A detective story that is yet to reach its conclusion was started in 1993 by a few flashes of light inside a bus-size tank of oil.
The lightest and most elusive of all known particles, the neutrinos, were being searched for by the LSND. Bill Louis, one of the experiment's leaders, said that they saw something.
They saw too many. Theorists had thought that the neutrinos might be different as they fly. LSND wanted to see if this idea could be tested by aiming a beam of muon neutrinos towards the oil tank and counting the number of electron neutrinos that arrived there. Louis and his team detected more electron neutrinos arriving in the tank than the simple theory predicted.
Dozens of more experiments have been built since then. Physicists have built cathedrals to these slippery particles in mountains, mining caverns, and the South Pole. The experiments yielded conflicting pictures of how the particles behave. Louis said the plot keeps getting more complicated.
It is a very confusing story. The Garden of Forking Paths is called by Carlos Argelles-Delgado, a neutrino physicist at Harvard University. In the 1941 short story, time branches into an infinite number of futures. Figuring out which data to trust and which might be leading them astray has been a problem with the results of the neutrinos. Sometimes you see clues and they throw you in the wrong direction.
Los Alamos National Laboratory reported a bounty of neutrino detections in 1993. The photomultiplier tubes that would detect light from neutrino interactions inside the tank were put in by Rick Bolton.
The simplest explanation of the LSND anomalies was the existence of a fourth type of neutrino called the sterile neutrino, which mixes up all the other types according to new rules. Sterile neutrinos would allow muon neutrinos to move more quickly towards the oil tank.
The results of other experiments didn't fit the results of the sterile neutrino. The problem was that other theories fail miserably. We needed to come out of the forest.
Forced to retrace their steps, physicists have been rethinking what happened. They have come up with new theories that are more complicated than the sterile neutrino, but which, if correct, would completely change physics. The new models theorize that dark matter is four times more abundant than normal matter.
Four analyses released yesterday by the MicroBooNE experiment at the Fermi National Accelerator Laboratory near Chicago and another study from the IceCube detector at the South Pole suggest that the more complex neutrino theories may be on the right track.
I feel like something is in the air, said Argelles-Delgado. It is a tense environment that points towards discovery.
A desperate remedy.
Wolfgang Pauli called the existence of the neutrino a desperation remedy when he made it known in 1930. His theoretical construct did not have a mass or electric charge, making him doubt an experiment could detect it. He wrote in his journal that it was something no theorist should ever do. In an experiment similar to LSND, there was the neutrino.
Physicists found less than half of the number predicted by theoretical models of stars' nuclear reactions when they detected neutrinos coming from the sun. By the 1990s, it was clear that neutrinos were acting strange. When the upper atmosphere collides with the sun's rays, solar neutrinos disappear.
