The Unbearable Lightness of Neutrinos Dave Casper University of California, Irvine

March 14, 2001 D. Casper, University of California Irvine 2 Lightness and Weight 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? Journal Publications: Fraction on neutrinos by year

March 14, 2001 D. Casper, University of California Irvine 3 Enrico Fermi A Desperate Remedy Wolfgang Pauli

March 14, 2001 D. Casper, University of California Irvine 4 Operation Poltergeist Clyde Cowan Fred Reines

March 14, 2001 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!

March 14, 2001 D. Casper, University of California Irvine 6 Last, but not least…

March 14, 2001 D. Casper, University of California Irvine 7 Three’s Company Number of light neutrinos can be measured! Lifetime (and width) of Z 0 vector boson depends on number of neutrino species Measured with high precision at LEP N = 3.02 ± 0.04 Probably no more families exist

March 14, 2001 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%)

March 14, 2001 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

March 14, 2001 D. Casper, University of California Irvine 10 Proton Decay Generic Prediction of most Grand Unified Theories Lifetime > yr! Requires comparable number of protons Colossal Detectors Neutrino background proved more interesting than the (non-existent) signal 00 e+e+ Proton Proton Decay 00 e–e– Neutron Neutrino e Proton

March 14, 2001 D. Casper, University of California Irvine 11 Muon Electron Water Cerenkov Technique 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

March 14, 2001 D. Casper, University of California Irvine 12 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!)

March 14, 2001 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%

March 14, 2001 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

March 14, 2001 D. Casper, University of California Irvine 15 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

March 14, 2001 D. Casper, University of California Irvine 16 Neutrino Oscillation Quantum mechanical interference effect Requires: Neutrinos have different masses (  m 2  0) Neutrino states of definite flavor are mixtures of several masses (and vice-versa) (  mixing  0) Simplest expression (2-flavor): Prob(oscillation) = sin 2 (2  ) sin 2 (  m 2  L/E) In three-neutrino case, three angles and two mass differences characterize the behavior

March 14, 2001 D. Casper, University of California Irvine 17 An Oscillation Analogy Consider a “clock” with two hands, A and B The hands advance with (in principle) different rates (corresponding to  m 2  0) We start out with hands at right angles The lengths of the hands obeys A 2 +B 2 =1 The oscillation probability is equal to A B Start A B Same speed (  m 2 =0) AB Different Speed (  m 2  0)

March 14, 2001 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

March 14, 2001 D. Casper, University of California Irvine 19 Accelerator, Reactor Searches

March 14, 2001 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

March 14, 2001 D. Casper, University of California Irvine 21 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 best fit: sin 2 2  =1.0  m 2 = 2.5  eV 2  2 = 142/152 DoF no oscillation:  2 = 344/154 DoF SuperK Preliminary 1289 days

March 14, 2001 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 sin 2 (L  E n ) Best fit at ~-1 (  L/E)

March 14, 2001 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

March 14, 2001 D. Casper, University of California Irvine 24 The K2K Experiment

March 14, 2001 D. Casper, University of California Irvine 25 K2K Results ObservedExpected (No osc) Expected (3  eV 2 ) All Fully Contained ± ±2.7 1-ring1522.7± ±2.0  -like ± ±1.9 e-like11.9±0.41.6±0.3 multi-ring1315.1± ±2.0 Outside Fiducial Vol

March 14, 2001 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

March 14, 2001 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

March 14, 2001 D. Casper, University of California Irvine 28 Sterile Neutrinos? Too many neutrino oscillation modes! Three neutrinos cannot accommodate  m 2 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m2 m2m2 m2m2 m2m2  m 2 LSND

March 14, 2001 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

March 14, 2001 D. Casper, University of California Irvine 30 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? h0h0 L m ~m e 2 /M N e e L NRNR h0h0 eLeL eReR m e ~ e e h0h0

March 14, 2001 D. Casper, University of California Irvine 31 2 nd Generation LongBaseline (MINOS,CNGS) 730 km baselines MINOS: Factor ~500 more events than K2K (at 3  distance) Disappearance and appearance ( e,  ) experiments

March 14, 2001 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

March 14, 2001 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.

March 14, 2001 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

March 14, 2001 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…

March 14, 2001 D. Casper, University of California Irvine 36 SuperBeams Small  m 2 (< 1 eV) values appear interesting Oscillation probability depends on  m 2  L/E Small  m 2 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/L 2 Solution: Increase beam luminosity and/or detector size

March 14, 2001 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

March 14, 2001 D. Casper, University of California Irvine 38 SuperBeam Sensitivity For this study, with realistic simulation and reconstruction, sensitivity to sin 2  13 down to (few)  10 -3

March 14, 2001 D. Casper, University of California Irvine 39 Neutrino Factory

March 14, 2001 D. Casper, University of California Irvine 40 Neutrino Factory Signatures 30 GeV  storage ring Baseline: FNAL/SK (7300 km) 450 kton water  decays (=1 yr) Look for wrong-sign muons (  appearance) 110K44K52K94K

March 14, 2001 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)

March 14, 2001 D. Casper, University of California Irvine 42 World-Wide Neutrino Web?

March 14, 2001 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

March 14, 2001 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?