Low energy astrophysical neutrino observations with megaton class detector ｓ

1. NNN05, April 7, 2005 Low energy astrophysical neutrino observations with megaton class detectorｓ M.Nakahata Kamioka observatory, ICRR, Univ. of Tokyo 8B solar neutrino measurement Supernova burst neutrino observation Supernova relic neutrino observation Conclusions

2. Current status of solar neutrino oscillations Expected P(ne ne) at best fit Vacuum osc. dominant P(ne ne) 99.73% Solar+KamLAND matter osc. 95% KamLAND (MeV) SSM spectrum pp Solar global 7Be pep 8B

3. Expected low energy upturn (8B) SK (ne scattering) SNO CC Dm2=8.3x10-5eV2 Dm2=8.3x10-5eV2 sin2(q) 0.22 0.28 0.35 Arbitrary unit sin2(q)=0.28 sin2(q)=0.28 Dm2 (eV2) 9.1 x 10-5 8.3 x 10-5 7.4 x 10-5 Arbitrary unit Total energy ~20% in SNO and ~10% in SK upturn is expected from 4 MeV to 15 MeV.

4. SNO pure D2O Measurements by SNO and SK so far SNO salt phase kinetic energy No spectrum distortion so far. Current SK and SNO measurements are limited by statistics and systematic. SK spectrum sin2q=0.28, Dm2 =8.3×10-5 eV2 Need more statistics, lower threshold and make systematic errors smaller to test spectrum distortion. Total energy

5. Expected solar n spectrum measurement by Mega-ton water cherenkov detector 8B spectrum distortion sin2q=0.28, Dm2 =8.3×10-5 eV2 Enough statistics to see distortion. Energy scale calibration should be better than ~0.3%. Correlated sys. error of SK 1/2 of SK Data/SSM 5 Mton·years (*)For the statistical error, SK background level above 5.5MeV and 70% reduction below 5.5 MeV are assumed. Ee (MeV) hep neutrino SSM(BP2004) flux: hep: 7.88(1±0.16)x103 /cm2/sec = ~1/700 of 8B Integral spectrum Statistically possible to measure hep neutrinos. Precise calibration of energy resolution is necessary. 5 Mton·years

6. 8B -- Day-Night effect (Night-Day) ANＤ= X100(%) (Night+Day)/2 SK SNO • Not yet convincingly seen either in SK nor SNO 2~3% effect 1~2% effect Solar+KamLAND Solar+KamLAND (pure D2O) Observations (salt phase)

7. Day/night measurement by mega-ton detector 1 Mton year(0.5 Day & 0.5 Night) for no BG (2.4 Mton year(1.2 Day & 1.2 Night) for SK spallation B.G.) Dm2=7.1x10-5eV2, sin2(q)=0.28 Day/night asymmetry Statistical significance 3s level can be achieved with 0.5% systematic error for this parameter.

8. Sensitivity of Day/Night Asymmetry 1.3% sys. error 0.5% sys. error Stat. only 1 Mton year(0.5 Day & 0.5 Night) for no BG (2.4 Mton year(1.2 Day & 1.2 Night) for SK spallation B.G.) Expected asymmetry sin2(q)=0.30 3s excess above this line Solar+KamLAND (95% CL) Systematic error must be less than 0.5%.

9. Supernova event rate in Mega-ton detector Livermore simulation Expected number of events (T.Totani et al., ApJ.496,216(1998)) 5MeV threshold

10. Time profile with neutrino oscillations νｅ＋ｐ→ｅ++n Total number of events in parentheses SN at 10kpc, 1mega-ton Time variation 200 log bins from 20msec to 18sec ν＋e－→ν＋ｅ－ Inverted hierarchy (PH=0) Nomal, Inv.(PH=1) No oscillation PH: crossing probability at H resonance (PH=0: adiabatic)

11. Neutronization burst (e-+pn+ne) SN at 10kpc, 1mega-ton Number of events from 20msec to 0.1 sec (1bin=10msec) ν＋e－ No oscillation Normal PH=1 or Inverted hierarchy Normal hierarchy PH=0 Neutronization burst can be observed even with neutrino oscillations.

12. neenergy spectrum measurement SN at 10kpc, 1mega-ton Visible energy spectrum in each time range range Time variation of average energy

13. Identification of ne scattering events by direction to supernova ne+p ne+p ne+p ne+p SN at 10kpc, 1 mega-ton n+e n+e ne scattering events can be statistically extracted using the direction to supernova. n+e n+e

14. ne+nx spectrum measurement by ne scattering 10kpc, 1 Mega-ton 56kpc, 1 Mega-ton Spectrum measurement up to ~40MeV. Spectrum measurement up to ~20MeV.

15. Search for supernova relic neutrinos(SRN) Reactor n Population synthesis (Totani et al., 1996) Constant SN rate (Totani et al., 1996) Cosmic gas infall (Malaney, 1997) Cosmic chemical evolution (Hartmann et al., 1997) Heavy metal abundance (Kaplinghat et al., 2000) LMA noscillation (Ando et al., 2002) Solar 8B Solar hep SRN predictions Atmospheric n Spallation B.G.

16. SRN search in SK-I Energy spectrum above 18 MeV 1496 days (SK-I) Total background Atmospheric nm → invisible m → decay e 90% CL limit of SRN Atmospheric ne

17. n SK limit (90% C.L.) SK-I upper limit

18. n Ando’s talk today SK limit (90% C.L.) SK-I upper limit

19. SRN event rate in Mega-ton detector SRN signal:~630 (Ando’s talk today) Relic model: S.Ando, K.Sato, and T.Totani, Astropart.Phys.18, 307(2003). 1 Mega-ton, 5 years SRN signal:246 Background: 2518 in 15-30 MeV Invisible muon BG must be reduced.

20. Possibility 2 n+Gd →~8MeV g DT = several 10th msec (→ M.Vagins’ talk) ne could be identified by delayed coincidence. Possibilities of netagging n g ne Possibility 1 p n+p→d + g 2.2MeV g-ray DT = ~ 200 msec e+ g Nhit=~6 for 40% coverage and 20% peak QE Positron and gamma ray vertices are within ~50cm.

21. Possibility of SRN detection Relic model: S.Ando, K.Sato, and T.Totani, Astropart.Phys.18, 307(2003). Signal yield will be factor ~2.6 larger if today’s Ando’s value is used. 1 Mega-ton, 5 years B.G. reduction by neutron tagging No B.G. reduction Statistically 4.6s excess (Evis > 15 MeV) Assuming 90% of invisible muon B.G. can be reduced by neutron tagging.

22. Conclusions • Precise measurements of 8B solar neutrino spectrum and day/night asymmetry by Mega-ton detectors are still important for further study of solar neutrino oscillation. • Quite high statistics of supernova events is expected for galactic supernova. It enables us to measure • Precise nu_e_bar spectrum and time variation • nu_e and nu_x spectrum measurement by ne scattering • Expected number of SRN event is ~250/5yr/Megaton. Delayed coincidence method to tag nu_e_bar is important to discover SRN neutrinos.