Transcript Slide 1

Light Sterile Neutrinos: The Evidence
Jonathan Link
Virginia Tech
Workshop on Neutrinos at the SNS
Oak Ridge National Lab
5/2/12
Jonathan Link
5/2/2012
Outline
1. Sterile Neutrinos in Theory and Phenomenology
2. Evidence from Particle Physics
3. Evidence from Cosmology
4. Proposed Future Experiments
Jonathan Link
5/2/2012
Sterile 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
Jonathan Link
5/2/2012
Sterile Neutrinos
A sterile neutrino is a lepton with no ordinary electroweak
interaction except those induced by mixing.
Three neutrinos allow only 2 independent Δm2 scales.
ν4
ν3
Dm2
ν2
ν1
LSND
2
Solar
Dm12
But there is evidence for 3
different Dm2 scales.
Dm32
mass2
mass2
Atmospheric
ν3
ν2
ν1
Jonathan Link
Atmospheric
Dm22
Solar
Dm12
5/2/2012
LSND νμ→ νe Appearance
Stopped π+ beam
30 meters baseline
The detector was downstream of the target.
Event Excess: 32.2 ±9.4 ±2.3
Jonathan Link
5/2/2012
KARMEN νμ→ νe Appearance Search
Stopped π+ beam
18 meters baseline
The detector was slightly behind the target.
Joint LSND &
KARMEN analysis
15 candidate events agree with background
expectation
Jonathan Link
5/2/2012
Bugey νe Disappearance
Reactor Neutrinos
15, 40 and 95 m baselines
Sensitivity from absolute rate and near/far comparisons
Jonathan Link
5/2/2012
Mixing with a Fourth, Mostly Sterile, Mass Eigenstate
Comparing appearance (like LSND) and disappearance probabilities
(like Bugey) requires some care…
ν4
Ue4
2
2
Uμ4 Uτ42
Sterile
Dm32
Us42
The appearance probability:
222θ
2 2(1.27Δm 2L/E)
sin
Pμe =4Usin
U
3
e4
μ4
The disappearance probability:
Pee ≈
=P
Pes =
+ 4U
Peμe4+2 PUeτs42 sin2(1.27Δm32L/E)
ν3
ν2
ν1
Atmospheric
Dm22
Solar
Dm12
If Ue4≈Uμ4 and Us4 ≈ 1 then Pee ≈ 2√Pμe
Jonathan Link
5/2/2012
Bugey νe Disappearance
Reactor Neutrinos
15, 40 and 95 m baselines
Sensitivity from absolute rate and near/far comparisons
Assuming Ue4=Uμ4
Jonathan Link
5/2/2012
MiniBooNE νμ→ νe Appearance Search
π+ decay in flight beam
~500 m baseline
Found No Significant Excess Consistent with LSND
Jonathan Link
5/2/2012
MiniBooNE νμ→ νe Appearance Search
π- decay in flight beam
Jonathan Link
~500 m baseline
5/2/2012
νμ and νμ Disappearance
Neutrino and antineutrino disappearance rates should be equal
(Assuming CPT is conserved)
Neutrinos
MiniBooNE and
SciBooNE Two
Baseline Analysis
Antineutrinos
Jonathan Link
5/2/2012
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 (Bahcall)
Or
R=0.76 +0.09
. (Haxton)
−0.08
Bahcall does not calculate
a theory uncertainty.
The deficit may be due to
sterile neutrino oscillations.
Jonathan Link
5/2/2012
Reactor Anomaly
New analyses (blue and red) of the reactor νe spectrum predict a 3%
higher flux than the existing calculation (black).
Huber
Meuller et al.
Jonathan Link
5/2/2012
Bugey Revisited
The constraint from rate goes away.
Jonathan Link
5/2/2012
Cosmology and the Number of Neutrinos
The energy density of neutrinos
is proportional to the number of
neutrino families, Neff :
  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
5/2/2012
Future 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.)
Jonathan Link
5/2/2012
Future Experiments: Reactors
Several ideas for new reactor experiments. Some, piggybacking 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.
Jonathan Link
5/2/2012
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.
Jonathan Link
5/2/2012
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.
Jonathan Link
5/2/2012
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. 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
5/2/2012
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.
Jonathan Link
5/2/2012
Supplemental Slides
Jonathan Link
5/2/2012
Sterile Neutrinos at the Crossroads Workshop
Virginia Tech, September 26-28, 2011
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.
~60 participants, 28 talks and 9 poster
with lots of time for discussion
Jonathan Link
5/2/2012
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 http://cnp.phys.vt.edu/white_paper/
Jonathan Link
5/2/2012