Direct Evidence for Neutrino Flavor Transformation from Neutral-Current Interactions in the Sudbury Neutrino Observatory

The heavy-water strategy

The power of SNO’s approach lay in the availability of three complementary detection channels in heavy water ($D_{2}O$):

${\nu }_e+d\to p+p+e^{-}\text{(CC, }{\nu }_e\text{ only)}$ ${\nu }_x+d\to p+n+{\nu }_x\text{(NC, all flavors equal)}$ ${\nu }_x+e^{-}\to {\nu }_x+e^{-}\text{(ES, }{\nu }_e\text{ weighted }\sim 6\times \text{)}$

Herb Chen had proposed this three-channel strategy in 1984 specifically to settle the solar neutrino problem: if the deficit is due to oscillation, NC should match the Standard Solar Model while CC should come out lower. If the deficit is due to an incorrect solar model, both channels should be proportionally depressed.

The 2002 measurement

The paper reports SNO’s first-phase results based on 306 live days of pure-D₂O data. Neutral-current events were identified by the 6.25 MeV gamma from neutron capture on deuterium. The extracted fluxes were: ${\Phi }_{CC}=(1.76\pm 0.10)\times 10^6{\text{ cm}}^{-2}{\text{s}}^{-1}$ ${\Phi }_{ES}=(2.39\pm 0.26)\times 10^6{\text{ cm}}^{-2}{\text{s}}^{-1}$ ${\Phi }_{NC}=(5.09\pm 0.44)\times 10^6{\text{ cm}}^{-2}{\text{s}}^{-1}$

The Standard Solar Model prediction was ${\Phi }_{SSM}=(5.05\pm 0.91)\times 10^6$ cm⁻² s⁻¹ — agreeing with the NC measurement at the percent level.

The ${\nu }_e$ survival probability extracted from the ratio was $P({\nu }_e\to {\nu }_e)=0.35\pm 0.02$.

Combined with earlier data

An earlier SNO paper (2001) had compared the CC flux with Super-Kamiokande’s ES flux — the latter sensitive partly to non-${\nu }_e$ flavors through neutral-current — and found a non-${\nu }_e$ component. The 2002 paper directly measured that non-${\nu }_e$ component within the same detector, eliminating cross-calibration uncertainties.

The result was a sharp, single-experiment demonstration that flavor transformation is the explanation for the solar neutrino deficit.

The MSW-LMA solution

The SNO data, combined with solar-neutrino measurements at Homestake, SAGE, GALLEX, and Super-K, selected the “large mixing angle” (LMA) region of MSW-oscillation parameter space: ${\tan }^2{\theta }_{12}\sim 0.4$ and $\Delta m_{21}^2\sim 5\times 10^{-5}$ eV². The subsequent KamLAND reactor measurement confirmed these parameters independently using antineutrinos.

The LMA region has since been refined with greater precision by KamLAND, Borexino, and Super-K; the solar and reactor determinations of $\Delta m_{21}^2$ and ${\theta }_{12}$ are now in good agreement at the percent level.

Legacy

The SNO result provided the second of the two independent oscillation discoveries — atmospheric at Super-K and solar at SNO — that established neutrino mass. The 2015 Nobel Prize was shared by Kajita and McDonald for these complementary achievements.

SNOLAB, the underground laboratory that grew out of SNO, now hosts SNO+, DEAP-3600, SuperCDMS, and other experiments at the 2 km depth originally excavated for SNO.