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

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

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

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

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

Last, but not least…

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

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

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

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+

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 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?

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

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 1983-1990

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

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!

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!)

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)

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

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 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)

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

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

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

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)

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

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

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

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 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)

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

Status of Proton Decay

The K2K Experiment

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

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

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

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!

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

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 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 < 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!