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

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

Jonathan LinkJonathan Link

Center for Neutrino PhysicsCenter for Neutrino PhysicsVirginia TechVirginia Tech

HQL 2012 HQL 2012

6/12/126/12/12

6/12/20126/12/2012Jonathan LinkJonathan Link

Sterile NeutrinosSterile NeutrinosA 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

Phys.Rept. 427, 257 (2006)


ν3

ν2ν1

m22

m12

mas

s2

But the LSND m2 is at a completely different 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.


The LSND ExperimentThe 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

LSND’s Signature

2.2 MeV neutron capture Čerenkov

Scintillation

+ +

e+ e

eepp e e+ + nnGolden ModeGolden Mode

Stopped Pion Beam

Inverse Inverse ββ-decay-decay

LSND LSND ννμμ→→ ν νee Appearance Appearance

Event Excess: 32.2 ± 9.4 ± 2.3


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


The KARMEN ExperimentThe KARMEN Experiment

LSND

Downstream of target

100°

Not downstream of the target

Only 18 meters baseline

Gadolinium for neutron capture

Karmen

Stopped π+ beam experiment like LSND

KARMEN KARMEN ννμμ→→ ν νee Appearance Search Appearance Search


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

15 candidate events that agree with the background expectation

KARMEN KARMEN ννμμ→→ ν νee Appearance Search Appearance Search

Joint LSND & KARMEN analysis


15 candidate events that agree with the background expectation

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

Church et al., Phys.Rev.D66 013001

(2002)


The Bugey ExperimentThe Bugey Experiment

6Li doped

Reactor antineutrinos observed at three baselines:15, 40 and 95 m

Sensitivity from absolute rate and near/far comparisons

Achkar et al., Nucl.Phys.B434, 503 (1995)

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 probabilities (like LSND) with 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)


(at oscillation maximum)

Bugey Bugey ννee Disappearance Disappearance


Assuming Ue4=Uμ4 and Us4≈1…


e?

The MiniBooNE ExperimentThe MiniBooNE Experiment

π+ (π−) decay in flight beam

500 m baseline

Mean ν energy of about 500 MeV

L/E of about 1 m/MeV

Primary objective was to look for νe appearance in a νμ beam

MiniBooNE MiniBooNE ννμμ→→ ν νee Appearance Search Appearance Search

Found No Significant Excess Consistent with LSND


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

MiniBooNE MiniBooNE ννμμ→→ ν νee Appearance Search Appearance Search


Event Excess: 54.9 ± 17.4 ± 16.3

Consistent with LSND

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


Mahn et al., Phys.Rev.D85, 032007 (2012)

Aguilar-Arevalo et al., Phys.Rev.Lett. 103,

061802 (2009)

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 .

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).

Huber, Phys.Rev.C84,024617 (2011)Meuller et al., Phys.Rev.C83,054615 (2011)Schreckenbach et al., Phys.Lett.B160,325 (1985)


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


Mention et al., Phys.Rev.D83 073006 (2011)

Rate only analysis

Bugey Revisited in Light of Reactor AnomalyBugey Revisited in Light of Reactor AnomalyThe constraint from rate goes away.



Global Fits to Particle Physics DataGlobal Fits to Particle Physics Data

3+1 model

Best Fit Parameter Values


Cosmology and the Number of NeutrinosCosmology and the Number of NeutrinosThe 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


Cosmology and the Number of NeutrinosCosmology 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.


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.

( — )

Future Experiments: Radioactive SourceFuture Experiments: Radioactive Source

Three different types of interaction channels have been proposedElastic Scattering: Borexino, SNO+CrCharged Current: LENS, Baksan, Ce-LAND, Borexino, Daya BayNeutral Current: RICOCHET

Potential for oscillometry (imaging the oscillation wave)


The SNO+Cr Concept:

Without Background With Background

Future Experiments: ReactorsFuture Experiments: ReactorsSeveral 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

Future Experiments: Stopped Future Experiments: Stopped ππ Beams BeamsWhat 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:

Future Experiments: Decay in Flight BeamsFuture Experiments: Decay in Flight BeamsProposed 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.


Golden ModeGolden Mode: νμ appearance in a νe beam

With a muon decay beam, νSTORM may not need a near detector.

νSTORM: ( — ) ( — )

Perspectives and ConclusionsPerspectives and Conclusions• There is a great deal of interest in sterile neutrinos lately

Workshop 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.



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.

Perspectives and ConclusionsPerspectives and Conclusions

Sterile Neutrino White PaperSterile Neutrino White PaperFor more information see the Light Sterile Neutrinos: A White Paper (arXiv: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/


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

Documents

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