Takaaki Kajita (ICRR, U.of Tokyo)

1. Production of atmospheric neutrinos • Some early history (Discovery of atmospheric neutrinos, Atmospheric neutrino anomaly) • Discovery of neutrino oscillations • Studies of atmospheric neutrino oscillations • Sub-dominant oscillations –present and future- Atmospheric neutrinos Takaaki Kajita (ICRR, U.of Tokyo)

2. Production of atmospheric neutrinos Atmosphere

4. Total nm+nm flux Flux × En2 En(GeV) Measured cosmic ray proton flux Calculating the atmospheric neutrino beam + geomagnetic field + (p+Nucleon) int. + decay of p or K + ……

5. (nm+ nm)/(ne+ ne) Some features of the beam (1) nm/ne ratio is calculated to an accuracy of better than 3% below ～5GeV.

6. Some features of the beam (2) ＠Kamioka (Japan) Zenith angle cosqzenith Up-going Down Up/down ratio very close to 1.0 and accurately calculated (1% or better) above a few GeV.

7. Comment: Geomagnetic field and the flux Assume a detector in Kamioka (Japan)  Calculate the minimum momentum of a cosmic ray proton directing to Kamioka arriving at the atmosphere. GeV/c Down-going Up-going For this location, flux(up) > flux(down) in the low-energy range

8. Comment: Flux in the horizontal direction now 10 years ago… 1D calculation 3D calculation

9. Horizontal enhancement C.R.(3D) C.R.(1D) ν Target area(1D, 3D) G. Battistoni et al., Astropart. Phys. 12, 315 (2000) 1D 3D

10. How accurate is the absolute normalization of the flux ? Syst error better than 5% Below 10GeV, the flux is predicted to better than 10%. Above 10GeV the flux calculation must be improved. (This statement is for Honda04 flux.)

11. Neutrino interactions Quasi-elastic CC total Deep inelastic 1p production Eν(GeV) Quasi-elastic 1p production Deep inelastic n n lepton lepton n lepton p p p N* N’ N N N N’ N’

12. Event classification Fully Contained (FC) (E ~1GeV) Partially Contained (PC) (E ~10GeV) Stopping  (E~10GeV) Through-going (E~100GeV)

13. Comment: upward-going muons s(nN) ∝En Range ∝Em, <Em> ∝ En Wide energy range

14. Some early history (discovery of atmospheric neutrinos)

15. Discovery of atmospheric neutrinos At the depth of 3200 meters (8800 meters water equivalent) in South Africa First observed on Feb. 23, 1965 By F.Reines et al. At the depth of 2400 meters (7500 meters water equivalent) in India (Kolar Gold Field) First published on Aug. 15, 1965 By C.V. Achar et al. photo of the South Africa experiment (nmNmX) Detector for the KGF experiment

16. Zenith angle distribution (updated data from the South Africa experiment) PRD18, 2239 (1978) Cosmic ray muons Neutrino induced muons Vertical (going up or down) Horizontal going

17. (Atmospheric neutrino anomaly)

18. The first hint ? (South Africa experiment, 1978) PRD18, 2239 (1978) Cosmic ray muons Neutrino induced muons Vertical Horizontal Deficit of muon data “We conclude that there is fair agreement between the total observed and expected neutrino induced muon flux …”

19. Proton decay experiments Grand Unified Theories  tp=1030±2 years Kamiokande (1000ton) IMB (3300ton) NUSEX (130ton) Frejus (700ton) These experiments observed many contained atmospheric neutrino events (background for proton decay).

20. Selection of atmospheric neutrinos Example: Kamiokande At 1000m underground, cosmic ray m: 0.3/sec/detector Atmospheric n: 0.3/day/1000ton n 2.7m 3.5m Fiducial region m anti-counter

21. Number of Ch. photons with λ=300-600 nm emitted by a relativistic particle per cm = 340. Need an efficient detection of the photons. Large PMTs Detecting Cherenkov photons Charged particle Photomultiplier tube (PMT) 50cm φ(Super-K) ν 20cm φ n (refractive index)=1.34 in water θ=42deg. for β=1

22. Charged particle PMT n Time: vertex position direction Pulse height (number of pe’s): energy Detecting Cherenkov photons and event reconstruction Super-K Color: timing Size: pulse height

23. Too few muon decays Proton decay background papers: IMB: PRL57, 1986 (1986) Kamiokande: J.Phys.Soc.Jpn 55, 711 (1986) vmNmX, m(t=2.2msec)enn or nNlepton+p++X, p+m+n, m+e+nn

24. On the other hand,… several “no evidence” for atmospheric neutrino oscillation papers Boiliev et al, (Baksan), Sov J. Nucl. Phys. 34, 787 (1981) J.M. LoSecco et al. (IMB), PRL 54, 2299 (1985) R.M. Bionta et al., (IMB), PRD 38, 768 (1988)

25. electronics m/e ratio measurement in Kamiokande Water system 1983 (Kamiokande construction)

26. Electrons and muons Kamiokande electron-like events muon-like events Super-K

27. Particle identification electron-like event muon-like event e: electromagnetic shower, multiple Coulomb scattering m: propagate almost straightly, loose energy by ionization loss Difference in the event pattern Particle ID

28. Particle ID performance (figures from Super-K) e from μ decay Cosmic ray μ ε=99%@Super-K (98% @Kamiokande)

29. First result on the m/e ratio (1988) “We are unable to explain the data as the result of systematic detector effects or uncertainties in the atmospheric neutrino fluxes. Some as-yet-unaccoundted-for physics such as neutrino oscillations might explain the data.” Kamiokande (3000ton Water Ch. 　　　　　　　　　　～1000ton fid. Vol.) 2.87 kton・year K. Hirata et al (Kamiokande）Phys.Lett.B 205 (1988) 416.

30. However, … Let’s write the atmospheric nm deficit by (m/e)data/(m/e)MC

31. First supporting evidence for small m/e IMB experiment also observed smaller (m/e) in 1991 and 1992.

32. However, … Let’s write the atmospheric nm deficit by (m/e)data/(m/e)MC

33. Atmospheric neutrinos and neutrino oscillations Cosmic ray p, He, …… Detector Detect down-going and up-goingn nmntoscillation Cosmic ray p, He, …… Down-going Atmosphere Up-going

34. Zenith angle distributions Kamiokande (Evis <1.3 GeV) IMB (<1.5GeV) Consistent with no zenith angle dependence...

35. Angular correlation n (CC ne samples) lepton (CC nm sample) Nucleon (MN= 1GeV/c2) q Lepton momentum (MeV/c)

36. No oscillation Dm2=1.6・10-2eV2 nmnt Up/Down=0.58 (2.9 σ) +0.13 -0.11 Next: zenith angle…(Kamiokande,1994) multi-GeV m-like events Up-going Down-going Deficit of upward-going m-like events

37. Discovery of neutrino oscillations

38. Super-Kamiokade detector 50,000 ton water Cherenkov detector (22,500 ton fiducial volume) Exit 11200 PMT(Inner detector) 1900 PMT(Outer detector) ４２ｍ ３９ｍ １０００ｍ underground

39. Around Super-K Entrance to the mine

40. Super-Kamiokande (under construction, Dec. 1994)

41. Super-Kamiokande detector under construction Early summer 1995

42. Kamiokande Water filling in Super-Kamiokande Jan. 1996

43. Event type and neutrino energy Fully Contained (FC) Partially Contained (PC) Stopping  Through-going 

44. Various types of atmospheric neutrino events (1) ・Both CC ne and nm (+NC) ・Need particle identification to separate ne andnm FC (fully contained) n Outer detector (no signal) Single Cherenkov ring electron-like event Single Cherenkov ring muon-like event Color: timing Size: pulse height

45. Various types of atmospheric neutrino events (2) Signal in the outer detector PC (partially contained) n ・97% CC nm

46. Various types of atmospheric neutrino events (3) Upward going muon ・ almost pure CC nm ν Upward stopping muon Upward through-going muon

47. Atmospheric neutrinos and neutrino oscillations Cosmic ray p, He, …… Detector Detect down-going and up-goingn nmntoscillation Cosmic ray p, He, …… Down-going Atmosphere Up-going

48. Super-Kamiokande @Neutrino98 Fully contained, 1-ring events with Evis > 1.33GeV plus partially contained events SK concluded that the observed zenith angle dependent deficit (and the other supporting data) gave evidence for neutrino oscillations.

49. Super-Kamiokande data now @Neutrino98 (535 day) Now (2293 day) No oscillation nmnt oscillation Down-going Up-going

50. Results from the other atmospheric neutrino experiments Soudan-2 MACRO MINOS (first data in 2005)