, and so must have mass. But the discovery raises many more questions about neutrinos. How quickly do they change identity? And just how much mass do they have? The answers to these questions should help physicists probe some of the other great mysteries of particle physics and cosmology.
Some of the biggest unknowns concern antiparticles – each subatomic particle’s mirror-image equivalent, which have identical mass but opposite charge (the electron’s antiparticle, for example, is the positron). By comparing how neutrinos change identity, versus how antineutrinos do it, researchers could have a unique way to shine a light on how the laws of physics differ when applied to matter versus antimatter. “Then you might have an explanation for why the Universe is all matter,” says Geoffrey Taylor, director of the ARC Centre for Particle Physics at the University of Melbourne who is working on experiments at the European Organisation for Nuclear Research (CERN). “Which is good because we exist.”
Another reason to want to know neutrinos’ precise mass is that although they are tiny – each one’s mass is dwarfed even by an electron’s mass – they are the second most abundant particle in the Universe (after photons). The combined mass of all the neutrinos in the Universe is estimated to be roughly equal to the mass of all visible stars. So, we need to know the mass of this tiny particle to describe the evolution of the Universe at the grandest scales.
At the same time, neutrinos give us a “unique window” on what may lie at the most fundamental scale, far smaller than an atomic nucleus, says Langacker. The biggest mystery of all is why neutrinos are so much lighter compared with all the other particles in nature – which hints there must be a whole new layer of particle physics to uncover. The answer to that, he says, “would point to all the physics beyond the standard model”.
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