Reactor Neutrino Oscillation Experiments: Status and Prospects

Transcription

1 6! 5! 4! 1 2! 3!! Reactor Neutrino Oscillation Experiments: Status and Prospects Survival Probability ! 15 Karsten M. Heeger University of Wisconsin Data - BG - Geo & e 99.73% C.L. Expectation based on osci. parameters best fit determined by KamLAND L 0 /E &e (km/mev) Karsten Heeger, Univ. of Wisconsin 10 5 ev 2 Penn and tan 2 State, θ 12 = 0.52 July , $# ) 2 (ev 2 21 $m KamLAND 95% C.L. 99% C.L % C.L. best fit Solar 95% C.L. 99% C.L. 2! 1! tan " 12 $# FIG. 2: Allowed region for neutrino oscillation parameters from KamLAND and solar neutrino experiments. The side-panels show the χ 2 -profiles for KamLAND (dashed) and solar experiments (dotted) individually, as well as the combination of the two (solid). we also expect geo-neutrinos. We observe 1609 events. Figure 1 shows the prompt energy spectrum of selected electron anti-neutrino events and the fitted backgrounds. The unbinned data is assessed with a maximum likelihood fit to two-flavor neutrino oscillation (with θ 13 = 0), simultaneously fitting the geo-neutrino contribution. The method incorporates the absolute time of the event to account for time variations in the reactor flux and includes Earth-matter oscillation effects. The best-fit is shown in Fig. 1. The joint confidence intervals give m 2 21 = (stat) (syst) 10 5 ev 2 and tan 2 θ 12 = (syst) for tan2 θ 12 <1. A 0.07 (stat)+0.10 scaled reactor spectrum without distortions from neutrino oscillation Penn is excluded State, morejuly than 5σ. 1, An independent 2010 analysis using cuts similar to Ref. [2] finds m 2 21 = ! 0.20 The allowed contours in the neutrino oscillation parame- N osc /N no_osc Events / 0.2 MeV KamLAND data best-fit osci. accidental (MeV) E p Data - BG - best-fit osci. Reference Geo " e [8] O C(!,n) best-fit Geo " e best-fit osci. + BG + best-fit Geo " e FIG. 3: 0.7The low-energy region of the ν e spectrum relevant for geoneutrinos. The main panel shows the data with the fitted background and geo-neutrino 0.6 contributions; the upper panel compares the background and reactor ν e subtracted data to the number of geo-neutrinos for the 0.5 decay chains of U (dashed) and Th (dotted) calculated from a geological reference model [8]. Survival Probability Data - BG - Geo! e 0.3 Expectation based on osci. parameters determined 1 by KamLAND Baseline (km) L 0 /E!e (km/mev) FIG. 4: Ratio of the background and geo-neutrino subtracted ν e spectrum to the expectation for no-oscillation as a function of L 0/E. L 0 is the effective baseline taken as a flux-weighted average (L 0 = 180 km); the energy bins are equal probability bins of the best-fit including all backgrounds (see Fig. 1). The histogram and curve show the expectation accounting for the distances to the indi- 4

2 Neutrino Physics at Reactors Next - Discovery and precision measurement of θ 13 Daya Bay Double Chooz Reno Precision measurement of Δm12 2. Evidence for oscillation Evidence for spectral distortion First observation of reactor antineutrino disappearance Nobel Prize to Fred Reines at UC Irvine KamLAND 1980s & 1990s - Reactor neutrino flux measurements in U.S. and Europe First observation of (anti)neutrinos Chooz Chooz Past Reactor Experiments Hanford Savannah River ILL, France Bugey, France Rovno, Russia Goesgen, Switzerland Krasnoyark, Russia Palo Verde Chooz, France Savannah River

3 Discovery of the Neutrino Observation of the Free Antineutrino by Reines and Cowan inverse beta decay ν e + p e + + n

4 Antineutrino Detection inverse beta decay ν e + p e + + n coincidence signature prompt e + and delayed neutron capture kev MeV Eν e Ee + + En + (Mn-Mp) + me + including E from e + annihilation, Eprompt=E ν MeV

5 Arbitrary Reactor Antineutrinos From Bemporad, Gratta and Vogel observed spectrum Observable! Spectrum mean energy of νe: 3.6 MeV only disappearance expts possible cross-section accurate to +/-0.2% Flux Cross Section time-dependent rate and spectrum calculated reactor spectrum threshold: neutrinos with E < 1.8 MeV are not detected only ~ 1.5 νe/fission can be detected

6 Measurement of Reactor Spectra Goesgen Experiment (1980ʼs) comparison of predicted spectra to observations two curves are from fits to data and from predictions based on Schreckenbach et al. 3 baselines with one detector flux and energy spectrum agree to ~ 1-2% reactors are calibrated source of νeʼs

7 Reactor and Accelerator Experiments Method 1: Accelerator Experiments P µe sin 2 2θ 13 sin 2 2θ 23 sin 2 Δm 31 2 L 4E ν +... p target horn π + decay pipe π + µ + absorber detector # appearance experiment ν µ ν e # measurement of ν µ ν e and ν µ ν e yields θ 13,δ CP # baseline O( km), matter effects present Method 2: Reactor Neutrino Oscillation Experiment # # P ee 1 sin 2 2θ 13 sin 2 Δm 31 2 L 4E ν Δm cos 4 θ 13 sin 2 2θ 12 sin E ν disappearance experiment ν e ν e look for rate deviations from 1/r 2 and spectral distortions observation of oscillation signature with 2 or multiple detectors baseline O(1 km), no matter effects 2 L ν e ν e ν e

8 Oscillation Experiments with Reactors experiments look for non-1/r 2 behavior of antineutrino interaction rate Δm 2 P ee 1 sin 2 2θ 13 sin 2 31 L Δm cos 4 θ 13 sin 2 2θ 12 sin E ν 4E ν 2 L for 3 active neutrinos, can study oscillation with two different oscillation length scales: Δm 2 12, Δm 2 13 Δm 2 12 ~ 8 x 10-5 ev 2 Δm 2 13 ~ 2.5 x 10-3 ev 2 L ~ 1.8 km L ~ 60 km what about reactor appearance experiments? Mean antineutrino energy is 3.6 MeV. Only disappearance experiments are possible.

9 Oscillation Searches with Reactor Antineutrinos ν e from n-rich fission products detection via inverse beta decay Measure flux and energy spectrum Chooz ~3000 events 335 days thermal power 8.5 GW ν e ν e 1 km baseline ν e νe νe ν e 5 ton target ν e + p e + + n No evidence for oscillation, absolute measurement with 1 detector

10 Measuring Reactor Antineutrinos in Japan Japanese Reactors Reactor Antineutrinos Kashiwazaki Takahama Ohi Japan Kamioka 235 U: 238 U: 239 Pu: 241 Pu = 0.570: 0.078: : reactors ~ 200 MeV per fission ~ 6 ν e per fission ~ 2 x ν e /GW th -sec reactor ν flux at KamLAND ~ 6 x 10 6 /cm 2 /sec

11 KamLAND 2003: First Direct Evidence for Reactor ν e Disappearance Reactor Neutrino Physics PRL 90: (2003) Observed ν e 54 events No-Oscillation 86.8 ± 5.6 events Background 1 ± 1 events Livetime: ton-yr Japan Thermal Power Flux (µw/cm 2 ) mean, flux-weighted reactor distance ~ 180km Many reactors, far away Distance (km) Survival Evis >2.6 MeV

12 KamLAND 2008: Precision Measurement of Oscillation Prompt event energy spectrum for νe number of events expected: 2179 ± 89 (syst) observed: 1609 bkgd: 276 ± 23.5 systematic uncertainties: fiducial volume reduced from 4.7% 1.8% total systematics: 4.1% (and mainly affecting θ ) is 4.1%. significance of disappearance (with 2.6 MeV threshold): 8.5σ no-osc χ 2 /ndf=63.9/17 significance of distortion: > 5σ best-fit χ 2 /ndf=21/16 (18% C.L.) Detector-related (%) Reactor-related (%) m 2 21 Energy scale 1.9 ν e -spectra [7] 0.6 Event rate Fiducial volume 1.8 ν e -spectra 2.4 Energy threshold 1.5 Reactor power 2.1 Efficiency 0.6 Fuel composition 1.0 Cross section 0.2 Long-lived nuclei 0.3

13 KamLAND 2008: Precision Measurement of Oscillation L/E Dependence 1 Data - BG - Geo & e Expectation based on osci. parameters determined by KamLAND L0=180km L 0 /E &e (km/mev) Phys.Rev.Lett.100:221803,2008 KamLAND+solar (combined under assumption of CPT invariance) tan 2 Θ= Δm 2 =7.59 x10-5 ev 2

14 1! Precision Measurement of Oscillation Parameters Neutrino Mass Splitting KamLAND 2008 ) 2 (ev KamLAND 95% C.L. 99% C.L % C.L. best fit $m 2 21 Solar 95% C.L. 99% C.L % C.L. best fit tan 2 " 12 normal inverted KamLAND provides most precise value of Δm12 2 (~2.8%)

15 Precision Measurement of Oscillation Parameters Neutrino Mixing Angles Δm 2 SNO solar Δm 2 SK atmospheric U e1 U e2 U e U e 3 U = U µ1 U µ2 U µ 3 = U τ1 U τ 2 U τ U MNSP Matrix tan 2 θ12 Maki, Nakagawa, Sakata, Pontecorvo cosθ 13 0 e iδ CP sinθ 13 cosθ 12 sinθ = 0 cosθ 23 sinθ sinθ 12 cosθ sinθ 23 cosθ 23 e iδ CP sinθ 13 0 cosθ 0 e iα / 2 0 iα / 2+iβ e atmospheric, K2K reactor and accelerator SNO, solar SK, KamLAND 0νββ θ 23 = ~ 45 θ 13 =? maximal? small? zero? θ 12 ~ 32 large, but not maximal! what symmetry explains this?

16 Karsten Heeger, Univ. of Wisconsin NUSS, July 13, May 2003 S. Glashow

17 Hints from Global Fits current best limit sin 2 2θ13 < CL Fogli, et al., arxiv:0905:3549 Also: A. B. Balantekin and D. Yilmaz, Karsten Heeger, Univ. of Wisconsin J. Phys. G 35, (2008) Penn State, July 1, 2010 sin 2 2θ 13 ~ ?

18 Precision Measurement of θ 13 with Reactor Antineutrinos Search for θ 13 in new oscillation experiment with multiple detectors Δm 2 P ee 1 sin 2 2θ 13 sin 2 31 L Δm cos 4 θ 13 sin 2 2θ 12 sin E ν 4E ν 2 L Daya Bay Reactors: Powerful ν e source, multiple cores 11.6 GW th now,17.4 GW th in 2011 Small-amplitude oscillation due to θ 13 integrated over E θ 13 Large-amplitude oscillation due to θ 12 ~1-1.8 km > 0.1 km νe N osc /N no_osc Δm 2 13 Δm 2 23 detector 1 detector Baseline (km)

19 Concept of Reactor θ13 Experiments Measure ratio of interaction rates in multiple detectors νe near distance L ~ 1.5 km far Measured Ratio of Detector Mass Ratio, H/C Detector Efficiency Ratio Rates sin 2 2θ 13 mass measurement calibration

20 Reactor θ 13 Experiment at Krasnoyarsk, Russia Original Idea: First proposed at Neutrino2000 Krasnoyarsk - underground reactor - detector locations determined by infrastructure ~1.5 x 10 6 ev/year ~20000 ev/year Reactor Ref: Marteyamov et al, hep-ex/

21 World of Proposed Reactor θ13 Neutrino Experiments Diablo Canyon, USA Braidwood, USA Chooz, France Krasnoyasrk, Russia Kashiwazaki, Japan RENO, Korea Daya Bay, China Angra, Brazil Daya Bay, Double Chooz, and Reno - international collaborations - started construction Angra - R&D - nuclear proliferation studies Daya Bay - most precise experiment - only experiment to reach sin 2 2θ13 < 0.01

22 Daya Bay, China experimental hall PMTs antineutrino detectors RPCs water pool Karsten Heeger, Univ. of Wisconsin Penn State, July 1, 2010 muon veto system multiple detectors per site cross-check efficiency

23 Daya Bay Antineutrino Detectors 8 identical, 3-zone detectors no position reconstruction, no fiducial cut calibration system liquid scintillator mineral oil ν e + p e + + n Gd-doped liquid scintillator steel tank acrylic tanks photomultipliers target mass: 20t per detector detector mass: ~ 110t photosensors: 192 PMTs energy resolution: 12%/ E

24 Antineutrino Detection Signal and Event Rates ν e + p e + + n 0.3 b + p D + γ (2.2 MeV) (delayed) Daya Bay near site 840 Ling Ao near site 760 Far site## 90 events/day per 20 ton module 49,000 b + Gd Gd* Gd + γʼs (8 MeV) (delayed) Prompt Energy Signal Delayed Energy Signal 1 MeV 6 MeV 10 MeV

25 Systematic Uncertainties Detector-Related Uncertainties Absolute measurement Relative measurement O( %) precision for relative measurement between detectors at near and far sites Ref: Daya Bay TDR

26 Antineutrino Detector Assembly Jan 2010 Karsten Heeger, Univ. of Wisconsin Penn State, July 1, 2010

27 Expected Precision and Sensitivity of Daya Bay Daya Bay Sensitivity to sin 2 2θ13 sin 2 2θ13 < 90% CL in 3 years of data taking 2011 start data taking with near site 2012 start data taking with full experiment

28 Expected Precision and Sensitivity of Daya Bay Expected Precision to νe Flux past reactor experiments = 1 detector past Daya Bay - projected uncertainty next generation of experiments > 2 detectors KamLAND

29 Search for θ13: A Possible Scenario precision measurement of θ13 for unambiguous discovery and combined analysis with T2K and NOvA GLOBES 2009 NoVA could provide unique sensitivity to mass hierarchy in next decade precision measurement to sin 2 2θ13 < 0.01 by Daya Bay first hint of θ13 by Double Chooz and T2K possible if θ13 large early measurement of θ13 will help make decision on future long-baseline experiments T2K may see early signature of νe appearance if θ13 0 Ref: Huber et al.

30 Reactor Neutrino Physics at Intermediate Baselines Precision Measurement of θ 12? P(ν e ν e ) 1-sin 2 (2θ 12 )sin 2 (Δm 2 21 L/4E) Bandyopadhyay et al., Phys. Rev. D67 (2003) Δm 2 atm Δm 2 sol Minakata et al., hep-ph/ Bandyopadhyay et al., hep-ph/ GW kt y exposure at km requires kt-sized detectors ~4% systematic error from near detector sin 2 (θ 12 ) measured with ~2% uncertainty

31 Reactor Neutrino Physics at Intermediate Baselines θ 13 and Mass Hierarchy? High-frequency amplitude in energy spectrum is θ 13 In L/E plot, a purely sinusoidal component Invites the use of Fourier Transform for analysis intermediate baseline ~ 60km good energy resolution required Fourier Power ev 2 peak due to nonzero θ 13 Learned et al., hep-ex/ Zhan et al size of peak proportional to θ 13 asymmetry encodes hierarchy

32 Reactor Neutrino Physics at Intermediate Baselines θ 13 and Mass Hierarchy? - analysis with Fourier transforms (sine, cosine) and Fourier power spectrum - mass hierarchy can be discriminated for sin 2 (2θ13) > Zhan et al

33 Intermediate/Long Baselines Longer baselines for reactor antineutrino flux measurements require - large νe source - large detectors - deep experimental site Ocean-based detectors: Adjustable baseline Ability to avoid reactor background in the geoneutrino studies Unique sensitivity to mantle geo-neutrinos Additional physics measurements achievable to higher precision, due to large size > 10kt LS detector Hanohano project

34 Summary and Conclusions Non-accelerator experiments were key in discovering neutrino mass and oscillations in the past decade ( ). Reactor experiments have made key contributions: KamLAND discovered reactor νe oscillation and has made most precise measurement of Δm 2 12 to < 3% Reactor experiments RENO, Double-CHOOZ, Daya Bay will study θ 13 during , with Daya Bay reaching sin 2 2θ 13 <0.01. The measurement of sin 2 2θ13 > 0.01 is a prerequisite for the search of leptonic CPV. Future intermediate/long-baseline reactor antineutrino experiments may be used for a precision measurement of θ12 (using baseline from Δm 2 12= Δm 2 sol). Determination of mass hierarchy with kt-size detectors is being explored.

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