Xin Qian Caltech For Daya Bay Collaboration Improved Measurement of Electron Antineutrino Disappearance at Daya Bay Xin Qian Caltech For Daya Bay Collaboration NuFact12

Design goal for systematic 0.2~0.4% (relative) The largest, deepest reactor Θ13 experiment in Town, aimed for a precision measurement of sin22θ13 <0.01 NuFact12

Key to reach 0.01 (Stat) Powerful reactors (17.6 GW) + Large Mass (80 ton) (Sys) Reactor related: using near/far to form ratio + baseline (near ~0.4 km, far ~1.7 km) (Sys) Detector related: “identical detectors” + “precise detector calibration” (Sys) Background related: deep underground to reduce cosmic+ active/passive shielding Distances from reactor Far/Near νe Ratio Oscillation deficit Detector efficiency Detector Target Mass NuFact12

Accurate Survey  reduced uncertainties in reactor flux. Baseline Survey Accurate Survey  reduced uncertainties in reactor flux. Detailed Survey: - GPS above ground - Total Station underground - Final precision: 2.8 cm Validation: - Three independent calculations - Cross-check survey - Consistent with reactor plant and design plans By Total station By GPS NuFact12

Daya Bay Antineutrino Detector Automated calibration system Reflectors at top/bottom of cylinder Prompt positron: Carries antineutrino energy Eprompt ≈ Eν – 0.8 MeV Delayed neutron capture: Efficiently tags antineutrino signal Photomultipliers Steel tank Radial shield Outer acrylic tank Inner acrylic tank Photosensors: 192 8”-PMTs Energy resolution: (empirical) total detector mass: ~ 110t inner: 20 tons Gd-doped LS (d=3m) mid: 20 tons LS (d=4m) outer: 40 tons mineral oil buffer (d=5m) Three ACUs, 6 “functionally identical”, 3-zone detectors reduces systematic uncertainties NuFact12

ACU (Caltech) Automated: weekly calibration Ge68, LED, Co60, Am-C MC 0.712 MeV in DYB > 6 MeV in DYB NuFact12

Detector Filling Detector target filled from GdLS in ISO tank. LAB + Gd (0.1%) + PPO (3 g/L) + bis-MSB (15 mg/L) Number of protons: (7.169±0034) × 1025 p per kg Gd-loaded liquid scintillator shows good stability with time Detector target filled from GdLS in ISO tank. Load cells measure 20 ton target mass to 3 kg (0.015%) A 1-m apparatus yielded attenuation length of ~15 m @ 430 nm. H/C ratio dominate uncertainy. Checmitry, Xray + Combustion Fill in pairs, within 2 weeks. 3 fluids filled simultaneously, with heights matched to minimize stress on acrylic vessels Gadolinium-doped Liquid Scintillator (GdLS) Liquid Scintillator (LS) Mineral Oil (MO) NuFact12

Muon Veto System Dual tagging systems: 2.5 meter thick two-section water shield and Resistance Plate Chambers (RPC) Outer layer of water veto (on sides and bottom) is 1m thick, inner layer >1.5m. Water extends 2.5m above ADs 288 8” PMTs in each near hall 384 8” PMTs in Far Hall 4-layer RPC modules above pool 54 modules in each near hall 81 modules in Far Hall Two Zone ultrapure water Cherenkov detector NuFact12

Hall I: AD1/2 Comparison Data taking began Aug. 15, 2011 NuFact12

Hall 2 and Hall 3 Hall 2: Began 1 AD operation on Nov. 5, 2011 on Dec. 24, 2011 2 more ADs still in assembly; installation planned for 2012 NuFact12

Data Period ATwo Detector Comparison: Sep. 23, 2011 – Dec. 23, 2011 Hall 1 Hall 2 Hall 3 A B C Data Period ATwo Detector Comparison: Sep. 23, 2011 – Dec. 23, 2011 Nucl. Inst. and Meth. A 685  (2012), pp. 78-97 BFirst Oscillation Result: Dec. 24, 2011 – Feb. 17, 2012 Phys. Rev. Lett. 108, 171803 (2012) CUpdated analysis: Dec. 24, 2011 – May 11, 2012 To be submitted to Chinese Physics C Data volume: 40TB DAQ eff. ~ 96% Eff. for physics: ~ 94% NuFact12

PMT Light Emission (Flashing) Flashing PMTs: ~ 5% of PMTs - Easily discriminated with patterns in charge and time. Neutrinos Flashers Quadrant = Q3/(Q2+Q4) MaxQ = maxQ/sumQ Relative PMT charge Inefficiency to anti-neutrinos signal: 0.024%  0.006%(stat) Contamination: < 0.01% Further suppressed by background subtraction. NuFact12

AD Energy Calibration (abs/rel+F.V./Point) 6 MeV Cut F.V. = Full Volume Sources Energy (MeV) Co60 (ACU) 2.5 (abs) AmC-n (ACU) 8.0 Ge68 (ACU) 1.0 Spallation-n (F.V.) 8.0 /2.2 IBD-n (F.V.) 8.0 / 2.2 K-40 1.3 Tl-208 2.6 Bi214-Po214-Pb210 (Alpha) 7.7 MeV Bi212-Po212-Pb208(Alpha) 8.8 MeV Rn219-Po215-Pb211(Alpha) 6.7+7.4 MeV PMT gain calibration NuFact12

Performance Energy scale vs. Position LS region NuFact12

Inverse Beta Decay Selection: Muon Veto on delay signal: Remove flashers 0.712 MeV prompt event 612 MeV delay event 1200 us time correlation Multiplicity Cut No prompt-like event 200us before prompt. No prompt-like events between prompt and delay. No prompt-like events 200 us after delay. Also alternative methods. Muon Veto on delay signal: Water Pool Muon: 600 us after WP muon AD Non-shower Muon: 1 ms after AD muon (20 MeV) Alternative: 1.4 ms after AD muon (3000 PE~18 MeV) AD Shower Muon: 1 s after AD Shower muon (2.5 GeV). Alternative: ~0.4s after AD Shower muon (3e5 PE~1.8 GeV) 20 MeV AD muon, Shiwer 1.8 GeV NuFact12

Neutron thermalization Ratio Erec (MeV) Spill In/Out Spill/out RENO Neutron thermalization Asy NuFact12 Δt (us) Erec (MeV)

Alternative Multiplicity Cut Goal of multiplicity cut is to remove ambiguity in the prompt energy One need to sync calculation of Efficiency Accidental Background Livetime Calculation Decoupled Multiplicity Cut (DMC) No additional prompt-like events 400 us before delay. No delay-like events 200 us after delay. Fixed window Cut  Easy to calculate live time NuFact12

Study to understand Efficiencies 6 MeV Cut Timing Cut Erec (MeV) Absolute Efficiency Comparison with MC (6 MeV, Timing, Spill-in/out) H/Gd ratio checked with data Spallation Neutron AmC Why absolute efficiency is cancelled. H/Gd Ratio Erec (MeV) NuFact12

Accidental Background Accidental Background: ~1.7% (Near) + ~ 4.6% (Far) Multiple methods to calculate Rprompt to get systematic uncertainties RdelayM is a direct measured quantity Single Spectra are well understood (Po-210, K-40, Tl-208) Cross check: coincidence vertex, off window coincidence Black: All triggers Red: isolated triggers Single spectrum + single neutron signal, reason NuFact12

ACU Am-C ACU Am-C Correlated Background (GEANT4 MC) Anchor Single Neutron rate with Data ~0.03% (Near) + ~0.3% (Far) n-like singles NuFact12

β-n decay: Eμ>4 GeV (visible) 9Li uncorrelated Time since last muon (s) Muon + shower  Li9 and He8 Analysis muon veto cuts control B/S to ~0.35% Near 0.2% Far NuFact12

RPC tagged Fast N background Fast neutrons Fast Neutrons: Energetic neutrons produced by cosmic rays (inside and outside of muon veto system) Mimics antineutrino (IBD) signal: - Prompt: Neutron collides/stops in target - Delayed: Neutron captures on Gd Constrain fast-n rate using IBD-like signals in 10-50 MeV Analysis muon veto cuts control B/S to 0.1% (0.13%) of far (near) signal RPC tagged Fast N background Muon interact of neutron Validate with fast-n events tagged by water pool. NuFact12 Prompt Energy MeV

Alpha-13C neutrons Alpha’s rate from data Potential alpha source: 238U, 232Th, 235U, 210Po: Each of them are measured in-situ: U&Th: cascading decay of Bi(or Rn) – Po – Pb 210Po: spectrum fitting Combining (α,n) cross-section, correlated background rate is determined. (1ms, 3ms) 238U 232Th (10ms, 160ms) Radioactive contamination alpha 227Ac Alpha’s rate from data <0.02% (Near) + < 0.1% (Far) (1ms, 2ms) Total NuFact12

Background Budgets Really small background contamination! Backgrounds Near Far + systematic uncertainties Accidental ~1.7% ~4.6% + negligible sys. ACU-N ~0.03% ~0.3% + 100% relative sys Li9/He8 ~0.35% ~0.2% + 50% relative sys Fast-N ~0.13% ~0.1% + 30% relative Alpha-N ~0.02% <0.1% + 50% relative sys Really small background contamination! Conservative estimation of systematics for background contamination! NuFact12

Reactor Flux Expectation Antineutrino flux is estimated for each reactor core Flux estimated using: Isotope fission rates vs. reactor burnup Reactor operators provide: - Thermal power data: Wth - Relative isotope fission fractions: fi Energy released per fission: ei V. Kopekin et al., Phys. Atom. Nucl. 67, 1892 (2004) Antineutrino spectra per fission: Si(Eν) K. Schreckenbach et al., Phys. Lett. B160, 325 (1985) A. A. Hahn et al., Phys. Lett. B218, 365 (1989) P. Vogel et al., Phys. Rev. C24, 1543 (1981) T. Mueller et al., Phys. Rev. C83, 054615 (2011) P. Huber, Phys. Rev. C84, 024617 (2011) simulation Flux model has negligible impact on far vs. near oscillation measurement (1/20 reduction with ratio) NuFact12

Antineutrino Rate vs. Time Detected rate correlated with reactor flux expectations. Predicted Rate: - Normalization is determined by data fit. - Absolute normalization is within a few percent of expectations. NuFact12

Uncertainty Summary For near/far oscillation, only uncorrelated uncertainties are used. Largest systematics are smaller than far site statistics (~0.6%) Combined, total efficiency is 79% Influence of uncorrelated reactor systematics significantly reduced by far vs. near measurement. NuFact12

Prompt Energy Spectra Near Site data contains AD1/2 comparison period. ~ 82k ~ 29k ~ 204k Erec (MeV) Erec (MeV) Erec (MeV) Near Site data contains AD1/2 comparison period. High-statistics reactor antineutrino spectra. B/S ratio is 5% (2%) at far (near) sites.

Discovery of a non-zero value of q13 R = 0.940 ± 0.011 (stat) ± 0.004 (syst) sin22θ13=0.092±0.017 A clear observation of far site deficit 5.2 s for non-zero value of q13 with the first 55 days’ data PRL 108 171803 (2012) NuFact12

Improved results R = 0.944 ± 0.007 (stat) ± 0.003 (syst) Increase stat to 7 sigma R = 0.944 ± 0.007 (stat) ± 0.003 (syst) sin22θ13=0.089±0.011 With 2.5x more statistics, an improved measurement to q13 Spectral distortion consistent with oscillation NuFact12

Summary Daya Bay Experiment is well on track. Six functionally identical ADs enabled a rapid discovery of Θ13 Calibration/Construction of ADs well achieved designed systematic goal. ADs are stable and backgrounds are well understood. 2 additional ADs are under construction (will deploy in this year). The Daya Bay reactor neutrino experiment has made an unambiguous observation of reactor electron-antineutrino disappearance at ~2 km: 8 AD full configuration! Completion! NuFact12

Outlook Y. F. Wang: Daya Bay II, WG1 25th Good progress in shape analysis. Understanding the Energy non-linearity. Measurement of Δm2ee ~ |Δm231| Measuring sin22θ13 ~ 5% level Measurement of absolute antineutrino Flux Precise measurement of antineutrino energy spectrum. Y. F. Wang: Daya Bay II, WG1 25th Expect improvement in Systematic uncertainties as well NuFact12

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Spill in/out NuFact12