1. Search for Sterile Neutrino Oscillations with MiniBooNE
Robert Cooper

2. 3-Flavor Neutrino Mixing Model
Mass splittings Dm212 ≅ 7.6 × 10-5 eV2 Dm312 ≅ 2.4 × 10-3 eV2
Mixing angles sin2 2q13 ≅ sin2 2q23 ≅ sin2 2q12 ≅ 0.86
Hierarchy, CP d, mass scale?
m12
m22
m32 m2
solar
atm ?
normal
inverted ne nm nt
R.L. Cooper

3. 3-Flavor Anomalies Radioactive source disappearance (SAGE/GALLEX)
Short baseline reactor disappearance, “Reactor Anomaly”
Sterile neutrinos Dm2 ~ 1 eV2 ?
3 n oscillations (new flux)
3+1-n oscillations
R.L. Cooper

4. LSND LSND searched for oscillations Decay At Rest source
Excess of oscillation events that are inconsistent solar and atmospheric oscillations
Excess = 87.9 ± 22.4 (sys) ± 6.0 (stat)  (3.8s)
Sterile neutrino(s) ?
… Enter MiniBooNE
R.L. Cooper

5. LSND LSND searched for oscillations Decay At Rest sourcem2 ?
LSND searched for oscillations
Decay At Rest source
Excess of oscillation events that are inconsistent solar and atmospheric oscillations
Excess = 87.9 ± 22.4 (sys) ± 6.0 (stat)  (3.8s)
Sterile neutrino(s) ?
… Enter MiniBooNE
R.L. Cooper

6. MiniBooNE Experiment, Neutrinos
Designed to look for and oscillations
8 GeV protons strike Be target
Magnetic horn selects charged mesons, leptons absorbed
<En> ~ 800 MeV
,  6.7×1020 POT
R.L. Cooper

7. MiniBooNE Experiment, Antineutrinos
Designed to look for and oscillations
8 GeV protons strike Be target
Magnetic horn selects charged mesons, leptons absorbed
<En> ~ 800 MeV
,  11.3×1020 POT
R.L. Cooper

8. MiniBooNE Detector
800 tons very pure mineral oil (CnH2n+2) Cherenkov detector
Small amount of natural scintillation
12 m diameter sphere
541 m from BNB target
Inner signal region 1280× 8” PMTs (10% coverage)
Outer veto region 240× 8” PMTs
R.L. Cooper

9. MiniBooNE Event Reconstruction
Oscillation signal is Charged Current Quasi-Elastic (CCQE)  e- final state
CCQE is kinematically reconstructed from lepton energy and angle
Lepton reconstruction (type, energy, direction) with PMT time and charge likelihood
p0  gg can be poorly reconstructed as a single electron
R.L. Cooper

10. Neutrino Results
6.7×1020 POT
Excess (En > 200 MeV) ± 28.4 ± 40.2 (3.8s)
NCg & NCp0 are biggest at low En
c2 probabilities for (null) hypothesis, best fit (bf) 2n oscillation hypothesis
R.L. Cooper

11. Antineutrino Results 11.3×1020 POT
Excess (En > 200 MeV) 77.8 ± 20.0 ± 23.4  (3.8s)
NCg & NCp0 are biggest at low En
c2 probabilities for (null) hypothesis, best fit (bf) 2n oscillation hypothesis
R.L. Cooper

12. Combined Fit Excess (En > 200 MeV) 240.3 ± 34.5 ± 52.6  (3.8s)
Consistent with LSND
3+1 n fits relatively poor
3+2 n fits better
Is a low En background likely?
NCp0 is directly measured
NCg is constrained by NCp0 (due to D  Ng)
Theory corroborates MiniBooNE estimates (e.g. Zhang & Serot)
R.L. Cooper

13. Combined Fit Excess (En > 200 MeV) 240.3 ± 34.5 ± 52.6  (3.8s)
Consistent with LSND
3+1 n fits relatively poor
3+2 n fits better
Is a low En background likely?
NCp0 is directly measured
NCg is constrained by NCp0 (due to D  Ng)
Theory corroborates MiniBooNE estimates (e.g. Zhang & Serot)
R.L. Cooper

14. Discriminating NC & CC with Neutrons
Excellent CCQE En Recon.
“Clean”
Expect ~1% (200 MeV) rising to ~10% (1 GeV) neutron production NC
Poor CCQE En Recon.
More energy to hadrons
Expect ~50% neutron production
Tagging neutrons can discriminate between NC backgrounds and CC oscillation signal
R.L. Cooper

15. Future: MiniBooNE+
Neutrons thermalize, n(p,d)g capture, release 2.2 MeV g
Need more light!
300 kg PPO will allow 2.2 MeV g to be detected
Measure neutron fraction
Use nm events to calibrate
3 years (6×1020 POT) neutrino mode run
If excess remains, could be combined with previous runs for 5s rejection of null
R.L. Cooper

16. Future: MiniBooNE+
Neutrons thermalize, n(p,d)g capture, release 2.2 MeV g
Need more light!
300 kg PPO will allow 2.2 MeV g to be detected
Measure neutron fraction
Use nm events to calibrate
3 years (6×1020 POT) neutrino mode run
If excess remains, could be combined with previous runs for 5s rejection of null
R.L. Cooper

17. Future: Low-Mass Dark Matter Search
Direct production of dark matter through SM-Dark portal vectors Phys. Rev. D86 (2012)
Beam off-target to reduce n signal
Elastic forward-scattering and beam timing sensitive to mc ~ 200 MeV
Currently collecting off-target data in test beam run
Full proposal soon
R.L. Cooper

18. Summary MiniBooNE completed running April 2012
Currently collecting off-target test beam data
Low-mass dark matter run
See a 3.8s excess in combined and oscillations
MiniBooNE+ will add scintillator to tag neutrons
Neutrons discriminate NC backgrounds from CC signal
R.L. Cooper

19. Summary MiniBooNE completed running April 2012
Currently collecting off-target test beam data
Low-mass dark matter run
See a 3.8s excess in combined and oscillations
MiniBooNE+ will add scintillator to tag neutrons
Neutrons discriminate NC backgrounds from CC signal
R.L. Cooper