1. Atmospheric neutrinos
III International Pontecorvo Neutrino Physics School
Alushta, Ukraine, Sep. 2007
Atmospheric neutrinos
-status and prospect-
Takaaki Kajita (ICRR, U.of Tokyo)
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-

2. Production of atmospheric neutrinos
Atmosphere

3. (Sign of the particles are neglected in this figure.)

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

5. Some features of the beam (1)
(nm+ nm)/(ne+ ne)
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
Let’s 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: Horizontal flux
now
10 years ago…
3D calculation
1D calculation

9. Horizontal enhancement
For simplicity, let’s assume that the direction of the secondary particles are isotropic (reasonable at low E.) ν
C.R.(1D)
Target area(1D, 3D)
C.R.(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
lepton n
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
Enhanced sensitivity to high energy neutrinos
Wide energy range

14. (discovery of atmospheric neutrinos)
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 (from the South Africa experiment 1978)
PRD18, 2239 (1978)
Cosmic ray muons
Neutrino induced muons
Vertical (going up or down)
Horizontal going

17. (Atmospheric neutrino anomaly)
Some early history
(Atmospheric neutrino anomaly)
Idea to study oscillations with atmospheric neutrinos:
Bilenky & Pontecorvo, Phys. Rep. 1978

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 (in the late 1970’s)  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 m

21. Detecting Cherenkov photons
Number of Ch. photons with λ= nm emitted by a relativistic particle per cm = 340.
Need an efficient detection of the photons Large PMTs
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. Detecting Cherenkov photons and event reconstruction
Super-K n
Charged particle
PMT
Time: vertex position
direction
Pulse height (number of pe’s): energy
Color: timing
Size: pulse height

23. Too few muon decays Proton decay background papers: or
IMB: PRL57, (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. m/e ratio measurement in Kamiokande
electronics
Water system
1983 (Kamiokande construction)

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

26. 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

27. Particle ID performance
(figures from Super-K)
e from μ decay
Cosmic ray μ

28. First result on the m/e ratio (1988)
Data
MC prediction
e-like (～CC ne) 93
88.5
m-like (～CC nm) 85
144.0
“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.

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

30. First supporting evidence for small m/e
IMB experiment also observed smaller (m/e) (1991, 1992).

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

32. Atmospheric neutrinos and neutrino oscillations
Cosmic ray p, He, ……
Detector
Detect down-going and up-going n
nmnt　oscillation
Cosmic ray p, He, ……
Down-going
Atmosphere
Up-going

33. Zenith angle distributions
Kamiokande (Evis <1.3 GeV)
IMB (<1.5GeV)
e-like
m-like
Consistent with no zenith angle dependence...

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

35. Next: zenith angle…(Kamiokande,1994)
multi-GeV m-like events
No oscillation
Dm2=1.6・10-2eV2
nmnt
Up-going Down-going
Up/Down= (2.9 σ)
Deficit of upward-going m-like events
Not high enough statistics to conclude …

36. Discovery of neutrino oscillations

37. 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

38. Around Super-K
Entrance to the mine

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

40. Super-Kamiokande under construction
Early summer 1995

41. Super-Kamiokande with pure water
Jan. 1996

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

43. Various types of atmospheric neutrino events (1)
・Both CC ne and nm (+NC)
・Need particle identification to separate ne and nm
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

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

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

46. Atmospheric neutrinos and neutrino oscillations
Cosmic ray p, He, ……
Detector
Detect down-going and up-going n
nmnt　oscillation
Down-going
Cosmic ray p, He, ……
Atmosphere
Up-going
Up/down ratio measurement is essentially free from systematics

47.
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.

48. Super-Kamiokande data now
@Neutrino98
(535 day)
Now
(2290 day)
No oscillation
nmnt
oscillation
Up-going
Down-going

49. Results from the other atmospheric neutrino experiments
Soudan-2
MACRO
MINOS
(Atmospheric
neutrinos only
in this lecture)

50. Soudan2 nm CC quasi-elastic 1 kton fine grain tracking calorimeter
ne CC
nm CC deep inelastic

51. Soudan2 e μ e μ 5.9 kton・yr exposure
Partially contained events included.
L/E analysis with the “high resolution” sample
Upward stopping muons included. hep-ex/
Phys.Rev. D68 (2003)
Zenith angle
Reconstructed Lν/ Eν dist. e μ e
Up-going
Down-going μ
No osc.
nm  nt osc.
Upward stopping
muons

52. Upward stopping m + down-going PC
MACRO
Upward through-going m
Upward-going PC
10m
77m
12m
Down-going cosmic ray m
Upward through-going m
Upward stopping m down-going PC

53. MACRO No osc. Oscillation Upward horizontal No osc. or Osc.
PLB 566 (2003) 35 EPJ C36(2004)323
No osc. or
Osc.
No osc.
Oscillation
Δm2 =2.5×10-3
Upward
horizontal

54. MINOS (atmospheric) 5.4 kton tracking detector with magnetic field
PRD73, (2006) kton・yr (418days)
nm zenith-angle
L/E
Separation of nm and anti-nm
hep-ex/

55. nmnt oscillation parameters
nmnt, 90%C.L.
MINOS
(Atmospheric)
Kamiokande
Soudan-2
Super-K
MACRO
Also, consistent results from long baseline experiments (K2K & MINOS)  Lecture by K.Nishikawa

56. Summary of Atmospheric Neutrino-1
Experimental studies of atmospheric neutrinos started in the mid. 1960’s.
Different type of atmospheric neutrino experiments started in the 1980’s (proton decay experiments).
Study of the background for proton decay found unexpected atmospheric nm deficit.
In 1998, the nm deficit was concluded as evidence for neutrino oscillations.
Various atmospheric neutrino experiments give consistent results.

57. End