Arthur B. McDonald

Canadian physicist and director of the Sudbury Neutrino Observatory (SNO), whose 2001–2002 results resolved the long-standing solar neutrino problem by showing that solar electron neutrinos change flavor in flight. Shared the 2015 Nobel Prize in Physics with Takaaki Kajita.

Contributions

SNO's flavor-transformation result (2001–2002)

Under McDonald's leadership SNO used heavy water (D₂O) to measure solar neutrinos through three distinct channels — charged current (νe-sensitive), neutral current (flavor-blind), and elastic scattering (νe-weighted). The NC flux matched the Standard Solar Model; the CC flux did not. The conclusion was direct and unambiguous: two-thirds of the solar νe had transformed into νμ or ντ in flight.

SNOLAB

McDonald was a driving force behind the expansion of SNO into the broader SNOLAB underground facility near Sudbury, Ontario, which now hosts experiments on neutrinoless double beta decay, dark matter, and geophysics at a depth of 2 km.

Precision solar neutrino spectroscopy

SNO's subsequent phases — salt loading, NCD deployment — progressively refined the measurement, confirming the MSW-LMA oscillation scheme and constraining CP and matter effects with increasing precision.

Legacy

McDonald's SNO result provided the second independent oscillation discovery, complementary to Kajita's atmospheric result: one from cosmic-ray atmospheric neutrinos traversing the Earth, one from solar neutrinos traversing the Sun. Together they made neutrino mass inescapable. The 2015 Nobel Prize was shared for this pair of discoveries. SNOLAB today is one of the premier underground physics facilities in the world.

Background

Arthur Bruce McDonald was born in Sydney, Nova Scotia in 1943. He earned his BSc and MSc at Dalhousie University and his PhD at Caltech in 1969. He held positions at the Chalk River Nuclear Laboratories and at Princeton before returning to Canada to lead the SNO project at Queen’s University in 1989.

The Sudbury Neutrino Observatory

SNO was designed to resolve the solar neutrino problem by an unambiguous method. The key idea — due originally to Herb Chen — was to use heavy water (deuterium) as the target, enabling three complementary reactions:

Charged current: $ν_{e} + d \to p + p + e^{-}$ — sensitive only to $ν_{e}$
Neutral current: $ν_{x} + d \to p + n + ν_{x}$ — sensitive to all active flavors
Elastic scattering: $ν_{x} + e^{-} \to ν_{x} + e^{-}$ — weighted mostly toward $ν_{e}$

By comparing the three rates, SNO could directly measure both the total flavor-summed solar neutrino flux and the electron-neutrino fraction. If oscillation was the cause of the deficit, NC should match the Standard Solar Model while CC should come out lower.

The definitive measurements

Results announced in 2001 and refined through 2002 gave: $Φ_{N C} = 5.0 9_{- 0.43}^{+ 0.44} \times 1 0^{6} cm^{- 2} s^{- 1}$ $Φ_{C C} = 1.7 6_{- 0.10}^{+ 0.10} \times 1 0^{6} cm^{- 2} s^{- 1}$ The NC flux matched Bahcall’s Standard Solar Model within uncertainties. The ratio $Φ_{C C} / Φ_{N C} \approx 0.35$ was consistent with MSW oscillation parameters in the “large mixing angle” (LMA) region later pinned down by KamLAND.

The solar neutrino problem, unresolved for more than three decades, was definitively resolved as a consequence of neutrino oscillations.

Later work and recognition

McDonald stepped down as SNO director in 2013. He has remained active in SNOLAB governance and in Canadian science policy.

He shared the 2015 Nobel Prize with Takaaki Kajita, specifically honoring the pair of independent oscillation discoveries — atmospheric at Super-K and solar at SNO — that together established neutrino mass.

In addition to the Nobel, McDonald has received the Sackler Prize in Physics, the Franklin Medal, the Companion of the Order of Canada, and numerous other honors. He continues to advocate for underground physics infrastructure and for international collaboration in the field.