Light Sterile Neutrinos: The Evidence Jonathan Link Virginia Tech

1. Light Sterile Neutrinos: The Evidence Jonathan Link Virginia Tech Workshop on Neutrinos at the SNS Oak Ridge National Lab 5/2/12 Jonathan Link

2. Outline Sterile Neutrinos in Theory and Phenomenology Evidence from Particle Physics Evidence from Cosmology Proposed Future Experiments Jonathan Link

3. Sterile Neutrinos A sterile neutrino is a lepton with no ordinary electroweak interaction except those induced by mixing. Active neutrinos: LEP Invisible Z0 Width is consistent with only three light active neutrinos Jonathan Link

4. Sterile Neutrinos A sterile neutrino is a lepton with no ordinary electroweak interaction except those induced by mixing. Three neutrinos allow only 2 independent Δm2 scales. ν4 ν3 Atmospheric LSND Dm22 mass2 Dm32 ν2 Solar Dm12 ν1 mass2 ν3 Atmospheric But there is evidence for 3 different Dm2 scales. Dm22 ν2 Solar Dm12 ν1 Jonathan Link

5. LSND νμ→ νe Appearance Stopped π+ beam 30 meters baseline The detector was downstream of the target. Event Excess: 32.2 ±9.4 ±2.3 Jonathan Link

6. KARMEN νμ→ νe Appearance Search Stopped π+ beam 18 meters baseline The detector was slightly behind the target. Joint LSND & KARMEN analysis 15 candidate events agree with background expectation Jonathan Link

7. Bugeyνe Disappearance Reactor Neutrinos 15, 40 and 95 m baselines Sensitivity from absolute rate and near/far comparisons Jonathan Link

8. ν4 Ue42 Uμ42 Dm32 Uτ42 Us42 ν3 Dm22 ν2 Solar Dm12 ν1 Pee= Pes + Peμ + Peτ Pee≈ Pes = 4Ue42Us42sin2(1.27Δm32L/E) Mixing with a Fourth, Mostly Sterile, Mass Eigenstate Comparing appearance (like LSND) and disappearance probabilities (like Bugey) requires some care… Sterile The appearance probability: sin2(1.27Δm32L/E) Pμe = sin22θ 4Ue42Uμ42 Atmospheric The disappearance probability: If Ue4≈Uμ4and Us4≈ 1 then Pee≈ 2√Pμe Jonathan Link

9. Bugeyνe Disappearance Reactor Neutrinos 15, 40 and 95 m baselines Sensitivity from absolute rate and near/far comparisons Assuming Ue4=Uμ4 Jonathan Link

10. MiniBooNEνμ→ νe Appearance Search π+ decay in flight beam ~500 m baseline Found No Significant Excess Consistent with LSND Jonathan Link

11. MiniBooNEνμ→ νe Appearance Search π- decay in flight beam ~500 m baseline Jonathan Link

12. νμ and νμ Disappearance Neutrino and antineutrino disappearance rates should be equal (Assuming CPT is conserved) Neutrinos MiniBooNE and SciBooNE Two Baseline Analysis Antineutrinos Jonathan Link

13. Gallium Anomaly (νe Disappearance) The solar radiochemical detectors GALLEX and SAGE used intense EC sources (51Cr and 37Ar) to “calibrate” the νeGa cross section. Giunti & Laveder The average ratio of measurement to theory is R=0.86±0.05 Or R=0.76 . Bahcall does not calculate a theory uncertainty. The deficit may be due to sterile neutrino oscillations. (Bahcall) (Haxton) +0.09 −0.08 Jonathan Link

14. Reactor Anomaly New analyses (blue and red) of the reactor νe spectrum predict a 3% higher flux than the existing calculation (black). Huber Meuller et al. Jonathan Link

15. Bugey Revisited The constraint from rate goes away. Jonathan Link

16. Cosmology and the Number of Neutrinos • The energy density of neutrinos • is proportional to the number of neutrino families, Neff : • The expansion rate of the radiation dominated era of the early universe depends on the density • of relativistic particles. • This can be measured in: • Large Scale Structure (LSS) • Cosmic Microwave Background (CMB) • Big-Bang Nucleosynthesis (BBN) Jonathan Link

17. Future Experiments: Sources • Intense radioactive sources (EC, fission fragments) plus low energy detectors to search for νe and νe disappearance. • Elastic Scattering: Borexino, SNO+Cr • Charged Current: LENS, Baksan, Ce-LAND, Borexino, Daya Bay • Neutral Current: RICOCHET • Most use existing detectors (low cost) • Potential for oscillometry (imaging the oscillation wave) • R&D needed on sources (especially in the U.S.) Jonathan Link

18. Future Experiments: Reactors Several ideas for new reactor experiments. Some, piggy-backing on safe guards measurements are under construction now. Nucifer, SCRAAM, Stereo, PIK… Need to be very close to the reactor core. Small cores are advantageous. Jonathan Link

19. Future Experiments: Stopped π Beams • Direct test of LSND anomaly. • OscSNS: Improved LSND (Off beam axis, lower duty factor, gadolinium?) • LSND-Reloaded: Gd-loaded Super-K plus cyclotron. Possibility of oscillometry. • What if LSND is new physics but not oscillations? This may be the only approach that is sensitive. Jonathan Link

20. Future Experiments: Decay in Flight Beams MiniBooNE was a decay in flight experiment. Proposed experiment in this class include MicroBooNE, BooNE, LArLAr NESSiE, νSTORM Most of these are two detector experiments which will fix the greatest difficulty with MiniBooNE. With a muon decay beam, νSTORM may not need a near detector. Jonathan Link

21. Conclusions and Perspectives • There is a great deal of interest in lately • Workshop on Beyond Three Family Neutrino Oscillations, LNGS, April 2011 • Short-Baseline Neutrino Workshop, Fermilab, May 2011 • Sterile Neutrinos at the Crossroads Workshop, Virginia Tech, Sept. 2011 • Future Short Baseline Neutrino Experiments −Needs & Options, Fermilab, March 2012 • Light Sterile Neutrinos: A White Paper, arXiv:1204.5379, April 2012 • There are many hints of sterile neutrinos in particle physics: LSND, MiniBooNE ν, Gallium, Reactor Flux • There are many null or ambiguous results as well: KARMEN, Bugey, MiniBooNE ν, Accelerator Disappearance • There are several proposals/concepts for new, hopefully definitive tests of the Δm ~ 1 eV2 sterile neutrino hypothesis. Jonathan Link

22. Conclusions and Perspectives The hints for light sterile neutrinos from particle physics and cosmology are certainly not definitive − individually or combined But They can’t just be ignored. This situation calls for further, definitive investigation. Jonathan Link

23. Supplemental Slides Jonathan Link

24. Sterile Neutrinos at the Crossroads Workshop Virginia Tech, September 26-28, 2011 Workshop Objective The goal of this workshop was to bring together experts in the various sub-disciplines − such as nuclear and particle theory and experiment, cosmology, and neutrino phenomenology − in order to critically review the evidence for and against sterile neutrinos and to discuss the need or otherwise to pursue dedicated new experiments and possibly new strategies. ~60 participants, 28 talks and 9 poster with lots of time for discussion Jonathan Link

25. Sterile Neutrino White Paper • Outline: • Theory and Motivation (editors Barenboim & Rodejohann) • Astrophysical Evidence (Abazajian & Wong) • Evidence from Oscillation Experiments (Koop & Louis) • Global Picture (Lasserre & Schwetz) • Requirements for Future Experiments(Fleming & Formaggio) • Appendix: Possible Future Experiments (Huber & Link) • Written from an international perspective for an audience including both the scientific community and funding agencies. • Visithttp://cnp.phys.vt.edu/white_paper/ Jonathan Link