Study the standard model of particle physics at any length, and you might find yourself musing about the quasi-mystical significance of the number three (see diagram).
Among the matter particles, there are three quarks 鈥� up, charm and top 鈥� with the same electric charge, differing only in mass. Then another group of three quarks 鈥� down, strange and bottom 鈥� for which the same pattern holds. Then three charged leptons, the electron, the muon and the tau: again, same charge, different mass. And finally, three distinct 鈥渇lavours鈥� of chargeless, near-massless neutrinos Except maybe not that last one.
Neutrinos are particles that interact only via the weak nuclear force that governs radioactive decay. Trillions of the lightest type, electron neutrinos, are generated in reactions in the sun and pass through you every second. They will also pass through Earth without a blip.
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Fiddly measurements over decades have revealed that a sizable portion of this flux is going missing. This 鈥渟olar neutrino problem鈥� was resolved conclusively only in 2001, when electron neutrinos were shown to be shape-shifting, or 鈥渙scillating鈥�, into muon and tau neutrinos on their way to us. That breakthrough was awarded .
At that point we might all have lived happily ever after, had not a series of detectors, starting with the Liquid Scintillator Neutrino Detector at Los Alamos Laboratory in New Mexico in the 1990s, begun to see more experimental blips. The blips suggested that there were not merely three types of neutrino, but four 鈥� or more. In 2011, an analysis of neutrinos emitted in nuclear reactors found there were 鈥� a shortfall by researchers from the reactor neutrino experiment in eastern China.
One possible explanation is that neutrinos briefly oscillate into unobserved 鈥渟terile鈥� neutrino states that don鈥檛 even interact through the weak force. That is uncomfortable not just because it breaks the rule of three. If sterile neutrinos exist, our best guess is that they should be very heavy. These ones, however, seem to be very light.
A few detectors around the world are looking for the sterile neutrinos now, and in February, CERN gave the go-ahead for the SHiP 鈥� 鈥� experiment that will seek out heavier versions.
Neutrino physicist of the University of Oxford does not deny the neutrino anomaly exists. 鈥淭he most recent measurements from Daya Bay confirm it,鈥� he says. But he is sceptical the new searches will find anything, because these results also reveal that our predictions for the overall numbers of neutrinos reactors should be pumping out don鈥檛 tally with what we see. This implies our models may be at fault, says Wark 鈥� rather than our basic understanding of physics.
That鈥檚 odd: the Pioneer anomaly
The unexpected slowing of the two Pioneer spacecraft as they crossed the solar system excited interest from the late 1990s as it indicated a possible deviation from general relativity鈥檚 predictions. Heat recoil from the spacecraft鈥檚 thermoelectric generators now seems to be the most likely explanation.
Read more: 鈥�The 6 biggest glitches in physics鈥�
This article appeared in print under the headline 鈥淭hat鈥檚 odd鈥� Missing neutrinos鈥�
