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Neutral Current Analysis in MINOS
Alexandre SousaUniversity of OxfordHarvard University*
DOE Review of theHarvard HEP Group August 15, 2008
*After December 01, 2008
DOE Review, 08/15/08 2Alex Sousa
• The MINOS detectors observe both neutral current (NC) and charged current (CC) interactions
• Reconstructed events are composed of tracks and showers
• MINOS is not optimized to measure the short showering NC events, but they are interesting!
• NC events allow us to look for sterile neutrinos
Neutrino Interactions in MINOS
Hadronic Interaction Length = 7 planesRange of 2 GeV muon = 50 planesEM Radiation Length = 0.7 steel planes
DOE Review, 08/15/08 3Alex Sousa
• Measurements of the Z0 decay width at LEP exclude more than 3 light active neutrinos
– A 4th neutrino cannot couple to Z0
– It does not participate in weak interactions – sterile neutrino
• Results from the short baseline LSND experiment suggested the existence of a fourth neutrino with large mass splitting
• Results from MiniBooNE and other experiments strongly disfavor sterile neutrinosas an explanation for LSND
• Searches on long baseline experiments are relevant as sterile neutrinos would
– Indicate presence of one or more additional mass eigenstates
– be possible dark matter candidates
• Sterile neutrinos => new physics!
Sterile Neutrinos
Phys. Rev. Lett. 98, 231801 (2007)
DOE Review, 08/15/08 4Alex Sousa
Looking for Sterile Neutrinos in MINOS• Oscillations in MINOS are driven by
Δm2Atm
– Oscillation is between νμ and ντ
– No effect on neutral current interactions
• Add a 4th neutrino– Extra mass ν4, extra flavor νs
– Oscillations can now occur between νμ and νs
• driven by Δm2Atm
• driven by a new mass scale
• Oscillations into νs reduce number of observed NC interactions as νs do not interact in the detector
• Look for NC disappearance at the Far Detector– Sterile neutrino mixing would
deplete NC Energy spectrum
Toy Simulation
No νs
With νs mixing
DOE Review, 08/15/08 5Alex Sousa
• Beam quality and detector quality cuts– Beam positioning, magnetic horns energized, detector running within
operational parameters
• Event vertex reconstructed within the fiducial volume of the detectors
• Fiducial volume is optimized for containment of hadronic showers
• Cut-based method is used to separate CC and NC events
NC Event Selection
Calorimeter Spectrometer
NEAR DETECTOR
FAR DETECTOR
DOE Review, 08/15/08 6Alex Sousa
• Event classified as NC-like if:
− event length < 60 planes
− has no reconstructed track or
− has one reconstructed track that does not protrude more than 5 planes beyond the shower
NC/CC Event Separation• NC events are typically shorter than CC events
• Expect showers and no tracks or very short tracks reconstructed for NC events
• Main background from inelastic (high-y) νμ CC events
Excluded
Excluded
Excluded
DOE Review, 08/15/08 7Alex Sousa
• The FD NC selection uses the same variables as the ND selection, with identical cut values
• MC oscillated with 2007 CC best fit: m2 = 2.38 x10-3 eV2,
sin2(223)=1
Far Detector NC Selection
Far Detector Data
Osc. Monte Carlo
Excluded Excluded
Excluded
DOE Review, 08/15/08 8Alex Sousa
NC Selected Events in Far Detector Data
DOE Review, 08/15/08 9Alex Sousa
• Far detector (FD) energy spectrum without oscillations is not the same as the Near detector (ND) spectrum– Decay angles for neutrinos to reach detector are different for ND and FD different
energy spectrum
• The measured ND energy spectrum is used to predict the unoscillated FD energy spectrum. Corrections for energy smearing and detector acceptance are obtained from MC
• Ratio of events in a given bin in FD relative to ND is the same for data and MC
• F/N Ratio method is used to cancel most systematic uncertainties on flux and cross-sections
Extrapolation to the Far Detector
FDDecay Pipe
π+Target
ND
p
E~ 0.43Eπ / (1+π2θ
2)
2
2 2 2
1 11
FluxL γ θ
( .)
MCpredicted Data ii i MC
i
f oscF N
n
DOE Review, 08/15/08 10Alex Sousa
NC Analysis Results - Rate• Compare the NC energy spectrum with the expectation
of standard 3-flavor oscillation physics
• Fix the oscillation parameter values– sin22Θ23 = 1
– Δm232
= 2.43x10-3 eV2
– Δm221 = 7.59x10-5 eV2, Θ12 = 0.61 from KamLAND+SNO
– Θ13 = 0 or 0.21 (normal MH, δ=3π/2) from CHOOZ Limit
• N.B. CC e are classified as NC by the analysis
• Make comparisons in terms of the R statistic:
• For different energy ranges– 0-3 GeV
– 3-120 GeV
– All events (0-120 GeV)
From MINOS 2008 CC measurement
Data CC
NC
N BR
S
Predicted CC background
from all flavors
Predicted NC interaction signal
DOE Review, 08/15/08 11Alex Sousa
NC Analysis Results - RateMINOS Far Detector NC Spectrum• Plot shows the selected FD
NC energy spectrum for Data and oscillated MC predictions
• Expect largest NC disappearance for E < 3 GeV if sterile mixing is driven by Δm2
32
• Depletion of total NC event rate (1-R) < 17% at 90% C.L. for the 0-120 GeV range
Data is consistent with no NC deficit at FD and thus with no sterile neutrino mixing
DOE Review, 08/15/08 12Alex Sousa
• Assume one sterile neutrino and that mixing between νμ, νs and ντ
occurs at a single Δm2
• Survival and sterile oscillation probabilities are then:
• Simultaneous fit to CC and NC energy spectra yields the fraction of νμ that oscillate to νs:
NC Analysis Results – fs Fit
0.250.28
( )0.28 (stat.+syst.)
1 ( )s
s
Pf
P
)/27.1(sin)(
)/27.1(sin1)(22
22
ELmP
ELmP
ss
0.68 (90% C.L.)sf
DOE Review, 08/15/08 13Alex Sousa
fs Sensitivities• fs fit sensitivities with and without νe appearance and with
and without including systematics for different exposures– Solid curves are at 90% C.L., dashed curves are at 68% C.L.
VERY PRELIMINARY
DOE Review, 08/15/08 14Alex Sousa
• MINOS has completed an analysis of neutral current neutrino interactions in 2.461020 POT of NuMI beam exposure:
• From 3-flavor analysis:– R = 0.99 ± 0.09 ± 0.07, 0 < E < 120 GeV
– 1-R < 17% at 90% C.L., 0 < E < 120 GeV
• From fit to a 3+1, single mass splitting, sterile oscillation model–
–
• Results consistent with no sterile neutrino mixing– Submitted to PRL (hep-ex:0807.2424)
• PRD paper in preparation, including fitting with more complex oscillation models and 3.201020 POT exposure.
• Next round of analysis to include improved reconstruction and MC
Conclusions and Outlook
0.250.280.28 (stat.+syst.)sf
0.68 (90% C.L.)sf
DOE Review, 08/15/08 15Alex Sousa
Backup Slides
DOE Review, 08/15/08 16Alex Sousa
• Relative Normalization: 4% – POT counting, Near/Far reconstruction
efficiency, fiducial mass• Relative Hadronic Calibration: 3%
– Inter-Detector calibration uncertainty • Absolute Hadronic Calibration: 11%
– Hadronic Shower Energy Scale(6%), Intranuclear rescattering(10%)
• Muon energy scale: 2%– Uncertainty in dE/dX in MC
• CC Contamination of NC-like sample: 15%
• NC contamination of CC-like sample: 25%
• Cross-section uncertainties:– mA (qe) and mA (res): 15%– KNO scaling: 33%
• Poorly reconstructed events: 10% • Near Detector NC Selection: 8% in 0-1
GeV bin• Far Detector NC Selection: 4% if E < 1
GeV, <1.6% if E > 1 GeV • Beam uncertainty: 1 error band around
beam fit results
Systematic Errors
Effect of the most relevant systematic uncertainties on R
DOE Review, 08/15/08 17Alex Sousa
Accumulated Beam Data
RUN I - 1.27x1020 POT Higher energy beam
0.15x1020 POT
RUN IIa1.23x1020 POT
RUN IIb0.71x1020 POT
RUN III1.2x1020 POT
2008 NC (PRL)
2009 NC (PRD)
DOE Review, 08/15/08 18Alex Sousa
• Good agreement between Data and Monte Carlo
• Discrepancies much smaller than systematic uncertainties
• NC events are selected with 90% efficiency and 60% purity
Neutral Current NC Energy Spectrum• NC selected Data and MC energy spectra for Near Detector
DOE Review, 08/15/08 19Alex Sousa
• Systematic errors studied using simulated Far Detector data histograms with oscillation parameters m2 = 2.38 x10-3 eV2, sin2223=1
• Left plot displays magnitude of shift in FD simulated data compared to nominal
• Ratio plots show shifted/nominal ratio for FD simulated data, overlaid with shifted/nominal MC FD prediction
– Displays ability of F/N extrapolation method to reproduce systematic shift
• F/N extrapolation method is robust to absolute systematic errors, which shift the energy spectra in both Near and Far detectors
• Most relevant systematics are relative, where shifts only applied to one detector
Systematic Errors
Simulated Data
DOE Review, 08/15/08 20Alex Sousa
Systematic Errors
Simulated Data
Simulated Data
DOE Review, 08/15/08 21Alex Sousa
to sterile in SuperK
• High energy experience matter effects which suppress oscillations to sterile – Matter effects not seen in up-
or high-energy PC data– Reduction in neutral current
interactions also not seen– constrains s component of
disappearance oscillations
• Pure ->s disfavored – s fraction < 20% at 90% c.l.
• Result published only in conference proceedings