Search for sterile neutrinos in MINOS

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Search for sterile neutrinos in MINOS

• Christopher Backhouse
• University of Oxford
• Fermilab Users Meeting
• 4 June 2009

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Search for sterile neutrinos in MINOS

• Overview of the MINOS experiment
• Sterile neutrinos
• Event Selection
• Prediction of FD spectrum from ND
• Results from 3.181020 protons-on-target
• 3-flavour expectation
• 4-flavour model
• ? decay model

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Main Injector Neutrino Oscillation Search

• 93 ?? beam from Fermilab Main Injector
• Two detectors
• Near Detector at Fermilab
• Far Detector in Soudan mine 735km away
• Measure flux at ND, compare to FD measurement

Near Detector
Far Detector

• Functionally identical magnetized
  steel-scintillator tracking calorimeters
• Fiducial masses
• ND 27t
• FD 3.8kt

See talk by G. Pawloski for more details
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3-Flavour Theory
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Sterile Neutrinos

• Z0 width (LEP) ? 3 light active neutrinos
• Additional sterile ??
• LSND suggests new large ?m2
• Bugey, Karmen and MiniBooNE dont support this
  interpretation
• Additional massive ? still interesting
• Astrophysics dark matter, supernovae
• Seesaw mechanism
• Sterile neutrinos ? new physics!

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Looking for Sterile Neutrinos in MINOS

• NC interactions same for all flavours
• Oscillations among active flavours dont affect
  NC spectrum
• Sterile neutrinos wouldnt interact weakly at all
• Sterile signal
• Depletion of FD NC spectrum
• Energy-dependent

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Event Topologies
?µ CC Event
NC Event

• Charged lepton track
• Hadronic shower

June 4, 2009
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Pre-Selection Cuts

• Beam and detector quality
• Veto cosmics using timing and steepness
• Event vertex not close to detector edge
• Ensures containment of hadronic shower

Near Detector
Far Detector
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ND Pre-Selection Cuts

• High event rate in ND can cause poor event
  reconstruction
• Split events
• Incorrect vertex
• Apply a series of cuts
• Time and spatial separation
• Total number of hit strips
• Event steepness
• Activity in edge region
• Reduce poorly reconstructed background lt1GeV from
  34 to 8

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ND NC Event Selection

• Discard events gt60 planes

• Discard events with a track gt5 planes longer than
  the shower

• Same selection applied at FD
• Variable distributions similar

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Neutral Current Selection

• NC events selected with 90 efficiency and 60
  purity

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Near Detector NC Spectrum

• Good agreement between data and MC
• Discrepancies smaller than systematic
  uncertainties

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Far/Near Ratio Method

• The F/N ratio method uses the ND data without
  relying on a specific parameterization
• Correct each energy bin in the FD MC using the ND
  data/MC differences as a scale factor
• Simple, robust to systematic errors
• FD data spectrum blinded so prediction unbiased

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3-Flavour Analysis

• Compare NC energy spectrum with expectation from
  standard 3-flavour oscillations
• Pick the oscillation parameter values
• Normal mass hierarchy
• sin22?23 1
  (Super Kamiokande)
• ?m232 2.43x10-3 eV2
  (MINOS CC result)
• ?m221 7.59x10-5 eV2, ?12 35
  (KamLANDSNO)
• ?13 0 or 12 from CHOOZ limit
• ?3p/2 (maximum ne appearance)
• Because CC ne are almost all selected as NC

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3-Flavour FD Energy Spectrum
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R statistic

• No ? disappearance ? R1

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4-Flavour Analysis

• Assume an additional sterile ? and an additional
  ?m2
• Extend mixing matrix with extra angles and phases
• Consider two mass scales
• ?m241 gtgt ?m231
• ?m241 ltlt ?m231
• Simplify oscillation formulae

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4-Flavour Analysis

• Start with general matrix
• UR34(?34) R24(?24,?2) R14(?14) R23(?23)
  R13(?13,?1)R12(?12)
• 11 parameters
• ?34, ?24, ?14, ?23, ?13, ?12
• ?m223, ?m241, ?m221
• ?1, ?2
• Need to simplify

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4-Flavour Analysis

• ?34, ?24, ?23, ?13, ?14, ?12
• ?m223, ?m241, ?m221
• ?1, ?2
• ?m221 ? 0
• ?14 ? 0
• ?2 ? 0
• Pick mass scale
• ? UR34(?34) R24(?24) R23(?23) R13(?13,?1)
• ?13 and ?1 as before

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4-Flavour Analysis

• ?34 lt 38 (56 ?e) (90 C.L.) (Both models)
• ?24 lt 10 (10.6 ?e) (90 C.L.) (?41 gtgt ?31)

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Sterile fraction

• Simple way to interpret result
• What fraction of ?µ that disappear go to steriles?

fs lt 0.51 (0.55 ?e) (90 C.L.)
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Future Sensitivity
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Neutrino Decay CC spectrum

• Deficit in FD CC energy spectrum versus
  expectation
• Decay disfavoured at 3.7s

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Neutrino Decay NC spectrum

• Decay would also deplete NC spectrum, improves
  limit
• Mixed model
• ? oscillations
• One mass state decays

Pure decay disfavoured at 5.4s
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Summary

• Analysis of neutral current neutrino interactions
  from 3.18?1020 POT NuMI beam exposure
• Consistent with no neutrino decay and no sterile
  neutrino admixture (R1.040.080.07-0.10(?e))
• fs lt 51 (90 C.L. no ?e appearance)
• Disfavour pure decay by 5.4s as alternative to
  oscillations
• Mass-corrected ? lifetime gt 2.1x10-12 s/eV (90
  C.L.)
• Statistically limited. More protons-on-target
  will continue to improve result

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Acknowledgements

• On behalf of the MINOS Collaboration, I would
  like to express our gratitude to the many
  Fermilab groups who provided technical expertise
  and support in the design, construction,
  installation and operation of the experiment
• We also gratefully acknowledge financial support
  from DOE, STFC(UK), NSF and thank the University
  of Minnesota and the Minnesota DNR for hosting us

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14 October 2009
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Backup Slides
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The MINOS Collaboration
140 scientists 28 institutions
Argonne Athens Benedictine Brookhaven
Caltech Cambridge Campinas Fermilab Harvard
Holy Cross IIT Indiana Minnesota-Twin
Cities Minnesota-Duluth Otterbein Oxford
Pittsburgh Rutherford Sao Paulo South
Carolina Stanford Sussex Texas
AM Texas-Austin Tufts UCL Warsaw William
Mary
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MINOS Physics Goals

• Measure Dm2atm and sin22qatm via nm
  disappearance
• Search for ne appearance (?13)
• Compare n, n oscillations
• Atmospheric ? and cosmic rays
• Cross sections using ND
• Search for or constrain exotic physics such as
  sterile n

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NuMI Beam

• 120GeV protons strike target
• Pions and Kaons focussed
• Decay into neutrinos (93 ?µ)
• Rock absorbs everything else

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Near Detector

• Magnetized steel-scintillator tracking
  calorimeter
• Alternate planes of steel and plastic
  scintillator strips
• Read out by PMTs
• 27t fiducial mass

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Far Detector

• Functionally equivalent to Near Detector
• 3.8kt fiducial mass
• 705m underground

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Weak Interactions

• Neutral current
• No track
• Unobserved energy
• Hadronic shower

• Charged current
• Charged lepton track
• Hadronic shower

June 4, 2009
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Muon Neutrino Disappearance

• Deficit in FD CC energy spectrum versus
  expectation

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Muon Neutrino Disappearance

• Worlds best measurement of ?m2

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ND Event Pre-Selection Cuts

• Beam quality and detector quality cuts
• Event vertex reconstructed within the fiducial
  volume of the detector
• Transverse vertex position gt 50 cm away from the
  edge of a partial plane or its outline on a full
  plane
• 30 planes lt z lt 80 planes

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FD Event Pre-Selection Cuts

• Beam quality and detector quality cuts
• Event vertex reconstructed within the fiducial
  volume of the detector
• Transverse vertex position gt50 cm from the edge
  of the detector and gt45 cm from the center of the
  detector (avoid uninstrumented coil hole region)
• z position gt25 cm from the front and gt1m from the
  back of each super-module

14 October 2009
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FD NC Event Selection

• Discard events gt60 planes

• Discard events with a track gt5 planes longer than
  the shower

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Systematic Errors

• Normalization ?4
• Livetime, Near/Far reconstruction efficiency,
  fiducial mass
• Relative Hadronic Calibration ?3
• Inter-detector calibration uncertainty
• Absolute Hadronic Calibration ?11
• Hadronic shower energy scale(?6)
• Modeling of intranuclear rescattering(?10)
• CC Contamination of NC-like sample ?15
• ND Preselection
• Evis lt 0.5GeV ?15.2
• Evis lt 1.0GeV ?2.9

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Blind Analysis

• Hide unknown fraction of FD events using unknown
  function of energy
• Decide analysis details using Monte Carlo and ND
  data
• Use open FD dataset for data quality checks
• Open the box
• Dont change any details of the analysis after
  this point
• Ensures result unbiased

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4-Flavor Contours
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4-Flavor Contours Sensitivity
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Decay Sensitivity
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