Light Sterile Neutrinos: The Evidence
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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

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 Z0 Width is consistent with only three light active neutrinos

Jonathan Link


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


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


LSND νμ→ νe Appearance

Aguilar-Arevalo et al., Phys.Rev. D64, 112007 (2001)

Event Excess: 32.2 ± 9.4 ± 2.3

Jonathan Link


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


KARMEN νμ→ νe Appearance Search

Armbruster et al., Phys.Rev.D65 112001 (2002)

15 candidate events that agree with the background expectation

Jonathan Link


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


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


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


Bugeyνe Disappearance

Assuming Ue4=Uμ4and Us4≈1…

Jonathan Link


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


MiniBooNEνμ→ νe Appearance Search

Aguilar-Arevalo et al., Phys.Rev.Lett. 98, 231801 (2007)

Found No Significant Excess Consistent with LSND

Jonathan Link


MiniBooNEνμ→ νe Appearance Search

Event Excess: 54.9 ± 17.4 ± 16.3

Consistent with LSND

Jonathan Link


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


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


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


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


Bugey Revisited in Light of Reactor Anomaly

The constraint from rate goes away.

Jonathan Link


Global Fits to Particle Physics Data

3+1 model

Best Fit Parameter Values

Jonathan Link


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


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


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


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


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


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


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


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


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


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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