Search for Sterile Neutrino

Oscillations with MiniBooNE

Robert Cooper

2R.L. Cooper

3-Flavor Neutrino Mixing Model

• Mass splittings Dm21

2 ≅ 7.6 × 10-5 eV 2

Dm312 ≅ 2.4 × 10-3 eV 2

• Mixing angles sin2 2q13 ≅ 0.1 sin2 2q23 ≅ 0.97 sin2 2q12 ≅ 0.86

• Hierarchy, CP d, mass scale?m1

2

m22

m32

m2 m2

m32

m12

m22

0 0

solar

atm

atm

solar

? ?

normal inverted

ne nm nt

3R.L. Cooper

3 n oscillations (new flux)

3+1-n oscillations

3-Flavor Anomalies• Radioactive source

disappearance (SAGE/GALLEX)• Short baseline reactor

disappearance, “Reactor Anomaly”

• Sterile neutrinos Dm 2 ~ 1 eV 2 ?

4R.L. Cooper

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

5R.L. Cooper

LSNDm2

0

?

• 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

6R.L. Cooper

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• 2002-2005, 2007

6.7×1020 POT

7R.L. Cooper

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• 2005-2006, 2008-2012

11.3×1020 POT

8R.L. Cooper

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

9R.L. Cooper

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

10R.L. Cooper

Neutrino Results• 6.7×1020 POT

• Excess (En > 200 MeV)146.3 ± 28.4 ± 40.2 (3.8s)

• NCg & NCp0 are biggest at low En

c2 probabilities for (null) hypothesis, best fit (bf) 2n oscillation hypothesis

11R.L. Cooper

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

12R.L. Cooper

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)

13R.L. Cooper

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)

14R.L. Cooper

Discriminating NC & CC with NeutronsCC• 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

15R.L. Cooper

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

16R.L. Cooper

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

17R.L. Cooper

Future: Low-Mass Dark Matter Search• Direct production of dark matter

through SM-Dark portal vectorsPhys. Rev. D86 (2012) 035022

• 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

18R.L. Cooper

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

19R.L. Cooper

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