In 1998, a detector under a Japanese mountain announced that half the muon neutrinos arriving from the far side of the Earth were simply gone, the first laboratory crack in the Standard Model.
Documented
Six sigma through the planet
At the Neutrino'98 conference in Takayama, Super-Kamiokande presented 535 days of data: 5,400 atmospheric neutrino events, with a clear deficit of upward-going muon neutrinos, the ones that had traveled up to 12,700 kilometers through Earth, at more than six standard deviations. The e-like distribution showed no such asymmetry. Oscillation was the only explanation, and oscillation requires mass.
Assessment: Documented: the zenith-angle distributions, the 6-sigma significance, and Kajita's own Nobel lecture account.
Stories
The Nobel split the atom of credit
The 2015 Nobel went to Kajita for Super-K and Arthur McDonald for the Sudbury Neutrino Observatory, which in 2001 proved solar neutrinos also switch identities, closing the decades-old solar neutrino problem: the Sun was never underproducing; the neutrinos were changing costume en route.
Assessment: Stories: two detectors, two hemispheres, two neutrino puzzles, one phenomenon. The Sun was innocent the whole time.
Interesting claims
The Standard Model's open door
Neutrino mass is the first and still the clearest laboratory evidence of physics beyond the Standard Model, the theory that otherwise passes every test. The most successful framework in science history has exactly one experimentally verified hole, and it was found by counting ghosts in a mountain.
Assessment: Interesting: the deepest crack in the deepest theory was found not by smashing particles but by noticing absences.
Sources and further reading
Takaaki Kajita, Nobel Lecture 2015: 535 live days, 5,400 events, the upward-going muon deficit past 6 sigma
Fukuda et al. (Super-Kamiokande), Phys. Rev. Lett. 81:1562 (1998), the oscillation evidence paper
Nobel Prize 2015 popular information: Kajita and McDonald, and SNO's 2001 closure of the solar neutrino problem
Worth asking
If the most numerous matter particles in the cosmos were misclassified for half a century, what else in the model is a placeholder?