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

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• 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

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• 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

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• 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

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NC Selected Events in Far Detector Data

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• 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

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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

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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

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• 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

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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

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Backup Slides

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• 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

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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)

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• 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

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Systematic Errors

Simulated Data

Simulated Data

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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