HQL 2004 Puerto Rico MayJune 2004 Takaaki Kajita

HQL 2004, Puerto Rico, May-June. 2004 Takaaki Kajita, ICRR, Univ. of Tokyo

Outline • Atmospheric neutrino beam • Atmospheric neutrinos: Past • Atmospheric neutrinos: Present L/E analysis • Atmospheric neutrinos: Future sub-dominant oscillations ? • Summary

Atmospheric neutrinos Cosmic Ray p, K Atmosphere μ e e e Neutrinos from the other side of the Earth.

Atmospheric neutrino beam Measured cosmic ray proton flux Zenith angle: Flux × En 2 Total nm+nm flux En(Ge. V) nm ne

Event classification Fully Contained (FC) (E ~1 Ge. V) Partially Contained (PC) (E ~10 Ge. V) Stopping (E ~10 Ge. V) Through-going (E ~100 Ge. V)

• 1988: First serious study of the atmospheric neutrino flavor ratio (Kamiokande) • 1991, 92: Confirmation of the atmospheric neutrino problem (IMB) • 1994: Indication for the zenith angle dependent deficit of atmospheric nm (Kamiokande). • 1998: Evidence for oscillation (Super. Kamiokande) Kam. 1988 Data MC e-like (CC ne) 93 88. 5 m-like (CC nm) 85 144. 0 e-like m-like Kam. 1994 SK, 1998 No osc. nm nt osc.

Super-Kamiokande (50, 000 ton water Ch. Detector) 76 m 9. 3 m Soudan-2 (1 kton tracking detector) 12 m MACRO (large muon detector)

SK atmospheric neutrino data 1489 day FC+PC data + 1646 day upward going muon data 1 -ring e-like 1 -ring m-like multi-ring m-like up-going m stopping < 1. 3 Ge. V No osc. Osc. > 1. 3 Ge. V Up-going Down-going Through going

Soudan 2 • 5. 9 kton・yr exposure • Partially contained events included. • L/E analysis with the “high resolution” sample Reconstructed Lν/ Eν dist. Phys. Rev. D 68 (2003) 113004 Zenith angle e Upgoing μ Downgoing e μ No osc. nm nt osc.

MACRO PLB 566 (2003) 35 Multiple scattering Oscillation Em Δm 2 =2. 5× 10 -3 En L /E or Upward horizontal

Neutrino oscillation parameters (90%CL) nm→nt Soudan-2 Kamiokande Super-K MACRO (Prelim. 90%CL) (Note: the final SK-1 nm nt oscillation results will be presented in 10 days. )

Ne w n oscillation SK collab. hep -ex/0404034 ! -like multi-Ge. V + PC n decoherence n decay Should observe this dip! Further evidence for oscillations Strong constraint on oscillation parameters, especially Dm 2

Selection criteria m Full osc. Half osc. m Select events with high L/E resolution (D(L/E) < 70%) Following events are not used: ★horizontally going events ★low energy events

L/E distribution 1489 days FC+PC (Super-K) Mostly up -going MC (no osc. ) Mostly down -going Best fit expectation w/ systematic errors First dip is observed as expected from neutrino oscillation

Allowed neutrino oscillation parameters 1. 9 x 10 -3 < Dm 232 < 3. 0 x 10 -3 e. V 2 0. 90 < sin 22 q 23 (90% C. L. ) c 2 min=37. 9/40 d. o. f @ Dm 2=2. 4 x 10 -3, sin 22 q=1. 00 (sin 22 q=1. 02, c 2 min=37. 8/40 d. o. f) 90% allowed regions Soudan 2 K 2 K L/E analysis MACRO Zenith angle analysis Consistent with that of the standard zenith angle analysis

Neutrino decay and decoherence models ? Oscillation Decay Decoherence c 2 min=37. 9/40 d. o. f c 2 min=49. 1/40 d. o. f Dc 2 =11. 3 c 2 min=52. 4/40 d. o. f Dc 2 =14. 5 n decay disfavored at 3. 4 s n decoherence at 3. 8 s First dip observed in the data cannot be explained by alternative hypotheses Evidence for oscillatory signature

Present: Study of dominant oscillation channel (nm nt) Future: Study of sub-dominant oscillations ★q 13? ★Mass hierarchy? ne nm ν mass ★Solar oscillation effects? nt n 3 n 2 n 1 Normal mass hierarchy is assumed.

Possible future atmospheric n detectors Very large water Cherenkov detector Hyper-K (1 Mton) Magnetized large tracking detector MONOLITH, INO (Indian Neutrino Observatory, …

Search for non-zero q 13 (Dm 122=0 assumed) M at te re ffe ct MC, SK 20 yrs 1+multi-ring, e-like, 2. 5 - 5 Ge. V SK Present result (Note: will be updated in 10 days) cos. Q Electron appearance s 213=0. 05 s 213=0. 00 null oscillation En(Ge. V) cos. Q Electron appearance in the 5 – 10 Ge. V upward going events. (And stronger muon disappearance in the 5 – 10 Ge. V upward going events. )

Sensitivity for non-zero q 13 Importance of s 2 q 23>0. 5; S. Pascoli et al. , hepph/0305152 Water Cherenkov detector 450 kton・yr (SK 20 years) 3 s 3 s ～ Present bound on sin 2 q 13 (Dc 2 ∝～ exposure) 3 s

Sign of Dm 23(13)2 ? How can we discriminate positive and negative Dm 2 ? Real Dm 232 = positive assumed Real Dm 232 = negative assumed n 2 n 1 P(nm ne) cos. Q P(nm ne) n 3 En(Ge. V) (No resonance for anti-neutrinos) En(Ge. V) (No resonance for neutrinos) n 2 n 1 n 3

2 Measurement of θ 13 and sign of Δm ? Matter effect Δm 2=2. 5× 10 -3 sin 2θ 13=0. 02 Charge identification (Magnetized tracking detector) Determination of sign of Δm 2 at 90%CL. NPB (proc suppl) 91 (2001) 147, hep-ex/0106252

Electron appearance for positive and negative Dm 2 in a water Chrenkov detector Single-ring e-like Relatively high anti-ne fraction Dm 2=0. 002 e. V 2 s 2 q 23 = 0. 5 s 2 q 13 = 0. 05 (SK 20 yrs) Multi-ring e-like Lower anti-ne fraction Positive Dm 2 Negative Dm 2 null oscillation cos. Q

c 2 difference (inverted-normal) True= normal mass hierarchy assumed. Dm 2: fixed, q 23: free, q 13: free Exposure: 1. 8 Mtonyr 80 yr or HK ～ 3. 3 yr) 3 s 3 s (SK 3 s

Solar oscillation effects Solar neutrino oscillation: LMA (Dm 122 = 7× 10 -5 e. V 2) Expected number of sub-Ge. V e-like events in SK. Peres, Smirnov NPB 680 (2004) 479 10 1 P. Lipari NOON 2004 The number of e-like events changes as a function of sin 2 q 23 (NOT sin 22 q 23). Discrimination of >45 and <45 q 23 might be possible.

• Atmospheric neutrinos have been playing major role in the neutrino oscillation studies. • The present data are nicely explained by nm nt oscillations with; Dm 2=1. 9 – 3. 0 × 10 -3 e. V 2 sin 22 q > 0. 90 • Recent L/E analysis has shown evidence for “oscillatory” signature. • Future atmospheric neutrino experiments is likely to continue to contribute to the neutrino oscillation physics (q 13, sign of Dm 232 …. ) (If (a) much larger detector, (b) relatively large q 13. )

End

Specials in L/E analysis 1. 5 m from top & bottom FC single-ring, multi-ring -like Expand fiducial volume More statistics for high energy muons 1 m from barrel 22. 5 kt → 26. 4 kt observed charge / expectation from through-going PC Classify PC events using OD charge I. OD stopping II. OD through going Different L/E resolution OD stopping MC OD stopping OD throughgoing MC

Energy and angular resolution of neutrinos

Resolution of L/E and cuts

Sensitivity to other models (determination of L/E resolution cut) 70% 80% n decoherence n decay L/E resolution cut at 70%

Event summary of L/E analysis Fractions of FC and PC samples in L/E distribution MC CC nm FC Data single-ring 1619 2105. 8 (98. 3%) multi-ring 502 813. 0 (94. 2%) PC stopping 114 137. 0 (95. 4%) through-going 491 670. 4 (99. 1%)

Check of the observed dip in L/E distribution (1) Other L/E resolution cuts

Check of the observed dip in L/E distribution (2) FC e-like (Flat L/E distribution is expected. )

Check of the observed dip in L/E distribution (3) zenith angle cosq -cosq (Zenith angle of each event is inverted. Because of the wrong assignment of L, no dip is expected. )

Sensitivities to alternative models and the data n decoherence obtained Dc 2 n decay L/E resolution cut at 70%

How can we discriminate neutrino and antineutrino interactions ? Simple answer: No. It is not possible to discriminate event by event in water Cherenkov experiments. However, s(total) and ds/dy are different. Single-ring e-like Multi-ring e-like Others CC ne Try to discriminate positive and negative Dm 2 using these events.

c 2 difference (normal – inverted) True= inverted mass hierarchy assumed. Dm 2: fixed, q 23: free, q 13: free Exposure: 1. 8 Mtonyr (SK 80 yr or HK ～ 3. 3 yr) 3 s 3 s 3 s