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Neutrino Masses and Oscillations: A View of the Next Steps

Neutrino Masses and Oscillations: A View of the Next Steps. M. H. Shaevitz Columbia University January, 2005. Neutrinos Have Been Surprising Us. Introduction to Neutrinos Neutrino Revolution over the Past Decade Solar and Atmospheric Osc. LSND Anomaly Possibilities for a Next Revolution

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Neutrino Masses and Oscillations: A View of the Next Steps

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  1. Neutrino Masses and Oscillations: A View of the Next Steps M. H. Shaevitz Columbia UniversityJanuary, 2005

  2. Neutrinos Have Been Surprising Us • Introduction to Neutrinos • Neutrino Revolution over the Past Decade • Solar and Atmospheric Osc. • LSND Anomaly • Possibilities for a Next Revolution • Sterile Neutrinos • CP Violation in the Neutrino Sector • Summary

  3. Standard Model of Particle Physics

  4. Neutrinos in the Standard Model • Neutrinos are the only fundamental fermions with no electric charge • 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. 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)

  6. 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 MeV Neutrinos: • s(ne) ~ 10-42and s(nn) ~ 10-39cm2compared to s(pp) ~ 10-24cm2 • A neutrino has a good chance of traveling through 3 million earths before interacting at all! • Mean free path length in Steel ~ 1013 meters! (Need big detectors and lots of n’s) MW ~ 80 GeVMZ ~ 91 GeV

  7. Structure formation n < ~0.02 Why Neutrino Mass Matters? • Massive neutrinos with osc. important for heavy element production in supernova • Light neutrinos effect galactic structure formation Cosmological Implications Window on Physics at High E Scales See-Saw Mechanism Heavy RHneutrino Typical Dirac Mass Set of very lightneutrinos Set of heavysterile neutrinos

  8. nm(keV) ne(eV) nt(MeV) Experimental Limits on the Neutrino Mass 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 < 170 keV < 18 MeV  Need to use “Neutrino Oscillations” to probe for smaller neutrino masses

  9. Neutrino Oscillation Experiments

  10. 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.

  11. Oscillation Formula Parameters nmDisappearance neAppearance

  12. Oscillation Plots • If you see an oscillation signal with Posc = P  dP then carve out an allowed region in (Dm2,sin22q) plane. • If you see no signal and limit oscillation with Posc < P @ 90% CLthen carve out an excluded region in the (Dm2,sin22q) plane.

  13. CP violating process can also occur if d0  3-Generation Oscillation Formalism • But we have 3-generations: ne , nm, and nt (and maybe even more ….. the sterile neutrino ns’s ) • Naively might expect the neutrino and quark matrix to look similar

  14. Types of Neutrino Oscillation Experiments n’s from sun (few MeV)or atmosphere (0.5-20 GeV) Use earthto shield detectorfrom cosmic rays(mainly muons) n’s from reactors (~3 MeV) Smaller the Neutrino Energy More depth (10 m – 2000 m) nmmake muonsnemake electrons  make light(Use pattern oflight to id type) n’s from pulsedaccelerator beams (~1 GeV)Also have timing Detector: Vat of 1 to 20 kton of oil, water, or liquid scintillator with light detectors (PMTs)

  15. The FirstNeutrinoRevolution

  16. Situation in mid-1990’s: 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%

  17. Three Signal Regions(Mid 1990’s) LargeMixingSolution SmallMixingSolution

  18. Theoretical Prejudices before 1995 • Natural scale for Dm2 ~ 10 – 100 eV2since needed to explain dark matter • Oscillation mixing angles must be small like the quark mixing angles • Solar neutrino oscillations must be small mixing angle MSW solutionbecause it is “cool” • Atmospheric neutrino anomaly must be other physics or experimental problembecause it needs such a large mixing angle • LSND result doesn’t fit in so must not be an oscillation signal

  19. Theoretical Prejudices before 1995 What we know now • Natural scale for Dm232 ~ 10 – 100 eV2Wrongsince needed to explain dark matter • Oscillation mixing angles must be small Wronglike the quark mixing angles • Solar neutrino oscillations must be Wrongsmall mixing angle MSW solutionbecause it is “cool” • Atmospheric neutrino anomaly must be Wrongother physics or experimental problembecause it needs such a large mixing angle • LSND result doesn’t fit in so must not ????be an oscillation signal

  20. Neutrino Revolution: 1995 - 2005 K2K Accelerator Neutrino Exp. • Atmospheric neutrino oscillations definitively confirmed • “Smoking Gun”  Super-K flux change with zenith angle • Accelerator neutrino confirmation with KEK to Super-K exp. (K2K) • Value of Dm2: 1.5 to 3.5 × 10-3 eV2 Super-K (SK)Atmospheric NeutrinoExperiment

  21. Neutrino Revolution: 1995 - 2005 SNO Solar  Exp. • Solar Neutrino Oscillations Confirmed and Constrained • Many different exp’s see deficit • SNO experiments sees that total neutrino flux correct from sun but just changing flavor • Kamland experiment using reactor neutrinos confirms solar oscillations • Combination of experiments  Large Mixing Angle MSW Solution Combination All Solar + KamLAND KamLAND Reactor Exp.

  22. Neutrino Revolution: 1995 - 2005 The LSND Experiment Saw an excess ofe:87.9 ± 22.4 ± 6.0 events. With an oscillation probability of (0.264 ± 0.067 ± 0.045)%. 3.8 s evidence for oscillation. Oscillations? LSND took data from 1993-98 - 49,000 Coulombs of protons - L = 30m and 20 < En< 53 MeV

  23. A Next NeutrinoRevolution?

  24. Possible New Surprises • Sterile Neutrinos: • New type of neutrino • No weak interactions (effectively no interactions) • Produced by mixing with normal neutrinos • Expected in many extensions to the standard model • They would give a whole new spectrum of mass states and mixings  MiniBooNE and follow-ups are key • Probing for CP violation • CP violation comes about when a process has a different rate for particles and anti-particles • CP violation in the neutrino mixing couldbe a key ingredient for explaining the matter-antimatter asymmetry in the universe • Then look at n versus n oscillations to measure d  New long baseline and reactor experiments are key

  25. N3 N3 n Mixing n Mixing Matter-Antimatter Asymmetry (B  0)from Leptogenesis • Hard to generate a baryon asymmetry (B  0) using quark matrix CP violation  Use Heavy Sterile Neutrinos and Neutrino CP Violation • Generate L  0 in the early universe from CP (or CPT) violation in heavy neutrino N3 vs.N3decays (only needs to be at the 10-6 level) • B-L processes then convert neutrino excess to baryon excess. • Sign and magnitude ~correct to generate baryon asymmetry in the universe with mN > 109 GeV and mn < 0.2 eV

  26. (M.Sorel, J.Conrad, M.Shaevitz, PRD 70(2004)073004 (hep-ph/0305255) ) One of the experimental measurements is wrong Additional “sterile” neutrinos involved in oscillations Why Sterile Neutrinos? Need better measurement in LSND region MiniBooNE

  27. Booster MainInjector Use protons from the Fermilab 8 GeV booster Neutrino Beam <En>~ 1 GeV Booster Neutrino Experiment(MiniBooNE) MiniBooNE designed to check LSND signal by searching for ne appearance in a nm beam at Fermilab. 12m sphere filled withmineral oil and PMTslocated 500m from source • Oscillation Search Method • Look for ne events in a pure nm beam

  28. p  m n 50m Decay Pipe MiniBooNE Neutrino Exp. At Fermilab Look for   e (and  e ) 8 GeV Proton Beam Transport 50 m One magnetic Horn, with Be target Detector

  29. MiniBooNE Collaboration Y. Liu, I. Stancu Alabama S. Koutsoliotas Bucknell E. Hawker, R.A. Johnson, J.L. Raaf Cincinnati T. Hart, R.H. Nelson, E.D. Zimmerman Colorado A. Aguilar-Arevalo, L.Bugel, L. Coney, J.M. Conrad,Z. Djurcic, J. Link, J. Monroe, K. McConnel, D. Schmitz, M.H. Shaevitz, M. Sorel, G.P. Zeller Columbia D. Smith Embry Riddle L.Bartoszek, C. Bhat, S J. Brice, B.C. Brown, D.A. Finley, R. Ford, F.G.Garcia, P. Kasper, T. Kobilarcik, I. Kourbanis, A. Malensek, W. Marsh, P. Martin, F. Mills, C. Moore, P. Nienaber, E. Prebys, A.D. Russell, P. Spentzouris, R. Stefanski, T. Williams Fermilab D. C. Cox, A. Green, H.-O. Meyer, R. Tayloe Indiana G.T. Garvey, C. Green, W.C. Louis, G.McGregor, S.McKenney, G.B. Mills, H. Ray, V. Sandberg, B. Sapp, R. Schirato, R. Van de Water, D.H. White Los Alamos R. Imlay, W. Metcalf, M. Sung, M.O. Wascko Louisiana State J. Cao, Y. Liu, B.P. Roe, H. Yang Michigan A.O. Bazarko, P.D. Meyers, R.B. Patterson, F.C. Shoemaker, H.A.Tanaka Princeton B.T. Fleming Yale MiniBooNE consists of about 70 scientists from 13 institutions.

  30. 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 (  e and  e )

  31. Particle Identification • Separation of nm from ne events • Exiting nm events fire the veto • Stopping nm events have a Michel electron after a few msec • Čerenkov rings from outgoing particles • 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

  32. Animation Each frame is 25 ns with 10 ns steps. Low High Early Late Muon Identification Signature:m  e nm ne after ~2msec Charge (Size) Time (Color)

  33. Dm2 = 1 eV2 Dm2 = 0.4 eV2 MiniBooNE Run Plan • At the current time have collected >4x1020 p.o.t. ( original goal is 10.0x1020 p.o.t) • Data collection rate is steadily improving • Many improvements in the Booster and Linac (these not only help MiniBooNE but also the Tevatron and NuMI in the future) • Blind analysis: Hide possible ne candidates; other ~80% events are openPlan is to “open the ne appearancebox” when the analysis has been substantiated and when sufficient data has been collected for a definitive result Current estimate is sometime in Fall 2005 Next Step “Signal” No Signal“Limit”

  34. If MiniBooNE sees no indications of oscillations with nm Need to run withnm since LSND signal wasnmne(Indications of CP violation) If MiniBooNE sees an oscillation signal Many m2 and mixing angles plus CP violation to determine  BooNE Experiment (with  and) Add another detector to MiniBooNE at 1-2 km distance The Next Step

  35. Question 2: CP Violation in Neutrino Mixing

  36. Unknown CP violation phase What do we know? Solar: 12 ~ 30° sin2 213 < 0.2 at 90% CL(or 13 < 13°) Atmospheric: 23 ~ 45° What is e component of 3 mass eigenstate? These two differentmass schemesare called:Mass HierarchyProblem

  37. Key questions • What is value of 13? • What is mass hierarchy? • Do neutrino oscillations violate CP symmetry? • May give hints about possible “Leptogenesis” CP violating phase sin  0  CP Violation sin 13 • Why are quark and neutrino mixing matrices so different? Value of 3 central to these questions; it sets the scale for experiments needed to resolve mass hierarchy and search for CP violation.

  38. Methods to measuresin2213 Reactor experiments allow direct measurement of sin22: no matter effects, no CP violation, almost no correlation with other parameters. • Long-Baseline Accelerators: Appearance (nmne) at m22.510-3 eV2 NOA:<E> = 2.3 GeV, L = 810 km T2K: <E> = 0.7 GeV, L = 295 km Proposed Offaxis Exps. • Reactors: Disappearance (nene) at m22.510-3 eV2

  39. Regions where the Offaxis Nova Experiment Can Discover CP Violation at 3 Reactor Exp. Best for Determining 13 Reactor Exp. Offaxis Exp. Reactor and Offaxis Exps. Are Complementary • Reactor experiment needed for determining q13  Is q13 large enough? • Then offaxis studies of n andn give sensitivity to CP violation sin22θ13 = 0.05, δCP=0, Δm2 = 2.5×10-3 eV2(3 yr reactor, 5yr T2K) Reactor experiment measures or limits the value of q13 Indicates if Offaxis experiments have CP violation discovery potential (Kendall McConnel and M. Shaevitz – hep-ex/0409028)

  40. Consensus Recommendation 2 (of 3): An expeditiously deployed multi-detector reactor experiment with sensitivity … sin22q13=0.01 … A timely accelerator experiment with comparable … sensitivity … A proton driver … with an appropriate very large detector …

  41. Long History of ReactorNeutrino Measurements The original neutrino discovery experiment, by Reines and Cowan, used reactor anti-neutrinos… e+ νe W n p Reines and Cowan at the Savannah River Reactor The first successful neutrino detector Theνe interacts with a free proton via inverse β-decay: Later the neutron captures giving a coincidence signal. Reines and Cowan used cadmium to capture the neutrons (modern exp. use Gadolinium)

  42. Nuclear reactors are a very intense sources ofνe with a well understood spectrum 3 GW → 6×1020ne/s700 events / yr / ton at 1500 m away Reactor spectrum peaks at ~3.7 MeV Oscillation Max. for Dm2=2.510-3 eV2at L near 1500 m From Bemporad, Gratta and Vogel Arbitrary Observable n Spectrum Cross Section Flux Reactor Measurements of 13 • Disappearance Measurement:Look for small rate deviation from 1/r2 measured at a near and far baselines • Counting Experiment • Compare events in near and far detector • Energy Shape Experiment • Compare energy spectrum in near and far detector

  43. e+ Liquid Scintillatorwith Gadolinium e = Photomultiplier Tube n Experimental Setup • The reaction process is inverse β-decay followed by neutron capture • Two part coincidence signal is crucial for background reduction. • Positron energy spectrum implies the neutrino spectrum • The scintillator will be doped with gadolinium to enhance capture Shielding Eν = Evis + 1.8 MeV – 2me 6 meters nmGd → m+1Gdg’s (8 MeV)

  44. Looking for a small change in the expected rate and/or shape of the observed event Past reactor measurements: Precision Reactor Disappearance Exp. Are Difficult • How to do better than previous reactor experiments? •  Reduce systematic uncertainties due to reactor flux and detector • Optimize baseline • Larger detectors  Reduce and control backgrounds

  45. How Do You Measure a Small Disappearance? • Use identical near and far detectors to cancel many sources of systematics.

  46. Sin22θ13 Reactor Experiment Basics νe νe νe νe νe νe sin22θ13 Well understood, isotropic source of electron anti-neutrinos Oscillations observed as a deficit of νe Eν≤ 8 MeV 1.0 Unoscillated flux observed here Probability νe Survival Probability Distance 1200 to 1800 meters

  47. How Do You Measure a Small Disappearance? • Use identical near and far detectors to cancel many sources of systematics. • Design detectors to allow simple analysis cuts that will have reduced systematic uncertainty.

  48. Detector Design Basics • Homogenous Volume • Viewed by PMT’s • Coverage of 20% or better • Gadolinium Loaded, Liquid Scintillator Target • Enhances neutron capture • Pure Mineral Oil Buffer • To shield the scintillator from radioactivity in the PMT glass.

  49. How Do You Measure a Small Disappearance? • Use identical near and far detectors to cancel many sources of systematics. • Design detectors to eliminate the need for analysis cuts that may introduce systematic error. • Detector cross calibration may be used to further reduce the near/far normalization systematic error. • Use events and sources to cross calibrate • For example, n capture peaks •  Move far detectors to near site for cross calibration

  50. How Do You Measure a Small Disappearance? • Use identical near and far detectors to cancel many sources of systematics. • Design detectors to eliminate the need for analysis cuts that may introduce systematic error. • Detector cross calibration may be used to further reduce the near/far normalization systematic error. • Reduce background rate and uncertainty

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