Neutrino Oscillations, Proton Decay and Grand Unified Theories

1
Neutrino Oscillations,Proton Decayand Grand
Unified Theories

• D. CasperUniversity 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
4
Operation Poltergeist
5
Two Kinds of Neutrinos

• Reines and Cowans 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!

6
Last, but not least
7
Threes 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!

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? lt 10-11 ? 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

• Maybe quarks and leptons arent different after
  all?
• Maybe the three subatomic forces arent different
  either?
• Maybe a more complete theory can predict particle
  masses?

13
Proton Decay

• Generic prediction of most Grand Unified Theories
• Lifetime gt 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

14
IMB

• Worlds first large, ring-imaging water detector
• Total mass 8000 tons
• Fiducial mass 3300 tons
• 2048 Photomultipliers
• Built to search for proton decay
• Operated 1983-1990

15
Water Cerenkov Technique
16
The Rise and Fall of SU(5)

• SU(5) grand-unified theory predicted proton decay
  to e?0 with lifetime 4.5?10291.7 years
• With only 80 days of data, IMB was able to set a
  limit gt 6.5?1031 years (90CL)
• SU(5) was ruled out!

17
Nova
18
Atmospheric Neutrinos

• Products of hadronic showers in atmosphere
• 21 µe 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
• 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 gt 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

25
Checking the Result (Again)

• Look for expected East/West modulation of
  atmospheric flux
• Due to earths 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

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)
• ?/? gt 5.7 1033 yrs (90 CL)

31
16O?15N ?K, K? ??
No candidates
16O
p
Present limit for ?K?/?gt2?1033 years
32
Status of Proton Decay
33
The K2K Experiment
34
K2K Results

• 56 events observed at Super-K, vs. 806 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 0.005 MW)
• Proposed
• Neutrino beamline to Kamioka
• Upgrade to 4 MW
• Outlook
• Completion of PS in 2006

37
Neutrino Factory

• The Ultimate Neutrino 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? lt 10-11 ? 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!