Takaaki Kajita: The Direction That Made the Difference
The atmospheric neutrino deficit had been visible in the data for years before 1998, and it had not convinced anyone. Kajita's contribution was to add the one variable that made the alternative explanations untenable: where each neutrino had come from, and therefore how far it had travelled before arriving.
The Anomaly Before the Answer
Cosmic rays hitting the upper atmosphere produce showers containing muon and electron neutrinos in a ratio theory predicts fairly precisely - roughly two muon neutrinos for every electron neutrino. Several experiments, Kamiokande among them, had measured fewer muon neutrinos than that.
The result was not widely believed. A ratio can be wrong for many uninteresting reasons: the atmospheric shower model might be off, the detector might mis-identify particle types, the cross-sections might be inaccurate. A deficit in a ratio is suggestive and no more.
What was needed was a variable that no plausible systematic error could mimic. That variable was distance. Neutrinos produced in the atmosphere directly overhead have travelled about fifteen kilometres when they reach a detector. Neutrinos produced on the opposite side of the planet have travelled roughly thirteen thousand.
Super-Kamiokande could reconstruct the direction of each event, because a muon leaves a long straight Cherenkov track whose orientation is measurable. That turned the question from how many into how many as a function of how far.
Takayama, June 1998
The answer was clean. Muon neutrinos arriving from overhead appeared in the expected numbers. Muon neutrinos that had crossed the Earth appeared at roughly half that rate. The deficit varied with path length exactly as oscillation predicts.
No detector systematic explains a dependence on arrival direction of that shape, because the detector does not know which way a neutrino came from until the reconstruction is done. The Earth is not absorbing them either - it is nearly transparent to neutrinos at these energies.
Kajita presented this at the Neutrino '98 conference in Takayama in June 1998, and it was accepted quickly, which is unusual for a result contradicting the Standard Model. It was accepted because the variable that carried the signal was the one variable that could not be faked.
Quantum mechanics permits oscillation only if the mass states differ in mass, which requires at least two of them to be non-zero. The Standard Model as originally written had no mechanism to give neutrinos mass at all, so the result was the first laboratory evidence of physics beyond it.
The Other Half of the Answer
Kajita's result showed that muon neutrinos were disappearing. It did not show what they were turning into, because Super-Kamiokande could not detect tau neutrinos efficiently at those energies.
The complementary measurement came from the Sudbury Neutrino Observatory, which used heavy water to count both electron neutrinos and the total of all three types arriving from the Sun. The total matched the solar model while the electron component was a third of it: the missing neutrinos had changed type rather than vanished.
Two experiments, different sources, different technologies, different systematic errors, one conclusion. That convergence is why the result held and why the 2015 Nobel Prize was shared between Kajita and Arthur McDonald.
It also resolved a thirty-year-old problem neither experiment had set out to solve: Raymond Davis had been measuring the solar neutrino flux correctly all along.
Since
Kajita is a professor at the University of Tokyo and directs its Institute for Cosmic Ray Research, which operates the Kamioka facilities. He has also led work on KAGRA, a gravitational-wave detector built in the same mine complex.
Super-Kamiokande continues to run, and its successor Hyper-Kamiokande is under construction with roughly five times the mass, aimed at measuring whether neutrinos and antineutrinos oscillate differently.
The 1998 measurement remains one of very few confirmed cracks in the Standard Model. Everything on the neutrino mass page, and the entire long-baseline programme that DUNE and JUNO belong to, follows from it.
Frequently asked questions
What did Kajita measure in 1998?
That muon neutrinos arriving from the far side of the Earth were about half as numerous as those arriving from overhead. Since the only difference was distance travelled, they must have been changing type in flight.
Why was direction the decisive variable?
Because no detector systematic can mimic a dependence on arrival direction. The detector does not know where a neutrino came from until reconstruction, and the Earth does not absorb neutrinos at these energies.
Why does oscillation imply mass?
Because the different mass states must accumulate quantum phase at different rates for the mixture to change identity in flight. That requires them to have different masses, and therefore at least two must be non-zero.
How does this relate to the Sudbury result?
Kajita showed muon neutrinos disappearing; Sudbury showed solar neutrinos arriving as other types rather than vanishing. Two sources, two technologies, two sets of systematics, one conclusion. They shared the 2015 Nobel Prize.
What does he work on now?
He is a professor at the University of Tokyo and directs its Institute for Cosmic Ray Research. He has also led work on KAGRA, a gravitational-wave detector in the same mine complex.
