Post on 06-Jan-2018
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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