Asking for Directions in Ireland... Continuing the Hunt for the Sterile Neutrino

1. Asking for Directions in Ireland...Continuingthe Hunt for the Sterile Neutrino Joseph A. Formaggio University of Washington NuFACT’03

2. Outline • Steriles from two points of view: • Light Steriles n’s: • Why look? • How can current and future experiments constrain them? • What can we say now? • Heavy Sterile n’s: • Signatures and Techniques • Previous experiments • n Factories

3. hierarchical degenerate Higgs Field n n n t µ e Window to new physics! Three Pieces • For a full understanding of neutrino properties, it is necessary to determine: • The mixing parameters and phases • The neutrino masses • The physics governing mass

4. Unresolved Questions: Why are neutrino masses so small? Why are neutrinos seen only as left-handed particles? Is this compatible with neutrino mass at all? Can neutrino mass tell us what is going on at a much higher energy scale? Sterile Neutrinos: Can be invoked as right-handed partners to the neutrino Naturally introduce mass into the (extended) SM and extend symmetry to quarks Arise naturally from more comprehensive theories (Grand Unified Theories, etc.) Steriles: Theoretical Motivation jL only jL and jR

5. Beyond the Standard Model • It is possible to introduce heavy neutral leptons (“sterile” neutrinos) into the Standard Model • Steriles exhibit certain properties in common with ordinary neutrinos: • Massive (if high enough, potentially unstable) • No electromagnetic couplings • No strong couplings • No direct weak couplings !

6. Light Probing Sterile Neutrinos

7. Steriles: Experimental Motivation • LSND sees oscillation signal nm to ne • Combined analysis with KARMEN experiment consistent with Dm2 < 1 eV2 Oscillation Probability: (0.264±0.067±0.045)% E. Church, K. Eitel, G. Mills, M. Steidl hep/ex 0203023

8. ? } LSND Doesn’t Add Up • If dealing with 3 neutrino species then… Dmsolar2 +Dmatmo2 +DmLSND2 = 0 • But if Dm122 andDm232 are both small then Dm312 must be small as well (constrained). • …but this is not the case. Dmsolar2 ~ 10 -5 eV2 Dmatm2 ~ 10 -3 eV2 DmLSND2 ~ 1 eV2 Atmospheric { Solar {

9. 2+2 Model: Assumes mass difference of LSND signal bridges between solar and atmospheric Model gives prediction of: 3+1 Model: Assumes LSND mass apart from near degenerate weak states. Two Phenomenological Models ne nt nm ns Psolar + Patmo = 1

10. Best fit favors no oscillations to sterile neutrinos Pure mixing ve -> vs ruled out at the 5s level Partial mixing (global fit) of mixing parameter to steriles (h): What is the problem? Sterile content driven by uncertainties in the flux. Is there a way to constrain it further? What About Sterile Neutrinos? John N. Bahcall, M. C. Gonzalez-Garcia, C. Pena-Garay hep/ph 0204194 No Sterile Mixing sin2 h < 0.70 (3s) Full Sterile Mixing 8B Flux

11. SNO provides an independent measure of the active content, but no constraint on the flux. KamLAND only reveals information about the mixing parameters, but not on the total active flux. Combining the two experiments provide an independent check on the active and sterile components If SNO improves NC measurement, this uncertainty will decrease even further Solar model independent. Combining KamLAND & SNO John N. Bahcall, M. C. Gonzalez-Garcia, C. Pena-Garay hep/ph 0202147 sin2 h < 0.52 (3s)

12. Day, Night, and Steriles? • One can use day and night rates for neutral current rates in the sun in order to limit sterile neutrinos. • Void of solar model systematic uncertainties • Results consistent with no sterile content at 1.5 s level. Signal Extraction in CC, NC, ES ASNOCC = +14.0 ± 6.3 % ASNONC = -20.4 ± 16.9 %

13. + 1% + 10% + 20, -16% Precision Solar pp Measurements • If one is able to access pp neutrinos from the sun, model is known extremely well. • Limit there stems from detector uncertainties • Need a dedicated experiment to probe this region and test both solar physics and steriles (but can you do both?)

14. 2+2 Model: Model gives prediction for solar and atmospheric constraints New data now available: SNO’s neutral current measurement Recent data analysis from SK and KamLAND Updated numbers from the LSND collaboration on oscillation parameters Do the Models Survive? Psolar + Patmo = 1 M. Maltoni, T.Schwetz, M. Tortola, J. Valle hep/ph 0209368

15. 3+1 Model: Model usually disfavored due to restrictions from reactor limits New oscillation probability from LSND recent analysis places more favor on the 3+1 scheme Most analyses place only marginal survival to the 3+1 scheme. Do the Models Survive? M. Maltoni, T.Schwetz, M. Tortola, J. Valle hep/ph 0209368

16. 3+2, 4+1, etc. Models • Can also look at constraints for model with more than one single sterile neutrino. • Constraints mostly come from short and long baseline experiments. • Region exists which is still compatible with current experimental observations. M. Sorel, J. Conrad, and M. Shaevitz. hep/ph 0305255

17. Probing Sterile Neutrinos Heavy

18. Conventional (p/K beams): Couples to n from two-body decay. Dominated by p decay (with contamination from K decay) Mass range limited by p/K mass e+ L0 m + n Neutral Heavy Lepton Production p+ n-Factory / m-Storage (m beams): • Produced solely from m+/m- decay • Mass range limited to <105 MeV.

19. GeV Scale Steriles • If sterile neutrinos possess a mass on the order of ~GeV scales, these can be produced and later decay to lighter particles. • Ideal of high energy neutrino beams for searching for such particles. • Various mass ranges and models ruled out by direct searches. Mass (GeV) A. Vaitaitis et al., Phys. Rev. Lett. 83, 4943 (1999)

20. Production Rates • Production rates and branching ratio both scale as coupling | U | 2 Detector Efficiency / Acceptance Branching Ratio

21. Production Rates • Maximum sensitivity independent of mass to first order: • Production rates and branching ratio both scale as coupling | U | 2 NL0seen~ U4 Qprod e QdecayL(tmin) Pure geometric factor

22. For pion and muon sources, three channels typically dominate: Ability to identify g / e+e- in detector. g-channel often a favorite channel for very small (~ few keV) sterile neutrino masses. Detection Channels L0n n n L0 e+e- n L0n g Typically smaller by ~1/20

23. Detection Techniques : Low Mass Targets • Disadvantages: • Low target requirements makes instrumentation difficult. • High intensity neutrino beams induce large background from surrounding material. • Advantages: • Makes use of existing system. • Cover large area with little instrumentation • Makes use of kinematic features of decay.

24. Can use timing a signature to distinguish potential signal events from background. One example, the KARMEN experiment was sensitive to steriles produced near the kinematic threshold for pion decay. Disadvantages: Sensitive to small mass range for neutral heavy lepton production. Lesson from KARMEN: Anomalous signal eventually ruled out as statistical or beam-induced background. Detection Techniques: Timing

25. Detection Techniques: Timing • Previous technique limited to slow-moving particles. • Combination of bunched timing and long baselines enable detecting particles out of sync with main beamline. • Opens sensitivity to high mass particles. • Mono-energetic beams, such as off-axis, ideal sources.

26. Case Example: NuTeV • NuTeV • High energy neutrino beam from 800 GeV protons. • Instrumented low mass (He) decay region to look for neutral heavy lepton decays. • Makes use of kinematic selection of events to reduce backgrounds in signal region to <<1 event.

28. Hint of Something Out There? • NuTeV has measured an anomalous rate of dimuon events in its apparatus. • Could be a massive (~5 GeV) sterile neutrino decaying to mmn • However, certain aspects of event topology consistent with background.

29. Neutrino factory settings provide a large neutrino beam flux for increased sensitivity to sterile neutrinos. Limited to below 105 MeV Sensitive to Ue and Um couplings Relatively insensitive to distance or energy of beam (w/o taking backgrounds into consideration). Neutrino Factory Sensitivities

30. Projected Sensitivity: Considered muon storage ring with 3 x 1019 and 3 x 1020m decays. Energy ranges between 20 and 50 GeV (for m) show very little differences. Improvement over limited mass range (below 105 MeV). Neutrino Factory Sensitivities Muon Decay SIN 1019m decays KEK LBL 1020m decays Muon Storage Ring

31. Light Steriles: Information for solar, atmospheric, and LBL experiments provide model tests on 3+n generation models. Current and upcoming solar, reactor, and atmospheric measurements to increase sterile sensitivity. MiniBooNE to explicitly test LSND signal. Heavy Steriles: Superbeams and n-factories provide ideal source for heavy sterile searches. To overcome background issues, combination of low-mass targets, kinematic selection, and timing information must be used. Often a complimentary program to oscillation physics. Conclusions

33. What the Future Holds

34. If MiniBooNE sees a signal in neutrino running: Confirmation of LSND Is a single sterile compatible with rest of oscillation data? If MiniBooNE sees a signal only in anti-neutrino running: CPT violation a reality? What other systems test this scenario ? What to Expect

35. Despite the progress made, the fundamental properties of neutrinos still remain to be explored: What is the full mixing of neutrinos? Do they exhibit CP violation? What is the neutrino mass scale? What physics governs the mass scale? Road Map to New Physics

36. Using Solar & Reactor: Direct measurement of 8B flux from sun from SNO measurement Use SNO + KamLAND to further constrain solar models Use lowest energy solar n’s to understand steriles Global Fits: Use combined fits to constrain 3+n models Direct verification of LSND: MiniBooNE Testing the Sterile Content