The Unbearable Lightness of Neutrinos

1. The Unbearable LightnessofNeutrinos Dave CasperUniversity of California, Irvine

2. Lightness and Weight Journal Publications:Fraction on neutrinosby year • Parmenides’ Query: Which is “positive”?Lightness or Weight? • Today we wonder: where to look for new physics? • High energy/mass? • Or the lightest particle known – the neutrino? D. Casper, University of California Irvine

3. Enrico Fermi Wolfgang Pauli A Desperate Remedy D. Casper, University of California Irvine

4. Clyde Cowan Fred Reines Operation Poltergeist D. Casper, University of California Irvine

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! D. Casper, University of California Irvine

6. Last, but not least… D. Casper, University of California Irvine

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 D. Casper, University of California Irvine

8. Weighing the Neutrino • Mass of e should alter endpoint of -decay spectrum • Extremely difficult measurement! • m(e) < 2.2 eV (90%) • Mass of  from -decay: • m() < 0.19 MeV (90%) • Mass of  from -decay: • m() < 18.2 MeV (90%) D. Casper, University of California Irvine

9. Neutrinos in the(Minimal) Standard Model • Three generations • Right handed neutrinos do not exist • Neutrinos unable to acquire mass through the Higgs mechanism without right-handed component D. Casper, University of California Irvine

10. 0 Proton Proton Decay e+ e 0 Neutron Neutrino e– Proton Proton Decay • Generic Prediction of most Grand Unified Theories • Lifetime > 1033 yr! • Requires comparable number of protons • Colossal Detectors • Neutrino background proved more interesting than the (non-existent) signal D. Casper, University of California Irvine

11. Electron Muon • 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 Water Cerenkov Technique D. Casper, University of California Irvine

12. 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!) Nova D. Casper, University of California Irvine

13. Atmospheric Neutrinos • Products of hadronic showers in atmosphere • 2:1 µ:e ratio from naive flavor counting • Normalization uncertainty ±20% • Flavor ratio (/e) uncertainty ± 5% D. Casper, University of California Irvine

14. The Atmospheric Neutrino Problem • Large water detectors measure significant deficit of  interactions • Smaller detectors do not • Need for larger and more sensitive experiments D. Casper, University of California Irvine

15. Ray Davis 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 D. Casper, University of California Irvine

16. Neutrino Oscillation • Quantum mechanical interference effect • Requires: • Neutrinos have different masses (m20) • Neutrino states of definite flavor are mixtures of several masses (and vice-versa) (mixing 0) • Simplest expression (2-flavor): • Prob(oscillation) = sin2(2) sin2(m2L/E) • In three-neutrino case, three angles and two mass differences characterize the behavior D. Casper, University of California Irvine

17. A B A A B B Different Speed (m20) Same speed (m2=0) Start An Oscillation Analogy • Consider a “clock” with two hands, A and B • The hands advance with (in principle) different rates (corresponding to m20) • We start out with hands at right angles • The lengths of the hands obeys A2+B2=1 • The oscillation probability is equal to D. Casper, University of California Irvine

18. Summary: 5 years ago • Neutrino lives happily within the Standard Model, but some there are some unconfirmed signs of trouble: • Solar neutrino problem • Confirmed, but no clear indication of new physics • Atmospheric neutrino problem • Partially confirmed, but conflicting evidence • LSND • Unconfirmed, conflicting evidence • New and much more sensitive experiments are coming D. Casper, University of California Irvine

19. Accelerator, Reactor Searches D. Casper, University of California Irvine

20. Super-Kamiokande Detector • 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 D. Casper, University of California Irvine

21. SuperK Preliminary1289 days best fit:sin22=1.0m2 = 2.5  10-3 eV22 = 142/152 DoF no oscillation: 2 = 344/154 DoF Evidence • 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 D. Casper, University of California Irvine

22. Double Checks • 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) D. Casper, University of California Irvine

23.  Appearance Signal? • Best proof of neutrino oscillation:  appearance • Very difficult in water detector • Find -like particle ID estimator • Look for excess in upward direction D. Casper, University of California Irvine

24. The K2K Experiment D. Casper, University of California Irvine

25. K2K Results D. Casper, University of California Irvine

26. 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 D. Casper, University of California Irvine

27. LSND and KARMEN • LSND • Reported signal of e oscillation(e appearance) • Possible background problems render result controversial • KARMEN • Experiment with lower backgrounds sees no evidence for effect • Entire parameter space not excluded D. Casper, University of California Irvine

28. m2 m2 Sterile Neutrinos? m2 • Too many neutrino oscillation modes! • Three neutrinos cannot accommodate m2 values of atmospheric, solar and LSND results • Two possibilities: • One (or more) experiment is wrong (or not due to oscillation) • Additional “sterile” neutrinos are needed • Is another “desperate remedy” needed? m2 m2LSND D. Casper, University of California Irvine

29. Summary: Today • Strong evidence for atmospheric neutrino oscillation from Super-Kamiokande • Effect further confirmed by Soudan-II, MACRO • Solar neutrino problem again confirmed, but still not proven as oscillation • LSND and KARMEN still disagree • Neutrinos seem to have mass! • New and much more sensitive experiments are coming D. Casper, University of California Irvine

30. eR eL e me~e <> h0 L L NR e e m~me2/MN h0 h0 Beyond the Standard Model • Masses: • Disparity between neutrino and charged lepton masses • “See-Saw” mechanism involves Unification-scale intermediary • Mixing: • Small in hadronic sector • Large in at least some of lepton sector • What about CP violation? D. Casper, University of California Irvine

31. 2nd Generation LongBaseline (MINOS,CNGS) • 730 km baselines • MINOS: • Factor ~500 more events than K2K (at 3 distance) • Disappearance and appearance (e, ) experiments D. Casper, University of California Irvine

32. SNO • Water detector with a difference: • Heavy water • Able to measure charged current (e) and neutral current (x) • Should determine (finally!) whether solar neutrinos are oscillating or not D. Casper, University of California Irvine

33. KAMLAND • Long-baseline experiment to study reactor neutrino disappearance • Uses about 2 dozen reactors spread through-out Japan • Very sensitive to LMA solar solution • Possibility to study solar neutrinos directly too. D. Casper, University of California Irvine

34. MiniBoone • Mineral oil Cerenkov detector to study LSND effect (e appearance) at Fermilab • Should avoid backgrounds which plagued LSND D. Casper, University of California Irvine

35. Summary: 5 year outlook • SNO will determine whether solar neutrinos oscillate • KAMLAND will find LMA solution if it is the right one • K2K and MINOS will check atmospheric oscillation with accelerator beams •  appearance may be observed • MiniBoone will determine whether LSND result is correct • End of “discovery” era? • Time for precision measurements… D. Casper, University of California Irvine

36. SuperBeams • Small m2 (< 1 eV) values appear interesting • Oscillation probability depends on m2L/E • Small m2 requires either small E or large L • Small E is problematic because neutrino detection probability is proportional to E • Large L is problematic because neutrino flux drops like 1/L2 • Solution: Increase beam luminosity and/or detector size D. Casper, University of California Irvine

37. SuperBeam Signatures • Study of beam designed to look for e appearance • Low beam energy has benefit of reducing backgrounds • Good sensitivity even for very small mixing angle D. Casper, University of California Irvine

38. SuperBeam Sensitivity • For this study, with realistic simulation and reconstruction, sensitivity to sin213 down to (few)10-3 D. Casper, University of California Irvine

39. Neutrino Factory D. Casper, University of California Irvine

40. Neutrino Factory Signatures • 30 GeV  storage ring • Baseline: FNAL/SK(7300 km) • 450 kton water • 1020  decays (=1 yr) • Look for wrong-sign muons ( appearance) D. Casper, University of California Irvine

41. Neutrino Factory Sensitivity • For this preliminary study, another 1-2 orders of magnitude improvement in sensitivity • Other studies are more optimistic • CP violation search depends critically on solution to solar neutrino problem (large or small angle mixing) D. Casper, University of California Irvine

42. World-Wide Neutrino Web? D. Casper, University of California Irvine

43. Summary: 10 year outlook • Precision measurement era of neutrino physics underway • Experiments of epic proportions will probably be in the works • A neutrino factory will very likely be built, especially if we get (or stay) lucky: • LMA solar solution • 13 not too small (10-4?) • Otherwise SuperBeams may be able to do the job D. Casper, University of California Irvine

44. Conclusions • Neutrinos are the final frontier of the Standard Model • An outpouring of creative energy is now devoted to finding new and more sensitive approaches to taming them • More surprises may lie ahead • What if LSND is right? • What if the solar neutrino problem is other new physics? • Major discoveries elsewhere (LHC) during this period? • Maybe Parmenides was right? D. Casper, University of California Irvine