Neutrino oscillations and neutrino mass

Settled in 1998 by Super-Kamiokande Collaboration (1998, atmospheric neutrinos); SNO Collaboration (2001-2002, solar neutrinos); Nobel Prize 2015 to Kajita and McDonald.

Ginzburg's final numbered item (#30) asked whether the long-standing 'solar neutrino problem' (fewer solar neutrinos detected than predicted) was due to oscillations, which would require nonzero neutrino mass. Just as his paper was going to print, Super-Kamiokande announced (June 1998) strong evidence for atmospheric neutrino oscillations; the Sudbury Neutrino Observatory then directly confirmed that solar electron neutrinos convert to other flavors (2001-2002), definitively resolving the solar neutrino problem. Kajita and McDonald shared the 2015 Nobel Prize in Physics for this discovery. Some sub-questions remain open (the absolute neutrino mass scale, the mass ordering, and whether neutrinos are their own antiparticles).

Neutrinos are so ghostly that trillions pass through your body every second unnoticed, yet by watching them change identity mid-flight, physicists proved for the first time that they have mass at all.

References

• What problems of physics and astrophysics seem now to be especially important and interesting (thirty years later, already on the verge of the XXI century)?, V L Ginzburg, Physics-Uspekhi 42 (4) 353-373, 1999
• Evidence for Oscillation of Atmospheric Neutrinos, Y. Fukuda et al. (Super-Kamiokande Collaboration), Physical Review Letters 81, 1562, 1998
• Direct Evidence for Neutrino Flavor Transformation from Neutral-Current Interactions in SNO, SNO Collaboration, Physical Review Letters 89, 011301, 2002
• The Nobel Prize in Physics 2015, NobelPrize.org press release, 2015