Findings from a neutrino detector in China have opened the door to using the ghostly subatomic particles to explore the fate of another mysterious substance in the Universe, antimatter.
The discovery from the Daya Bay detector of how these elusive particles transform into each other will allow physicists to probe for the answer to a question that has perplexed physicists for decades: Why is the world made of matter, and what happened to antimatter thought to have been created during the birth of the Universe?
The Universe is made up of matter – if it wasn’t, you wouldn’t be reading this. But in the early moments of the Universe, according to theoretical speculations, equal parts of matter and antimatter coexisted before matter triumphed over antimatter in the rapidly expanding primordial universe.
The new findings at Daya Bay confirm that neutrinos, and their corresponding antineutrinos, can be used to study how nature favors matter over antimatter. The particles have long puzzled scientists because they appear to be partners to electrons yet they weigh much less. Observations of neutrinos from cosmic rays demonstrated that neutrinos can transform, or oscillate, from one type of neutrino to another.
The latest finding, announced March 8 by an international team of scientists, puts in place the last piece of the puzzle of what happens to neutrinos when they oscillate. Scientists had already determined two of the ways they transform, and with this new finding, scientists now know how all three ‘flavors’ of neutrinos mix together and oscillate from one type to another.
The detection of the third way that the particles oscillate makes it highly likely that successful studies of antimatter can be accomplished, said A.J. Stewart Smith, Princeton University’s Dean for Research and the Class of 1909 Professor of Physics. “The results from Daya Bay are surprising and extremely exciting because they open up an entirely new realm of experimental possibilities for exploring antimatter,” he said.
The new results enable researchers to use neutrinos and antineutrinos to tackle the question of exactly how matter prevailed over antimatter. Scientists propose that the answer lies in slight variations in the decay of matter and antimatter particles. These variations lead to a phenomenon called charge-parity (CP) violation, in which particles decay in ways that defy the rules governing their behavior.
CP violation was first detected in short-lived elementary particles called neutral K mesons in 1964 by Princeton physics professors Val Fitch and James Cronin, who earned the Nobel Prize in physics for their discovery. Since then, CP violation has been detected in another elementary particle, the B meson, in two experiments in 2001, one at the Stanford Linear Accelerator (SLAC) led by Princeton’s Smith with Princeton's Daniel Marlow, the Evans Crawford 1911 Professor of Physics, and the other at KEK, the High Energy Accelerator Research Organization, in Japan.
Although the K meson and B meson experiments detected CP violation, the amount of CP violation detected was small, leaving a lot of room for further studies of matter and antimatter.
The new Daya Bay results yield the opportunity for scientists to look for CP violation in the decay of neutrinos and their antiparticles, known as antineutrinos. “We want to compare the behavior of neutrinos and antineutrinos,” said Kirk McDonald, professor of physics at Princeton University, “and in that comparison we hope to find CP violation.”
McDonald and Changguo Lu, a detector physicist at Princeton, are members of the Daya Bay collaboration and were instrumental in constructing some of the antineutrino detectors.
Neutrinos are invisible, extremely light particles that are born in stars including our Sun and flow through the atmosphere and penetrate the Earth and even our bodies without causing any harm.
Antineutrinos are produced in great quantities at the Daya Bay nuclear power plant facility in Guangdong Province about 30 miles northeast of Hong Kong. Radioactive nuclear "beta decay" emits the electrons and electron antineutrinos.
To detect the antineutrinos, a multinational collaboration of scientists buried detectors in the mountains surrounding the nuclear power plant. The use of multiple detectors at varying distances from the nuclear reactors reduces the chances of erroneous measurements. The first six of eight planned detectors, the furthest placed about 1.2 miles from the reactors, began operating in December and by the end of February had detected antineutrinos oscillating into other flavors.
Neutrinos come in three flavors, known as electron, tau, and muon (named after their charged-particle partners). Each of these has an antiparticle. At Daya Bay, scientists measured the number of electron antineutrinos that streamed out from the Daya Bay reactor but never made it to the detector because they oscillated into other flavors along the way. They used this measurement to calculate a key parameter, a value called mixing angle θ13, pronounced "theta one-three," that describes the oscillation from the electron flavor to the tau flavor.
θ13 must be nonzero if physicists hope to study the possible matter-antimatter difference of neutrinos versus antineutrinos, and the study will only be practical if θ13 is large.
The Daya Bay results demonstrate that θ13 is both nonzero and large. "The results were really surprising to everyone because we didn't expect so see such a strong result and we didn't expect it so quickly," said Lu.
The large oscillation of electron antineutrinos to tau antineutrinos implies that a new generation of experimental studies will be feasible, said McDonald. “The study of possible differences in the behavior of neutrinos and antineutrinos in particular will have tremendous implications for the large observed difference in the amount of matter and antimatter in the Universe,” he said.
The next step in harnessing neutrinos and antineutrinos to investigate CP violation is to look at the particles traveling over hundreds of miles. These "long-baseline" studies are needed because the CP violation is most easily detected over large distances.
The Daya Bay collaboration consists of scientists from China, the United States, Russia, the Czech Republic, Hong Kong, and Taiwan. The Princeton Daya Bay effort is supported by the U.S. Department of Energy's Office of Science.
The collaborating institutions of the Daya Bay Reactor Neutrino Experiment are Beijing Normal University, Brookhaven National Laboratory, California Institute of Technology, Charles University in Prague, Chengdu University of Technology, China Guangdong Nuclear Power Group, China Institute of Atomic Energy, Chinese University of Hong Kong, Dongguan University of Technology, Joint Institute for Nuclear Research, University of Hong Kong, Institute of High Energy Physics, Illinois Institute of Technology, Iowa State University, Lawrence Berkeley National Laboratory, Nanjing University, Nankai University, National Chiao-Tung University, National Taiwan University, National United University, North China Electric Power University, Princeton University, Rensselaer Polytechnic Institute, Shandong University, Shanghai Jiao Tong University, Shenzhen University, Siena College, Tsinghua University, University of California at Berkeley, University of California at Los Angeles, University of Cincinnati, University of Houston, University of Illinois at Urbana-Champaign, University of Science and Technology of China, Virginia Polytechnic Institute and State University Blacksburg, University of Wisconsin-Madison, College of William and Mary, and Sun Yat-Sen (Zhongshan) University.