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Light Sterile Neutrinos: The Evidence Jonathan Link Center for Neutrino Physics Virginia Tech

Light Sterile Neutrinos: The Evidence Jonathan Link Center for Neutrino Physics Virginia Tech HQL 2012 6/12/12. Sterile Neutrinos. A sterile neutrino is a lepton with no ordinary electroweak interaction except those induced by mixing. Phys.Rept. 427, 257 (2006). Active neutrinos:

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Light Sterile Neutrinos: The Evidence Jonathan Link Center for Neutrino Physics Virginia Tech

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  1. Light Sterile Neutrinos: The Evidence Jonathan Link Center for Neutrino Physics Virginia Tech HQL 2012 6/12/12

  2. Sterile Neutrinos A sterile neutrino is a lepton with no ordinary electroweak interaction except those induced by mixing. Phys.Rept. 427, 257 (2006) Active neutrinos: LEP Invisible Z0 Width is consistent with only three light active neutrinos Jonathan Link

  3. 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 the LSND Dm2 is at a completely different scales. Dm22 ν2 Solar Dm12 ν1 Jonathan Link

  4. The LSND Experiment LSND took data from 1993-98 The full dataset represents nearly 49,000 Coulombs of protons on target. Baseline of 30 meters Energy range of 20 to 55 MeV L/E of about 1 m/MeV Golden Mode p+ m+ nm nep e+ n e+ nmne Stopped Pion Beam Inverse β-decay LSND’s Signature Scintillation Čerenkov 2.2 MeV neutron capture Jonathan Link

  5. LSND νμ→ νe Appearance Aguilar-Arevalo et al., Phys.Rev. D64, 112007 (2001) Event Excess: 32.2 ± 9.4 ± 2.3 Jonathan Link

  6. The KARMEN Experiment Stopped π+ beam experiment like LSND 100° LSND Downstream of target Karmen Not downstream of the target Only 18 meters baseline Gadolinium for neutron capture Jonathan Link

  7. KARMEN νμ→ νe Appearance Search Armbruster et al., Phys.Rev.D65 112001 (2002) 15 candidate events that agree with the background expectation Jonathan Link

  8. KARMEN νμ→ νe Appearance Search Armbruster et al., Phys.Rev.D65 112001 (2002) Joint LSND & KARMEN analysis Church et al., Phys.Rev.D66 013001 (2002) 15 candidate events that agree with the background expectation Jonathan Link

  9. The Bugey Experiment Reactor antineutrinos observed at three baselines:15, 40 and 95 m Sensitivity from absolute rate and near/far comparisons 6Li doped Achkar et al., Nucl.Phys.B434, 503 (1995) Jonathan Link

  10. 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 probabilities (like LSND) with disappearance probabilities (like Bugey) requires some care… Sterile The appearance probability: sin2(1.27Δm32L/E) Pμe = sin22θ 4Ue42Uμ42 Atmospheric The disappearance probability: (at oscillation maximum) If Ue4≈Uμ4 and Us4 ≈ 1 then Pee≈ 2√Pμe Jonathan Link

  11. Bugeyνe Disappearance Assuming Ue4=Uμ4and Us4≈1… Jonathan Link

  12. The MiniBooNE Experiment Primary objective was to look for νe appearance in a νμ beam nmne? π+ (π−) decay in flight beam 500 m baseline Mean ν energy of about 500 MeV L/E of about 1 m/MeV Jonathan Link

  13. MiniBooNEνμ→ νe Appearance Search Aguilar-Arevalo et al., Phys.Rev.Lett. 98, 231801 (2007) Found No Significant Excess Consistent with LSND Jonathan Link

  14. MiniBooNEνμ→ νe Appearance Search Event Excess: 54.9 ± 17.4 ± 16.3 Consistent with LSND Jonathan Link

  15. νμ and νμ Disappearance Neutrino and antineutrino disappearance rates should be equal (Assuming CPT is conserved) Neutrinos MiniBooNE and SciBooNE Two Baseline Analysis Antineutrinos Aguilar-Arevalo et al., Phys.Rev.Lett. 103, 061802 (2009) Mahn et al., Phys.Rev.D85, 032007 (2012) Jonathan Link

  16. 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 . The deficit may be due to sterile neutrino oscillations. (Bahcall) (Haxton) +0.09 −0.08 Jonathan Link

  17. Reactor Anomaly New analyses (blue and red) of the reactor νe spectrum predict a 3% higher flux than the existing calculation (black). Huber, Phys.Rev.C84,024617 (2011) Meuller et al., Phys.Rev.C83,054615 (2011) Schreckenbach et al., Phys.Lett.B160,325 (1985) Jonathan Link

  18. Reactor Anomaly New analyses (blue and red) of the reactor νe spectrum predict a 3% higher flux than the existing calculation (black). Rate only analysis Mention et al., Phys.Rev.D83 073006 (2011) Jonathan Link

  19. Bugey Revisited in Light of Reactor Anomaly The constraint from rate goes away. Jonathan Link

  20. Global Fits to Particle Physics Data 3+1 model Best Fit Parameter Values Jonathan Link

  21. 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

  22. Cosmology and the Number of Neutrinos • There are large parameter degeneracies in the cosmological fits. • The preferred 4 light neutrinos are actually light degrees of freedom: they don’t have to be neutrinos, an axion would work as well. • BBN disfavors 5 neutrinos, while fits to all of the particle data seem to require 5 neutrinos. • Bottom Line: • While all the hints (from particle physics and cosmology) are tantalizing none is definitive. There is away around or back door out for each individual hint. This is also true of the null results. Jonathan Link

  23. Ideas for Future Experiments • Many groups are interested in pursuing new experiments to search for and perhaps study sterile neutrino oscillations. • Broadly there are four types of experiments being considered: • Radioactive Neutrino Sources: Electron capture or fission fragment sources plus low-energy detectors to search for νe disappearance • Reactor Neutrinos: Compact detectors placed very near a nuclear reactor looking for νe disappearance • Stopped π Beams: Direct test of the LSND anomaly • Decay in Flight Beams: Like MiniBooNE, but with two detectors, some with innovative ideas for detectors and beam lines. ( — ) Jonathan Link

  24. Future Experiments: Radioactive Source • Three different types of interaction channels have been proposed • Elastic Scattering: Borexino, SNO+Cr • Charged Current: LENS, Baksan, Ce-LAND, Borexino, Daya Bay • Neutral Current: RICOCHET • Potential for oscillometry (imaging the oscillation wave) The SNO+Cr Concept: With Background Without Background Jonathan Link

  25. Future Experiments: Reactors Several ideas for new reactor experiments. Nucifer, SCRAAM, Stereo, PIK, NIST… Some, piggy-backing on safe guards measurements are under construction now. Small cores are advantageous. Nucifer: SCRAAM: 24 m Jonathan Link

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

  27. Future Experiments: Decay in Flight Beams Proposed experiment in this class include MicroBooNE, BooNE, LArLAr, NESSiE, νSTORM Most of these are two detector experiments which will fix one of the greatest difficulties of MiniBooNE. νSTORM: ( — ) ( — ) Golden Mode: νμ appearance in a νe beam With a muon decay beam, νSTORM may not need a near detector. Jonathan Link

  28. Perspectives and Conclusions • There is a great deal of interest in sterile neutrinos 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

  29. Perspectives and Conclusions 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

  30. Sterile Neutrino White Paper • For more information see the Light Sterile Neutrinos: A White Paper (arXiv:1204.5379 [hep-ph]) • 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

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