Are Neutrinos Different?

1. Are Neutrinos Different? Michael Shaevitz Columbia University • Neutrinos and the Standard Model • Do neutrinos fit in like the other particles? • NuTeV High-Energy Neutrino Experiment • Neutrino Masses and Mixing • What is the mass spectrum and mixing of the different neutrinos? • Are there more than three types of neutrinos? • MiniBooNE

2. Introduction to Neutrino Physics

3. Standard Model of Particle Physics

4. Neutrinos in the Standard Model • Neutrinos have no electric charge (neutral) • Neutrinos only interact through the “weak force” • Neutrino interaction thru W and Z bosons exchange is (V-A) • Neutrinos are left-handed(Antineutrinos are right-handed) • Neutrinos are massless • Neutrinos have three types • Electron ne e • Muon nm m • Tau nt t

5. But we now have indications that the picture is more complicated • Neutrinos may have interactions “Beyond-the Standard-Model” • Modifications including new particles such as Z bosons, lepto-quarks, … • Neutrinos have (very?) small masses • Must come from mechanism different from other particles (e,quarks…) • Neutrinos can change flavor • Oscillations of nmne etc. • Neutrinos may be Majoranna particles • Since neutrinos are neutral  n =  • Models add heavy right-handed neutrinos

6. 1st Observedpmn decay Highlights of Neutrino History Reines & Cowann Detector Nobel 2002 Observation of neutrinos from the sun and supernovae Davis (Solar n’s in1970) and Koshiba (Supernova n’s1987)

7. Neutrino Interactions • W exchange gives Charged-Current (CC) events and Z exchange gives Neutral-Current (NC) events In CC events the outgoing lepton determines if neutrino or antineutrino

8. n Earth Neutrino Cross Section is Very Small • Weak interactions are weak because of the massive W and Z boson exchange  s weak GF2 (1/MW or Z)4 • For 100 GeV Neutrinos: • s(ne) ~ 10-40and s(np) ~ 10-36cm2compared to s(pp) ~ 10-26cm2 • Mean free path length in Steel ~ 3109 meters! (Need big detectors and lots of n’s) MW ~ 80 GeVMZ ~ 91 GeV

9. Helicity is projection of spin along the particle’s direction Frame dependent (if massive) left-helicity right-helicity Neutrinos in the Standard Model Are Left-Handed(Helicity and Handedness) • Handedness (or chirality) is Lorentz-invariant • Only same as helicity for massless particles. • If neutrinos have mass then left-handed neutrino is: • Mainly left-helicity • But also small right-helicity component  m/E • Neutrinos only interact weakly with a (V-A) interaction • All neutrinos are left-handed • All antineutrinos are right-handed Right-handed neutrinos do not interact in the Standard Model   “Sterile” neutrinos

10. A Little Electroweak Theory (For NuTeV)

11. Maxwell (1873) Unification of Electricity and Magnetism Glashow/Weinberg/Salam (1960’s) Unification of the Weak and Electromagnetism SU(2) group: “weak isospin” U(1) group: “weak hypercharge” Predicts “weak neutral current” with Z0 Weak Interaction: W , Z0Photon:  (couples to electric charge) Higgs Mechanism makes W , Z0 heavy One parameter “weak mixing angle” qW g’ = g tanqW Standard Model Electroweak Unification g g’

12. Higgs Mechanism at a Cocktail Party Higgs Field gives mass to particles Higgs Field gives itself mass

13. Electroweak Theory • Standard Model • Charged Current mediated by W with (V-A) Neutral Current mediated by Z0 with couplings below • One parameter to measure! • Weak / electromagnetic mixing parameter sin2qW(Related to weak/EM coupling ratio g’=g tanW) • Neutrinos are special in SM • Only have left-handed weak interactions  W and Z boson exchange

14. History of EW Measurements Gargamelle CCFR, CDHS, CHARM, CHARM IIUA1 , UA2 , Petra , Tristan , APV, SLAC eD • Discovery of the Weak Neutral Current (1973 CERN) • First Generation EW Experiments (late 1970’s) • Precision at the 10% level • Tested basic structure of SM  MW,MZ • Second Generation EW Experiments (late 1980’s) • Discovery of W,Z boson in 1982-83 • Precision at the 1-5% level • Radiative corrections become important • First limits on the Mtop • Current Generation Experiments • Precision below 1% level • Discovery of the top quark • Constrain MHiggs •  Predict light Higgs boson • (and possibly SUSY) • Use consistency to search for new physics ! GargamelleHPWF CIT-F NuTeV, D0, CDF, LEP1 SLDLEPII, APV , SLAC-E158

15. Current Era of Precision EW Measurements • Precision parameters define the SM: • aEM-1 = 137.03599959(40) 45ppb (200ppm@MZ) • Gm = 1.16637(1)10-5GeV-2 10ppm • MZ= 91.1871(21) 23ppm • Comparisons test the SM and probe for new physics • LEP/SLD (e+e-), CDF/D0 (p-pbar), nN , HERA (ep) , APV • Radiative corrections are large and sensitive to Mtop and MHiggs • MHiggs constrained in SM to be less than196 GeV at 95%CL

16. Summary of Electroweak Measurements(Before NuTeV) • Most data suggest a light Higgs except AFBb • Global fit has c2=23/15 (9%) • AFBb is off about 3s

17. From Bruce Knuteson’s Columbia Colloquium X Precision Electroweak measurements

18. NuTeV Electroweak Measurements

19. NuTeV Experiment at Fermilab • High Energy (En~100 GeV) High Statistics (several million events)Neutrino Experiment (1996-97) using the Fermilab 800 GeV Proton Beam Charged-Current(CC) Neutral-Current(NC) • For a target nucleus with equal numbers of of u,d quarks:

20. Before and After NuTEV • nN experiments before NuTeV had hit a brick wall in precision Due to systematic uncertainties (i.e. mc ....) • NuTeV Technique Cross section differences remove these systematic uncertainties (from mainly sea quark contributions) But need to makemeasurement for bothneutrinos and antineutrinos

21. Use magnets after target to pick:p+ for neutrino modep- for antineutrino mode NuTeV Detector (690 tons of steel) Make ’s and K’s Make ’s fromp,K m n decays Slow down and stopall m’s in shielding Only n’s left !

22. 690 ton n-target Toroid Spectrometer Target / Calorimeter NuTeV Detector

23. NuTeV Collaboration K. S. McFarland Cincinnati1, Columbia2, Fermilab3, Kansas State4, Northwestern5, Oregon6, Pittsburgh7, Rochester8(Co-spokepersons: B.Bernstein, M.Shaevitz)

24. Neutral Current / Charged CurrentEvent Separation • Separate NC and CC events statistically based on the “event length”defined in terms of # counters traversed

25. Events LcutRegion Events LcutRegion Determine Rexp: The Short to Long Ratio: Event Length DistributionNeutrinos NC CC Event Length DistributionAntineutrinos Cuts:- Ehad > 20 GeV- Fiducial Vol.

26. From Rexp to Rn Need detailed Monte Carlo simulation to relate Rexp to Rn and sin2qWA few CC events look like NC events • Short nm CC’s(20% n , 10% n) • muon exits, range outat high y • Short ne CC’s (5%) • ne N  e X • Cosmic Rays (0.9%/4.7%) A few NC events look like CC events • Long nm NC’s (0.7%) • punch-through effects

27. Result from Fit to Rn and Rn • NuTeV result: • (Previous neutrino measurements gave 0.2277  0.0036) Standard model fit (LEPEWWG): sin2qW = 0.2227  0.00037A 3s discrepancy ... Phys.Rev.Lett. 88,91802 (2002) Discrepancy is neutrinosnot antineutrinos Discrepancy is left-handedcoupling to u and d quarks n NC coupling is too small

28. Using sin2qW = 1-mW2/mZ2 relation with known mZ Compare to other mW measurements Comparison to Other EW Measurements Each electroweak measurement also indicates a range of Higgs masses

29. ? ? Old and New Physics Interpretations(Davidson et al. hep-ph/0112302) • Changes in Standard Model Fits • Change assumed quark distributions • Change Mhiggs  Need > 1000 GeV ! •  Constrained by other measurements • “Old Physics” Interpretations: QCD • Violations of “isospin” symmetry •  Size of violation too big • Strange vs anti-strange quark asymmetry •  Directly measured to be ~ symmetric • “New Physics” Interpretations • New Z’ or lepto-quark exchanges • New particle loop corrections •  Hard but possible to fit all data

30. Apparent n NC coupling to the Z is too small  Maybe this reduced coupling is due to neutrino mixing or oscillations “The NuTeV Anomaly, Neutrino Mixing and a Heavy Higgs”,W. Loinaz et al. (hep-ph/0210193) Znn coupling suppression occurs due to muon neutrino mixing with a heavy “sterile” neutrino Everything fits together if: Mixed neutrinos: q = 0.055  0.010 Heavy Higgs: Mhiggs 200 GeV LEP Z Are n’s Different? • “ne ns oscillations with large neutrino mass in NuTeV?”,C.Giunti and M.Laveder (hep-ph/0202152) • Sizeable probability for ne ns oscillations (~20%) • This reduces the real ne background NuTeV oversubtracts the background giving a lower apparent Znn coupling

31. Future Measurements • Unfortunately, the high-energy neutrino beam at Fermilab has been terminated Need to rely on other experiments for progress • Upcoming new measurements: • SLAC E-158 Polarized electron-electron scattering (Low Q2) • Fermilab Tevatron CDF/D0 Run 2 and LHC searches for Z’ and leptoquarks • Neutrino mixing and oscillations • Next section of the talk NuTeV AtomicParityViolation SLACE 158Region Tevatron(D0,CDF) LHC(Atlas,CMS) (Plot from J.Erler and P.Langacker)

32. Neutrino Mass and Oscillation Phenomenology

33. Making Neutrinos Matter • Standard Model assumes that neutrinos are massless • No symmetry property or theoretical reason for mn = 0 • Neutrinos are partners of the massive charged leptons • Could imply right-handed n ’s, Majorana n =nor sterile n’s • Neutrino mass hierarchy ? t m entnm ne • Cosmological Consequences • Neutrinos fill the universe from the Big Bang (109n / m3) Even a small mass (~1 eV) will have effects • Models have hot (n) and coldDark Matter • Massive neutrino affect structure formation such as galaxies and clusters

34. Neutrino Mass: Theoretical Ideas • No fundamental reason or symmetry for why neutrinos must be massless • But why are they much lighter than other particles? • Grand Unified Theories • Dirac and Majorana MassLeft and Right – handed neutrinos  See-saw Mechanism • Modified Higgs sector to accommodate neutrino mass • Extra Dimensions • Neutrinos live outside of 3 + 1 space Many of these models have at least one non-interacting type of n • Right-handed partner of the left-handed n • Mass uncertain from light (< 1 eV) to heavy (>1016 eV) • Would be “sterile” – Doesn’t couple to standard W and Z bosons

35. Dirac Neutrinos Neutrino and Antineutrino are distinct particles (like their charged lepton partners) Lepton number conserved Neutrino m- Antineutrino  m+ Dirac Mass Term Need to have a right-handed neutrino (Not in the Standard Model) Mass term like e, m, t Majorana Neutrinos Neutrinos and Antineutrinos are the same particle (This can only happen since neutrinos have no charge!) Only difference is “handedness” Neutrinos are left-handed n m- Antineutrinos are right-handed n m+ Lepton number not conserved Neutrino  Antineutrino with spin flip Majorana Mass Term New type of mass Dirac and Majorana Neutrinos

36. See – Saw Mechanism • If only the nL exist, then neutrinos only can have a Majorana mass • But if both nL and nR exist then can have both Majorana and Dirac mass components • If postulate that mL=0, mD=me,m,t << mR= very heavy (since right-handed )and diagonalize the mass matrix Explains? why neutrinos have small mass but predicts very small mixing

37. Direct Neutrino Mass Experiments Direct decay studies have made steady progress but limited • Electron neutrino: • 3H3He + ne + e- • Muon neutrino: • pmnm decays • Tau neutrino: • t (np) nt decays < 2 eV nm(keV) < 170 keV ne(eV) nt(MeV) < 18 MeV

38. Neutrino Oscillations • Direct measurements have difficulty probing small neutrino masses  Use neutrino oscillations • If we postulate: • Neutrinos have (different) mass  Dm2 = m12 – m22 • The Weak Eigenstates are a mixture of Mass Eigenstates Then a pure nm beam at L=0, will develop a ne component as it travels a distance L.

39. Oscillation Formula Parameters nmDisappearance neAppearance

40. Current Situation: Three Experimental Indications for Neutrino Oscillations Atmospheric NeutrinosL = 15 to 15,000 km E =300 to 2000 MeV LSND ExperimentL = 30m E = ~40 MeV Solar NeutrinosL = 108 km E =0.3 to 3 MeV Dm2 = ~ 2 to 8 10-5eV2ProbOSC = ~100% Dm2 = .3 to 3 eV2ProbOSC = 0.3 % Dm2 = ~ 1 to 7 10-3 eV2 ProbOSC = ~100%

41. Oscillation Parameter Plot(Dm2 vs. sin22q)

42. Too Many n-Oscillation Signals • With the three known neutrinos nenm nt Cannot explain three different Dm2 values. • Explanation could be experimental or theoretical (or both) • Experimental ideas • Not all of the three signals are neutrino oscillations • Unknown uncertainties give false signals • Theoretical ideas • More than three types of neutrinos – extra “sterile” neutrino types • Neutrinos and antineutrinos have different masses

43. MiniBooNE: 1 GeV Acc. n’s 500 m to Detector Booster NuMI: 3 GeV Acc. n’s 750 km to MINOS Det. Kamland (Japan): Reactor n’s 1000 m3 Liquid Scint. MainInjector Also K2K (Japan)CNGS (CERN to Italy) Probing the Experimental Explanations • Need new experiments that probe these three signals with different neutrino sources, detectors, parameters ….. LSND signal Atmospheric signal Solar signal

44. MiniBooNE Oscillation Exp

45. The LSND Experiment Saw an excess of:87.9 ± 22.4 ± 6.0 events. With an oscillation probability of (0.264 ± 0.067 ± 0.045)%. 4 s evidence for oscillation. Oscillations? LSND took data from 1993-98 - 49,000 Coulombs of protons - L = 30m and 20 < En< 55 MeV  Need a definitive measurement in this region MiniBooNE

46. The MiniBooNE Collaboration The BooNE Collaboration Y.Liu, I.Stancu University of Alabama, Tuscaloosa S.Koutsoliotas Bucknell University, Lewisburg E.Church, C.Green, G.J.VanDalen University of California, Riverside E.Hawker, R.A.Johnson, J.L.Raaf, N.Suwonjandee University of Cincinnati, Cincinnati T.Hart, E.D.Zimmerman University of Colorado, Boulder L.Bugel, J.M.Conrad, J.Formaggio, J.M.Link, J.Monroe, M.H.Shaevitz, M.Sorel Columbia University, Nevis Labs, Irvington D.Smith Embry Riddle Aeronautical University C.Bhat, S.Brice, B.Brown, B.T.Fleming, R.Ford, F.G.Garcia, P.Kasper, T.Kobilarcik, I.Kourbanis, A.Malensek, W.Marsh, P.Martin, F.Mills, C.Moore, P.J. Nienaber, E.Prebys, A.D.Russell, P.Spentzouris, R.Stefanski, T.William Fermi National Accelerator Laboratory D.C.Cox, J.A.Green, S.McKenney, H.Meyer, R.Tayloe Indiana University, Bloomington G.T.Garvey, W.C.Louis, G.McGregor, G.B.Mills, E.Quealy, V.Sandberg, B.Sapp, R.Schirato, R.Van de Water, D.H.White Los Alamos National Laboratory R.Imlay, W.Metcalf, M.Sung, M.Wascko Louisiana State University, Baton Rouge J.Cao, Y.Liu, B.P.Roe University of Michigan, Ann Arbor A.O.Bazarko, P.D.Meyers, R.B.Patterson, F.C.Shoemaker Princeton University, Princeton MiniBooNE collaboration was formed to search for ne appearance in a nm beam at Fermilab. MiniBooNE consists of about 62 scientists from 13 institutions. Co-spokespersons:Janet Conrad and Bill Louis

47. Booster MainInjector MiniBooNE Experiment Use protons from the 8 GeV booster Neutrino Beam <En>~ 1 GeV 12m sphere filled withmineral oil and PMTslocated 500m from source

48. 50m Decay Pipe Magnetic Horn MiniBooNE Neutrino Beam Variable decay pipe length (2 absorbers @ 50m and 25m) 8 GeV Proton Beam Transport p  m n One magnetic Horn, with Be target Detector

49. The MiniBooNE Detector • 12 meter diameter sphere • Filled with 950,000 liters (900 tons) of very pure mineral oil • Light tight inner region with 1280 photomultiplier tubes • Outer veto region with 241 PMTs. • Oscillation Search Method:Look for ne events in a pure nm beam

50. Particle Identification By Phototube Hit Pattern • Charged particles produce Čerenkov photons at a constant angle as they go through the oil • This shows up as a ring of hits in the phototubes mounted inside the MiniBooNE sphere • Pattern of phototube hits tells the particle type Stopping muon event