Measurements from the Sudbury Neutrino Observatory in Canada were thought to have settled the mysteries of neutrinos - but new calculations have raised new questions. Credit: Roy Kaltschmidt / Lawrence Berkeley Nat'l Lab

Neutrinos have long perplexed physicists with their uncanny ability to evade detection, with as many as two-thirds of the ghostly particles apparently going missing en route from the Sun to Earth. Now a refined version of an old calculation is causing a stir by suggesting that researchers have also systematically underestimated the number of the particles' antimatter partners — antineutrinos — produced by nuclear reactor experiments.

The deficit could be caused by the antineutrinos turning into so-called 'sterile antineutrinos', which can't be directly detected, and which would be clear evidence for effects beyond the standard model of particle physics.

In the 1960s, physicist Ray Davis, working deep underground in the Homestake gold mine in South Dakota, found that the flux of solar neutrinos hitting Earth was a third of that predicted by calculations of the nuclear reactions in the Sun by theorist John Bahcall. Davis later received a Nobel prize for his contributions to neutrino astrophysics. That puzzle was considered solved in 2001, when the Sudbury Neutrino Observatory (SNO) in Canada found the missing two-thirds through an alternative means of detection. The SNO's results were taken as evidence that neutrinos have a mass, which allows them to oscillate between three flavours: electron, muon and tau. Davis had only detected the electron neutrinos.

Experiments that measure the rate of antineutrino production from the decay of uranium and plutonium isotopes have so far produced results roughly consistent with this theory. But the revised calculation1 accepted this week by Physical Review D suggests that it's not the whole story. While waiting for the Double Chooz neutrino experiment in France to become fully operational, Thierry Lasserre and his colleagues at the French atomic energy commission(CEA) in Saclay set out to check predictions of the rate of antineutrino production by nuclear reactors. They repeated a calculation first done in the 1980s by Klaus Schreckenbach at the Technical University of Munich, using more modern techniques that allowed them to be much more precise.

Their new estimate of the rate of production is around 3% more than previously predicted. This means that several generations of neutrino and antineutrino experiments have unknowingly missed a small fraction of the particles. "It was completely a surprise for us," says Lasserre.

Double Chooz consists of two detectors measuring the flux of antineutrinos produced by the Chooz nuclear power plant in the French Ardennes, one detector about 400 metres away from the plant and the other 1 kilometre away. The far detector became operational this year.

Stefan Schönert, a neutrino physicist at the Technical University of Munich, says the calculation is solid, and has been checked with Schreckenbach. "They can reproduce each other's results. There's no way around this result. It's very solid."

Art McDonald of Queen's University in Kingston, Canada and the SNO says that people have to look carefully at the calculation, which may itself have a systematic error. But, he adds, "there's no doubt it would have significance as a physics result if it can be shown with more accuracy."

The result may be pointing to evidence of neutrinos and antineutrinos oscillating into a fourth kind of neutrino or antineutrino, a so-called 'sterile' version that doesn't interact with ordinary matter, says Carlo Giunti, a physicist at the University of Turin in Italy. Other experiments have previously seen evidence for sterile particles, including the Liquid Scintillator Neutrino Detector at Los Alamos National Laboratory in New Mexico and the Mini Booster Neutrino Experiment, or MiniBooNE, at Fermilab in Batavia, Illinois, and the search to confirm their existence is a hot area of physics.

Giunti says that the magnitude of the anomaly uncovered by Lasserre is not statistically significant on its own, but that it points promisingly in the same direction as another anomaly found by the SAGE collaboration, which studied neutrinos from a radioactive source at the Baksan Neutrino Observatory in the Caucasus in 20052. "Before this, there used to be a contradiction between [reactor and radioactive source] experiments but now they are in agreement," says Giunti.

Schönert says that one key experiment everyone is waiting for is a measurement showing that the rate of disappearance of antineutrinos from a source increases with the distance from it. "This would be the smoking gun," he says.