Neutrino Experiments with Reactors

Transcription

1 Neutrino Experiments with Reactors 1 Ed Blucher, Chicago Lecture 2 Reactors as antineutrino sources Antineutrino detection Reines-Cowan experiment Oscillation Experiments Solar Δm 2 (KAMLAND) Atmospheric Δm 2 -- θ 13 (CHOOZ, Double-Chooz, Daya Bay) Conclusions June 2008 NUFACT 08

2 2 Atmospheric Δm 2 : Searching for θ 13 with Reactors Importance of θ 13 Experimental approaches to θ 13 ; motivation for a precise reactor experiment Designing an ideal experiment Planned experiments Conclusions

3 Neutrino mixing and masses 3 Ue1 Ue2 Ue3 Big Big Small? U = U U U = Big Big Big μ1 μ2 μ3 Uτ1 Uτ2 U τ3 Big Big Big iδcp cosθ12 sinθ12 0 cosθ13 0 e sinθ = sinθ12 cosθ cosθ23 sinθ23 iδcp e sinθ13 0 cosθ 13 0 sinθ23 cosθ 23 θ 12 ~ 30 sin 2 2θ 13 < 0.15 at 90% CL θ 23 ~ 45 What is ν e component of ν 3 mass eigenstate? normal inverted

4 Key questions in neutrino mixing 4 What is value of θ 13? What is mass hierarchy? Do neutrino oscillations violate CP symmetry? P(ν μ ν e ) P(ν μ ν e ) = 16s 12 c 12 s 13 c 2 13 s 23 c 23 sinδ sin Δm E L sin Δm E L sin Δm E L Why are quark and neutrino mixing matrices so different? Big Big Small? 1 Small Small U MNSP ~ Big Big Big vs. VCKM ~ Small 1 Small Big Big Big Small Small 1 Value of θ 13 central to these questions; it sets the scale for experiments needed to resolve mass hierarchy and search for CP violation.

5 Methods to measure sin 2 2θ 13 5 Accelerators: Appearance (ν μ ν e ) at Δm ev Δm L 13 2 P( νμ νe) = sin θ23 sin 2θ13 sin + not small terms ( δcp, sign( Δm13)) 4E NOνA: <E ν > = 2.3 GeV, L = 810 km T2K: <E ν > = 0.7 GeV, L = 295 km Reactors: Disappearance (ν e ν e ) at Δm ev ΔmL 13 P( νe νe) = 1 sin 2θ13 sin + very small terms 4E Use reactors as a source of ν e (<E ν >~3.5 MeV) with a detector 1-2 kms away and look for non-1/r 2 behavior of the ν e rate Reactor experiments provide the only clean measurement of sin 2 2θ 13 : no matter effects, no CP violation, almost no correlation with other parameters.

6 Reactor Measurement of sin 2 2θ 13 6 Past measurements: 2 2 Δm atm Δm solar θ 13 : Search for small oscillations at km distance (corresponding to Δm atm ) Δm13L Δm12L P( νe νe) 1 sin 2θ13 sin c13 sin 2θ12 sin 4E 4E P ee Δ m = sin 2 13 = 0.04 E ν θ = 3.5MeV ev Goals: Small: sin 2 2θ 13 <0.03 Medium: sin 2 2θ 13 <0.01 Large: sin 2 2θ 13 <0.005 Distance to reactor (m)

7 Both reactor and accelerator experiments have sensitivity to sin 2 2θ 13, but accelerator measurements have ambiguities Example: T2K. ΔP(ν μ ν e )= Δsin 2 2θ 13 = normal inverted δ cp (5 yr ν) +/ Δm 2 = ev 2

8 8 Reactor and accelerator sensitivities to sin 2 2θ 13 90% CL exluded regions with no osc.signal 90% CL allowed regions with osc.signal med react med react δ CP =0, Δm 2 = ev 2 (3 yr reactor, 5 yr Nova) sin 2 2θ 13 = 0.05, δ CP =0, Δm 2 = ev 2 (3 yr reactor, 5 yr T2K)

9 Resolving the θ 23 Degeneracy 9 ν μ disappearance experiments measure sin 2 2θ 23, while P(ν μ ν e ) sin 2 θ 23 sin 2 2θ 13. If θ 23 45, ν μ disappearance experiments, leave a 2-fold degeneracy in θ 23 it can be resolved by combination of a reactor and ν μ ν e appearance experiment. Green: Nova Only Blue: Medium Reactor plus Nova Red: Small reactor plus offaxis Example: sin 2 2 θ 23 = 0.95 ± 0.01 Δm 2 = ev 2 sin 2 2θ 13 = % CL Δm 2 = ev 2 sin 2 2θ 13 = 0.05 Medium react (3 yrs) + Nova Nova only (3yr + 3yr) Small react (3yrs) + Nova

10 CP Violation and the Mass Hierarchy 10 T2K Nova P(ν μ ν e ) sin 2 2θ 13 =0.1 δ CP δ CP

11 Example: Reactor + T2K ν running 11 T2K ν - 5 years P(ν μ ν e ) Δsin 2 2θ 13 =±0.01 from reactor sin 2 2θ 13 =0.1 Neutrino, normal hierarchy Neutrino, inverted hierarchy δ CP

12 Chooz: Current Best θ 13 Experiment 12

13 Chooz Experiment 13 P=8.4 GW th L=1.05 km D=300mwe m = 5 tons, Gd-loaded liquid scintillator

14 CHOOZ 14

15 Gadolinium Loaded Scintillator 15 Small amount of Gd added to liquid scintillator to improve neutron detection: shorter capture time and higher energy. Element σ (barns) Isotopic abundance (%) 155 Gd 61, Gd 255, Gd (natural) 49, H Neutrino detection by + ν e + p e + n, n + m Gd m+1 Gd* m+1 Gd γs (8 MeV); τ=30 μsec (Compared to n+ p d +γ (2.2 MeV); τ ~ 200μsec) For 0.1% Gd, about 85% of neutrons are captured by Gd

16 16 Degradation of Chooz Scintillator Attenuation degrades by ~0.4% per day.

17 17 Summary of Chooz run: 4/97-7/98 ~2.2 evts/day/ton with bkg evts/day/ton ~total sample included 3600 ν events Chooz started data collection before reactor began operating. UNIQUE possibility to measure backgrounds

18 Final Chooz Data Sample 18

19 19 CHOOZ Systematic errors Reactor ν flux Detect. Acceptance 2% 1.5% Total 2.7% sin 2 2θ 13 < 0.15 for Δm 2 = ev 2

20 How can one improve on Chooz Experiment? Add an identical near detector Eliminate dependence on reactor flux; only relative acceptance of detectors needed Optimize baseline Larger detectors; improved detector design Reduce backgrounds (Go deeper and use active veto systems) Stable scintillator 20 ~200 m ~1300 m

21 Kr2Det: Reactor θ 13 Experiment at Krasnoyarsk 21 Features - underground reactor - existing infrastructure Detector locations constrained by existing infrastructure ~1.5 x 10 6 ev/year ~20000 ev/year Reactor Ref: Marteyamov et al, hep-ex/

22 What is the best baseline? It depends 22 What is Δm 2? For rate measurement, you must consider competition between 1/R 2 and sinusoidal term. For shape measurement, distortion is different at different baselines: L=1100 m L=1700 m E ν (MeV)

23 Best baseline also depends on relative size of statistical and systematic errors. 23 (From J. Link) No systematic errors σ norm =6 σ stat Δm13L 1.27Δm13 L180 P( νe νe) 1 sin 2θ13 sin ; Kinematic Phase E 3.6 MeV π

24 Combined Rate and Shape Analysis 24

25 Sensitivity Using Rate and Energy Spectrum (Huber et al. hep-ph/ ) 25 Shape only σ norm = σ norm = 0.8% Statistics only σ norm = 0 Δm 2 = ev 2

26 Sensitivity Using Rate and Energy Spectrum (Huber et al. hep-ph/ ) 26 Shape only σ norm = σ norm = 0.8% Statistics only σ norm = 0 Small Medium Large Δm 2 = ev 2

27 Different Scales of Experiments 27 Small: sin 2 2θ 13 ~ 0.03 (e.g., Double-Chooz, KASKA,Reno) Double-Chooz: 10 ton detector at L-1.05 km. Mostly rate information, fixed detectors, non-optimal baseline Medium: sin 2 2θ 13 ~ 0.01 (e.g., Braidwood, Daya Bay) ton detectors, optimized baseline, optimized depths, rate and shape info, perhaps movable detectors to check calibration, multiple far detector modules for additional cross checks Large: sin 2 2θ 13 ~ (e.g., Angra) ~500 ton fiducial mass; sensitivity mainly through E spectrum distortion

28 Acceptance Issues 28 Must know: (relative) number of protons in fiducial region (relative) efficiency for detecting IBD events Known volume of stable, identical Gd-loaded liquid scintillator in each detector Well understood efficiency of positron and neutron energy requirements

29 Detectors and analysis strategy designed to minimize relative acceptance differences 29 Central zone with Gd-loaded scintillator surrounded by buffer regions; fiducial mass determined by volume of Gd-loaded scintillator Shielding Neutrino detection by + ν e + p e + n, n m Gd m+1 Gd γs (8 MeV); τ=30μsec Events selected based on coincidence of e + signal (E vis >0.5 MeV) and γs released from n+gd capture (E vis >6 MeV). ν e n e + No explicit requirement on reconstructed event position; little sensitivity to E requirements. Mineral oil buffer 6 meters Gd-loaded liquid scintillator To reduce backgrounds: depth + active and passive shielding

30 30 Events selected based on coincidence of e + signal (E vis >0.5 MeV) and γs released from n+gd capture (E vis >6 MeV). Reconstructed e + and n-capture energy n Capture on H n Capture on Gd

31 31 Gd LS Neutron Capture Energy as a Function of R 3.5 m Mineral Oil 2.6 m

32 32 Acceptance as a function of R Acrylic boundary

33 33 Events selected based on coincidence of e + signal (E vis >0.5 MeV) and γs released from n+gd capture (E vis >6 MeV). Reconstructed e + and n-capture energy n Capture on H n Capture on Gd

34 2-zone versus 3-zone detectors 34 I. Gd-loaded liquid scintillator II. Non-scintillating buffer I. Gd-loaded liquid scintillator II. γ catcher: liquid scintillator (no Gd) III. Non-scintillating buffer I I II III II

35 3-zone versus 2-zone detectors 35 I. Gd-loaded liquid scintillator II. γ catcher: liquid scintillator (no Gd) III. Non-scintillating buffer I III II

36 Questions What should the detectors look like? 36 To achieve a certain detector mass, is it better to have lots of small detectors or fewer big detectors?

37 Questions What should the detectors look like? 37 To achieve a certain detector mass, is it better to have lots of small detectors or fewer big detectors? Larger, spherical detectors minimize surface area to volume ratio, simplify reconstruction, and make it possible to study radial dependence of signal and background. Multiple detectors allow additional cross checks, and systematic errors could be reduced by N det if sys errors are independent.

38 Acceptance cross checks: Movable Detectors 38 Take data with Near and Far detectors simultaneously at near site. High flux a near site allows precise check of acceptance in ~1 month. Taken to extreme: swap near and far detectors (Daya Bay)

39 Backgrounds 39 Uncorrelated backgrounds from random coincidences Reduced by limiting radioactive materials Directly measured from rates and random trigger setups Correlated backgrounds Neutrons that mimic the coincidence signal Cosmogenically produced isotopes that decay to a beta and neutron: 9 Li (τ 1/2 =178 ms) and 8 He (τ 1/2 =119 ms); associated with showering muons. Reduced by shielding (depth) and veto systems

40 How deep should detector be? 40 Should the near and far detectors be at the same depth? Not necessary since signal rates are very different, but same depth offers systematic advantages.

41 Veto (Tagging) System Strategy: tag muons that pass near the detector. Use shielding to absorb neutrons produced by muons that miss the veto system. 41 Veto Detectors Shielding Residual n background: 1. Veto inefficiency μ p n 6 meters n μ 2. Fast neutron created outside the shielding Dead time: E.g., with μ rate in the veto system of 20 Hz and the tag window of 100 μs 0.2% dead time Muon identification should allow in situ determination of the residual background rate

42 42 Features of Ideal Experiment: multiple large, spherical detectors that minimize boundary effects all detectors protected by an equal and well-understood overburden so cosmic ray backgrounds are similar detectors on the reactor symmetry axis to eliminate reactor flux effects a robust shielding system to reduce and measure backgrounds in situ (+ reactor-off time to measure backgrounds) What do real experiments look like?

43 Collaboration of ~150 physicists from France, Germany, Spain, Japan, U.K., Russia, Brazil, and U.S. Double Chooz Experiment m ν ν ν ν ν ν ν ν 1051 m

44 300 m.w.e. Shielding Chooz Far Detector Hall 44

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47 Double Chooz will begin data taking in April 2009 with far detector only. The near detector will be installed months later. 47 σ sys =2.5% σ sys =0.6% Far det. only Near + far det.

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50 Daya Bay Site 50

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52 Daya Bay Projected Sensitivity 52 Datataking with all 8 detectors by December 2010.

53 Conclusions 53 Reactor experiments have played an important role in investigating the properties of the neutrino. The worldwide program to understand ν oscillations and determine the mixing parameters, CP violating effects, and mass hierarchy will require a broad range of measurements a reactor experiment to measure θ 13 is a key part of this program. A reactor experiment will provide the most precise measurement of θ 13 or set the most restrictive limit. An observation of θ 13 will open the door to searching for CP violation in neutrino oscillations. Many new results to look forward to