Homestaka

Homestaka, a mine-dwelling tank-keeper dwarf of the Paradox Parlour, counts argon atoms in a huge tank of cleaning fluid and finds only a third of the neutrinos he expected.

The Physics

Raymond Davis Jr.'s experiment in the Homestake gold mine, South Dakota, 1478 m underground, used a tank of about 615 tonnes of perchloroethylene (tetrachloroethylene, C₂Cl₄). A solar electron neutrino can turn a chlorine-37 nucleus into argon-37 (threshold 0.814 MeV), and the few argon atoms were extracted and counted. From 1970 to 1994 it measured about a third of the rate predicted by John Bahcall's standard solar model.

measured 2.56 ± 0.16 ± 0.16 SNU vs predicted 7.6^+1.3_−1.1 SNU

• SNU: solar neutrino unit: 10⁻³⁶ captures per target atom per second
• 2.56: final Homestake result (Cleveland et al., ApJ 496, 505, 1998), statistical and systematic errors
• 7.6: chlorine prediction of the BP2000 standard solar model (Bahcall, Pinsonneault and Basu, 2001)

The ratio is about 0.34. The answer came from SNO in 2001–2002: by comparing charged-current events (electron neutrinos only) with neutral-current events (all flavours), it showed the total ⁸B flux matched the solar model while most of it reached Earth as muon and tau neutrinos. Neutrinos change flavour, which means they have mass. Davis shared the 2002 Nobel Prize with Masatoshi Koshiba, and the 2015 Prize went to Takaaki Kajita and Arthur McDonald for neutrino oscillations.

Honest caveat: Neither the experiment nor the Sun was faulty; Homestake could detect only electron neutrinos. The “one third” is partly a coincidence: for the high-energy ⁸B neutrinos the survival probability of about 1/3 comes from flavour conversion in the Sun's matter (the MSW effect), not simple vacuum oscillation, and lower-energy neutrinos survive at a higher rate.

On the sticker

Homestaka holds a glowing green tank-flask deep in the mine. Printed text: HOMESTAKA with the line 2.56 vs 7.6 SNU. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.