They come in three flavors and can transform among these different types as they travel. They pass through most matter undetected and uninterrupted. Tens of trillions of them are passing through your body in the time it takes to read this sentence. But don’t worry – they are harmless. They are produced by the sun, within the Earth, at nuclear reactors, by exploding stars, and by cosmic rays interacting with Earth’s atmosphere, among other sources.
While many experiments, past and present, have taught us much about them, they remain the source of many unanswered questions and unsolved mysteries in science. A string of ambitious new experiments seek to fill in some of the most important gaps in our knowledge.
Each of their three types also has an antimatter counterpart, and it’s not yet clear whether these counterparts are essentially the same as their normal forms – the answer to this could help explain whether they played a substantial role in the abundance of matter vs. antimatter in the universe.
We know they have three different masses, too, but we aren’t yet sure which of these three is the lightest or heaviest.
They were discovered in 1956, and they were proposed to exist 90 years ago, on Dec. 4, 1930.
“They,” as it happens, are subatomic particles called neutrinos, and the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has a long history of participating in neutrino experiments and discoveries in locations ranging from a site 1.3 miles deep at a nickel mine in Ontario, Canada, to an underground research site near a nuclear power complex northeast of Hong Kong, and a neutrino observatory buried in ice near the South Pole.
In a Dec. 4, 1930, letter proposing the existence of these particles, Austrian physicist Wolfgang Pauli said they were an “almost improbable” and “desperate remedy” to preserve a fundamental physics law while explaining an apparent energy deficit in nuclear decay processes occurring in some atomic nuclei. Pauli had first referred to the particles as “neutrons,” though physicist Enrico Fermi in 1934 renamed these hypothesized particles “neutrinos” – Italian for “little neutral ones.”
To celebrate how far neutrino science has come in the 90 years since Pauli made his particle prediction, Berkeley Lab has prepared this list highlighting some of the landmark neutrino experiments and results its scientists have contributed to.
Follow the #neutrino90 hashtag on Twitter to learn more about the neutrino experiments and achievements that Berkeley Lab and other institutions have participated in.
Sudbury Neutrino Observatory (SNO), Canada
SNO provided the first direct evidence that neutrinos produced by the sun can change flavors, essentially shape-shifting among three different types. It was a Nobel Prize-winning revelation. The experiment consisted of a spherical acrylic detector, measuring about 40 feet in diameter, suspended from the ceiling of an underground cavern at an active mine site in Ontario, Canada. The sphere was filled with heavy water that emitted flashes of light during certain particle interactions, and these flashes were detected and converted into electronic signals by an array of 9,456 devices called photomultiplier tubes.
Starting in 1989, Kevin Lesko, a Berkeley Lab physicist who is now a spokesperson for the LUX-ZEPLIN dark matter experiment in South Dakota, led a team of Berkeley Lab engineers and nuclear scientists that contributed to the design and development of SNO. Berkeley Lab’s team conceptualized a design for a collection of 751 panels, in five different shapes, that surrounded the sphere to ensure adequate coverage by the PMT array. The components were designed to be portable so that they could fit inside the elevators used to deliver supplies to the underground site. Berkeley Lab also played a leading role in the physics analysis of SNO’s data.
“2015 Nobel Prize in Physics Has Berkeley Lab Roots,” Oct. 14, 2015
“Two Years of SNO Prove the Case: Solar Neutrinos Really Do Change,” April 22, 2002
“Astrophysics: Let There Be SNO,” Berkeley Lab Highlights, Fall 2000
Kamioka Liquid-scintillator Antineutrino Detector (KamLAND), Japan
While SNO focused on solar neutrinos, KamLAND was designed to measure nuclear-reactor-produced neutrinos. It was the first experiment to show that a quantum mechanical effect known as neutrino oscillation was responsible for the neutrino flavor transformation seen by SNO. It also conducted the first study of “geoneutrinos,” or neutrinos produced naturally in the Earth’s interior via the decay of radioactive elements, and set limits on a neutrino mixing angle – mixing angles relate to the oscillation rate of neutrinos.
As with SNO, KamLAND’s detector was filled with fluid that produces flashes of light in particle interactions, and the experiment also featured an array of photomultiplier tubes to gather and convert those flashes. Berkeley Lab scientists outfitted KamLAND with electronics that converted the signals from analog to digital. The technology was originally developed by Berkeley Lab scientists working on a South Pole neutrino experiment called IceCube. The late physicist Stuart Freedman, who died in November 2012, had led U.S. and Berkeley Lab involvement in KamLAND.
IceCube Neutrino Observatory, South Pole
Rather than use a liquid-filled detector, the IceCube Neutrino Observatory, near the South Pole, uses ice as its detector medium. Neutrinos colliding with oxygen nuclei in the ice turn into particles called muons, and these and other fast-moving particles generate blue light – an effect known as Cherenkov radiation – that IceCube detects and measures with its array of buried Digital Optical Modules, or DOMs. The light signals provide information about the neutrinos that stimulated this effect.
Berkeley Lab’s Dave Nygren and Stuart Kleinfelder had led the pioneering R&D for these spherical DOMs and related circuitry. The DOMs were lowered into boreholes in the ice on a series of cables, called strings, and then sealed in ice at a depth up to about 1.55 miles (2.5 kilometers). The original experiment, completed in 2010, featured 86 strings, with 60 of these optical sensors attached to each string. Berkeley Lab scientists also participated in a predecessor high-energy neutrinos experiment at the South Pole, dubbed AMANDA.
IceCube has found the first evidence for the source of high-energy cosmic neutrinos, which can travel uninterrupted for billions of light years through space. Based on IceCube data, the experiment’s collaboration pinpointed a blazar – a large galaxy with a fast-spinning black hole at its center – as the possible source of high-energy neutrinos. IceCube has also provided an analysis of the share of high-energy neutrinos that are absorbed by the Earth vs. those that pass through it.
“Into the Ice: Completing the IceCube Neutrino Observatory,” Dec. 17, 2010
“IceCube Neutrinos Point to Long-Sought Cosmic Ray Accelerator,” July 12, 2018
Daya Bay Reactor Neutrino Experiment, China
A first-of-its-kind equal partnership agreement between the U.S. and China launched the Daya Bay Reactor Neutrino Experiment northeast of Hong Kong, and it didn’t take long for the experiment to experience its first success. Just 55 days after it began taking data, the Daya Bay experiment reported that it had successfully measured the third of three neutrino mixing angles for the first time at high precision. And in the eight years since, the Daya Bay experiment has greatly improved the precision of that measurement as more data has become available.
Berkeley Lab and Brookhaven National Laboratory have led the U.S. participation in the experiment, which is hosted by the Beijing-based Institute of High Energy Physics. Berkeley Lab researchers played a major role in scientifically sleuthing why several experiments had reported fewer antineutrino measurements than expected from around reactor sites around the globe – the so-called “reactor antineutrino anomaly.” The analysis found that a miscalculation for a particular radioactive isotope in the nuclear reactor fuel was a likely contributor to this anomaly. Berkeley Lab physicist Kam-Biu Luk is the U.S. spokesperson and principal investigator for the Daya Bay experiment, and has received several honors for his role. Berkeley Lab’s Dan Dwyer had led an early analysis of the reactor antineutrino anomaly.
“International Physicists Join Forces in Hunt for Sterile Neutrinos,” Aug. 10, 2020
DUNE (Deep Underground Neutrino Experiment) at LBNF (Long-Baseline Neutrino Facility), Illinois, South Dakota
The Deep Underground Neutrino Experiment and the associated Long-Baseline Neutrino Facility are an effort by more than 1,000 scientists from 30 countries to build the most intense neutrino beam in the world, and to construct nearby and faraway detectors to study the properties of those beamed neutrinos, and also to study neutrinos produced by other sources in space such as an exploding star, and possibly the birth of a neutron star or black hole.
The neutrino beam, produced by the PIP-II particle accelerator at Fermi National Accelerator Laboratory, will be directed at the Sanford Underground Research Facility (Sanford Lab) in South Dakota, where a four-story, 70,000-ton detector will be installed about a mile underground to intercept its neutrinos. It will require the excavation of about 800,000 tons of rock – more than twice the weight of the Empire State Building.
A Berkeley Lab team is leading the work on the Near Detector, which will be installed 200 feet below ground at Fermilab, and developed the enabling technology for the pixelated readout of a Near Detector component known as the liquid argon time projection chamber. Berkeley Lab’s Dan Dwyer and Matthaeus Leitner have played significant roles in this effort. Berkeley Lab’s Carl Grace is on a team that is contributing to the R&D on cold electronics for the project’s Far Detector at the Sanford Lab site, which will operate at low temperatures.
Other neutrino science resources
- “Neutrinos 101,” Symmetry Magazine
- “All Things Neutrino,” Fermilab
- “Neutrino invention,” Symmetry Magazine, March 1, 2007
- “The neutrino turns 60,” Symmetry Magazine, June 14, 2016
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