In 1993, deep underground at Los Alamos National Laboratory in New Mexico, just a few flashes of sunshine inside a bus-size tank of oil kicked off a detective story that’s but to achieve its conclusion.
The Liquid Scintillator Neutrino Detector (LSND) was trying to find bursts of radiation created by neutrinos, the lightest and most elusive of all identified elementary particles. “Much to our amazement, that’s what we saw,” mentioned Bill Louis, one of many experiment’s leaders.
The drawback was that they noticed too many. Theorists had postulated that neutrinos may oscillate between sorts as they fly alongside—a speculation that defined varied astronomical observations. LSND had got down to take a look at this concept by aiming a beam of muon neutrinos, one of many three identified sorts, towards the oil tank, and counting the variety of electron neutrinos that arrived there. Yet Louis and his group detected much more electron neutrinos arriving within the tank than the easy concept of neutrino oscillations predicted.
Since then, dozens extra neutrino experiments have been constructed, every grander than the final. In mountains, disused mining caverns, and the ice beneath the South Pole, physicists have erected cathedrals to those notoriously slippery particles. But as these experiments probed neutrinos from each angle, they stored yielding conflicting photos of how the particles behave. “The plot keeps thickening,” mentioned Louis.
“It’s a very confusing story. I call it the Garden of Forking Paths,” mentioned Carlos Argüelles-Delgado, a neutrino physicist at Harvard University. In Jorge Luis Borges’ 1941 quick story of that title, time branches into an infinite variety of attainable futures. With neutrinos, contradictory outcomes have despatched theorists down a wide range of paths, uncertain which knowledge to belief and which is likely to be main them astray. “Like any detective story, sometimes you see clues and they throw you in the wrong direction,” Argüelles-Delgado mentioned.
The easiest rationalization of the LSND anomaly was the existence of a brand new, fourth form of neutrino, dubbed the sterile neutrino, that mixes up all of the neutrino sorts in response to new guidelines. Sterile neutrinos would enable muon neutrinos to oscillate extra readily into electron neutrinos over the quick distance to the oil tank.
But as time went on, the sterile neutrino didn’t match the outcomes of different experiments. “We had our champion theory, but the problem was that elsewhere it fails miserably,” Argüelles-Delgado mentioned. “We were very deep in the forest, and we needed to come out.”
Forced to retrace their steps, physicists have been rethinking what’s behind the muddle of hints and half outcomes. In current years, they’ve devised new theories which can be extra sophisticated than the sterile neutrino, however which, if appropriate, would totally revolutionize physics—resolving anomalies in neutrino oscillation knowledge and different main mysteries of physics on the identical time. Not least, the brand new fashions posit heavy extra neutrinos that would account for darkish matter, the invisible stuff enshrouding galaxies that appears to be 4 instances extra considerable than regular matter.
Now, 4 analyses launched yesterday by the MicroBooNE experiment on the Fermi National Accelerator Laboratory close to Chicago and one other current examine from the IceCube detector on the South Pole each recommend that these extra complicated neutrino theories could also be heading in the right direction—although the longer term stays removed from clear.
“I feel like something’s in the air,” mentioned Argüelles-Delgado. “It’s a very tense environment that points toward discovery.”
A Desperate Remedy
When Wolfgang Pauli postulated the existence of the neutrino in 1930 to elucidate the place power was disappearing to throughout radioactive decay, he referred to as it a “desperate remedy.” His theoretical assemble had no mass or electrical cost, making him doubt an experiment might ever detect it. “It is something no theorist should ever do,” he wrote in his journal on the time. But in 1956, in an experiment not in contrast to LSND, there the neutrino was.
Triumph quickly veered into confusion when physicists detected neutrinos coming from the solar, a pure supply of the particles, and located fewer than half the quantity predicted by theoretical fashions of stars’ nuclear reactions. By the Nineteen Nineties, it was clear that neutrinos have been behaving oddly. Not solely did photo voltaic neutrinos appear to mysteriously disappear, however so too did the neutrinos that fall to Earth when cosmic rays collide with the higher ambiance.