Present status of oscillation studies by atmospheric neutrino experiments

1. NuFact02, July 2002, London Status and prospects of neutrino oscillations: Atmospheric • Present status of oscillation studies by atmospheric neutrino experiments νμ→ντ 2 flavor oscillations 3 flavor analysis Non-standard explanations Search for CC ντ events • Future prospects Possible detectors Physics • Summary Takaaki Kajita ( ICRR, Univ. of Tokyo )

2. Present status of oscillation studies by atmospheric neutrino experiments Soudan-2 Super-Kamiokande MACRO

3. Present status of atmospheric neutrino experiments Plastic container Soudan-2 : stopped data taking. Top Side MACRO Super-Kamiokande

4. New (almost final) data from Soudan-2 • 5.9 kton・yr exposure • Partially contained events included. • L/E analysis with a “high resolution” sample • Total number of events: 403.6 (high resolution sample: 245.5 events, PC: 39.0) Zenith angle L/E distribution e μ Up-going Down-going

5. (Final) MACRO data νμ→ντ Δm2 = 2.5×10-3 Consistent with oscillation. L/E analysis with momentum measurement is also consistent with osc. or

6. Super-Kamiokande data 1489day FC+PC data + 1678day upward going muon data 1-ring e-like 1-ring μ-like multi-ring μ-like up-going μ • Whole SK-1 data have been analyzed. stopping < 1.3GeV No osc. Through going Osc. > 1.3GeV Up-goingDown-going

7. νμ→ντ oscillation results Kamiokande Soudan-2 Super-K sin22θ> 0.92 Δm2=(1.6 – 3.9)×10-3eV2 MACRO

8. 3 flavor analysis Matter effect ! ●Assumption / Approximation mν3 mν2 mν1 2 Δm12=0 2 2 2 Δm13 = Δm23 =Δm Δm , θ13, θ23 2

9. Allowed parameter region (3 flavor, 1 mass scale dominance, normal mass hierarchy) Super-K 99%CL Pure, maximal νμ→ντ 90%CL No evidence for non-zero θ13. Consistent with reactor exp.

10. Oscillation to sterile neutrinos? Pureνμ→νsoscillation: (1) NC deficit &(2) Matter effect High E. PC Super-K (1) NC deficit (2) Matter effect NC enriched multi-ring events Vertical / Horizontal ratio (through going μ) Super-K 79ktyr MACRO Through going μ νμ→ντ νμ→νs νμ→νsis disfavored > 99%.

11. Oscillation to sterile neutrinos? nmg cosxnt + sinxn s • Use all the SK data (including NC, up-through-going-muons and High-E PC). • . pure nmgnt pure nmgns sin2x

12. Neutrino decay ? ★Scenario (V.Barger et al., PLB 462 (1999) 109): νμ=cosθν2 +sinθν3 decay X For Δm2→0; P(ν→ν) = (cos2θ+ sin2θe-αL/2E)2 decay Oscillation α=m/τ Log10[L/E(km/GeV)] Decay scenario can explain the CC data well. c2min=141.5/152 dof @sin2q = 0.33 m3/t3=1.0x10-2GeV/km

13. Neutrino decay vs. NC data Allowed and excluded parameter regions FC multi-ring NC enriched sample • NC data should also decrease due to decay into sterile state. Excluded (by NC data) Allowed (by CC data) Use Up/Down to test decay scenario The 99%CL allowed region by FC 1-ring+PC+up-m samples is almost excluded at 99%CL by the NC enriched sample.

14. Search for CC ντ events CC ντ events Only ～ 1.0 CC ντ FC events/kton・yr (BG (other ν events) ～ 130 ev./kton・yr) ντ τ ντ hadrons ● Many hadrons ．．．． (But no big difference with other events ．) BADτ- likelihood analysis ● Upward going only GOOD Zenith angle

15. Tau likelihood analysis • total energy • number of rings • number of decay electrons • max(Ei)/ΣEi • distance between n interaction point and decay-e point • max(Pm) • Pt/Evis3/4 • PID likelihood of most energetic ring • Selection Criteria • multi-GeV, multi-ring • most energetic ring is e-like • log(likelihood) > 0 (multi-ring) > 1 (single-ring) Multi-ring Down-ward Multi-ring Up-ward BG MC t+BG MC τ-like τ-like

16. Tau analysis results Independent analysis by Neural Network Max. likelihood analysis t+BG B.G. Nτ= 145±44+11/-16 Nτ= 99±39+13/-21 Nτexpected=86 Consistent with νμ→ντ.

17. Future atmospheric neutrino experiments Topics ★ Really “oscillation”? ★ How accurate can sin2θ23 and Δm23 be determined ? ★ Is θ13 measurable ? ★ Sign of Δm2 ? 2 2

18. Possible future atmospheric neutrino detectors Magnetized large tracking detector (MONOLITH, ….) Very large water Cherenkov detector (UNO, Hyper-Kamiokande, …..)

19. Really oscillation ? Use up-going events ⇒ L = 2Rcosθz Large L ⇒ Need to measure high-energy events Very large detector Magnetized detector 0.14 Mton・yr (MONOLITH) 2.8 Mton・yr (UNO) Assume; Δm2=2×10-3eV2

20. Super-K may not be too small….. Use only high L/E resolution events 70 year MC (1.6Mtonyr) First osc. mim.

21. Accuracy of sin22θ measurement Standard SK analysis with the present SK systematics 90%C.L. 0.11 Mton・yr 0.23 0.9 Up sin22θ Down 2 = 1 - +ε Systematic error related to Up/Down is small (2% @SK) Precise determination ofsin22θ 3% Exposre(Mtonyr) δ (sin22θ) = 90%

22. Accuracy of Δm2 measurement L/E analysis Magnetized tracking detector 0.14 Mton・yr (MONOLITH) 0.14 Mton・yr First minimum Δm2 δ(Δm2) = 6%

23. Measurement of θ13 ? Matter effect !

24. Measurement of θ13 ? Large water Ch. detector (e-like)osc (e-like)no-osc Matter effect ! 0.9 Mton・yr Water Ch. 2 Down 2 sinθ13=0.026 CosΘ cosΘ < －0.2 (up going) Up 1 1 10 1 10 Reconstructed momentum (GeV/c) Reconstructed momentum (GeV/c) ～4σ effect in 0.9 Mton・yr

25. 2 Measurement of θ13 and sign of Δm ? Matter effect Charge identification(Magnetized tracking detector needed) Δm2=2.5×10-3 sin2θ =0.02 13 Determination of sign of Δm2 at 90%CL.

26. Summary Present status • All the data are consistent with pure νμ→ντ oscillations. • No evidence for θ13. • No evidence for physics beyond standard neutrino osci. • Hint of τ appearance. Future prospects • If much larger detectors and/or magnetized tracking detectors are constructed, our understanding of neutrino masses and mixing will be improved significantly: L/E, determination of oscillation parameters (23), θ13, sign of Δm , …. sin22θ > 0.92 Δm2 = (1.6 – 3.9)×10-3 eV2 (SK, 90%CL) 2