Nobel Prize in Physics 2015: Neutrino Oscillations and the Mass of Ghost Particles

This note covers the Nobel Prize in Physics 2015: who won it, what neutrino oscillations are, how two underground detectors in Japan and Canada solved a decades-old puzzle about missing neutrinos, how the discovery unfolded, why it matters for physics and cosmology, and quick facts for exams.

What was the Nobel Prize in Physics 2015 awarded for?

The official citation reads: "for the discovery of neutrino oscillations, which shows that neutrinos have mass".

In plain words, the two laureates led teams that caught neutrinos, tiny ghost-like particles, changing from one "type" to another as they travelled through space. Physicists call this switching behaviour neutrino oscillation.

The important consequence is that for a particle to change identity in this way, it cannot be completely massless, so neutrinos must carry some mass, however small.

The award is formally called the Nobel Prize in Physics, and the 2015 prize was given by the Royal Swedish Academy of Sciences.

It recognised a puzzle that had troubled particle physicists for decades: experiments on Earth kept finding far fewer neutrinos than theory predicted, and nobody could say where the missing ones had gone.

Who are the laureates?

Takaaki Kajita

Takaaki Kajita was born on 9 March 1959 in Higashimatsuyama, Japan. At the time of the award he was affiliated with the University of Tokyo, Kashiwa, Japan, where he was Director of the Institute for Cosmic Ray Research and a professor.

He received one half of the prize. Kajita led the analysis at the Super-Kamiokande detector, and around 1998 he presented the discovery that neutrinos produced when cosmic rays strike the atmosphere were switching identities on their way to the detector.

Arthur B. McDonald

Arthur B. McDonald was born on 29 August 1943 in Sydney, Nova Scotia, Canada. At the time of the award he was affiliated with Queen's University, Kingston, Canada. He received one half of the prize.

McDonald led the research group at the Sudbury Neutrino Observatory (SNO), which in 2001 showed that neutrinos streaming from the Sun were not vanishing but were being captured in a different identity than the one in which they were produced.

What problem were they trying to solve?

Neutrinos are extremely light, electrically neutral particles that barely interact with anything. According to the press release, after photons (particles of light) neutrinos are the most numerous particles in the entire cosmos, and the Earth is constantly bombarded by them.

Many are created when cosmic radiation hits the atmosphere, others come from nuclear reactions inside the Sun, and the popular information page notes that thousands of billions of neutrinos stream through our bodies every second without us noticing.

The Standard Model of particle physics describes three types (or "flavours") of neutrino: electron-neutrinos, muon-neutrinos and tau-neutrinos. For a long time physicists assumed neutrinos were massless, because the Standard Model works that way.

But since the 1960s, scientists calculating how many neutrinos the Sun should produce found that, when they actually measured neutrinos arriving on Earth, up to two thirds were missing compared with the theoretical prediction. This gap became known as the solar neutrino puzzle.

One proposed explanation was that the Sun only produces electron-neutrinos, but on the long journey to Earth some of them might be changing into muon- or tau-neutrinos, which the older detectors could not see.

This remained only a speculation until much larger detectors, built deep underground to shield them from interference, became powerful enough to test it directly.

How did Super-Kamiokande catch neutrinos switching identity?

Super-Kamiokande became operational in 1996 in a zinc mine in Japan, built 1,000 metres below the surface to keep out noise from cosmic radiation.

The detector consisted of a giant tank, 40 metres high and equally wide, filled with 50,000 tonnes of ultra-pure water, watched over by more than 11,000 light sensors.

The detector studied atmospheric neutrinos, created when cosmic rays collide with molecules in the atmosphere.

The detection method works roughly like this:

1. A neutrino occasionally collides with a water molecule, creating a fast, electrically charged particle (a muon from a muon-neutrino, or an electron from an electron-neutrino).
2. That charged particle moves through the water faster than light travels in water (though still slower than light in a vacuum), producing a cone of faint blue light called Cherenkov radiation.
3. The light sensors lining the tank record the shape and intensity of this flash, which reveals what type of neutrino caused it and roughly which direction it came from.
4. Scientists compare neutrinos arriving from directly above (which travelled only a short distance through the atmosphere) with those arriving from below (which had travelled thousands of kilometres through the entire globe).

Kajita's team found that muon-neutrinos coming from directly above matched expectations, but far fewer muon-neutrinos arrived from below after the long journey through the Earth.

Since electron-neutrino numbers were unaffected, the missing muon-neutrinos had most likely turned into tau-neutrinos during their longer trip, a change the detector could not directly see but could infer.

Draw a large cylindrical tank buried deep underground, filled with water, with light sensors lining its inner walls.

Show one neutrino arriving from directly above after a short atmospheric path, and another arriving from below after passing through the entire Earth, with a label noting that the long-path neutrinos show a deficit.

How did the Sudbury Neutrino Observatory confirm the change?

The Sudbury Neutrino Observatory (SNO) in Ontario, Canada, began observations in 1999 inside a nickel mine about two kilometres underground.

Unlike Super-Kamiokande's ordinary water, SNO used 1,000 tonnes of heavy water, in which each hydrogen atom carries an extra neutron, forming deuterium. This special water allowed two different kinds of measurement in the same tank.

1. Some reactions in the heavy water responded only to electron-neutrinos, giving a count of electron-neutrinos alone.
2. Other reactions responded to all three neutrino flavours together, giving a combined count of every type arriving from the Sun.
3. Since the Sun's nuclear reactions only produce electron-neutrinos, both counts should, in theory, give the same number if nothing strange were happening.
4. McDonald's team found the electron-neutrino count alone was only about a third of the expected number, while the combined count of all three flavours matched the theoretical prediction closely.

The conclusion was that the "missing" electron-neutrinos had not disappeared at all; they had simply changed into muon- or tau-neutrinos during their roughly 150-million-kilometre journey from the Sun, and so were still detected, just counted differently. Together with the Super-Kamiokande result, this gave firm proof of neutrino oscillation.

Why does changing identity mean neutrinos have mass?

The explanation needs quantum physics. In the quantum world, a particle is also described as a wave, and a particle's energy is linked to the frequency of that wave.

According to the popular information page, the electron-, muon- and tau-neutrino states are represented as combinations of underlying waves corresponding to neutrino states of slightly different masses.

While these waves travel together, their relative phases drift apart because waves of different frequencies do not stay perfectly in step.

This shifting combination changes the probability of finding the neutrino in one flavour versus another as it travels, so the particle appears to switch identity at different points along its journey.

Crucially, if all the underlying mass states were identical (that is, if neutrinos were truly massless), there would be no difference in wave frequency, the waves could never drift out of phase, and no oscillation could occur at all.

This is why the laureates' discovery was so significant for the Standard Model of particle physics, the theory describing the fundamental particles and forces.

The Standard Model had resisted experimental challenges for over twenty years, but it assumes neutrinos are massless.

The press release states that the new observations "clearly showed that the Standard Model cannot be the complete theory of the fundamental constituents of the universe." The combined weight of all the universe's neutrinos, though each one is tiny, is estimated to be roughly comparable to the combined weight of all visible stars.

How did the discovery unfold?

Why does this discovery matter?

The discovery resolved a puzzle that had stood for decades: the apparent deficit of neutrinos measured on Earth compared with theoretical calculations.

By showing that neutrinos oscillate between flavours, the two experiments proved that neutrinos carry mass, even though that mass is extremely small and, according to the scientific background material, has still never been measured directly.

This matters for two broad reasons. First, it revealed what the popular information page calls "the first apparent crack in the Standard Model", meaning physicists now know for certain that this highly successful theory of particle physics is incomplete and must eventually be extended to explain why neutrinos have mass at all.

Second, because neutrinos are so numerous throughout the universe, their collective mass has real consequences for cosmology, the study of the universe's structure and history.

The popular information page notes that intense experimental activity continues worldwide to pin down open questions: what the exact masses of the three neutrino types are, why they are so lightweight, whether more types exist beyond the three known, and whether neutrinos are their own antiparticles.

Scientists expect future discoveries about neutrinos to reshape understanding of the universe's past, present structure and eventual fate.

How does this connect to what you study?

Students learning about atoms and radioactivity in school science meet the idea of particles like electrons, protons and neutrons, and the way an unstable nucleus can throw off radiation.

Neutrinos extend this picture directly. According to the popular information page, the neutrino's existence was first proposed in 1930 by the physicist Wolfgang Pauli, who was trying to explain a puzzle in beta decay, a type of radioactive decay in which an atomic nucleus appeared to lose energy that nobody could account for.

Pauli suggested that an almost undetectable, electrically neutral and very light particle was quietly carrying away the missing energy, and this proposed particle later came to be called the neutrino. It took more than two decades before Frederick Reines and Clyde Cowan actually confirmed the particle was real, in 1956.

The idea of wave-particle duality, where a single particle also behaves as a wave with its own frequency, underlies the whole explanation of why neutrino oscillation can happen only if neutrinos carry some mass. The waves linked to the different neutrino mass states drift gradually out of step with one another as the particle travels, and this drifting is what makes the particle appear to switch flavour.

This connects directly to introductory ideas about quantum physics and particle behaviour that appear in senior school and undergraduate physics courses, where students first meet the notion that very small particles do not behave like simple everyday objects.

What questions about neutrinos remain open?

Even after this Nobel Prize in Physics 2015, neutrinos keep a lot of their secrets hidden. The popular information page lists several key questions that physicists still cannot answer with certainty.

One basic question is simply what the exact masses of the three neutrino flavours are. The two experiments showed that the different mass states must differ from one another, and other work has measured the size of that difference quite precisely, but nobody has yet measured the actual mass of any single neutrino type directly.

A second question asks why neutrinos are so lightweight compared with other particles, since their combined mass is estimated to be almost nothing next to particles like the electron, let alone the heavier particles in the Standard Model.

A third question is whether there might be more types of neutrino beyond the three known flavours, since some experiments have hinted at extra, harder-to-detect kinds that interact even more weakly than ordinary neutrinos.

A fourth, much-discussed question asks whether neutrinos are their own antiparticles, something that is not true of particles like electrons, which have a distinct antiparticle, the positron.

The source material notes that intense experimental activity is under way worldwide to chase these answers, and that new discoveries about neutrinos are expected to reshape scientific understanding of the history, structure and eventual fate of the universe.

Quick facts for exams

The Nobel Prize in Physics 2015 was awarded jointly to Takaaki Kajita of Japan and Arthur B. McDonald of Canada, announced on 6 October 2015 by the Royal Swedish Academy of Sciences, for the discovery of neutrino oscillations, which proved that neutrinos have mass.

Kajita worked at the University of Tokyo using the Super-Kamiokande detector to study atmospheric neutrinos, while McDonald worked at Queen's University leading the Sudbury Neutrino Observatory team studying solar neutrinos.

Each laureate received one half of the prize, worth 8,000,000 Swedish kronor in total. The discovery showed that the Standard Model of particle physics, which assumes massless neutrinos, is an incomplete description of nature.

Note: Source. The prize facts in this note are from the Nobel Prize's official site, nobelprize.org.

Glossary

• Neutrino — an extremely light, electrically neutral particle that barely interacts with matter.
• Neutrino oscillation — the phenomenon of a neutrino changing from one flavour to another as it travels.
• Flavour — one of the three types of neutrino: electron, muon or tau.
• Standard Model — the theory describing the known fundamental particles and forces, which originally assumed neutrinos are massless.
• Cherenkov radiation — the faint light produced when a charged particle moves through a medium like water faster than light travels in that medium.
• Atmospheric neutrinos — neutrinos created when cosmic rays collide with molecules in the Earth's atmosphere.
• Solar neutrinos — neutrinos produced by nuclear fusion reactions inside the Sun.
• Heavy water — water in which the hydrogen atoms contain an extra neutron, forming deuterium.
• Solar neutrino puzzle — the long-standing mismatch between the predicted and measured numbers of neutrinos from the Sun.
• Beta decay — a type of radioactive decay in which a nucleus emits an electron (or positron) along with a neutrino.
• Super-Kamiokande — the underground water detector in Japan used to study atmospheric neutrinos.
• Sudbury Neutrino Observatory (SNO) — the underground heavy-water detector in Canada used to study solar neutrinos.

Common errors and misconceptions

• Misconception: Neutrinos were proven to have a specific, measured mass. Correct: The sources state that the mass itself has never been measured directly; only the differences between masses of the flavours are known.
• Misconception: The discovery disproved the Standard Model entirely. Correct: It showed the Standard Model is incomplete, not wrong about everything; it needs to be extended to include massive neutrinos.
• Misconception: Kajita and McDonald worked together in the same team. Correct: They led two separate research groups, Super-Kamiokande in Japan and SNO in Canada, which independently reached complementary conclusions.
• Misconception: Super-Kamiokande and SNO studied the same source of neutrinos. Correct: Super-Kamiokande studied atmospheric neutrinos from cosmic rays, while SNO studied solar neutrinos from the Sun.
• Misconception: Neutrino oscillation means neutrinos are destroyed or created. Correct: The neutrinos are not lost; they change flavour, so the total number across all three types stays the same.
• Misconception: Cherenkov radiation violates the speed limit set by relativity. Correct: It only means the charged particle moves faster than light travels in water, not faster than light in a vacuum, so relativity is not broken.

Exam-style questions with model answers

Q1. What was the official citation for the Nobel Prize in Physics 2015? [2 marks]

1. The citation was "for the discovery of neutrino oscillations, which shows that neutrinos have mass."
2. It was awarded jointly to Takaaki Kajita and Arthur B. McDonald.

Q2. Name the two laureates of the Nobel Prize in Physics 2015 and their affiliations at the time of the award. [2 marks]

1. Takaaki Kajita was affiliated with the University of Tokyo, Kashiwa, Japan.
2. Arthur B. McDonald was affiliated with Queen's University, Kingston, Canada.

Q3. Explain how the Super-Kamiokande detector identified that muon-neutrinos were changing identity. [4 marks]

1. Super-Kamiokande was a huge tank of ultra-pure water buried underground, lined with light sensors, built to catch atmospheric neutrinos produced when cosmic rays strike the upper atmosphere.
2. When a neutrino collided with a water molecule, it produced a fast charged particle that generated a cone of Cherenkov light, whose shape and brightness revealed the neutrino's flavour and direction.
3. Kajita's team compared muon-neutrinos arriving directly from above, after a short journey through the atmosphere, with those arriving from below after travelling through the whole Earth.
4. Far fewer muon-neutrinos arrived from the long path than expected, while electron-neutrino numbers stayed normal, showing that the muon-neutrinos had changed into tau-neutrinos during the longer journey.

Q4. Describe how the Sudbury Neutrino Observatory confirmed that solar neutrinos change flavour. [5 marks]

1. The Sudbury Neutrino Observatory was built about two kilometres underground in a nickel mine in Ontario, Canada, and used 1,000 tonnes of heavy water, in which hydrogen atoms carry an extra neutron.
2. The heavy water allowed two kinds of reaction to be measured: one sensitive only to electron-neutrinos, and one sensitive to all three neutrino flavours combined.
3. Since the Sun's fusion reactions produce only electron-neutrinos, both measurements should match if nothing unusual were happening.
4. McDonald's team found that the electron-neutrino count alone was only about a third of the predicted number, while the combined count of all three flavours matched the theoretical prediction well.
5. This proved that the "missing" electron-neutrinos had not vanished but had changed into muon- or tau-neutrinos on their journey from the Sun, confirming neutrino oscillation and showing, together with Super-Kamiokande's results, that neutrinos must have mass.

Q5. Discuss why the discovery of neutrino oscillations was significant for the Standard Model of particle physics. [5 marks]

1. The Standard Model is the theory that successfully described the known fundamental particles and forces for more than twenty years, resisting all experimental challenges.
2. However, the Standard Model as originally formulated requires neutrinos to be completely massless.
3. Neutrino oscillation can only happen if the underlying mass states of the different neutrino flavours are not identical, because identical masses would mean identical wave frequencies that could never drift out of phase.
4. The observation of oscillation at both Super-Kamiokande and the Sudbury Neutrino Observatory therefore proved that at least some neutrinos carry non-zero mass, directly contradicting the massless assumption.
5. According to the popular information page and the press release, this revealed the first apparent crack in the Standard Model, showing it "cannot be the complete theory of the fundamental constituents of the universe," and opened questions about new physics needed to explain where neutrino mass comes from.

Q6. What is Cherenkov radiation and why was it important for detecting neutrinos? [3 marks]

1. Cherenkov radiation is faint light produced when a charged particle moves through a medium, such as water, faster than light itself travels through that same medium.
2. When a neutrino collided with a water molecule inside the detectors, it created a fast charged particle that produced a cone of this light.
3. Light sensors lining the detector tanks recorded the shape and brightness of the Cherenkov light, letting scientists work out the neutrino's type and direction.

Key takeaways

• Takaaki Kajita and Arthur B. McDonald jointly won the Nobel Prize in Physics 2015 for discovering neutrino oscillations.
• Neutrino oscillation is the switching of a neutrino between its electron, muon and tau flavours as it travels.
• Super-Kamiokande in Japan studied atmospheric neutrinos; the Sudbury Neutrino Observatory in Canada studied solar neutrinos from the Sun.
• Both experiments found fewer neutrinos of the expected flavour than predicted, resolving the long-standing solar neutrino puzzle and the atmospheric neutrino anomaly.
• Oscillation can only happen if neutrinos have mass, overturning the earlier assumption that they are massless.
• The discovery revealed that the Standard Model of particle physics is an incomplete theory of nature.
• The exact mass of the neutrino has still never been measured directly, only the differences between flavour masses.
• Because neutrinos are extremely numerous across the universe, their mass matters for cosmology and the structure of the universe.

Test yourself

When was the Nobel Prize in Physics 2015 announced?

It was announced on 6 October 2015 by the Royal Swedish Academy of Sciences.

Where was Takaaki Kajita affiliated at the time of the award?

Takaaki Kajita was affiliated with the University of Tokyo in Kashiwa, Japan, at the time of the award.

Where was Arthur B. McDonald affiliated at the time of the award?

Arthur B. McDonald worked at Queen's University, Kingston, Canada, at the time of the award.

What liquid filled the Super-Kamiokande tank?

The Super-Kamiokande tank was filled with 50,000 tonnes of ultra-pure ordinary water.

What made the water in the Sudbury Neutrino Observatory special?

SNO used heavy water, in which each hydrogen atom has an extra neutron, forming deuterium, unlike ordinary water.

Why does neutrino oscillation prove neutrinos have mass?

Oscillation needs the neutrino flavours to correspond to different wave frequencies, which requires them to have different, non-zero masses.

What fraction of expected solar neutrinos were found missing before the discovery?

Up to two thirds of the theoretically predicted number of solar neutrinos were found to be missing in earlier measurements.

What theory did the discovery show to be incomplete?

The discovery showed that the Standard Model of particle physics, which assumes massless neutrinos, cannot be the complete theory of matter.