Changgen Yang Institute of High Energy Physics, Beijing for the Daya Bay Collaboration Daya Bay Neutrino Experiment International UHE Tau Neutrino Workshop, April 24-26, 2006

 13  The Last Unknown Neutrino Mixing Angle U MNSP Matrix ? atmospheric, K2K reactor and accelerator 0  SNO, solar SK, KamLAND  12 ~ 32°  23 = ~ 45°  13 = ? Large and maximal mixing! ? What is  e fraction of 3 ? U e3 is a gateway to CP violation in neutrino sector: P(   e ) - P(     e )  sin(2  12 )sin(2  23 )cos 2 (  13 )sin(2  13 )sin 

 13 from Reactor and Accelerator Experiments - Clean measurement of  13 - No matter effects CP violation mass hierarchy matter reactor accelerator - sin 2 2  13 is missing key parameter for any measurement of  CP

Current Knowledge of  13 Direct search At  m 2 31 = 2.5  10  3 eV 2, sin 2 2   < 0.15 allowed region Fogli etal., hep-ph/ Sin 2 (2  13 ) < 0.09 Sin 2 2  13 < 0.18 Best fit value of  m 2 32 = 2.4  10  3 eV 2 Global fit

Limitations of Past and Current Reactor Neutrino Experiments Palo Verde, CHOOZ Typical precision is 3-6% due to limited statistics reactor-related systematic errors: - energy spectrum of  e (~2%) - time variation of fuel composition (~1%) detector-related systematic error (1-2%) background-related error (1-2%)

Daya Bay: Goals And Approach Utilize the Daya Bay nuclear power facilities to: - determine sin 2 2  13 with a sensitivity of 1% - measure  m 2 31 Adopt horizontal-access-tunnel scheme: - mature and relatively inexpensive technology - flexible in choosing overburden and changing baseline - relatively easy and cheap to add experimental halls - easy access to underground experimental facilities - easy to move detectors between different locations with good environmental control. Employ three-zone antineutrino detectors.

How To Reach A Precision of 0.01 ? Powerful nuclear plant Larger detectors “Identical” detectors Near and far detectors to minimize reactor-related errors Optimize baseline for best sensitivity and smaller residual reactor-related errors Interchange near and far detectors – cancel many detector systematic errors Sufficient overburden/shielding to reduce background Comprehensive calibration/monitoring of detectors

Ling Ao II NPP: 2  2.9 GWth Ready by Ling Ao NPP: 2  2.9 GW th Daya Bay NPP: 2  2.9 GW th 1 GW th generates 2 ×  e per sec 55 km 45 km The Daya Bay Nuclear Power Facilities 12th most powerful in the world (11.6 GW) Top five most powerful by 2011 (17.4 GW) Adjacent to mountain, easy to construct tunnels to reach underground labs with sufficient overburden to suppress cosmic rays

Where To Place The Detectors ? Place near detector(s) close to reactor(s) to measure raw flux and spectrum of  e, reducing reactor-related systematic Position a far detector near the first oscillation maximum to get the highest sensitivity, and also be less affected by  12 Since reactor  e are low-energy, it is a disappearance experiment: Large-amplitude oscillation due to  12 Small-amplitude oscillation due to  13 near detector far detector

Baseline optimization and site selection Neutrino spectrum and their error Neutrino statistical error Reactor residual error Estimated detector systematical error: total, bin-to-bin Cosmic-rays induced background (rate and shape) taking into mountain shape: fast neutrons, 9Li, … Backgrounds from rocks and PMT glass

Daya Bay NPP Ling Ao NPP Ling Ao-ll NPP (under const.) Entrance portal Empty detectors: moved to underground halls through access tunnel. Filled detectors: swapped between underground halls via horizontal tunnels. Total length: ~2700 m 230 m (15% slope) 290 m (8% slope) 730 m 570 m 910 m Daya Bay Near 360 m from Daya Bay Overburden: 97 m Ling Ao Near 500 m from Ling Ao Overburden: 98 m Far site 1600 m from Ling Ao 2000 m from Daya Overburden: 350 m Mid site ~1000 m from Daya Overburden: 208 m

A Versatile Site Rapid deployment: - Daya Bay near site + mid site - 0.7% reactor systematic error Full operation: (A) Two near sites + Far site (B) Mid site + Far site (C) Two near sites + Mid site + Far site Internal checks, each with different systematic

Geophysical profile (Daya–mid--far)

Bore Samples Zk4 (depth: 133 m) Zk2 (depth: ~180 m) Zk3 (depth: ~64 m) Zk1 (depth: 210 m) At tunnel depth

Findings of Geotechnical Survey No active or large fault Earthquake is infrequent Rock structure: massive and blocky granite Rock mass: most is slightly weathered or fresh Groundwater: low flow at the depth of the tunnel Quality of rock mass: stable and hard Good geotechnical conditions for tunnel construction

Detecting Low-energy  e  e  p  e + + n (prompt)  + p  D +  (2.2 MeV) (delayed)  + Gd  Gd*  Gd +  ’s (8 MeV) (delayed) Time- and energy-tagged signal is a good tool to suppress background events. Energy of  e is given by: E   T e+ + T n + (m n - m p ) + m e+  T e MeV keV The reaction is the inverse  -decay in 0.1% Gd-doped liquid scintillator: Arbitrary Flux Cross Section Observable Spectrum From Bemporad, Gratta and Vogel 0.3b 50,000b

What Target Mass Should Be? Systematic error Black : 0.6% DYB: B/S = 0.5% LA: B/S = 0.4% Far: B/S = 0.1%  m 2 31 = 2  eV 2 tonnes (3 year run) Red : 0.25% (baseline goal) Blue : 0.12%

Design of Antineutrino Detectors Three-zone structure: I. Target: 0.1% Gd-loaded liquid scintillator II. Gamma catcher: liquid scintillator, 45cm III. Buffer shielding: mineral oil, ~45cm Possibly with diffuse reflection at ends. ~200 PMT’s around the barrel: Isotopes (from PMT) Purity (ppb) 20cm (Hz) 25cm (Hz) 30cm (Hz) 40cm (Hz) 238 U(>1MeV) Th(>1MeV) K(>1MeV) Total Oil buffer thickness buffer 20 tonne s Gd-LS gamma catcher

Why three zones ? 3-ZONE2-ZONE n capture on Gd yields 8 MeV with 3-4  ’ s Chooz background 3 zones provides increased confidence in systematic error associated with detection efficiency and fiducial volume 2 zones implies simpler design/construction, some cost reduction but with increased risk to systematic error

Gd-loaded Liquid Scintillator For Daya Bay Absorbance at 430 nm Calendar Date 507 days (1.2% Gd in PC) 455 days (0.2% Gd in PC) 367 days (0.2% Gd in 20% PC + 80% C 12 H 26 ) 130 days (0.2% Gd in LAB) Require stable Gd-loaded liquid scintillator with - high light yield - long attenuation length BNL/IHEP/JINR nuclear chemists study on metal-loaded liquid scintillator (~1% Gd diluted to ~0.1% Gd) for Daya Bay: - technology of 1% Gd in pseudocumene (PC) is mature - need R&D for 1% Gd in mixture of PC and dodecane, and with linear alkyl benzene (LAB) Attenuation lengths > 15 m BNL samples

Design of Shield-Muon Veto Detector modules enclosed by 2m of water to shield neutrons and gamma-rays from surrounding rock Water shield also serves as a Cherenkov veto Augmented with a muon tracker: scintillator or RPCs Combined efficiency of Cherenkov and tracker > 99.5% 2 m of water Neutron background vs thickness of water Fast neutrons per day water thickness (m) tunnel

Active Water Shield and Muon Tracker Specifications –High efficiency muon tracker; less than 0.3% inefficiency when combined with the muon water Cherenkov –Good (ns) timing resolution to reduce accidentals due to ambient radioactivity background –Muon tracker can be deployed in water pool –Robust, good long-term stability PMT's for water Cherenkov

Moving Detectors in Horizontal Tunnels Aircraft Pushback Tractors are Ideal Zero emission vehicles available Low-speed towing Forward and reverse towing Vehicle ballasted OK for incline (<8%)

Prototype setup at IHEP LED Cables Flange to put Source Purposes: Test reflection, energy resolution, LS performance … Inner acrylic vessel: 1m in diameter and 1m tall, filled with normal liquid scintillator(70% mineral oil + 30% mesitylene). Outer stainless steel vessel: 2m in diameter and 2m tall, filled with mineral oil. PMTs mounted and immerged in oil. 45 MACRO PMT, 15 PMT/Ring

Attenuation Length and Light Yield L attn = 8.5+/-0.3m PMT XP2020 PMT Glass Tube Source Cs137 or Sr90 Liquid Scint. Or An Crystal 61% relative to Anthracene

Very Preliminary Resolution:

Backgrounds

~350 m ~97 m ~98 m ~210 m Cosmic-ray Muon Apply modified Gaisser parametrization for cosmic-ray flux at surface Use MUSIC and mountain profile to estimate muon flux & energy DYBLAMidFar Elevation (m) Flux (Hz/m 2 ) Mean Energy (GeV)

Summary of Background Near SiteFar Site Radioactivity (Hz)<50 Accidental B/S<0.05% Fast neutron background B/S0.15%0.1% 8 He/ 9 Li B/S 0.41% ± 0.18% 0.02% ± 0.08% Use a modified Palo Verde-Geant3-based MC to model response of detector: (neutrino signal rate 560/day 80/day) Further rejection of background may be possible by cutting showering muons.

Detector-related Uncertainties Baseline: currently achievable relative uncertainty without R&D Goal: expected relative uncertainty after R&D Absolute measurement Relative measurement → 0 → → 0.06% w/Swapping → 0 Swapping: can reduce relative uncertainty further

Summary of Systematic Errors Reactor-related systematic errors are: 0.09% (4 cores) 0.13% (6 cores) Relative detector systematic errors are: 0.36% (baseline) 0.12% (goal) 0.06% (with swapping) These are input to sensitivity calculations

90% confidence level 2 near + far (3 years) near (40t) + mid (40 t) 1 year Near-mid Use rate and spectral shape Sensitivity of Daya Bay in sin 2 2  13 Daya Bay near hall (40 t) Tunnel entrance Ling Ao near hall (40 t) Far hall (80 t)

Synergy Between Reactor and Accelerator Experiments Before 2011: Daya Bay provides basis for early decision on future program beyond NO A for CP and mass hierarchy After 2011: Daya Bay will complement NO A and T2K for resolving  23, mass hierarchy, and CP phase

Overall Project Schedule

Summary The Daya Bay nuclear power facility in China and the mountainous topology in the vicinity offer an excellent opportunity for carrying out a reactor neutrino program using horizontal tunnels. The Daya Bay experiment has excellent potential to reach a sensitivity of 0.01 for sin 2 2  13. The Daya Bay Collaboration continues to grow. Will complete detailed design of detectors, tunnels and underground facilities in Plan to commission the Fast Deployment scheme in 2009, and Full Operation in 2010.

The Daya Bay Collaboration: China-Russia-U.S. X. Guo, N. Wang, R. Wang Beijing Normal University, Beijing L. Hou, B. Xing, Z. Zhou China Institute of Atomic Energy, Beijing M.C. Chu, W.K. Ngai Chinese University of Hong Kong, Hong Kong J. Cao, H. Chen, J. Fu, J. Li, X. Li, Y. Lu, Y. Ma, X. Meng, R. Wang, Y. Wang, Z. Wang, Z. Xing, C. Yang, Z. Yao, J. Zhang, Z. Zhang, H. Zhuang, M. Guan, J. Liu, H. Lu, Y. Sun, Z. Wang, L. Wen, L. Zhan, W. Zhong Institute of High Energy Physics, Beijing X. Li, Y. Xu, S. Jiang Nankai University, Tianjin Y. Chen, H. Niu, L. Niu Shenzhen University, Shenzhen S. Chen, G. Gong, B. Shao, M. Zhong, H. Gong, L. Liang, T. Xue Tsinghua University, Beijing K.S. Cheng, J.K.C. Leung, C.S.J. Pun, T. Kwok, R.H.M. Tsang, H.H.C. Wong University of Hong Kong, Hong Kong Z. Li, C. Zhou Zhongshan University, Guangzhoz Yu. Gornushkin, R. Leitner, I. Nemchenok, A. Olchevski Joint Institute of Nuclear Research, Dubna, Russia V.N. Vyrodov Kurchatov Institute, Moscow, Russia B.Y. Hsiung National Taiwan University, Taipei M. Bishai, M. Diwan, D. Jaffe, J. Frank, R.L. Hahn, S. Kettell, L. Littenberg, K. Li, B. Viren, M. Yeh Brookhaven National Laboratory, Upton, New York, U.S. R.D. McKeown, C. Mauger, C. Jillings California Institute of Technology, Pasadena, California, U.S. K. Whisnant, B.L. Young Iowa State University, Ames, Iowa, U.S. W.R. Edwards, K. Heeger, K.B. Luk University of California and Lawrence Berkeley National Laboratory, Berkeley, California, U.S. V. Ghazikhanian, H.Z. Huang, S. Trentalange, C. Whitten Jr. University of California, Los Angeles, California, U.S. M. Ispiryan, K. Lau, B.W. Mayes, L. Pinsky, G. Xu, L. Lebanowski University of Houston, Houston, Texas, U.S. J.C. Peng University of Illinois, Urbana-Champaign, Illinois, U.S. 20 institutions, 89 collaborators