Neutrino Oscillations,Proton Decay
and Grand Unified Theories
D. CasperUniversity of California, Irvine
OutlineA brief history of neutrinosHow neutrinos fit into the “Standard Model”Grand Unified Theories and proton decayRecent neutrino oscillation discoveriesFuture prospects for neutrino oscillation and proton decay
Enrico Fermi
A Desperate Remedy
Wolfgang Pauli
Operation Poltergeist
Clyde Cowan Fred Reines
n e p
p e n
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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 largeProposal: Conserved “lepton” number and two different types of neutrinos(e and )Produce beam with neutrinos from
Neutrinos in beam should not produce electrons!
Two Kinds of Neutrinos
Last, but not least…
Three’s CompanyNumber 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 ± 0.04Probably no more families exist
Particles of “The Standard Model”
Three “families” of particlesFamilies behave identically, but have different massesKeeping 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 itNeutrinos must be massless, or the theory must have something new added to it.
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Quarks Leptons
Forces of The Standard Model
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Gravity – the weakest, not included in Standard ModelElectromagnetism – charged particles exchange massless photonsStrong 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
Weakly Interacting NeutrinosNeutrinos interact only via the two weakest forces:
GravityWeak nuclear force
W and Z particles extremely massive
W mass ~ Kr atom!Force extremely short-rangedThis makes the weak force weak
Neutrinos pass through light-years of lead as easily as light passes through a pane of glass!
µµ
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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 < 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?
Grand Unified TheoriesMaybe 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?
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Proton DecayGeneric prediction of most Grand Unified TheoriesLifetime > 1033 yr!
Requires comparable number of protonsColossal 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
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ProtonProton
Proton Decay
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NeutronNeutron
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Proton
IMBWorld’s first large, ring-imaging water detector
Total mass 8000 tonsFiducial mass 3300 tons2048 Photomultipliers
Built to search for proton decayOperated 1983-1990
Muon ElectronCheap target materialSurface instrumentationVertex from timingDirection from ring edgeEnergy from pulse height, range and opening angleParticle ID from hit pattern and muon decay
Water Cerenkov Technique
The Rise and Fall of SU(5)SU(5) grand-unified theory predicted proton decay to e+0 with lifetime 4.51029±1.7 yearsWith only 80 days of data, IMB was able to set a limit > 6.51031 years (90%CL)SU(5) was ruled out!
February 1987: Neutrino pulse from Large Magellanic Cloud observed in two detectorsConfirmed astrophysical modelsNeutrino mass limits comparable to the best laboratory measurements of that time (from 19 events!)
Nova
Atmospheric NeutrinosProducts of hadronic showers in atmosphere2:1 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)
Primary cosmic ray
/K
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Neutrino Interactions“Contained” (e , )
Fully-Contained (FC)Partially-Contained (PC)
“Upward-Muon” ()StoppingThrough-going
Difficult to detect Not enough energy in most atmospheric neutrinos to produce a heavy particle
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
Neutrino OscillationQuantum mechanical interference effect:
Start with one type of neutrino and end up with another!
Requires:Neutrinos have different masses (m20)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(m2L/E)
Checking the ResultA 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
Super-KamiokandeTotal Mass: 50 ktFiducial Mass: 22.5 ktActive Volume:
33.8 m diameter36.2 m height
Veto Region: > 2.5m11,146 50 cm PMTs 1,885 20 cm PMTs
Evidence for OscillationSuperK also sees deficit of interactions
Also clear angular (L) and energy (E) effects
Finally a smoking gun!
All data fits oscillation perfectlySurprise:
Maximal mixing between neutrino flavors
best fit:sin22=1.0m2 = 2.5 10-3 eV2
2 = 142/152 DoFno oscillation:2 = 344/154 DoF
SuperK Preliminary1289 days
Checking the Result (Again)
Look for expected East/West modulation of atmospheric flux
Due to earth’s B fieldIndependent of oscillation
Fit the data to a function of sin2(LEn)
Best fit at ~-1 (L/E)
The Solar Neutrino ProblemHomestake experiment first to measure neutrinos from Sun, finds huge deficit (factor of 3!)Anomaly confirmed by SAGE, GALLEX, Kamiokande experiments Ray Davis
SuperK Solar NeutrinosReal-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
SNOWater 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
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
SuperK pe+0
Require 2-3 showering rings, 0 e0 mass cut if 3 ringsOverall Detection Efficiency: 43%No candidates (0.2 background expected)/ > 5.7 × 1033 yrs (90% CL)
16O15N* + K+, K+ +
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Prompt6.3 MeV
K+ (~
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236 MeV/c+
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No candidates
Present limit for K+:/>21033 years
Status of Proton Decay
The K2K Experiment
K2K Results56 events observed at Super-K, vs. 80±6 expectedEnergy spectrum of observed events consistent with oscillationAppears completely consistent with SuperK
More data next year
2nd Generation LongBaseline (MINOS,CNGS)
730 km baselinesMINOS:
Factor ~500 more events than K2K (at 3 distance)Disappearance and appearance (e, ) experiments
CNGSHigher-energy beam from CERN to look for appearance at Gran SassoOnly a handful of signal events expected
JHF/SuperK ExperimentApproved:
50 GeV PS0.77 MW
(K2K is 0.005 MW)
Proposed: Neutrino beamline to KamiokaUpgrade to 4 MW
Outlook:Completion of PS in 2006
Neutrino FactoryThe Ultimate Neutrino Beam:
Produce an intense beam of high-energy muonsAllow to decay in a storage ring pointed at a distant detector
Perfectly known beamTechnically very challenging!
UNO (and Hyper-Kamiokande)Fiducial Mass: 450 kton
20 Super-Kamiokande
Sensitive to proton decay up to 1035 yr lifetimeAble to study leptonic CP violation (with neutrino beam)Hyper-Kamiokande
1 Mton Japanese version
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
Solving the MysteriesWhy three “families” of quarks and leptons?
Quark and lepton family mixing seems very differentOnly beginning to measure lepton mixings in detail
Why are do particles have masses?Why are the masses so different?
m < 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 TheoryObservation of proton decay would be direct evidence for it!