BEIJING; BERKELEY, CA; and UPTON, NY - The Daya Bay Reactor Neutrino
Experiment has begun its quest to answer some of the most puzzling
questions about the elusive elementary particles known as neutrinos. The
experiment's first completed set of twin detectors is now recording
interactions of antineutrinos (antipartners of neutrinos) as they travel
away from the powerful reactors of the China Guangdong Nuclear Power Group
in southern China.
Neutrinos are uncharged particles produced in nuclear reactions, such as
in the sun, by cosmic rays, and in nuclear power plants. They come in
three types or "flavors" - electron, muon, and tau neutrinos - that morph,
or oscillate, from one form to another, interacting hardly at all as they
travel through space and matter, including people, buildings, and planets
like Earth.
The start-up of the Daya Bay experiment marks the first step in the
international effort of the Daya Bay Collaboration to measure a crucial
quantity related to the third type of oscillation, in which the
electron-flavored neutrinos morph into the other two flavored neutrinos.
This transformation is due to the least-known neutrino "mixing angle,"
denoted by theta_one-three, and could reveal clues leading to an
understanding of why matter predominates over antimatter in the universe.
"This is a remarkable achievement after eight years of effort - four years
of planning and four years of construction - by hundreds of physicists and
engineers from around the globe," says Yifang Wang of the Institute of
High Energy Physics (IHEP) of the Chinese Academy of Sciences,
co-spokesperson for the Daya Bay Collaboration. "We worked collectively to
build the underground experimental facility to detect antineutrinos from
reactors, in order to find a new type of neutrino oscillation and measure
it quantitatively."
"The first data from Daya Bay sets us on a path that will lead to
measurement of the amplitude of the oscillation due to the least-known
mixing angle to within one percent," says Daya Bay co-spokesperson Kam-Biu
Luk, of the U.S. Department of Energy's Lawrence Berkeley National
Laboratory (Berkeley Lab) and the University of California at Berkeley.
"That precision is an order of magnitude better than present measurements
and much more precise than other experiments now in progress. The results
will be a major contribution to understanding the role of neutrinos in the
evolution of basic kinds of matter in the earliest moments after the big
bang, and why there is more matter than antimatter in the universe today."
The Daya Bay Experiment is well positioned for a precise measurement of
the poorly known value of theta_one-three because it is close to some of
the world's most powerful nuclear reactors - the Daya Bay and Ling Ao
nuclear power reactors, located some 55 kilometers from Hong Kong - and it
will take data from a total of eight large, virtually identical detectors
in three experimental halls deep under the adjacent mountains.
Experimental Hall #1, a third of a kilometer from the twin Daya Bay
reactors, is the first to start operating. Hall #2, about a half kilometer
from the Ling Ao reactors, will come online this fall. Hall #3, the
farthest hall, about two kilometers from the reactors, will be ready to
take data in the summer of 2012.
The Daya Bay experiment is a "disappearance" experiment, powered by the
enormous quantities of electron antineutrinos produced in the nearby
reactors. The detectors in the two closest halls will measure the raw flux
of electron antineutrinos from the reactors. The detectors at the far hall
will look for a depletion in the expected antineutrino flux.
The cylindrical antineutrino detectors are filled with clear liquid
scintillator, which reveals antineutrino interactions by the very faint
flashes of light they emit. Sensitive photomultiplier tubes line the
detector walls, ready to amplify and record the telltale flashes.
A relatively small number of these interactions, over a thousand a day out
of the millions of quadrillions of antineutrinos produced by the reactors
every second, will be captured by the twin detectors in each near hall.
Due to their greater distance, the four detectors in the far hall will
measure only a few hundred a day. To measure theta_one-three, the
experiment records the precise difference in flux and energy distribution
between the near and far detectors.
The experimental halls are dug deep under the mountain to shield the
detectors from cosmic rays. The antineutrino detectors are submerged in
pools of water to shield them from radioactive decays in the surrounding
rock. Despite shielding, some energetic cosmic rays make it all the way
through; their trajectories are tracked by photomultiplier tubes in the
walls of the water pool and "muon trackers" in the roof over the pool.
Events of this kind are ignored in collecting the antineutrino data.
After two to three years of collecting data with all eight detectors, the
Daya Bay Reactor Neutrino Experiment intends to meet the goal of measuring
the oscillation amplitude of theta_one-three with a sensitivity of one
percent.
China and the United States lead the Daya Bay Reactor Neutrino Experiment,
which includes participants from Russia, the Czech Republic, Hong Kong,
and Taiwan. The Chinese effort is led by project manager Yifang Wang of
the Institute of High Energy Physics, and the U.S. effort is led by
project manager Bill Edwards of Lawrence Berkeley National Laboratory and
chief scientist Steve Kettell of Brookhaven National Laboratory.
Contact information:
Yifang Wang, Co-spokesperson, IHEP, +86-10-88236076, yfwang@ihep.ac.cn
Kam-Biu Luk, Co-spokesperson, Berkeley Lab and UC Berkeley,
+1-510-486-7054, +1-510-642-8162, k_luk@lbl.gov
Tongzhou Xu, IHEP Public Affairs, +86-10-88236421, xutz@ihep.ac.cn
Kendra Snyder, Brookhaven Public Affairs, +1-631-344-8191, ksnyder@bnl.gov
Lynn Yarris, Berkeley Lab Public Affairs, +1-510-486-5375,
lcyarris@lbl.gov
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, Kurchatov Institute, 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.
Copyright © by the contributing authors, 2007-2024.