Download oscillation physics, exotic event rates, cross sections

Geoffrey Mills
Status of the
MiniBooNE Experiment
Purpose
Operation
Survey of Processes
Analysis Progress
Outlook
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Neutrino Oscillation Primer
Oscillatio n Probabilit y  sin 2 2θ
sin 2 (1.27m2 L / E )
Geoffrey Mills
Surprisingly…
• Evidence for neutrino oscillations is observed in (too?)
many settings:
–
–
–
–
in the sun
in cosmic ray interactions in the atmosphere
in reactors
at accelerators
Purpose: Confirm or deny LSND (as oscillations)
LSND found an excess of ne events in a nm beam
Geoffrey Mills
4.1 s evidence for oscillations.
Geoffrey Mills
If LSND is confirmed…
• Some possible explanations
– Light (variable mass?) sterile neutrinos
• Could explain dark matter and dark energy? hep-ph/0507235
• Could explain pulsar kicks?
• Could explain SN synthesis of heavy elements
– Lorentz invariance violation(??)
– Quantum decoherence(???)
– …
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Enter MiniBooNE...
Fermilab
Batavia, IL
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The MiniBooNE Neutrino Beam
nmne?
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Fermilab 8-GeV Booster
2.5 KV
170 KA
143 ms
5 Hz
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The MiniBooNE Detector
12 meter diameter sphere
Filled with 950,000 liters of
pure mineral oil — 20+ meter
attenuation length
Light tight inner region with
1280 photomultiplier tubes
Outer veto region with 240
PMTs.
Neutrino interactions in oil
produce:
• Prompt Čerenkov light
• Delayed scintillation light
Čerenkov:scintillation ~ 5:1
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Geoffrey Mills
Geoffrey Mills
MiniBooNE Operation
• Began recording data on September 1, 2002
• Finished neutrino mode running in February, 2006
– Booster provided ~ 5.6 E20 protons on target
– Recorded ~ 700,000 contained CC interactions
• Switched to anti-neutrino mode Feb. 2006
– Continuing to run this way until near detector
SciBooNE/MiniBooNE are ready to switch back to neutrino
mode
Geoffrey Mills
Neutrino Event Rates
Contains everything interesting: oscillation
physics, exotic event rates, cross sections, etc.
 observed event rate  cross section flux of neutrinos  detection efficiency 
One must understand all three factors in order to reach new physics
Geoffrey Mills
Neutrino Event Rates
Contains everything interesting: oscillation
physics, exotic event rates, cross sections, etc.
 observed event rate  cross section flux of neutrinos  detection efficiency 
Geoffrey Mills
Neutrino Beam Fluxes
•
Neutrino beams are produced in the laboratory by the weak decays of
nuclei, nucleons, and m, , and K mesons.
•
The spectrum of neutrinos from these decays is known extremely well
•
The only significant flux uncertainty comes from the production cross
section of the parent particle and its subsequent scattering in target
materials (ignoring neutrino oscillation parameters that is!)
•
In decay-at-rest beams, this is simply an overall normalization factor. A
single well-understood neutrino cross section is enough to completely
determine the neutrino flux (e.g. ne elastic process, or an inverse -decay
transition)
However, in decay-in-flight beams, the complete differential
production cross sections of the parent particles are needed,
along with their interactions in material along their flight paths
Geoffrey Mills
Decay-in-Flight n Beams
• The calculation of secondary-production cross sections of  and K mesons
in proton-nucleus collisions is not reliable, although new data is
challenging modelers to make improvements
• Phenomenological parameterizations can be valid over limited energy and
angle ranges, more useful at higher energies (> ~15 GeV) (e.g. SanfordWang or others)
• There are large discrepancies in the various hadron production models
used in MC generators (MARS, FLUKA, MCNP, GHEISHA, etc.), although
the situation is has been improving
In order to predict fluxes with uncertainties less than ~10%, direct
measurements in the appropriate energy and angular ranges are
necessary
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Variation in MC Predictions of n Flux
(MiniBooNE)
(old)

Conclusion: the neutrino beam is sensitive to poorly
understood, forward (small angle), pion production rates
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Mitigation of the Problem
• Experimental design helps in neutrino oscillation measurements
(although not in cross section measurements)
• near/far ratio (two detectors): (K2K/MINOS)
– This is the simplest solution, although somewhat more costly. Useful in
both appearance and disappearance measurements, a large signal helps!
• ne /nm “ratio”: (MiniBooNE)
– In appearance experiments, the source neutrino channel (usually nm) can
be used to measure the expected oscillated flux under a particular
oscillation hypothesis. This assumes only “lepton-universality” in the
reaction cross sections, but accurate background measurements are
important.
Nevertheless, flux uncertainties can result in much poorer experimental
sensitivities and better flux predictions are warranted
Geoffrey Mills
HARP hep-ex/0702024
Geoffrey Mills
Neutrino Event Rates
Contains everything interesting: oscillation
physics, exotic event rates, cross sections, etc.
 observed event rate  cross section flux of neutrinos  detection efficiency 
Geoffrey Mills
MiniBooNE Events (no beam)
Tank is inner region (1280
tubes)
"
Veto is optically isolated
outer region (240 tubes)
"
Cosmics enter outer veto
region first
"
"
"
Rate consistent with
known measurement.
Check of veto efficiency
Michel electrons decay
from cosmic muons
" Important tool for
calibration
" Well-understood.
"
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Michel Decay Events for Calibration
0
 Reconstruction:
constrains important mis-ID background
Neutral Current 0
nm C  nm 0 X
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Geoffrey Mills
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Oscillatio n Probabilit y  sin 2 2θ
sin 2 (1.27m2 L / E )
Geoffrey Mills
Geoffrey Mills
Backgrounds
• Misidentified neutral current 0
– 0 rate is well measured
– background comes from events where single decay photon
happens to go forward
• Misidentified CC nm interactions
– Have a large sample of Michel-tagged events to determine
backrounds
• Intrinsic ne from kaon and muon decays
– Daughter muon decays are directly linked to primary pion decays
and hence constrained by nm CCQE event rates
– Kaons are sufficiently constrained by measured high energy (>1.5
GeV) interactions
Geoffrey Mills
MiniBooNE Conclusions
• All the necessary ingredients are in place, flux, cross
sections, backgrounds, detector response
•We have constructed a “final fit” to our CCQE data (nm
and ne ) for an appearance signal
• We will proceed to “unblind” the data when we are
satisfied that our systematic errors are understood
•Stay tuned…