Overview of the Daya Bay experiment Yifang Wang Institute of High Energy Physics

Neutrino oscillation: PMNS matrix EXO Genius CUORE NEMO A total of 6 parameters: 2  m 2, 3 angles, 1 phases + 2 Majorana phases If Mass eigenstates  Weak eigenstates  Neutrino oscillation Oscillation probability ： P  sin 2 (1.27  m 2 L/E) Atmospheric solar  decays crossing ： CP &  13 Super-K K2K Minos T2K Daya Bay Double Chooz NOVA Homestake Gallex SNO KamLAND

Evidence of Neutrino Oscillations Unconfirmed: LSND:  m 2 ~ eV 2 Confirmed: Atmospheric:  m 2 ~ 2  eV 2 Solar:  m 2 ~ 8  eV 2 P      sin 2 2  sin 2 (1.27  m 2 L/E) 2 flavor oscillation in vacuum: Values of theta12 and theta23.

Current Knowledge of  13 Global fit fogli etal., hep-ph/ Sin 2 (2  13 ) < 0.09 Sin 2 (2  13 ) < 0.18 Direct search PRD 62, Allowed region

Importance to know  13 1 ） A fundamental parameter 2 ） important to understand the relation between leptons and quarks, in order to have a grand unified theory beyond the Standard Model 3 ） important to understand matter-antimatter asymmetry –If sin 2 2    ， next generation LBL experiment for CP –If sin 2 2    next generation LBL experiment for CP ??? 4 ） provide direction to the future of the neutrino physics: super-neutrino beams or neutrino factory ?

Measuring sin 2 2   at  reactors Clean signal, no cross talk with  and matter effects Relatively cheap compared to accelerator based experiments Provides the direction to the future of neutrino physics Rapidly deployment possible Small-amplitude oscillation due to  13 Large-amplitude oscillation due to  12 at reactors: P ee  1  sin 2 2   sin 2 (1.27  m 2  L/E)  cos 4   sin 2 2   sin 2 (1.27  m 2  L/E) at LBL accelerators: P  e ≈ sin 2   sin 2 2   sin 2 (1.27  m 2  L/E) + cos 2   sin 2 2   sin 2 (1.27  m 2 12 L/E)  A(  )  cos 2  13 sin    sin(  )

Recommendation of APS study report: APS

10-40 keV Neutrino energy: Neutrino Event: coincidence in time, space and energy neutrino detection: Inverse-β reaction in liquid scintillator 1.8 MeV: Threshold   or  s(0.1% Gd) n + p  d +  (2.2 MeV) n + Gd  Gd* +  ’s  (8 MeV)

Reactor Experiment: comparing observed/expected neutrinos: Palo Verde CHOOZ KamLAND Typical precision: 3-6%

How to reach 1% precision ? Increase statistics: –Use more powerful nuclear reactors –Utilize larger target mass, hence larger detectors Reduce systematic uncertainties: –Reactor-related: Optimize baseline for best sensitivity and smaller residual errors Near and far detectors to minimize reactor-related errors –Detector-related: Use “Identical” pairs of detectors to do relative measurement Comprehensive program in calibration/monitoring of detectors Interchange near and far detectors (optional) –Background-related Go deeper to reduce cosmic-induced backgrounds Enough active and passive shielding

Daya Bay nuclear power plant 4 reactor cores, 11.6 GW 2 more cores in 2011, 5.8 GW Mountains near by, easy to construct a lab with enough overburden to shield cosmic-ray backgrounds

Convenient Transportation, Living conditions, communications DYB NPP region - Location and surrounding 55 km

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

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

Best location for far detectors Rate only Rate + shape

Total Tunnel length ~2700 m Detector swapping in a horizontal tunnel cancels most detector systematic error. Residual error ~0.2% Backgrounds B/S of DYB,LA ~0.5% B/S of Far ~0.2% Fast Measurement DYB+Mid, Sensitivity (1 year) ~0.03 Full Measurement DYB+LA+Far, from 2010 Sensitivity (3 year) <0.01 The Layout LA: 40 ton Baseline: 500m Overburden: 112m Muon rate: 0.73Hz/m 2 Far: 80 ton 1600m to LA, 1900m to DYB Overburden: 350m Muon rate: 0.04Hz/m 2 DYB: 40 ton Baseline: 360m Overburden: 98m Muon rate: 1.2Hz/m 2 Access portal 8% slope 0% slope Mid: Baseline: ~1000m Overburden: 208m

Site investigation completed

Tunnel construction A feasibility study and two conceptual designs have been completed by professionals The tunnel length is about 3000m Cost is estimated to be about ~ 3K $/m Construction time is ~ months A similar tunnel on site as a reference

Baseline detector design: multiple neutrino modules and multiple vetos Redundancy is a key for the success of this experiment

Neutrino detector: multiple modules Multiple modules for side-by-side cross check Reduce uncorrelated errors Smaller modules for easy construction, moving, handing, … Small modules for less sensitivity to scintillator aging, details of the light transport, … Two modules at near sites Four modules at far site: Side-by-side cross checks

Central Detector modules Three zones modular structure: I. target: Gd-loaded scintillator  -ray catcher: normal scintillator III. Buffer shielding: oil Cylindrical module for easy construction Reflection at top and bottom for cost saving Module: 5 m high, 5 m diameter ~ 200 8”PMT/module Photocathode coverage: 5.6 %  10%(with reflector) Performance:  energy resolution  position resolution ~ 14 cm I II III Position resolution  ~14cm

Detector dimension  Catcher thickness IsotopesPurity (ppb) 20cm (Hz) 25cm (Hz) 30cm (Hz) 40cm (Hz) 238 U(>1MeV) Th(>1MeV) K(>1MeV) Total Oil buffer thickness Mass(t)Radius(m)Height(m) Target  -catcher Buffer

Calibration and Monitoring Source calibration: energy scale, resolutions, … –Deployment system Automatic: quick but limited space points Manual: slow but everywhere –Choices of sources: energy(0.5-8 MeV), activity(<1KHz),  /n,… –Cleanness Calibration with physics events: –Neutron capture –Cosmic-rays LED calibration: PMT gain, liquid transparency, … Environmental monitoring: temp., voltage, radon, … Mass calibration and high precision flow meters Material certification

The muon detector and the shielding Water shield also serves as a Cherenkov counter for tagging muons Water Cherenkov modules along the walls and floor Augmented with a muon tracker: RPCs Combined efficiency of Cherenkov and tracker > 99.5% with error measured to better than 0.25%

Alternative Design: Aquarium Tunnel Dry detectors Easier to deploy detectors Can access detectors Radon from rock can easily diffuse into detectors during data taking Less temperature regulation

Water Buffer & VETO At least 2m water buffer to shield backgrounds from neutrons and  ’s from lab walls Cosmic-muon VETO Requirement: –Inefficiency < 0.5% –known to <0.25% Solution: Two active vetos –Active water buffer, Eff.>95% –Muon tracker, Eff. > 90% RPC Water tanks scintillator strips –total ineff. = 10%*5% = 0.5% Two vetos to cross check each other and control uncertainty Neutron background vs water shielding thickness 2.5 m water

Background related error Need enough shielding and active vetos How much is enough ?  error < 0.2% –Uncorrelated backgrounds: U/Th/K/Rn/neutron Single gamma 0.9MeV < 50Hz Single neutron rate < 1000/day 2m water + 50 cm oil shielding –Correlated backgrounds: n  E  0.75 Neutrons: >100 MWE + 2m water Y.F. Wang et al., PRD64(2001) He/ 9 Li: > 250 MWE(near), >1000 MWE(far) T. Hagner et al., Astroparticle. Phys. 14(2000) 33 Nearfar Neutrino signal rate(1/day)56080 Natural backgrounds(Hz)45.3 Accidental BK/signal0.04%0.02% Correlated fast neutron Bk/signal0.14%0.08% 8 He+ 9 Li BK/signal0.5%0.2%

Systematic error comparison Chooz Palo VerdeKamLANDDaya Bay Reactor power % Reactor fuel/  spectra cross section No. of protonsH/C ratio  0 Mass  0 EfficiencyEnergy cuts Position cuts Time cuts P/Gd ratio n multiplicity0.5- <0.1 backgroundcorrelated uncorrelated <0.1 Trigger02.90 <0.1 livetime

Setting Up The Experiment Tunnel entrance 1 Detect  Measure bkg Cross calib. sin 2 2  13 Yes No 2 Detect  Measure bkg Cross calib. sin 2 2  13 Yes (1)Start in Jun 2009 Bring up a pair of detectors at a time Begin to observe  One month: N   17,500/module Begin to measure background, cross calibrate detectors at Daya Bay near site Move detectors 1 and 3 to Mid Hall Keep detectors 2 & 4 at Daya Bay near site Begin data taking with the Near-Mid configuration Tunnel entrance (2) When Mid Hall is ready in Sept 2009

Setting Up The Experiment (cont) Tunnel entrance Detect  Measure bkg Cross calib. sin 2 2  13 Yes (3) When Ling Ao & Far Halls are ready in Jun Move detector 5 to Far Hall, and detector 6 to Ling Ao Near Hall Begin to take data with the default Near-Far configuration 1 Tunnel entrance Detect  Measure bkg Cross calib. sin 2 2  13 Yes (4) Deployment is completed 243 Move detector 7 to Far Hall and detector 8 to Ling Ao Near Hall to complete the configuration

Sensitivity to Sin 2 2  13 Reactor-related correlated error:  c ~ 2% Reactor-related uncorrelated error:  r ~ 1-2% Calculated neutrino spectrum shape error:  shape ~ 2% Detector-related correlated error:  D ~ 1-2% Detector-related uncorrelated error:  d ~ 0.5% Background-related error: fast neutrons:  f ~ 100%, accidentals:  n ~ 100%, isotopes( 8 Li, 9 He, …) :  s ~ 50-60% Bin-to-bin error:  b2b ~ 0.5% Many are cancelled by the near-far scheme and detector swapping

Sensitivity to Sin 2 2  13 Other physics capabilities: Supernova watch, Sterile neutrinos, …

North America (13) LBNL, BNL, Caltech, UCLA Univ. of Houston, Iowa state Univ. Univ. of Wisconsin, Illinois Inst. Tech., Princeton, RPI, IIT, Virginia Tech., Univ. of Illinois Asia (13) IHEP, CIAE,Tsinghua Univ. Zhongshan Univ.,Nankai Univ. Beijing Normal Univ., Nanjing Univ. Shenzhen Univ., Hong Kong Univ. Chinese Hong Kong Univ. Taiwan Univ., Chiao Tung Univ., National United Univ. Europe (3) JINR, Dubna, Russia Kurchatov Institute, Russia Charles University, Czech Republic Daya Bay collaboration ~ 110 physicists

Organization of Daya Bay Collaboration Governed by Bylaws. Institutional board: with one representative from each member institution and two spokespersons. Executive board (two-year term): Y.F. Wang (China)A. Olshevski (Russia) C.G. Yang (China)K.B. Luk (U.S.) M.C. Chu (Hong Kong)R. McKeown (U.S.) Y. Hsiung (Taiwan) Scientific spokespersons (two-year term): Y.F. Wang (China), K.B. Luk

Project management Project managers: Y.F. Wang, W.Edwards Chief engineers: H.L. Zhuang, R. Brown Sub-system managers: –Anti-neutrino detector: J.W. Zhang, K. Heeger –Muon detector: L. Littenberg, C.G. Yang –Calibration: R. McKeown –Trigger/DAQ/electronics: X.N. Li, C. White –Offline/computing: J. Cao, C. Tull –Civil construction: H.Y. Zhang

Funding Daya Bay is the first US-China collaboration with equal partnership in nuclear and particle physics China plans to provide civil construction and ~half of the detector systems; U.S. plans to bear ~half of the detector cost Committed funding from the Chinese Academy of Sciences, the Ministry of Science and Technology, and Natural Science Foundation of China, China Guangdong Nuclear Power Group, Shenzhen municipal and Guangdong provincial government Gained strong support from: China Guangdong Nuclear Power Group China atomic energy agency China nuclear safety agency Supported by BNL/LBNL seed funds and $800K R&D fund from DOE

Schedule begin civil construction April 2007 Bring up the first pair of detectorsJun 2009 Begin data taking with the Near-Mid configurationSept 2009 Begin data taking with the Near-Far configurationJun 2010

Summary The Daya Bay experiment will reach a sensitivity of ≤ 0.01 for sin 2 2  13 The Daya Bay Collaboration continues to grow –Bylaws approved; –Working groups established; –Management structure introduced Design of detectors is in progress and R&D is ongoing Engineering design of tunnels and infrastructures will begin soon Continue to receive strong support from Chinese agencies Plan to start deploying detectors in 2009, and begin full operation in 2010

No good reason(symmetry) for sin 2 2  13 =0 Even if sin 2 2  13 =0 at tree level, sin 2 2  13 will not vanish at low energies with radiative corrections Theoretical models predict sin 2 2  13 ~ An experiment with a precision for sin 2 2  13 Better than 1% is desired An improvement of an order of magnitude over previous experiments

How Neutrinos are produced in reactors ? The most likely fission products have a total of 98 protons and 136 neutrons, hence on average there are 6 n which will decay to 6p, producing 6 neutrinos Neutrino flux of a commercial reactor with 3 GW thermal : 6   s

Prediction of reactor neutrino spectrum Reactor neutrino rate and spectrum depends on: –The fission isotopes and their fission rate, uncorrelated ~ 1-2% –Fission rate depends on thermal power, uncorrelated ~ 1% –Energy spectrum of weak decays of fission isotopes, correlated ~ 1% Three ways to obtain reactor neutrino spectrum: –Direct measurement at near site –First principle calculation –Sum up neutrino spectra from 235 U, 239 Pu, 241 Pu and 238 U 235 U, 239 Pu, 241 Pu from their measured  spectra 238 U(10%) from calculation (10%) They all agree well within 3%