# Presentation: 325732

## Asking for Directions in Ireland...Continuing the Hunt for the Sterile Neutrino

*Asking for Directions in Ireland...*Continuing the Hunt for the Sterile Neutrino

- Joseph A. Formaggio
- University of Washington
- NuFACT’03

## Steriles from two points of view:

Light Steriles ’s:

Why look?
How can current and future experiments constrain them?
What can we say now?

Heavy Sterile ’s:

Signatures and Techniques
Previous experiments
  Factories 

- Steriles from two points of view:
- Light Steriles ’s:
- Why look?
- How can current and future experiments constrain them?
- What can we say now?
- Heavy Sterile ’s:
- Signatures and Techniques
- Previous experiments
-   Factories

- Outline

## Three Pieces

- For a full understanding of neutrino properties, it is necessary to determine:
- The mixing parameters and phases
- The neutrino masses
- The physics governing mass

## Steriles: Theoretical Motivation

- Unresolved Questions:
- Why are neutrino masses so small?
- Why are neutrinos seen only as left-handed particles?  Is this compatible with neutrino mass at all?
- Can neutrino mass tell us what is going on at a much higher energy scale?

- Sterile Neutrinos:
- Can be invoked as right-handed partners to the neutrino
- Naturally introduce mass into the (extended) SM and extend symmetry to quarks
- Arise naturally from more comprehensive theories (Grand Unified Theories, etc.)

- L only

- L and R

## Beyond the Standard Model

- It is possible to introduce heavy neutral leptons (“sterile” neutrinos) into the Standard Model
- Steriles exhibit certain properties in common with ordinary neutrinos:
- Massive (if high enough, potentially unstable)
- No electromagnetic couplings
- No strong couplings

-  No direct weak couplings !

## Probing            Sterile Neutrinos

- Light

## Steriles: Experimental Motivation

- LSND sees oscillation signal       to e
- Combined analysis with KARMEN experiment consistent with m2 < 1 eV2

- Oscillation Probability: (0.264±0.067±0.045)%

- E. Church, K. Eitel, G. Mills, M. Steidl
- hep/ex 0203023

## Doesn’t Add Up

- If dealing with 3 neutrino species then...
- msolar2 + matmo2 + mLSND2 = 0
- But if m122 and m232 are both small then m312  must be small as well (constrained).
- ...but this is not the case.
- msolar2  ~ 10 -5 eV2
- matm2   ~ 10 -3 eV2
- mLSND2 ~ 1 eV2

- Solar {

- Atmospheric {

## Two Phenomenological Models

- 2+2 Model:
- Assumes mass difference of LSND signal bridges between solar and atmospheric
- Model gives prediction of:

- 3+1 Model:
- Assumes LSND mass apart from near degenerate weak states.

- Psolar + Patmo = 1

- e

- 

- 

- s

## What About Sterile Neutrinos?

- Best fit favors no oscillations to sterile neutrinos
- Pure mixing ve  -> vs ruled out at the 5 level
- Partial mixing (global fit)  of mixing parameter to steriles ():
- What is the problem?
- Sterile content driven by uncertainties in the flux.  Is there a way to constrain it further?

-  John N. Bahcall, M. C. Gonzalez-Garcia, C. Pena-Garay
- hep/ph 0204194

- sin2

- 8B Flux

- No Sterile
- Mixing

- Full Sterile
- Mixing

## Combining KamLAND & SNO

- SNO provides an independent measure of the active content, but no constraint on the flux.
- KamLAND only reveals information about the mixing parameters, but not on the total active flux.
- Combining the two experiments provide an independent check on the active and sterile components
- If SNO improves NC measurement, this uncertainty will decrease even further
- Solar model independent.

- sin2

-  John N. Bahcall, M. C. Gonzalez-Garcia, C. Pena-Garay
- hep/ph 0202147

## Day, Night, and Steriles?

- One can use day and night rates for neutral current rates in the sun in order to limit sterile neutrinos.
- Void of solar model systematic uncertainties
- Results consistent with no sterile content at 1.5  level.

- Signal Extraction in CC, NC, ES
- ASNOCC = +14.0 ± 6.3 %
- ASNONC = -20.4 ± 16.9 %

## Precision Solar pp Measurements

- If one is able to access pp neutrinos from the sun, model is known extremely well.
- Limit there stems from detector uncertainties
- Need a dedicated experiment to probe this region and test both solar physics and steriles (but can you do both?)

## Do the Models Survive?

- 2+2 Model:
- Model gives prediction for solar and atmospheric constraints
- New data now available:
- SNO’s neutral current measurement
- Recent data analysis from SK and KamLAND
- Updated numbers from the LSND collaboration on oscillation parameters

- Psolar + Patmo = 1

-  M. Maltoni, T.Schwetz, M. Tortola, J. Valle
- hep/ph 0209368

## Do the Models Survive?

- 3+1 Model:
- Model usually disfavored due to restrictions from reactor limits
- New oscillation probability from LSND recent analysis places more favor on the 3+1 scheme
- Most analyses place only marginal survival to the 3+1 scheme.

-  M. Maltoni, T.Schwetz, M. Tortola, J. Valle
- hep/ph 0209368

## 3+2, 4+1, etc. Models

- Can also look at constraints for model with more than one single sterile neutrino.
- Constraints mostly come from short and long baseline experiments.
- Region exists which is still compatible with current experimental observations.

-  M. Sorel, J. Conrad, and M. Shaevitz.
- hep/ph 0305255

## Probing                 Sterile Neutrinos

- Heavy

## Neutral Heavy Lepton Production

- Conventional (/K beams):
- Couples to  from two-body decay.
- Dominated by  decay (with contamination from K decay)
- Mass range limited by /K mass

********

- -Factory / -Storage ( beams):
- Produced solely from  decay
- Mass range limited to  _<_105 MeV.

## GeV Scale Steriles

- If sterile neutrinos possess a mass on the order of ~GeV scales, these can be produced and later decay to lighter particles.
- Ideal of high energy neutrino beams for searching for such particles.
- Various mass ranges and models ruled out by direct searches.

- Mass (GeV)

- A. Vaitaitis et al., Phys. Rev. Lett. 83, 4943 (1999)

## Production Rates

- Production rates and branching ratio both scale as coupling *| U | **2*

- Detector Efficiency /
- Acceptance

- Branching Ratio

## Production Rates

- Production rates and branching ratio both scale as coupling *| U | **2*

- Maximum sensitivity independent of mass to first order:

*N**L**0**seen** ****~**** U**4 ****prod ****decay** **(****min**)***

- Pure geometric factor

## Detection Channels

- For pion and muon sources, three channels typically dominate:
- Ability to identify / e+e- in detector.
-  channel often a favorite channel for very small (~ few keV) sterile neutrino masses.

*L**0**          ***
*L**0**          e**+**e**-***
*L**0**               ***

- Typically smaller by ~1/20

## Detection Techniques : Low Mass Targets

- Advantages:
- Makes use of existing system.
- Cover large area with little instrumentation
- Makes use of kinematic features of decay.

- Disadvantages:
- Low target requirements makes instrumentation difficult.
- High intensity neutrino beams induce large background from surrounding material.

## Detection Techniques: Timing

- Can use timing a signature to distinguish potential signal events from background.
- One example, the KARMEN experiment was sensitive to steriles produced near the kinematic threshold for pion decay.
- Disadvantages:
- Sensitive to small mass range for neutral heavy lepton production.
- Lesson from KARMEN:
- Anomalous signal eventually ruled out as statistical or beam-induced background.

## Detection Techniques: Timing

- Previous technique limited to slow-moving particles.
- Combination of bunched timing and long baselines enable detecting particles out of sync with main beamline.
- Opens sensitivity to high mass particles.
- Mono-energetic beams, such as off-axis, ideal sources.

## Case Example: NuTeV

- NuTeV
- High energy neutrino beam from 800 GeV protons.
- Instrumented low mass (He) decay region to look for neutral heavy lepton decays.
-  Makes use of kinematic selection of events to reduce backgrounds in signal region to <<1 event.

## Slide 27

##  Hint of Something Out There?

- NuTeV has measured an anomalous rate of dimuon events in its apparatus.
- Could be a massive (~5 GeV) sterile neutrino decaying to 
- However, certain aspects of event topology consistent with background.

## Neutrino Factory Sensitivities

- Neutrino factory settings provide a large neutrino beam flux for increased sensitivity to sterile neutrinos.
- Limited to below 105 MeV
- Sensitive to Ue and U couplings
- Relatively insensitive to distance or energy of beam (w/o taking backgrounds into consideration).

## Neutrino Factory Sensitivities

- Projected Sensitivity:
- Considered muon storage ring with 3 x 1019 and 3 x 1020  decays.
- Energy ranges between 20 and 50 GeV (for ) show very little differences.
- Improvement over limited mass range (below 105 MeV).

- KEK

- LBL

- SIN

- Muon Decay

- Muon Storage Ring

- 1019  decays

- 1020  decays

## Conclusions

- Light Steriles:
- Information for solar, atmospheric, and LBL experiments provide model tests on 3+n generation models.
- Current and upcoming solar, reactor, and atmospheric measurements to increase sterile sensitivity.
- MiniBooNE to explicitly test LSND signal.

- Heavy Steriles:
- Superbeams and -factories provide ideal source for heavy sterile searches.
- To overcome background issues, combination of low-mass targets, kinematic selection, and timing information must be used.
- Often a complimentary program to oscillation physics.

## Slide 32

## What the Future Holds

## What to Expect

- If MiniBooNE sees a signal in neutrino running:
- Confirmation of LSND
- Is a single sterile compatible with rest of oscillation data?
- If MiniBooNE sees a signal only in anti-neutrino running:
- CPT violation a reality?
- What other systems test this scenario ?

## Road Map to New Physics

- Despite the progress made, the fundamental properties of neutrinos still remain to be explored:
- What is the full mixing of neutrinos? Do they exhibit CP violation?
- What is the neutrino mass scale?
- What physics governs the mass scale?

## Testing the Sterile Content

- Using Solar & Reactor:
- Direct measurement of 8B flux from sun from SNO measurement
- Use SNO + KamLAND to further constrain solar models
- Use lowest energy solar ’s to understand steriles

- Global Fits:
- Use combined fits to constrain 3+n models
- Direct verification of LSND:
- MiniBooNE