Light Sterile Neutrinos The Evidence Jonathan Link Center

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: LEP Invisible Z 0 Width is consistent with only three light active neutrinos Jonathan Link 6/12/2012

Sterile Neutrinos A sterile neutrino is a lepton with no ordinary electroweak interaction except those induced by mixing. Three neutrinos allow only 2 independent Δm 2 scales. ν 4 ν 3 Dm 2 ν 1 Solar Dm 12 Dm 2 But the LSND is at a completely different scales. LSND 2 Dm 32 mass 2 Atmospheric ν 3 ν 2 ν 1 Jonathan Link Atmospheric Dm 22 Solar Dm 12 6/12/2012

The LSND Experiment LSND took data from 1993 -98 The full dataset represents nearly 49, 000 Coulombs of protons on target. p + m+ n m Stopped e+ nmne Pion Beam Golden Mode ne p e+ n Inverse β-decay Energy range of 20 to 55 Me. V LSND’s Signature L/E of about 1 m/Me. V Scintillation Čerenkov Baseline of 30 meters 2. 2 Me. V neutron capture Jonathan Link 6/12/2012

LSND νμ→ νe Appearance Aguilar-Arevalo et al. , Phys. Rev. D 64, 112007 (2001) Event Excess: 32. 2 ± 9. 4 ± 2. 3 Jonathan Link 6/12/2012

The KARMEN Experiment Stopped π+ beam experiment like LSND Downstream of target Karmen 100° Not downstream of the target Only 18 meters baseline Gadolinium for neutron capture Jonathan Link 6/12/2012

KARMEN νμ→ νe Appearance Search Armbruster et al. , Phys. Rev. D 65 112001 (2002) 15 candidate events that agree with the background expectation Jonathan Link 6/12/2012

KARMEN νμ→ νe Appearance Search Armbruster et al. , Phys. Rev. D 65 112001 (2002) Joint LSND & KARMEN analysis Church et al. , Phys. Rev. D 66 013001 (2002) 15 candidate events that agree with the background expectation Jonathan Link 6/12/2012

The Bugey Experiment Reactor antineutrinos observed at three baselines: 15, 40 and 95 m Sensitivity from absolute rate and near/far comparisons 6 Li doped Achkar et al. , Nucl. Phys. B 434, 503 (1995) Jonathan Link 6/12/2012

Mixing with a Fourth, Mostly Sterile, Mass Eigenstate Comparing appearance probabilities (like LSND) with disappearance probabilities (like Bugey) requires some care… ν 4 Ue 4 2 Uμ 42 Uτ42 Sterile Dm 32 Us 42 The appearance probability: 22 U sin 2 2(1. 27Δm 2 L/E) Pμe =4 Usin e 4 2θ μ 4 3 The disappearance probability: Pee ≈ = Pes = + 4 U Peμe 4+2 PUeτs 42 sin 2(1. 27Δm 32 L/E) ν 3 ν 2 ν 1 Atmospheric Dm 22 Solar Dm 12 If Ue 4≈Uμ 4 and Us 4 ≈ 1 then Pee ≈ 2√Pμe (at oscillation maximum) Jonathan Link 6/12/2012

Bugey νe Disappearance Assuming Ue 4=Uμ 4 and Us 4≈1… Jonathan Link 6/12/2012

The Mini. Boo. NE 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 Me. V L/E of about 1 m/Me. V Jonathan Link 6/12/2012

Mini. Boo. NE νμ→ νe Appearance Search Aguilar-Arevalo et al. , Phys. Rev. Lett. 98, 231801 (2007) Found No Significant Excess Consistent with LSND Jonathan Link 6/12/2012

Mini. Boo. NE νμ→ νe Appearance Search Event Excess: 54. 9 ± 17. 4 ± 16. 3 Consistent with LSND Jonathan Link 6/12/2012

νμ and νμ Disappearance Neutrino and antineutrino disappearance rates should be equal (Assuming CPT is conserved) Neutrinos Mini. Boo. NE and Sci. Boo. NE Two Baseline Analysis Antineutrinos Aguilar-Arevalo et al. , Phys. Rev. Lett. 103, 061802 (2009) Mahn et al. , Phys. Rev. D 85, 032007 (2012) Jonathan Link 6/12/2012

Gallium Anomaly (νe Disappearance) The solar radiochemical detectors GALLEX and SAGE used intense EC sources (51 Cr and 37 Ar) to “calibrate” the νe. Ga cross section. Giunti & Laveder The average ratio of measurement to theory is R=0. 86± 0. 05 (Bahcall) Or R=0. 76+0. 09. (Haxton) − 0. 08 The deficit may be due to sterile neutrino oscillations. Jonathan Link 6/12/2012

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. C 84, 024617 (2011) Meuller et al. , Phys. Rev. C 83, 054615 (2011) Schreckenbach et al. , Phys. Lett. B 160, 325 (1985) Jonathan Link 6/12/2012

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. D 83 073006 (2011) Jonathan Link 6/12/2012

Bugey Revisited in Light of Reactor Anomaly The constraint from rate goes away. Jonathan Link 6/12/2012

Global Fits to Particle Physics Data 3+1 model Best Fit Parameter Values Jonathan Link 6/12/2012

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 6/12/2012

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 a way around or back door out for each individual hint. This is also true of the null results. Jonathan Link 6/12/2012

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 Mini. Boo. NE, but with two detectors, some with innovative ideas for detectors and beam lines. Jonathan Link 6/12/2012

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: Without Background Jonathan Link With Background 6/12/2012

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 6/12/2012

Future Experiments: Stopped π Beams What if LSND is new physics but not oscillations? This may be the only approach that is sensitive. Osc. SNS: Improved LSND with off beam axis, lower duty factor, gadolinium(? ) LSND-Reloaded: Gd-loaded Super-K plus cyclotron. Possibility of oscillometry. Osc. SNS: Jonathan Link 6/12/2012

Future Experiments: Decay in Flight Beams Proposed experiment in this class include Micro. Boo. NE, LAr, NESSi. E, νSTORM Most of these are two detector experiments which will fix one of the greatest difficulties of Mini. Boo. NE. νSTORM: (—) Golden Mode: Mode νμ appearance in a νe beam With a muon decay beam, νSTORM may not need a near detector. Jonathan Link 6/12/2012

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, ar. Xiv: 1204. 5379, April 2012 • There are many hints of sterile neutrinos in particle physics: LSND, Mini. Boo. NE ν, Gallium, Reactor Flux • There are many null or ambiguous results as well: KARMEN, Bugey, Mini. Boo. NE ν, Accelerator Disappearance • There are several proposals/concepts for new, hopefully definitive tests of the Δm ~ 1 e. V 2 sterile neutrino hypothesis. Jonathan Link 6/12/2012

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 6/12/2012

Sterile Neutrino White Paper For more information see the Light Sterile Neutrinos: A White Paper (ar. Xiv: 1204. 5379 [hep-ph]) 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 http: //cnp. phys. vt. edu/white_paper/ Jonathan Link 6/12/2012