Tens of billions of neutrinos are passing through every square centimeter of the Earth’s surface right now.
Neutrinos, ghostlike particles that flooded the universe just moments after the Big Bang, are born in the hearts of stars and other nuclear reactions. Untouched by electromagnetism and nearly as fast as light, neutrinos pass practically unhindered through everything from planets to people, only rarely responding to the weak nuclear force and the even weaker gravity. In fact, at any given moment, tens of billions of neutrinos are passing through every square centimeter of the Earth’s surface, undetected. This ability to sail unhindered and unnoticed through almost anything earned neutrinos the nickname “ghost” particles. But despite their imperceptibility, neutrinos could be the key to understanding how our universe evolved just after the Big Bang and why the world is made of matter.
Brookhaven Lab’s first major contribution to neutrino research occurred in 1957, when Maurice Goldhaber performed an experiment that revealed neutrinos to be "left-handed." That is, a property of neutrinos known as "spin" is always oriented counter-clockwise to the direction of their linear momentum.
In 1962, a new type of neutrino, the muon neutrino, was discovered by scientists using the Alternating Gradient Synchrotron at Brookhaven. Leon Lederman, Mel Schwartz, and Jack Steinberger took home the 1988 Nobel Prize for this work, which established that there was more than one flavor of neutrino.
In the late 1960s, Brookhaven chemist Ray Davis discovered the solar neutrino problem. At the Homestake Mine in South Dakota, deep underground in order to shield the detector from cosmic rays, Davis was the first person able to directly detect the electron neutrinos being produced by the sun. But he only observed about one-third of the expected amount — this deficit would eventually become known as the solar neutrino problem (and the “missing” neutrinos would later turn out to be those that had changed to forms undetectable by Davis’ experiment while en route to Earth).
Brookhaven’s Maurice Goldhaber was also a founding member of a pioneering experiment built in the Morton salt mine in Ohio in the early 1980s that became famous for observing neutrinos from Supernova 1987A (along with the Kamioka detector in Japan). Originally designed to study proton decays, the experiment was a six-story cube lined with black plastic and a network of phototubes and filled with 2.5 million gallons of ultra-pure water. The neutrinos showed up by way of their interactions with protons in the water.
From the 1990s through the mid-2000s, Brookhaven's neutrino group played important roles in the GALLEX (Gallium Experiment) and SNO (Sudbury Neutrino Observatory) experiments in Italy and Canada, respectively. Brookhaven chemist Richard Hahn and his group were integral to the SNO experiment, which proved that neutrinos do oscillate between three forms — electron, muon, and tau.
Meanwhile, the Super-Kamiokande experiment in Japan, in which Brookhaven was represented by physicist and former Lab Director Maurice Goldhaber, had confirmed that neutrinos do indeed oscillate and have mass. Davis’s problem was solved: he had observed only the fraction of electron neutrinos from the sun that reached Earth without changing into muon or tau neutrinos. In 2002, his work was acknowledged with the Nobel Prize in Physics, shared with Masatoshi Koshiba of Japan and Riccardo Giacconi of the U.S.
Brookhaven then became involved in the ongoing MINOS (Main Injector Neutrino Oscillation Search) experiment based at Fermi National Accelerator Laboratory in Illinois, which began taking data in 2005 and has since provided measurements of mixing angles and oscillation frequency that describe how muon and tau neutrino types oscillate between one form and another.
In addition, Brookhaven is integral to the Daya Bay Neutrino Project, which began taking data in 2011. This experiment aims to measure the final unknown mixing angle that describes how neutrinos oscillate — another chapter in Brookhaven’s long history of neutrino research over the last several decades.
Many Brookhaven scientists played leading roles in the design and construction of the MicroBooNE cryogenic neutrino detector, located at Fermilab. MicroBooNE represents the latest development of massive liquid argon Time-Projection-Chamber detectors for neutrino physics—a particle physics detector design pioneered by physicists in Brookhaven Lab’s Instrumentation Division. Brookhaven physicists also built MicroBooNE’s custom electronics, designed to operate at cryogenic temperatures. The experiment is expected to start collecting data in mid-2015.
In the early 2000s, Brookhaven Lab scientists conceived an experiment to produce an intense collimated beam of neutrinos that would travel hundreds of miles through the Earth and strike a distant target to help unravel the mysteries of matter. Traveling over such a long distance would give the particles time to exhibit one of their strangest and most exciting quirks: quantum mechanical flavor transformations. Understanding details of these oscillations is one of the most important puzzles in fundamental physics. The project was eventually approved as the Long-Baseline Neutrino Experiment (LBNE), with the beam initiating at Fermi National Accelerator Laboratory (Fermilab) and striking a very large precision, underground detector capable of identifying and measuring neutrino events at the Sanford Underground Research Facility in Lead, South Dakota. Brookhaven Lab scientists were principal collaborators in this experiment, from fundamental neutrino science to beam and detector design, prototyping and construction. This program has now evolved into a large international enterprise known as the Long-Baseline Neutrino Facility (LBNF) and the Deep Underground Neutrino Experiment (DUNE).
The Deep Underground Neutrino Experiment (DUNE) is a growing international collaboration with currently more than 750 scientists. This collaboration will build and operate a huge liquid argon Time-Projection-Chamber detector in the Sanford Underground Research Facility (SURF) in Lead, SD, as well as a smaller detector on the Fermilab site. This experiment will build on Fermilab’s existing accelerator complex to supply its neutrinos. Fermilab’s Main Injector Ring will produce an intense collimated beam of neutrinos that will travel 800 miles through the Earth before striking its target at SURF (based in the same Homestake Mine in which Ray Davis did his famous experiment). Brookhaven Lab is one of the principal collaborators in the planning, design, and operation of this experiment. From fundamental neutrino science to beam and detector design, prototyping and construction, Brookhaven Lab has had a foundational role in DUNE.