Neutrinos Disappearing at Daya Bay?

This guest post is by Brookhaven Lab physicist Steve Kettell, the Chief Scientist for the U.S. Daya Bay Neutrino Project in southern China. Kettell received his Ph.D. in 1990 from Yale University and is the leader of Brookhaven’s Electronic Detector Group.

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Steve Kettell

Neutrinos are downright weird!

Produced in prodigious numbers in the sun, supernovae, nuclear reactors and particle accelerators, neutrinos are extremely hard to detect because they hardly interact with other material at all.

If we think about photons from the sun hitting blacktop during the summer, it is quite obvious that they interact and that their energy is absorbed by the blacktop (making it hot to the touch).

But even though 10s of billions of neutrinos pass through each square centimeter of that blacktop per second, most of them do not interact. In fact most pass through the Earth and through much of the universe without interacting with anything.

In order to study these mysterious particles, we need large detectors, and we have to reduce backgrounds from cosmic rays by placing those detectors deep underground.

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The two antineutrino detectors in Daya Bay Hall #1, shown here prior to the pool being filled with ultrapure water. The pool is lined with photomultiplier tubes to track any “stiff” (highly energetic) cosmic rays that make it all the way through the overlying rock. (Courtesy of Roy Kaltschmidt, Lawrence Berkeley National Laboratory)

Under a mountain in southern China, a new experiment is trying to answer key questions about neutrinos and their impact on the world around us. The Daya Bay Neutrino Experiment started taking data this month, recording interactions of antineutrinos, a neutrino’s counterpart with the same mass and opposite spin, as they travel away from powerful reactors of the China Guangdong Nuclear Power Group.

But before I explain Daya Bay in more detail, let me first provide a little background.

Neutrinos were first postulated by Wolfgang Pauli in the 1930s to explain the energy of radioactive decays, and were given their name, which means “little neutral one,” by Enrico Fermi. But it was not until the 1950s that they were first discovered, in experiments by Frederick Reines and Clyde Cowan at nuclear reactors in Washington state and South Carolina, leading to a Nobel Prize in 1995.

In 1962, a new type of neutrino, the muon neutrino, was discovered at Brookhaven Lab, a finding that led to the 1988 Nobel Prize. This discovery established that there was more than one flavor of neutrino.

In the 1990s, the Large Electron-Positron collider at CERN established that there were exactly three flavors of massless, weakly interacting neutrinos. These correspond to the three flavors of charged leptons (electron, muon, and tau) and the three flavors of quarks.

In the 1960-70s the Standard Model, which explains elementary particles and their interactions, was developed with three flavors of quarks and leptons. These quarks of different flavors can mix with each other (one flavor turns into another). This mixing of three flavors leads to CP violation (in which particles and their antiparticles interact differently) and this insight led to a Nobel Prize for Makoto Kobayashi and Toshihide Maskawa in 2008. The experimental discovery of CP violation at Brookhaven by Val Fitch and James Cronin in 1964 led to the 1980 Nobel Prize in physics. CP violation is essential for understanding why our universe is almost completely made up of matter, but the amount of CP violation in quarks is not enough to explain the dominance of matter in the universe. Therefore, it is natural to seek other sources of CP violation in nature.

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About a mile under solid rock in South Dakota’s Homestake Gold Mine, BNL researcher and Nobel Laureate, Raymond Davis, Jr. set up a chlorine-filled detector in 1967 to measure neutrinos from the sun.

Neutrinos from the sun were first detected in 1960s by BNL’s Ray Davis (who won a Nobel Prize for this work in 2002) nearly a mile underground in the Homestake Mine in South Dakota. However, only one third of the expected number was observed. Since then (late 1990s) the SuperK experiment in Japan observed a deficit of muon neutrinos from cosmic ray interactions in the earth’s atmosphere. Supporting evidence from SNO in Canada, KamLAND and K2K in Japan, and MINOS at Fermilab have allowed us to understand that neutrinos have small masses and that they mix, or oscillate, from one flavor to another.

The oscillation between electron and muon neutrinos (referred to as θ12) has been well measured from the sun and long-baseline reactor experiments, and the oscillation between muon and tau neutrinos (θ23) has been well measured from atmospheric and long-baseline accelerator experiments. Recently, the T2K experiment in Japan provided the first hint of oscillation between electron and tau neutrinos (θ13). However, in order to observe CP violation in neutrinos, it is critically important to conclusively establish this last oscillation.

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A pair of antineutrino detectors will be positioned in each of the two experimental halls near the reactors, while another set of four is positioned in the far hall almost two kilometers away. The first detectors to start up are at the Daya Bay Near Hall.

The Daya Bay experiment is seeking to measure θ13 with world-leading precision. The construction of the facility started in 2008 and we just recently completed the first (and nearest) of three experimental halls. We expect to have all of the facility’s detectors (eight in total) up and running by next summer.

The near halls will allow us to measure the number of neutrinos produced by the reactors, and with the far hall, we can see if we observe fewer neutrinos than predicted. Such an observation would indicate that some of our electron antineutrinos had oscillated into tau antineutrinos, which we cannot physically observe, and that θ13 is greater than 0.

The start of operations of the first hall has been the culmination of a lot of hard work by many people in the collaboration to design, assemble and install these detectors. Now we have the excitement of turning on the detectors, calibrating them and studying their performance. Our main goal from the near site is to verify the two detectors’ performance and determine how identical they are — in order to take accurate measurements, we must account for even the slightest difference between them.

A year from now, we will have all eight detectors operating and we anticipate that it will take about three years of running to get our desired sensitivity of below 1 percent.

This measurement will fill an important gap in our understanding of how to improve the Standard Model. It is also a critical step toward the possible discovery of CP violation in neutrinos and may lead to an understanding of why our universe is made up mostly of matter.