Search for Sterile Neutrinos with Reactor Antineutrinos

1. Search for Sterile Neutrinos with Reactor Antineutrinos An Opportunity for NIST?

2. 50 Years of Reactor Neutrino Physics

3. ? Daya Bay Double Chooz Reno Neutrino Physics at Reactors Next - Discovery and precision measurement of θ13 2008 - Precision measurement of Δm122 . Evidence for oscillation 2003 - First observation of reactor antineutrino disappearance 1995 - Nobel Prize to Fred Reines at UC Irvine KamLAND 1980s & 1990s - Reactor neutrino flux measurements in U.S. and Europe Past Reactor Experiments Hanford Savannah River ILL, France Bugey, France Rovno, Russia Goesgen, Switzerland Krasnoyark, Russia Palo Verde Chooz, France 1956- First observation of (anti)neutrinos Chooz Chooz 55 years of liquid scintillator detectors a story of varying baselines... Savannah River

4. inverse beta decay νe + p → e+ + n Discovery of the Neutrino 1956 - “Observation of the Free Antineutrino” by Reines and Cowan

5. Antineutrino Detection inverse beta decay νe + p → e+ + n coincidence signature prompt e+ and delayed neutron capture powerful background suppression technique 10-100 keV 1.805 MeV Eνe ≅ Ee+ + En + (Mn-Mp) + me+ including E from e+ annihilation, Eprompt=Eν - 0.8 MeV

6. νe from n-rich fission products Reactor Antineutrinos observed spectrum threshold: neutrinos with E < 1.8 MeV are not detected calculated reactor spectrum ~ 200 MeV per fission ~ 6 νe per fission ~ 2 x 1020νe/GWth-sec mean energy of νe: 3.6 MeV only disappearance experiments possible cross-section accurate to +/-0.2% time-dependent rate and spectrum only ~ 1.5 νe/fission can be detected rule of thumb: ~ 1 event per day per ton of LS per GWth at 1km

7. νe from n-rich fission products Reactor Antineutrinos time-dependent rate and spectrum observed spectrum threshold: neutrinos with E < 1.8 MeV are not detected calculated reactor spectrum threshold: neutrinos with E < 1.8 MeV are not detected mean energy of νe: 3.6 MeV only disappearance experiments possible only ~ 1.5 νe/fission are detected cross-section accurate to +/-0.2% only ~ 1.5 νe/fission can be detected rule of thumb: ~ 1 event per day per ton of LS per GWth at 1km

8. L/E oscillation effect provides measurement of Δm2 amplitude of oscillation provides measurement of θ Oscillation Experiments with Reactors Looking for non-1/r2 behavior of νe interaction rate for 3 active neutrinos, can study oscillation with two different oscillation length scales: Δm212, Δm213 baseline intermediate short Δm212 ~ 8 x 10-5 eV2 Δm213 ~ 2.5 x 10-3 eV2 L ~ 60 km L ~ 1.8 km Only disappearance experiments are possible since mean antineutrino energy is 3.6 MeV.

9. Oscillation Experiments with Reactors P(νe→νe)≈1-sin2(2θ)sin2(Δm2L/4E) Disappearance at Different Baselines Δm2atm Δm2sol past experiments θ13 θ12 L ~ 1-2 km L ~ 60 km

10. νe νe νe νe νe νe νe Oscillation Searches with Reactor Antineutrinos 1980-1990s Chooz 8.5GW power 1 km baseline ~3000 events 335 days 5 ton target νe + p → e+ + n absolute measurement with 1 detector, σdet ~ 2.7% time with reactors off critical for background studies No evidence for oscillation

11. Japan Kamioka Measuring Reactor Antineutrinos with KamLAND KamLAND Reactors in Japan Kashiwazaki Takahama Ohi νe + p → e+ + n through inverse β-decay 55 reactors 235U:238U:239Pu:241Pu = 0.570: 0.078: 0.0295: 0.057 reactor ν flux at KamLAND ~ 6 x 106/cm2/sec

12. PRL 90:021802 (2003) Observed νe 54 events No-Oscillation 86.8 ± 5.6 events Background 1 ± 1 events Livetime: 162.1 ton-yr KamLAND 2003: Evidence for Reactor νe Disappearance Reactor Neutrino Physics 1956-2003 mean, flux-weighted reactor distance ~ 180km

13. KamLAND 2010: Precision Measurement of Oscillation Prompt Energy Spectrum L/E Dependence L0=180km reactor phase of KamLAND is completed KamLAND → KamLAND-Zen in July 2011 continues to record antineutrino data KamLAND, Phys.Rev. D83 (2011) 052002

14. Neutrino Oscillation Mixing Angles & Mass Splittings UMNSP Matrix Maki, Nakagawa, Sakata, Pontecorvo reactor and accelerator atmospheric, K2K SNO, solar SK, KamLAND 0νββ Schwetz et al arXiv:0808.2016

15. Neutrino Oscillation Mixing Angles UMNSP Matrix Maki, Nakagawa, Sakata, Pontecorvo reactor and accelerator atmospheric, K2K SNO, solar SK, KamLAND 0νββ large, but not maximal! maximal? small? zero? because of small sin22θ13, solar & atmospheric ν oscillations almost decouple

16. νe νe,x νe,x θ13 Δm213≈Δm223 νe/MeV/fisson Neutrino Energy (MeV) Energy (MeV) detector 2 detector 1 Measuring θ13 with Reactor Experiments Near-Far Concept distance L ~ 1.5 km near far

17. νe νe,x νe,x θ13 Δm213≈Δm223 Neutrino Energy (MeV) detector 2 detector 1 Measuring θ13 with Reactor Experiments Near-Far Concept distance L ~ 1.5 km near far νe/MeV/fisson Energy (MeV) a clean measurement of θ13 at optimized distances without parameter degeneracies

18. Reactor Anomaly and 4th Neutrino Hypothesis

19. Reactor evolution codes Reactor data Thermal power, δPth <1% Fraction of fissions from isotope k, δαk=few % Nuclear databases ν spectrum per fission E released per fissions of isotope k, δEk≈0.3% Antineutrino Spectrum Emitted by a Reactor The prediction of reactor ν spectrum is the dominant source of systematic error for single detector reactor neutrino experiments

20. Antineutrino Spectrum Emitted by a Reactor = Σ all fission products

21. Reactor Anomaly Comparison of measured antineutrino fluxes and spectra with predictions. Recent re-evaluation of all reactor neutrino experiments. Average = 0.943 ± 0.023 (χ2=19.6/19) Phys. Rev. D83, 073006, 2011 G. Mention, M. Fechner, T. Lasserre, M. Cribier, Th. Mueller D. Lhuillier, A. Letourneau

22. Reactor Anomaly A Deficit in the Reactor Antineutrino Flux Laserre, HEP2011 Lindner, TAUP2011

23. A Deficit at Short Baselines Oscillation into New Neutrino State at Short Baselines? L ~ 1-2 km L ~ 60 km Δm2atm Δm2sol L ~ 10-100m θ13 θ12

24. 4th Neutrino Hypothesis Combine all rate measurements, no spectral-shape information Fit to anti-νe disappearance hypothesis Absence of oscillations disfavored at 98.6% C.L. allowed excluded area data suggest neutrino with Δm2 ~ 1 eV2

25. Other Laboratory Indications of 4th Neutrino from Y. Wong

26. Indications in Cosmology Current precision cosmological data show a preference for extra relativistic degrees of freedom (beyond 3 neutrinos). from Y. Wong

27. 4th Neutrino Hypothesis What does the hypothesis of 4th (or sterile) neutrino imply? neutrino mass spectrum 3 active neutrinos in color sterile neutrinos in white Δm2 ~ 1 eV2 new mass splitting, Δm2, defined by sterile neutrino

28. Short Baseline Searches for Sterile Neutrinos

29. Short Baseline Searches for Sterile Neutrinos What’s Next? • LSND, MiniBoone, reactor data and indications of Neff>3 in cosmology require definitive laboratory experiment to confirm or refute anomalies • Oscillation signature is critical for unambiguous evidence of 4th neutrino. Oscillation signatures include • baseline dependence of rate • spectral distortion as a signature of energy dependence spectrum oscillatory measurement 3-4m oscillation cycle E (MeV) length (m)

30. Short Baseline Searches for Sterile Neutrinos What’s Next? • LSND, MiniBoone, reactor data and indications of Neff>3 in cosmology require definitive laboratory experiment to confirm or refute anomalies • Oscillation signature is critical for unambiguous evidence of 4th neutrino. Oscillation signatures include • baseline dependence of rate • spectral distortion as a signature of energy dependence • What do we need? • powerful “point source” of antineutrinos (extended source washes out neutrino oscillation effect) • access to range of baselines from source, L ~ O(1-20m) • antineutrino detector at different baselines • background suppression through active and/or passive shielding

31. Baselines and Reactor Core Dimensions • extended neutrino source washes out neutrino oscillation effect From: N. Bowden, LLNL

32. Portable Antineutrino Detectors Antineutrino Detectors for Reactor Monitoring & Non-Proliferation Hawaii Saclay LLNL

33. Search for Sterile ν with Reactor Neutrinos SCRAAM: The Southern California Reactor Antineutrino Anomaly Monitor core ∅: ~3m, baseline: 24m Only one baseline, extended core proposal in preparation Bowden, SNAC 2011

34. Search for Sterile ν with Reactor Neutrinos NUCIFER at Osiris core: σ~0.3m baseline: 7m Only one baseline, small core Lhuillier, AAP2011

35. Search for Sterile ν with Reactor Neutrinos An Opportunity for NIST? - Close access to reactor core. Closest distance to reactor core compared to other experimental efforts. - Relatively compact core. Burnup well-understood. Detailed predictions for antineutrino flux possible. - Reactor-off periods would allow detailed studies of backgrounds. Not possible at San Onofre, or other commercial reactors. - Place m3-sized antineutrino detector at multiple baselines

36. A m3 sized Antineutrino Detector at NIST? May need to add muon veto. For a 1.5-ton Gd-doped liquid scintillator target mass with 40% efficiency, ~ 610 signal events/day at 5m distance.  < 1% stat error in 1 month of data taking.

37. A m3 sized Antineutrino Detector at NIST? May need to add muon veto. L~14m If detector movable over 3-4m, we can map out oscillation cycle. Can build multiple modules at different baselines and map out oscillation 2 6 10 14 distance from core (m)

38. Summary • Long-standing anomalies in accelerator and reactor neutrino experiments suggest possible existence of 4th (sterile) neutrino with Δm2 ~ O(1-2 eV2) • Definitive oscillation experiment needed. For reactor antineutrinos with <E> ~ 3-4 MeV we need baselines of 1-10m. Oscillation length L ~3 m • Significant progress in reactor monitoring with antineutrinos over past decade. Several m3-sized detectors deployed and taking data. Detectors on or near surface under little overburden. • Small but powerful reactor core, close distance to core, and variable baseline key. Reactor-off periods critical for measuring backgrounds. For typical parameters data taking time 1-2 months per baseline. NIST may have unique space for such an experiment.