Present status of oscillation studies by atmospheric neutrino experiments ν μ → ν τ 2 flavor oscillations 3 flavor analysis Non-standard explanations Search for CC ν τ events Future prospects Possible detectors Physics Summary NuFact02, July 2002, London Takaaki Kajita ( ICRR, Univ. of Tokyo )

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

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

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

(Final) MACRO data or νμ→ντνμ→ντ Δm 2 = 2.5×10 -3 Consistent with oscillation. L/E analysis with momentum measurement is also consistent with osc.

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

ν μ →ν τ oscillation results Kamiokande Soudan-2 MACRO Super-K sin 2 2θ> 0.92 Δm 2 =(1.6 – 3.9)×10 -3 eV 2

3 flavor analysis ●Assumption / Approximation mν3mν3 mν2mν2 mν1mν1 Δ m 12 =0 2 Δ m 13 = Δ m 23 = Δ m 222 Δ m, θ 13, θ 23 2 Matter effect !

Allowed parameter region Pure, maximal ν μ →ν τ 90%CL 99%CL No evidence for non-zero θ 13. Consistent with reactor exp. Super-K (3 flavor, 1 mass scale dominance, normal mass hierarchy)

Oscillation to sterile neutrinos? Pure ν μ → ν s oscillation: (1) NC deficit & (2) Matter effect NC enriched multi-ring events Super-K 79ktyr High E. PC Through going μ Super-K Vertical / Horizontal ratio (through going μ) MACRO ν μ→ ν τ ν μ→ ν s ν μ → ν s is disfavored > 99%. (1) NC deficit(2) Matter effect

Oscillation to sterile neutrinos? Use all the SK data (including NC, up-through-going-muons and High-E PC)..    cos    sin  s pure     pure     s sin 2 

Neutrino decay ? Decay scenario can explain the CC data well.  2 min =141.5/152 2  = 0.33 m 3 /  3 =1.0x10 -2 GeV/km Oscillation deca y Log 10 [L/E(km/GeV)] ★ Scenario (V.Barger et al., PLB 462 (1999) 109) : ν μ =cos θ ν 2 +sin θ ν 3 decay X For Δm 2 →0; P(ν→ν) = (cos 2 θ+ sin 2 θe -αL/2E ) 2 α=m/τ

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

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

Tau likelihood analysis Multi-ring Down-ward Multi-ring Up-ward BG MC  +BG MC Selection Criteria multi-GeV, multi-ring most energetic ring is e-like log(likelihood) > 0 (multi-ring) > 1 (single-ring) total energy number of rings number of decay electrons max(Ei)/ΣEi distance between  interaction point and decay-e point max(P  ) Pt/Evis 3/4 PID likelihood of most energetic ring τ-like

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

Future atmospheric neutrino experiments ★ Really “oscillation”? ★ How accurate can sin 2θ 23 and Δm 23 be determined ? ★ Is θ 13 measurable ? ★ Sign of Δm 2 ? Topics 22

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

Really oscillation ? Assume; Δm 2 =2×10 -3 eV Mton ・ yr (UNO) 0.14 Mton ・ yr (M ONOLITH ) Use up-going events ⇒ L = 2Rcosθ z Large L ⇒ Need to measure high- energy events Magnetized detector Very large detector

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

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

Accuracy of Δm 2 measurement L/E analysis 0.14 Mton ・ yr 0.14 Mton ・ yr (M ONOLITH ) Magnetized tracking detector First minimum Δ m 2 δ ( Δ m 2 ) = 6%

Measurement of θ 13 ? Matter effect !

Measurement of θ 13 ? Matter effect ! Reconstructed momentum (GeV/c) CosΘ Up Down 2 1 (e-like) osc (e-like) no-osc 1 10 Water Ch. Reconstructed momentum (GeV/c) 0.9 Mton ・ yr cosΘ < － 0.2 (up going) sin θ 13 = ～ 4σ effect in 0.9 Mton ・ yr 1 10 Large water Ch. detector

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

Summary Present status All the data are consistent with pure ν μ → ν τ oscillations. No evidence for θ 13. 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), θ 13, sign of Δm, …. sin 2 2θ > 0.92 Δm 2 = (1.6 – 3.9)×10 -3 eV 2 (SK, 90%CL) 2