Light Sterile Neutrinos: The Evidence Jonathan Link Virginia Tech

Light Sterile Neutrinos: The Evidence Light Sterile Neutrinos: The Evidence

Jonathan LinkJonathan Link

Virginia TechVirginia Tech

Workshop on Neutrinos at the SNS Workshop on Neutrinos at the SNS

Oak Ridge National Lab Oak Ridge National Lab

5/2/125/2/125/2/20125/2/2012Jonathan LinkJonathan Link

5/2/20125/2/2012Jonathan LinkJonathan Link

OutlineOutline

1. Sterile Neutrinos in Theory and Phenomenology

2. Evidence from Particle Physics

3. Evidence from Cosmology

4. Proposed Future Experiments


Sterile NeutrinosSterile 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


ν3

ν2ν1

m22

m12

mas

s2

But there is evidence for 3 different m2 scales.

Atmospheric

Solar

ν3

ν2ν1

m22

m12

mas

s2

ν4

m32

Atmospheric

Solar

LSND

Sterile NeutrinosSterile Neutrinos

Three neutrinos allow only 2 independent Δm2 scales.

A sterile neutrino is a lepton with no ordinary electroweak interaction except those induced by mixing.

LSND LSND ννμμ→→ ν νee Appearance Appearance

Event Excess: 32.2 ±9.4 ±2.3

Stopped π+ beam 30 meters baseline The detector was downstream of the target.


KARMEN KARMEN ννμμ→→ ν νee Appearance Search 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


Bugey Bugey ννee Disappearance Disappearance

Reactor Neutrinos 15, 40 and 95 m baselines

Sensitivity from absolute rate and near/far comparisons


Pee = Pes + Peμ + Peτ

Atmospheric

Sterile

ν1m1

2

ν3

ν2

m22

ν4

m32

Solar

Mixing with a Fourth, Mostly Sterile, Mass EigenstateMixing with a Fourth, Mostly Sterile, Mass Eigenstate

Ue42 Uμ4

2 Uτ42 Us4

2

Pμe

= sin2(1.27Δm32L/E)sin22θ4Ue4

2 Uμ42

The appearance probability:

The disappearance probability:

Comparing appearance (like LSND) and disappearance probabilities (like Bugey) requires some care…

If Ue4≈Uμ4 and Us4 ≈ 1 then Pee ≈ 2√Pμe

Pee ≈ Pes = 4Ue42 Us4

2 sin2(1.27Δm32L/E)


Bugey Bugey ννee Disappearance Disappearance

Reactor Neutrinos 15, 40 and 95 m baselines

Sensitivity from absolute rate and near/far comparisons


Assuming Ue4=Uμ4

MiniBooNE MiniBooNE ννμμ→→ ν νee Appearance Search Appearance Search

Found No Significant Excess Consistent with LSND

π+ decay in flight beam ~500 m baseline


MiniBooNE MiniBooNE ννμμ→→ ν νee Appearance Search Appearance Searchπ- decay in flight beam ~500 m baseline


ννμμ and and ννμμ Disappearance DisappearanceNeutrino and antineutrino disappearance rates should be equal

(Assuming CPT is conserved)

MiniBooNE and SciBooNE Two

Baseline Analysis

Neutrinos

Antineutrinos


Gallium Anomaly (Gallium Anomaly (ννee Disappearance) Disappearance)The solar radiochemical detectors GALLEX and SAGE used intense EC sources (51Cr and 37Ar) to “calibrate” the νeGa 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.

+0.09−0.08

(Bahcall)

(Haxton)

Giunti & Laveder


Reactor AnomalyReactor AnomalyNew analyses (blue and red) of the reactor νe spectrum predict a 3% higher flux than the existing calculation (black).

HuberMeuller et al.


Bugey RevisitedBugey RevisitedThe constraint from rate goes away.



Cosmology and the Number of NeutrinosCosmology 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)

effN

Future Experiments: SourcesFuture 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.)


Future Experiments: ReactorsFuture 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.


Future Experiments: Stopped Future Experiments: Stopped ππ Beams 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.


Future Experiments: Decay in Flight BeamsFuture 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.


Conclusions and PerspectivesConclusions and Perspectives

• There is a great deal of interest in latelyWorkshop on Beyond Three Family Neutrino Oscillations, LNGS, April 2011Short-Baseline Neutrino Workshop, Fermilab, May 2011Sterile Neutrinos at the Crossroads Workshop, Virginia Tech, Sept. 2011Future Short Baseline Neutrino Experiments −Needs & Options, Fermilab, March 2012Light 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.



Conclusions and PerspectivesConclusions 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.


Supplemental Slides

Sterile Neutrinos at the Crossroads WorkshopSterile Neutrinos at the Crossroads Workshop

Workshop ObjectiveThe 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


Sterile Neutrino White PaperSterile 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 http://cnp.phys.vt.edu/white_paper/


http://cnp.phys.vt.edu/white_paper/

Light Sterile Neutrinos: The Evidence Jonathan Link Virginia Tech

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