The Daya Bay Experiment
By Steve Kettell
BNL is one of the host labs for US participation in the Daya Bay experiment which will provide the most precise measurement of last unobserved neutrino mixing angle, θ13, with an expected sensitivity to sin22θ13 of better than 0.01 at 90% CL after three years of operation. This is an exciting opportunity to study physics beyond the Standard Model. The existence of neutrino oscillations requires an extension of the Standard Model and raises several interesting questions: why neutrinos have mass and why they are so small (more than 12 orders of magnitude smaller than the top quark)? why two of the mixing angle are large and one is small? and can neutrino CP violation explain the matter anti-matter asymmetry in the universe?
Compelling evidence from a variety of sources indicates that neutrinos have mass and that neutrino flavor states mix (oscillate). Neutrino oscillations are described by two mass splittings, three mixing angles and one CP violating phase. The smaller mass difference and mixing angle (first and second generation) have been measured by solar and very long baseline reactor experiments. The larger mass difference and mixing (second and third generations) have been measured by atmospheric and accelerator experiments. Both mixing angles are large, whereas the as of yet unobserved mixing angle θ13 between the first and third generations is known to be small. If θ13 is zero no CP violating phase will be observable; its non-zero value will provide important insights for the field in order to pursue the next generation of experiments to measure CP violation. BNL has played a pivotal role in neutrino physics, with a Nobel Prize to Lederman, Schwartz and Steinberger for establishing that there are distinct flavors of neutrinos and to Davis for the discovery of neutrinos from the sun, whose deficit led to the realization that neutrino flavors mix.
The Neutrino Scientific Assessment Group (NuSAG) February 2006 report and the APS multi-divisional study's report The Neutrino Matrix, both recommend with high priority a reactor antineutrino experiment to measure sin22θ13 at the level of 0.01. The Particle Physics Project Prioritization Panel (P5) recommended construction of the Daya Bay Experiment as part of its October 2006 Roadmap for Particle Physics.
The experiment will be located in Shenzhen in Southern China about 60km northeast of Hong Kong on the site of the Daya Bay Nuclear Power Plant. Two new reactor cores will come online by 2011 for a total of six with more than 17 GW of thermal power at which point this will be among the five most powerful reactor complexes in the world. The plant is located adjacent to a 700m high mountain that provides rock overburdens ranging from 100-350m against cosmic rays for the two near and one far underground experimental sites.
Eight identical cylindrical detectors each consisting of three nested zones will detect antineutrinos from the reactors via inverse beta decay (in which an antineutrino interacts with a hydrogen nucleus producing a positron and a neutron). Four detectors will be located at the far site about 2km from the reactors at the first oscillation maximum and two will be located at each near site. Each detector contains 20 tons of gadolinium doped liquid scintillator antineutrino target surrounded by 20 tons of undoped liquid scintillator to contain the gamma rays from positron annihilation and neutron capture. The outermost region contains 192 photomultiplier tubes along the circumference of the cylinder and 40 tons of mineral oil to provide shielding of the scintillator against external gamma rays. The complete 100 ton detectors (5m diameter and 5m high) are submerged in water pools to provide further shielding against external radiation from the rock and to shield against neutrons from cosmic ray muons. These water Cherenkov pools serve as active muon identification, as the primary backgrounds (0.2-0.3% of the signal) are cosmogenic in origin. The experiment looks for an energy dependent variation in the ratio of rates in the near and far detectors as a signature of the θ13 oscillation. The identical detectors are filled with the same Gd-LS whose mass is carefully measured. The baseline expectation for the systematic error in the relative uncertainty is 0.38% per detector module. This compares to a statistical uncertainty of less than 0.2% after three years (dominated by the far site). More details can be found at arXiv:hep-ex/0701029.
The Daya Bay Collaboration is made up of thirty-five institutions with a total of over 190 collaborators from three continents. The project is supported by the Ministry of Science and Technology of China, the Chinese Academy of Sciences, the National Natural Science Foundation of China, the Guangdong Provincial Government, the Shenzhen Municipal Government, the China Guangdong Nuclear Power Group, and the Office of High Energy Physics of the U.S. Department of Energy. The Daya Bay Reactor Neutrino Experiment is one of the largest cooperative scientific projects between China and the U.S., and is establishing a model for future cooperation in the realm of scientific research.
The Civil Construction of the facility will be financed by China, whereas the construction of the experimental apparatus will be shared approximately equally by China and the US. The US Daya Bay Project is hosted by BNL and LBNL. BNL provides the Chief Scientist, Chief Engineer, Safety Officer and the L2 managers for the Muon System, Installation, and Integration. BNL also provides the L3 managers for Liquid Scintillator, Muon Water Cherenkov supports, and Simulation and Analysis. We are the largest US group on the experiment and second largest overall, after IHEP. The experiment is moving forward rapidly, having received CD-0 in November 2005 (Statement of mission need), passed a Physics Review in October 2006 and a CD-1 Review in April 2007 (Site selection). We have scheduled a CD-2/3a review at BNL on January 8-10, 2008 (Baseline review and start of limited construction). The entire construction will be completed in three years, with the first two detectors collecting data in 2009 and all eight by 2011.
Groundbreaking for the Daya Bay Experiment was celebrated on October 13, 2007. The ceremony marks the beginning of civil construction for the experimental facilities on the Daya Bay nuclear power plant site. The ceremony was attended by representatives of the Chinese and US funding agencies. Robin Staffin, Director of the Office of High Energy Physics of U.S. Department of Energy, said, "At the ground where we now stand, scientists will measure with the world's greatest precision, one of the most important parameters of nature."