Nobel Prize in Physics 2015 – The Discovery That Proved Neutrinos Have Mass

Nobel Prize in Physics 2015 – The Discovery That Proved Neutrinos Have Mass

When and Where Was It Announced?

The 2015 Nobel Prize in Physics was announced on October 6, 2015, at the Royal Swedish Academy of Sciences, Stockholm.

The award ceremony took place on December 10, 2015, at the Stockholm Concert Hall, where King Carl XVI Gustaf of Sweden personally presented the medals and diplomas to the laureates.

Who Won and Why?

Laureates:

1. Takaaki Kajita, University of Tokyo, Japan

2. Arthur B. McDonald, Queen’s University, Canada

Awarded “for the discovery of neutrino oscillations, which shows that neutrinos have mass.”

This discovery solved a long-standing cosmic mystery: Why are fewer neutrinos observed from the Sun than theory predicts?

The Scientific Mystery: The Solar Neutrino Problem

Neutrinos are extremely light, neutral particles produced during nuclear reactions, such as those in the Sun or nuclear reactors on Earth.

The Standard Model of Particle Physics, which successfully explained fundamental forces and particles, assumed neutrinos were massless.

However, in the 1960s, American chemist Raymond Davis Jr. and Russian theorist John Bahcall noticed a puzzling result:

• Detectors deep underground recorded only about one-third of the expected solar neutrinos.

For decades, this “Solar Neutrino Problem” haunted physicists.
Were the calculations wrong? Was something missing in our understanding of neutrinos?

Kajita and McDonald finally answered it — and their answer shook physics to its core.

The Science Behind the Discovery: Neutrino Oscillations

Neutrinos come in three “flavors” — electron, muon, and tau.

According to quantum mechanics, these flavors are not fixed identities. Instead, each flavor is a quantum superposition of three distinct mass states.

Because these mass states travel at slightly different speeds, the neutrino’s flavor oscillates (changes) as it moves through space.

This phenomenon is called neutrino oscillation — and it can only occur if neutrinos have non-zero mass.

The Quantum Explanation (simplified)

When a neutrino is produced — say, as an electron neutrino in the Sun — it is actually a mixture of three mass eigenstates (ν₁, ν₂, ν₃). As it travels, the wave functions of these mass states evolve differently.

By the time it reaches a detector, the probability of observing it as an electron neutrino has changed — sometimes it’s detected as a muon or tau neutrino instead.

Mathematically, this behavior is described using a mixing matrix (the PMNS matrix — named after Pontecorvo, Maki, Nakagawa, and Sakata). Kajita and McDonald provided the experimental evidence that confirmed this theory.

The Experiments — Underground Windows to the Universe

Super-Kamiokande (Japan) — Kajita’s Breakthrough

Deep in the Kamioka mine in Japan, 1,000 meters below Mount Ikeno, lies a cylindrical tank 40 meters high, filled with 50,000 tons of ultra-pure water. This is Super-Kamiokande, one of the world’s most sensitive neutrino detectors.

• How it works:
  When a neutrino interacts with a water molecule, it occasionally knocks an electron or muon out at nearly light speed.
  This charged particle emits a faint blue flash called Cherenkov radiation — like a sonic boom of light.
  The tank’s walls are lined with 11,000 photomultiplier tubes (PMTs) that detect these tiny flashes and reconstruct the neutrino’s direction and energy.

• What Kajita’s team observed:
  By comparing neutrinos coming from the sky (short distance) and through the Earth (long distance), they found that muon neutrinos decreased with travel distance — evidence that they were transforming into other types during flight.
  This was the first solid experimental proof of neutrino oscillation in atmospheric neutrinos.

Sudbury Neutrino Observatory (SNO, Canada) — McDonald’s Discovery

Almost 2 km underground in a nickel mine near Sudbury, Ontario, Canada, stood the SNO detector, a 12-meter acrylic sphere filled with 1,000 tons of heavy water (D₂O).

Heavy water allowed SNO to detect all three types of neutrinos through two types of nuclear reactions:

1. Charged-current (CC) interaction – Sensitive only to electron neutrinos.

2. Neutral-current (NC) interaction – Equally sensitive to all neutrino types.

• McDonald’s key finding:
  When comparing the CC and NC data, they found that while electron neutrinos were fewer, the total number of neutrinos matched the theoretical prediction.
  This meant that solar neutrinos weren’t disappearing — they were changing flavor during their journey to Earth.

Together, Super-Kamiokande and SNO provided irrefutable evidence that neutrinos oscillate, and therefore must have mass.

Why This Was Revolutionary

1. Challenged the Standard Model:
   The Standard Model treated neutrinos as massless. Proving otherwise meant new physics exists beyond it.

2. Opened the Door to Neutrino Physics:
   Experiments worldwide — like T2K (Japan), NOvA (USA), and JUNO (China) — are now probing the neutrino mass hierarchy, CP violation, and the origin of matter-antimatter asymmetry.

3. Deepened Our Understanding of the Universe:
   Since neutrinos are the second most abundant particles after photons, their mass influences the evolution of galaxies and cosmic structures.

About the Laureates

Takaaki Kajita

• Born: 1959, Higashimatsuyama, Japan

• PhD: University of Tokyo, 1986

• Position: Director, Institute for Cosmic Ray Research, University of Tokyo

• Contribution: Discovery of atmospheric neutrino oscillations using Super-Kamiokande.

Arthur B. McDonald

• Born: 1943, Sydney, Nova Scotia, Canada

• PhD: California Institute of Technology, 1969

• Position: Professor Emeritus, Queen’s University

• Contribution: Discovery of solar neutrino transformation at SNO.

Simple Analogy

Think of neutrinos like travelers wearing colored shirts (red, blue, green).
They start the journey wearing red, but as they move, the colors gradually change — sometimes blue, sometimes green — depending on how far and fast they travel.
When you check at the destination, not all are red anymore — that’s neutrino oscillation in a nutshell!

Prize Details

FAQs

Q1. What are neutrinos made of?
Neutrinos are fundamental particles — they aren’t made of anything smaller. They belong to the lepton family, like electrons, but are neutral and extremely light.

Q2. How small is a neutrino’s mass?
Neutrino masses are less than one-millionth that of an electron — so tiny that even the most sensitive instruments can’t measure them directly yet.

Q3. How many types of neutrinos exist?
Three: electron neutrino (νe), muon neutrino (νμ), and tau neutrino (ντ).

Q4. Why are neutrinos important for cosmology?
Because they were produced abundantly in the early universe and can still affect how galaxies and large-scale structures formed.

Q5. What new questions did this discovery raise?
Scientists now want to know the absolute neutrino mass, whether neutrinos are their own antiparticles, and if neutrino behavior explains why matter dominates over antimatter.

Conclusion

The 2015 Nobel Prize in Physics honored not just two scientists, but a half-century journey of curiosity — from puzzling solar data to underground detectors capturing ghostly particles.

Kajita and McDonald’s discovery that neutrinos oscillate and have mass reshaped the foundations of particle physics. It proved that the universe still hides surprises, and even the smallest, most elusive particles can rewrite the biggest scientific theories.