1. Neutrino Oscillations, Proton Decay and Grand Unified Theories
D. Casper University of California, Irvine

2. Outline A brief history of neutrinos
How neutrinos fit into the “Standard Model”
Grand Unified Theories and proton decay
Recent neutrino oscillation discoveries
Future prospects for neutrino oscillation and proton decay

3. A Desperate Remedy Enrico Fermi Wolfgang Pauli
Example of crisis of beta decay
Pauli’s proposal

4. Operation Poltergeist
Clyde Cowan
Fred Reines
Ideas to detect neutrinos with A-bomb
Ideas to detect neutrinos with reactor
Description of R&C experiment
The first neutrinos

5. Two Kinds of Neutrinos
Reines and Cowan’s neutrinos produced in reaction:
Observed reaction was:
Muon decay was known to involve two neutrinos:
If only one kind of neutrino, the rate for the unobserved process: much too large
Proposal: Conserved “lepton” number and two different types of neutrinos(e and )
Produce beam with neutrinos from
Neutrinos in beam should not produce electrons!
Idea of two neutrinos
The first neutrino beam

6. Last, but not least…

7. Three’s Company Number of light neutrinos can be measured!
Lifetime (and width) of Z0 vector boson depends on number of neutrino species
Measured with high precision at LEP
N = 3.02 ± 0.04
Probably no more families exist

8. Particles of “The Standard Model”
Three “families” of particles
Families behave identically, but have different masses
Keeping it “in the family”?
Quarks from different families have a small mixing – do the neutrinos also mix?
Each quark comes in three “colors”
The electron and each of its “copies” has a neutrino associated with it
Neutrinos must be massless, or the theory must have something new added to it.
Quarks
Leptons

9. Forces of The Standard Model
 Z
 Z
 Z
 Z
Four known forces hold everything together:
Gravity – the weakest, not included in Standard Model
Electromagnetism – charged particles exchange massless photons
Strong force – holds quarks together, holds protons and neutrons together inside nucleus; particles exchange massless “gluons”
Weak force – responsible for radioactivity; particles exchange W and Z particles
W
W
W
gluons

10. Weakly Interacting Neutrinos
Neutrinos interact only via the two weakest forces:
Gravity
Weak nuclear force
W and Z particles extremely massive
W mass ~ Kr atom!
Force extremely short-ranged
This makes the weak force weak
Neutrinos pass through light-years of lead as easily as light passes through a pane of glass! µ
µ d u W+

11. Mysteries of the Standard Model
Why three “families” of quarks and leptons?
Why are do particles have masses?
Why are the masses so different?
m <  mt
Are neutrinos the only type of matter without mass?
Can quarks turn into leptons?
Are there really three subatomic forces, or just one?

12. Grand Unified Theories
X,Y
Maybe quarks and leptons aren’t different after all?
Maybe the three subatomic forces aren’t different either?
Maybe a more complete theory can predict particle masses?

13. 0 e+
Proton
Proton Decay
Proton Decay
Generic prediction of most Grand Unified Theories
Lifetime > 1033 yr!
Requires comparable number of protons
Colossal Detectors
Proton decay detectors are also excellent neutrino detectors (big!)
Neutrino interactions are a contamination which proved more interesting than the (as yet unobserved) signal 0 e–
Neutron
Neutrino e
Proton

14. IMB World’s first large, ring-imaging water detector
Total mass 8000 tons
Fiducial mass 3300 tons
2048 Photomultipliers
Built to search for proton decay
Operated

15. Water Cerenkov Technique
Muon
Electron
Cheap target material
Surface instrumentation
Vertex from timing
Direction from ring edge
Energy from pulse height, range and opening angle
Particle ID from hit pattern and muon decay

16. The Rise and Fall of SU(5)
SU(5) grand-unified theory predicted proton decay to e+0 with lifetime 4.51029±1.7 years
With only 80 days of data, IMB was able to set a limit > 6.51031 years (90%CL)
SU(5) was ruled out!

17. Nova
February 1987: Neutrino pulse from Large Magellanic Cloud observed in two detectors
Confirmed astrophysical models
Neutrino mass limits comparable to the best laboratory measurements of that time (from 19 events!)

18. Atmospheric Neutrinos
Products of hadronic showers in atmosphere
2:1 µ:e ratio from naive flavor counting
Flavor ratio (/e) uncertainty ± 5%
Neutrinos produced above detector travel ~15 km
Neutrinos produced below detector travel all the way through the Earth (13000 km)

19. Neutrino Interactions
“Contained” (e , )
Fully-Contained (FC)
Partially-Contained (PC)
“Upward-Muon” ()
Stopping
Through-going
Difficult to detect 
Not enough energy in most atmospheric neutrinos to produce a heavy  particle

20. The Atmospheric Neutrino Problem
Early large water detectors measured significant deficit of  interactions
What happened to these neutrinos?
Smaller detectors did not see the effect
Needed larger and more sensitive experiments, improved checks

21. Neutrino Oscillation Quantum mechanical interference effect: Requires:
Start with one type of neutrino and end up with another!
Requires:
Neutrinos have different masses (m20)
Neutrino states of definite flavor are mixtures of several masses (and vice-versa) (mixing 0, like quarks mix)
Simplest expression (2-flavor):
Oscillation probability = sin2(2) sin2(m2L/E)

22. Checking the Result
A number of incorrect “discoveries” of neutrino oscillation made over the years
Atmospheric neutrino problem was treated with (appropriate) skepticism
Less exotic explanations were explored:
Incorrect calculation of expected flux?
Many comparisons of calculations failed to find any mistake
Systematic problem with particle ID?
Beam tests of water detector particle ID performed at KEK lab in Japan – proved that water detectors can discriminate e and 
Conclusive confirmation required with higher statistics, improved sensitivity

23. Super-Kamiokande Total Mass: 50 kt Fiducial Mass: 22.5 kt
Active Volume:
33.8 m diameter
36.2 m height
Veto Region: > 2.5m
11,146  50 cm PMTs
1,885  20 cm PMTs

24. Evidence for Oscillation
SuperK also sees deficit of  interactions
Also clear angular (L) and energy (E) effects
Finally a smoking gun!
All data fits   oscillation perfectly
Surprise:
Maximal mixing between neutrino flavors
best fit: sin22=1.0 m2 = 2.5  10-3 eV2 2 = 142/152 DoF
no oscillation:
2 = 344/154 DoF
SuperK Preliminary 1289 days

25. Checking the Result (Again)
Look for expected East/West modulation of atmospheric flux
Due to earth’s B field
Independent of oscillation
Fit the data to a function of sin2(LEn)
Best fit at ~-1 (L/E)

26. The Solar Neutrino Problem
Homestake experiment first to measure neutrinos from Sun, finds huge deficit (factor of 3!)
Anomaly confirmed by SAGE, GALLEX, Kamiokande experiments
Ray Davis

27. SuperK Solar Neutrinos
Real-time measurement allows many tests for signs of oscillation:
Day/Night variation
Spectral distortions
Seasonal variation
Allowed oscillation parameter space is shrinking
SMA is disfavored by SK data

28. SNO Water detector with a difference:
Heavy water
Able to measure charged current (e) and neutral current (x)
Can determine (finally!) whether solar neutrinos are oscillating or not

29. Resolving the Solar Neutrino Problem
In July, 2000 SNO published their first results
Measured the rate of D charged-current scattering (only e)
Compare with SuperK precision measurement of e scattering (x)
Significant difference between flux of e and x implies non-zero  +  flux from the Sun: oscillation!
Combined flux of all neutrinos agrees well with solar model

30. SuperK pe+0 Require 2-3 showering rings, 0 e
0 mass cut if 3 rings
Overall Detection Efficiency: 43%
No candidates (0.2 background expected)
/ > 5.7 × 1033 yrs (90% CL)

31. 16O15N* + K+, K+ +  Present limit for K+: />21033 years 
236 MeV/c +
Prompt 6.3 MeV  
K+(~12 ns)
No candidates
16O p
Present limit for K+: />21033 years

32. Status of Proton Decay

33. The K2K Experiment

34. K2K Results 56 events observed at Super-K, vs. 80±6 expected
Energy spectrum of observed events consistent with oscillation
Appears completely consistent with SuperK
More data next year

35. 2nd Generation LongBaseline (MINOS,CNGS)
730 km baselines
MINOS:
Factor ~500 more events than K2K (at 3 distance)
Disappearance and appearance (e, ) experiments
CNGS
Higher-energy beam from CERN to look for  appearance at Gran Sasso
Only a handful of signal events expected

36. JHF/SuperK Experiment
Approved:
50 GeV PS
0.77 MW
(K2K is MW)
Proposed:
Neutrino beamline to Kamioka
Upgrade to 4 MW
Outlook:
Completion of PS in 2006

37. Neutrino Factory The Ultimate Neutrino Beam: Perfectly known beam
Produce an intense beam of high-energy muons
Allow to decay in a storage ring pointed at a distant detector
Perfectly known beam
Technically very challenging!

38. UNO (and Hyper-Kamiokande)
Fiducial Mass: 450 kton
20  Super-Kamiokande
Sensitive to proton decay up to 1035 yr lifetime
Able to study leptonic CP violation (with neutrino beam)
Hyper-Kamiokande
1 Mton Japanese version

39. A World-Wide Neutrino Web?
Enormous interest in future long-baseline oscillation experiments world-wide!
Some theoretical indications that proton decay may be within reach

40. Solving the Mysteries Why three “families” of quarks and leptons?
Quark and lepton family mixing seems very different
Only beginning to measure lepton mixings in detail
Why are do particles have masses?
Why are the masses so different?
m <  mt
Are neutrinos the only type of matter without mass?
It now seems clear that neutrinos have (very tiny) masses
Can quarks turn into leptons?
Are there really three subatomic forces, or just one?
Mixing between families, and the small neutrino masses may tell us a lot about a Grand Unified Theory
Observation of proton decay would be direct evidence for it!