Neutrino mass hierarchy and  13 Determination by Remote Detection of Reactor Antineutrinos

1. Neutrino mass hierarchy and 13 Determination by Remote Detection of Reactor Antineutrinos Mikhail Batygov, On behalf of UH Hanohano group, September 14, Sendai TAUP 2007

2. Outline • Neutrino oscillation parameters • Current knowledge • Parameters to be estimated at higher accuracy • Methods • Requirements and the Hanohano project • Other physics goals • Current status and conclusion

3. Oscillation Parameters: present • KamLAND (with SNO) analysis: tan2(θ12)=0.40(+0.10/–0.07) Δm221=(7.9+0.4/-0.35)×10-5 eV2 Araki et al., Phys. Rev. Lett. 94 (2005) 081801. (UPDATED: talk by I. Shimizu at this conference) • SuperK and K2K: Δm231=(2.5±0.5)×10-3 eV2 Ashie et al., Phys. Rev. D64 (2005) 112005 Aliu et al., Phys. Rev. Lett. 94 (2005) 081802 • CHOOZ limit: sin2(2θ13) ≤ 0.20 Apollonio et al., Eur. Phys. J. C27 (2003) 331-374.

4. Oscillation parameters to be measured 2 mass diffs, 3 angles, 1 CP phase • Precision measurement of mixing parameters needed • World effort to determine θ13 (=θ31) • Determination of mass hierarchy

5. 3- mixing Pee=1-{ cos4(θ13) sin2(2θ12) [1-cos(Δm212L/2E)] + cos2(θ12) sin2(2θ13) [1-cos(Δm213L/2E)] + sin2(θ12) sin2(2θ13) [1-cos(Δm223L/2E)]}/2 • Survival probability: 3 oscillating terms each cycling in L/E space (~t) with own “periodicity” (Δm2~ω) • Amplitude ratios ~13.5 : 2.5 : 1.0 • Oscillation lengths ~110 km (Δm212) and ~4 km (Δm213~Δm223) at reactor peak ~3.5 MeV Two possible approaches: • ½-cycle measurements can yield • Mixing angles, mass-squared differences • Less statistical uncertainty for same parameter and detector • Multi-cycle measurements can yield • Mixing angles, precise mass-squared differences • Mass hierarchy • Less sensitivity to systematic errors

6. 12 precise measurement • Reactor experiment- νe point source • P(νe→νe)≈1-sin2(2θ12)sin2(Δm221L/4E) • 60 GW·kt·y exposure at 50-70 km • ~4% systematic error from near detector • sin2(θ12) measured with ~2% uncertainty Ideal spot Bandyopadhyay et al., Phys. Rev. D67 (2003) 113011. Minakata et al., hep-ph/0407326 Bandyopadhyay et al., hep-ph/0410283

7. 3-flavor oscillations • “High-frequency” amplitude is 13 • In L/E plot, a purely sinusoidal factor • Invites the use of Fourier Transform for analysis

8. Fourier Transformed Spectrum • The size of the peak proportional to 13. • The peak’s asymmetry tells about hierarchy • Method developed at UH Δm232 < Δm231 normalhierarchy 0.0025 eV2 peak due to nonzero θ13 Preliminary- 50 kt-y exposure at 50 km range sin2(2θ13)≥0.02 Δm231=0.0025 eV2 to 1% level Learned, Dye,Pakvasa, Svoboda hep-ex/0612022 Includes energy smearing

9. Hierarchy Discrimination • Uses the difference in spectra • Efficiency depends heavily on energy resolution Perfect E resolution E = 6%*sqrt(Evis) E, MeV E, MeV

10. Estimation of the statistical significance • Thousands of events necessary for reliable discrimination, even at 1  CL • Longer baselines more sensitive to energy resolution; may be beneficial to adjust for actual detector performance Neutrino events to 1  CL < 3%: desirable but maybe unrealistic E resolution KamLAND: 0.065 MeV0.5 Detector energy resolution, MeV0.5

11. Additional goal: neutrino geophysics • Antineutrinos produced in -decays of 232Th and 238U decay series isotopes • A substantial (but not known precisely) part of Earth heat flux of 40 (31) TW • In continent-based detectors, flux dominated by continental crust • Ocean-based detectors allow to measure geo-neutrino flux from mantle

12. Requirements • Baseline on the order of 50 km; better variable for different studies • Big number of events (large detector) • For Hierarchy: • Good to excellent energy resolution • sin2(213)  0 • No full or nearly full mixing in 12 (almost assured by SNO and KamLAND) • For Geo-neutrinos: ability to “switch off” reactor background

13. MeV-Scale Electron Anti-Neutrino Detection Key: 2 flashes, close in space and time, 2nd of known energy, eliminate background Production in reactors and natural decays Detection Evis=Eν-0.8 MeV prompt delayed Evis=2.2 MeV • Standard inverse β-decay coincidence • Eν > 1.8 MeV • Rate and precise spectrum but no direction Reines & Cowan

14. Hanohano detector • 10-kt LS detector • Primary detection method: inverse-beta decay • Ocean-based, with 2 key advantages: • Adjustable baseline • Ability to avoid reactor background in the geo-neutrino studies Barge 112 m long x 23.3 wide

15. Additional Physics/AstrophysicsHanohano will be biggest low energy neutrino detector • Nucleon Decay: (SUSY-favored kaon modes may be also possible) • Supernova Detection: special νe ability • Relic SN Neutrinos • GRBs and other rare impulsive sources • Long list of ancillary, non-interfering science, with strong discovery potential

16. Current status • Several workshops held (’04, ’05, ’06) and ideas developed • Study funds provided preliminary engineering and physics feasibility report (11/06) • Strongly growing interest in geology community • Work proceeding and collaboration in formation • Upcoming workshops in Washington DC (10/07) and Paris (12/07) for reactor monitoring • Funding request for next stage (’06) in motion • Ancillary proposals and computer studies continue

17. Summary • Better precision for sin2(212) and sin2(213) along with the determination of hierarchy possible for reactor-based antineutrino experiment • Variable baseline desirable • particular measurements require individual tuning • optimal placement dependent on unknown parameters • minimize systematic errors (esp. in energy scale) • Needs large statistics → big detector • Requires precise e energy measurement Hanohano designed to meet those goals and also provides: • Unique sensitivity to mantle geo-neutrinos • Ability to avoid reactor background when needed • Additional physics measurements achievable to higher precision