Light Sterile Neutrinos: The Evidence Jonathan Link Virginia Tech Workshop on Neutrinos at the SNS Oak Ridge National Lab 5/2/12 5/2/12 5/2/2012Jonathan Link

5/2/2012Jonathan LinkOutline 1.Sterile Neutrinos in Theory and Phenomenology 2.Evidence from Particle Physics 3.Evidence from Cosmology 4.Proposed Future Experiments

5/2/2012Jonathan Link Sterile Neutrinos A sterile neutrino is a lepton with no ordinary electroweak interaction except those induced by mixing. Active neutrinos: LEP Invisible Z 0 Width is consistent with only three light active neutrinos

5/2/2012Jonathan Link ν3ν3 ν2ν2 ν1ν1 m22m22 m12m12 mass 2 But there is evidence for 3 different  m 2 scales. Atmospheric Solar ν3ν3 ν2ν2 ν1ν1 m22m22 m12m12 mass 2 ν4ν4 m32m32 Atmospheric Solar LSND Sterile Neutrinos Three neutrinos allow only 2 independent Δm 2 scales. A sterile neutrino is a lepton with no ordinary electroweak interaction except those induced by mixing.

LSND ν μ → ν e Appearance Event Excess: 32.2 ±9.4 ±2.3 Stopped π + beam 30 meters baseline The detector was downstream of the target. 5/2/2012Jonathan Link

KARMEN ν μ → ν e Appearance Search Stopped π + beam 18 meters baseline The detector was slightly behind the target. 15 candidate events agree with background expectation Joint LSND & KARMEN analysis 5/2/2012Jonathan Link

Bugey ν e Disappearance Reactor Neutrinos 15, 40 and 95 m baselines Sensitivity from absolute rate and near/far comparisons 5/2/2012Jonathan Link

P ee = P es + P eμ + P eτ Atmospheric Sterile ν1ν1 m12m12 ν3ν3 ν2ν2 m22m22 ν4ν4 m32m32 Solar Mixing with a Fourth, Mostly Sterile, Mass Eigenstate U e4 2 Uμ42Uμ42 Uτ42Uτ42 U s4 2 P μe = sin 2 (1.27Δm 3 2 L/E) sin 2 2θ 4U e4 2 U μ4 2 The appearance probability: The disappearance probability: Comparing appearance (like LSND) and disappearance probabilities (like Bugey) requires some care… If U e4 ≈U μ4 and U s4 ≈ 1 then P ee ≈ 2√P μe P ee ≈ P es = 4U e4 2 U s4 2 sin 2 (1.27Δm 3 2 L/E) 5/2/2012Jonathan Link

Bugey ν e Disappearance Reactor Neutrinos 15, 40 and 95 m baselines Sensitivity from absolute rate and near/far comparisons 5/2/2012Jonathan Link Assuming U e4 =U μ4

MiniBooNE ν μ → ν e Appearance Search Found No Significant Excess Consistent with LSND π + decay in flight beam ~500 m baseline 5/2/2012Jonathan Link

MiniBooNE ν μ → ν e Appearance Search π - decay in flight beam ~500 m baseline 5/2/2012Jonathan Link

ν μ and ν μ Disappearance Neutrino and antineutrino disappearance rates should be equal (Assuming CPT is conserved) MiniBooNE and SciBooNE Two Baseline Analysis Neutrinos Antineutrinos 5/2/2012Jonathan Link

Gallium Anomaly (ν e Disappearance) The solar radiochemical detectors GALLEX and SAGE used intense EC sources ( 51 Cr and 37 Ar) to “calibrate” the ν e Ga cross section. 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 −0.08 (Bahcall) (Haxton) Giunti & Laveder 5/2/2012Jonathan Link

Reactor Anomaly New analyses (blue and red) of the reactor ν e spectrum predict a 3% higher flux than the existing calculation (black). Huber Meuller et al. 5/2/2012Jonathan Link

Bugey Revisited The constraint from rate goes away. 5/2/2012Jonathan Link

5/2/2012Jonathan Link Cosmology and the Number of Neutrinos The energy density of neutrinos is proportional to the number of neutrino families, N eff : 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)

Future Experiments: Sources Intense radioactive sources (EC, fission fragments) plus low energy detectors to search for ν e and ν e 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.) 5/2/2012Jonathan Link

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. 5/2/2012Jonathan Link

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. 5/2/2012Jonathan Link

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. 5/2/2012Jonathan Link

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 Future Short Baseline Neutrino Experiments −Needs & Options, Fermilab, March 2012 Light Sterile Neutrinos: A White Paper, arXiv: , 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 eV 2 sterile neutrino hypothesis. 5/2/2012Jonathan Link

5/2/2012Jonathan Link 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.

5/2/2012Jonathan Link Supplemental Slides

Sterile Neutrinos at the Crossroads Workshop 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. Virginia Tech, September 26-28, 2011 ~60 participants, 28 talks and 9 poster with lots of time for discussion 5/2/2012Jonathan Link

Sterile Neutrino White Paper Outline: 1.Theory and Motivation (editors Barenboim & Rodejohann) 2.Astrophysical Evidence (Abazajian & Wong) 3.Evidence from Oscillation Experiments (Koop & Louis) 4.Global Picture (Lasserre & Schwetz) 5.Requirements for Future Experiments (Fleming & Formaggio) 6.Appendix: Possible Future Experiments (Huber & Link) Written from an international perspective for an audience including both the scientific community and funding agencies. Visit 5/2/2012Jonathan Link