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

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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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WFour known forces hold everything together:

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!

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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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Proton Decay

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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

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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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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!