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How can a detector establish that an unseen particle changes identity? The Super-Kamiokande atmospheric-neutrino experiment compares different travel paths; the Sudbury Neutrino Observatory compares different interaction channels. Both infer neutrino flavor from reaction products and calibrated responses. Their complementarity connects particle detection to neutrino oscillation, neutrino mass and neutrino astronomy.

Takaaki Kajita and Arthur B. McDonald shared the 2015 physics Nobel Prize. These were collaboration achievements: Yoichiro Suzuki's Super-Kamiokande work included the solar neutrino analysis, and George Ewan helped establish and lead the Sudbury Neutrino Observatory.^[ref][ref]

The older Kamiokande account explains water Cherenkov neutrino reconstruction; Raymond Davis Jr.'s Homestake experiment explains why a deficit in an electron neutrino channel alone leaves a source-versus-propagation ambiguity. Complementary channels can break that ambiguity.

TODO: a numerical estimate from the missing MINOS spectrum cannot be supplied. On source page 7 the graph is absent: PDF Form /Im3 invokes undefined image /Im1 with no image resources, and the archived bundle has the same defect. In particular, no measured minimum energy survives. For maximal mixing angle, the first minimum of muon neutrino survival obeysA restored spectrum or fitted data are needed to choose $E_{\min }$. This is a relation for the survival minimum; a minimum in the raw event spectrum also depends on flux, cross section and smearing. For context only, accelerator and atmospheric results put the splitting at a few $10^{-3}{\mathrm{eV}}^2$, versus the solar splitting about $7.5\times 10^{-5}{\mathrm{eV}}^2$, roughly thirty times smaller. This known scale is not a reconstruction of the absent graph. ^[ref]

Finite event statistics, hadronic shower energy resolution, the muon scale, near-to-far flux extrapolation, interaction and nuclear modeling, backgrounds and detector calibration limit the precision; the baseline itself is comparatively well known. In a two-flavour vacuum fit the sign of $\Delta m^2$ is inaccessible. Evidence for dominant ${\nu }_{\mu }\to {\nu }_{\tau }$ includes the zenith-angle-dependent atmospheric deficit in Super-Kamiokande, approximately preserved active neutral current rates, small electron-flavour appearance at this scale, atmospheric tau appearance and direct tau appearance in OPERA. The latter had already reported four candidates and $4.2\sigma$ evidence in 2014; a later five-candidate discovery result is not needed for this June 2015 answer. ^[ref] These tests constrain a dominant sterile conversion as well as conversion into electron neutrinos.

Nuclear fusion in the Sun produces predominantly electron neutrinos, through the pp chain and subsidiary reactions. The historical solar-neutrino deficit could have reflected either a source-model error or flavour transformation. In a two-flavour approximation, mixing gives the vacuum survival law derived in a. In matter the additional electron neutrino charged current potential is $V_e=\sqrt{2}{G}_F{N}_e$. Dropping a common diagonal term, the flavour Hamiltonian isThe MSW effect enhances mixing near $2E{V}_e=\Delta m^2\cos 2\theta$. An initially electron-like high-density state follows its instantaneous mass eigenstate approximately adiabatically as solar density falls. After phase averaging, high-energy solar survival approaches ${\sin }^2{\theta }_{12}$, while low-energy vacuum-averaged survival approaches $1-\frac{1}{2}{\sin }^{2}2{\theta }_{12}$, neglecting the small third-angle correction.

The Sudbury Neutrino Observatory used about a kilotonne of heavy water viewed by photomultipliers. Its three channels separate flavour from total active flux:The charged current channel measures electron flavour, the neutral current channel measures all active flavours nearly equally, and elastic scattering is more sensitive to electron flavour but also detects the others. Cherenkov radiation gives the charged-particle energy and direction. Neutrons were detected through capture gamma rays, with later salt loading and dedicated counters improving the neutral-current separation. The different angular patterns, light isotropy and energy spectra permit a combined channel fit, with radioactive backgrounds and calibrations controlled.

The measured electron flux was lower than the total active flux, while the latter agreed with solar fusion expectations. This directly established a non-electron component and removed a simple overall source-flux error as the explanation. ^[ref] Super-Kamiokande supplied high-statistics directional elastic-scattering data; radiochemical chlorine/gallium experiments supplied complementary thresholds, and reactor antineutrino disappearance independently confirmed the solar mass-squared difference. Together these support large-angle mixing at $\Delta m_{21}^2\sim 7.5\times 10^{-5}{\mathrm{eV}}^2$ rather than the atmospheric splitting in c.

In 2001, $240.95$ live days, reconstructed radius below $5.50\mathrm{m}$ and effective electron kinetic energy above $6.75\mathrm{MeV}$ gave a CC neutrino flux of $1.75\times 10^6{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ for the assumed boron-8 spectrum. Source calibrations measured $1.4\%$ instrumental-cut signal loss. SNO's own CC–ES difference was $1.6\sigma$; comparison with Super-Kamiokande's precise ES flux gave $3.3\sigma$ evidence for non-electron active neutrinos. This was not an NC measurement.^[ref]

In 2002 the pure-heavy water analysis used $306.4$ live days and a $5\mathrm{MeV}$ reconstructed kinetic-energy threshold. NC neutrons captured on deuterons emitted $6.25\mathrm{MeV}$ gamma rays, producing secondary electron light. The NC reaction threshold was $2.2\mathrm{MeV}$, a different quantity. Californium-252 calibrated $29.9\%$ capture efficiency, but only $14.4\%$ detection efficiency after fiducial and energy cuts. Assays constrained backgrounds from radioactive decay and photodisintegration.^[ref]

Energy, radius and direction relative to the Sun statistically separated channels. Assuming the standard boron-8 spectral shape, the NC total active neutrino flux was $5.09_{-0.43}^{+0.44}{(stat.)}_{-0.43}^{+0.46}(syst.)\times 10^6{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, consistent with stellar structure predictions. The non-electron active component differed from zero by $5.3\sigma$. This established active flavour conversion, without individually tagging muon neutrinos and tau neutrinos. It did not exclude every sterile admixture.^[ref]

Super-Kamiokande is an underground water Cherenkov detector in Japan. Its roughly 50,000-ton water target is viewed by photomultipliers around the tank. A neutrino is observed indirectly when an interaction produces charged secondaries that emit Cherenkov radiation.^[ref]

Photon arrival times and detected charges constrain interaction position and outgoing directions. Ring appearance helps distinguish showering electrons from more track-like muons, enabling the Super-Kamiokande atmospheric-neutrino experiment. The water mass is both target and optical medium: transparency, sensor timing and background rejection affect reconstruction. The useful fiducial volume is smaller than the full water volume.