Search for 13 at Daya Bay On behalf

Search for 13 at Daya Bay On behalf of the Daya Bay Collaboration Deb Mohapatra Virginia Tech

Outline • • The neutrino mixing matrix and the mixing angle θ 13 Reactor neutrino experiments Daya Bay experimental setup Expected signal and background rates Systematics and sensitivity Current status Summary

The neutrino mixing (MNS) matrix • The MNS matrix relates the mass eigenstates ( 1, 2 and 3) to the flavor eigenstates ( e, m and t) Last unknown matrix element • It can be described by three 2 D rotations Atmospheric Reactor Solar • If θ 13 is zero there is no CP violation in neutrino mixing Majorana Phases

Existing limit on 13 Hints for 13 ≠ 0 Global Fit Results Sin 2 13 = 0. 016 ± 0. 010 or Sin 22 13 = 0. 06 ± 0. 04 [ E. Lisi, et al. , ar. Xiv: 0905. 3549 ] allowed region

Nuclear reactors as antineutrino source • The observable antineutrino spectrum is the product of the flux and the cross section Arbitrary • Fission process in nuclear reactor produces huge number of low-energy antineutrino • A typical commercial reactor, with 3 GW thermal power, produces 6× 1020 νe/s • Daya Bay reactors produce 11. 6 GWth now, 17. 4 GWth in 2011 From Bemporad, Gratta and Vogel Antineutrino spectrum x Flu Cr o S ss e n o i ct

Measuring 13 with reactor antineutrinos Reactor anti-neutrinos survival probability: Small-amplitude oscillation due to 13 integrated over E θ 13 Δm 213≈ Δm 223 far detector near detector Solar oscillation due to 12

Daya Bay: Experimental setup Total tunnel length ~ 3000 m Far site Overburden: 355 m 900 m Empty detectors: moved to underground halls via access tunnel. Filled detectors: transported between halls via horizontal tunnels. Ling Ao Near Overburden: 112 m 465 m 810 m Water hall Liquid Scintillator hall Entrance Daya Bay Near Overburden: 98 m Ling Ao II cores Construction tunnel Ling Ao cores 295 m Daya Bay cores 7

Daya Bay: Experimental setup Far site Overburden: 355 m • 8 identical anti-neutrino detectors ( two at each near site and four at the far site) to cross-check detector efficiency • Two near sites sample flux from reactor groups 9 different baselines under the assumption of point size reactor cores and detectors Ling Ao Near Overburden: 112 m Ling Ao II cores (Starting 2011) Ling Ao cores Daya Bay Near Overburden: 98 m Daya Bay cores Halls Cores Daya Bay Near (m) Ling Ao Near (m) Far (m) Daya Bay 363 1347 1985 Ling Ao I 857 481 1618 Ling Ao II 1307 526 1613

Antineutrino Detector (AD) ~ 12% / E 1/2 Calibration System Mineral Oil LS Gd-Loaded LS 1. 55 m 1. 99 m 2. 49 m PMT 5 meters • Three-zone cylindrical design ü Target: 20 ton 0. 1% Gd-doped Liquid Scintillator (LS) ü Gamma catcher: 20 ton LS ü Buffer : 40 ton (mineral oil) • 192 low-background 8” PMTs • Reflectors at top and bottom • AD sits in a pool of ultra-pure water

Muon veto system Cerenkov Water Pool (2 Zone) RPC’s PMTs (962) • Two tagging systems to detect cosmic ray and fast neutron background: 2. 5 meter thick two-section water shield and RPCs • Efficiency 99. 5% with uncertainty <0. 25%

Antineutrino event signature in AD Inverse b-decay • Two part coincidence is crucial for background reduction • Neutron capture on Gd provides a secondary burst of light approximately 30 μs later e p e+ + n (prompt) 0. 3 b 50, 000 b + p D + (2. 2 Me. V) (delayed) + Gd Gd* Gd + ’s(8 Me. V) (delayed)

Measuring 13 with reactor antineutrinos at Daya Bay Measured Proton Number Ratio of Ratio Rates ± 0. 3% Storage Tank Near Far + flow & mass measurement sin 22 13

Target mass measurement 200 -ton Gd-LS reservoir ISO Gd-LS weighing tank filling platform with clean room pump stations 20 -ton ISO tank detector load cell accuracy < 0. 02% Coriolis mass flowmeters < 0. 1% filling “pairs” of detectors

Measuring 13 with reactor antineutrinos at Daya Bay Measured Proton Number Ratio of Ratio Rates ± 0. 3% Storage Tank Near Far + flow & mass measurement Detector Efficiency Ratio ± 0. 2% Calibration systems sin 22 13

AD calibration system Automated calibration system → routine weekly deployment of sources LED light sources → monitoring optical properties e+ and n radioactive sources (=fixed energy) → energy calibration automated calibration system • 68 Ge source • Am-13 C + 60 Co source • LED diffuser ball

Energy calibration Prompt Energy Signal 1 Me. V Delayed Energy Signal 8 Me. V 6 Me. V 10 Me. V e+ threshold: stopped positron signal using 68 Ge 6 Me. V threshold: n capture signals at 8 and 2. 2 source (2 x 0. 511 Me. V) Me. V (n source, spallation) e+ energy scale: 2. 2 Me. V neutron capture signal (n source, spallation) 1 Me. V cut for prompt positrons: >99%, uncertainty negligible 6 Me. V cut for delayed neutrons: 91. 5%, uncertainty 0. 22% assuming 1% energy uncertainty

Backgrounds • Fast neutron ─ fast neutron enters detector, creates prompt signal, thermalizes, and is captured • β+n decays of 9 Li and 8 He created in AD via μ - 12 C spallation 9 Li Antineutrino Signal • Random coincidence ─ two unrelated events happen close together in space and time

Signal, background and systematic Signal rates: (1%) far site < 90 events/det/day Daya Bay site < 840 events/det/day Ling Ao site < 740 events/det/day Total expected background rates: far site < 0. 4 events/det/day Daya Bay site < 6 events/det/day Ling Ao site < 4 events/det/day Systematic and statistical budgets summary Source Uncertainty Reactor power 0. 13% Detector (per module) 0. 38% (baseline) 0. 18% (goal) Signal statistics 0. 2%

Daya Bay sensitivity to sin 22θ 13 Sin 22θ 13 < 0. 01 @ 90% CL in 3 years of data taking 2011 start data taking with full experiment nominal running period: 3 years

Site preparation tunnel cleanroom surface assembly building assembly pit staging area assembly pittest assembly

Fabrication and delivery of detector components detector tank acrylic target vessels

Gd-Liquid scintillator test production Daya Bay experiment uses 200 ton 0. 1% gadolinium-loaded liquid scintillator (Gd-LS). 4 -ton test batch production 0. 1% Gd-LS in 5000 L tank Gd-TMHA + LAB + 3 g/L PPO + 15 mg/L bis-MSB Gd-LS will be produced in multiple batches but mixed in reservoir on-site, to ensure identical detectors.

Summary • Daya Bay will reach a sensitivity of ≤ 0. 01 for sin 22 13 • Daya Bay is most sensitive reactor 13 experiment under construction • Civil and detector construction are progressing. Data taking will begin in summer 2010 with 2 detectors at near site. • Full experiment will start taking data in 2011.

The Daya Bay Collaboration Europe (3) (9) JINR, Dubna, Russia Kurchatov Institute, Russia Charles University, Czech Republic North America (14)(73) BNL, Caltech, George Mason Univ. , LBNL, Iowa state Univ. Illinois Inst. Tech. , Princeton, RPI, UC-Berkeley, UCLA, Univ. of Houston, Univ. of Wisconsin, Virginia Tech. , Univ. of Illinois-Urbana-Champaign ~ 210 collaborators Asia (18) (125) IHEP, Beijing Normal Univ. , Chengdu Univ. of Sci. and Tech. , CGNPG, CIAE, Dongguan Polytech. Univ. , Nanjing Univ. , Nankai Univ. , Shandong Univ. , Shenzhen Univ. , Tsinghua Univ. , USTC, Zhongshan Univ. , Hong Kong Univ. , Chinese Hong Kong Univ. , National Taiwan Univ. , National Chiao Tung Univ. , National United Univ. Thank You