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New results from a search for nm nt and ne nt
oscillation with the CHORUS experiment
Belgium (Brussels, Louvain-la-Neuve), CERN,
Germany (Berlin, Münster), Israel (Haifa), Italy
(Bari, Cagliari, Ferrara, Naples, Rome,
Salerno), Japan (Toho, Kinki, Aichi, Kobe,
Nagoya, Osaka, Utsunomiya) ,Korea
(Gyeongsang), The Netherlands (Amsterdam),
Russia (Moscow), Turkey (Adana, Ankara, Istanbul)
Takayuki Kawamura
INFN - Sezione di Napoli, ITALY
PHENO 2001 SYMPOSIUM
University of Wisconsin, Madison
7-9 May 2001
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CHORUS experiment
(CERN Hybrid Oscillation Research ApparatUS)
Physics motivation

• Neutrino masses and mixings have profound
  implications for particle physics, astrophysics
  and cosmology.

• Physics beyond the standard model
• A candidate for the Dark Matter

CHORUS neutrino oscillation search

• nt appearance in a pure nm beam
• Direct observation of t decay in nuclear
  emulsions
• Electronic detectors for event reconstruction and
  kinematical study

(spatial resolution lt 1 mm)
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Neutrino beam
West Area Neutrino Facility at CERN SPS
450 GeV
CHORUS, NOMAD

• Wide Band Beam
• 5.06 ? 1019 POTs (1994-1997)
• ltEnmgt 27 GeV
• ltLgt 0.6 km
• Prompt nt negligible ( 0.1 bg events)

ltLgt/ltEgt 2 ? 10-2 km/GeV
? Dm2 gt 1 eV2
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CHORUS detector

• Active target
• nuclear emulsion target
• scintillating fiber tracker

Calorimeter
Air-core magnet

• Air-core magnet
• Dp/p 0.035 p (GeV/c) ? 0.22
• 1996-97 upgrade ET, HC

• Calorimeter
• DE/E 32 /? E (hadrons)
• 14 /? E (electrons)
• Dqhadrons 60 mrad _at_ 10 GeV
• Muon spectrometer
• Dp/p 10 15 (p lt70 GeV/c)

Spectrometer
n
Active target
Veto
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Target region

• Fiber target tracker (TT)
• angle resolution 2 mrad
• position resolution 150 mm

Interface emulsion sheets (CS and SS)
Emulsion target (BULK)

• Interface emulsion sheets
• Changeable Sheets (CS)
• Special Sheets (SS)

n
800 mm
plastic base
100 mm
emulsion on both side

• Emulsion target
• 4 stacks each made of 36 plates
• Total mass 770 kg

90 mm
plastic base
350 mm
Fiber target tracker (TT)
emulsion on both side
(2 stacks are shown in figure)
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Procedure of analysis
1) Predictions 2) Scan-back Vertex
location 3) Kink search 4) Post scanning
Pre-selection of events
Momentum cuts
Pm- lt 30 GeV/c,
1 lt Ph- lt 20 GeV/c
Angle cut
q lt 0.4 rad
Follow-up of scan-back track in emulsion to reach
the vertex
Automatic decay topology search on scan-back track
Computer assisted eye-scan
Kinematical selection
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Emulsion data acquisition
take 16 tomographic images and shift them
accordingly to the angle of the track
Principle
frames acquisition
512x512 pixels 120x150 mm view
CCD camera
microscope stroke
CCD camera
shifting
x50 magnification 3mm focal depth
100 mm
350 mm
90 mm
emulsion plate
350 mm
pile-up
Total time 0.3 sec, Efficiency gt 98
1 cm2/hour to search all tracks with slopes up to
400 mrad
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Scanning procedure

• Automatic vertex location

Fiber tracker Interface emulsion Bulk
emulsion Vertex plate
the plates where a scan-back track disappeared

• Kink finding (parent search)

Look for track segments around the scan-back
position
Segments with small impact parameters w.r.t. the
follow-up track are candidates for the parent
track
About 8 of located events are automatically
selected as t decay candidates
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Eye-Scan

• The operators make a computer assisted
  measurement of the candidate kink topology
• Scan-back track
• Parent track
• Other tracks from the primary vertex
• To be confirmed as a decay there must be
• No black prongs or blobs
• No recoil
• No Auger electrons

Low momentum BG track (80)
Distortion(4)
No angle difference
Backward going nuclear fragment (12)
Hadron secondary interaction (2.5)
PT gt 250 MeV/c to reject p and K decays
decay (1.5)
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Summary of t search
Emulsion triggers 2,305K
0µ
1µ
Initial sample (CC contamination)
335,000 (140,000)
713,000
Initial sample
³1 negative tracks Momentum cut angle cut
122,400
477,600
Momentum cut angle cut
Events scanned
85,211
355,395
Events scanned
Vertex located
20,081
143,742
Vertex located
2,282
Selected for eye-scan
11,398
Selected for eye-scan
Kink candidates after eye-scan
Kink candidates after eye-scan
0
0
1m events with one negative charged muon 0m
events without muon
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Background sources

• Charm from n CC interactions with missed
  primary lepton
• Charm from n CC with µ/h wrong sign
• Associated charm production in NC
• Prompt nt
• Hadronic white kinks

nµ/e N D- X µ/e µ-/h- neutrals
D/D0 missed, associated to D µ-/h- neutrals
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White kink background
WK

• 1-prong nuclear interaction with no ionising
  activity at the interaction point (fake t decay
  topology)
• CHORUS measured
• lWK(PT gt 250 MeV/c) 24.0 8.5 m

t ? h
WK
PtGeV
Post-scanning WK rejection
t ? h
Ft cut and Ldecay cut t flight length shorter and
correlated with phad
Fkink rad
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Sensitivity maximization
Ft cut
4 6 1 1 0 0
2 2 0 0 0 0
Cuts optimisation by the a-priori criteria of
maximizing the sensitivity, independently from
data. A posteriori, DATA behave as expected in
the hypothesis of NO signal.
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Limit evaluation
Oscillation probability
At large Dm2
loc
loc
0.53 143,742 0.97 0.39 20,081
2.3 0.13
A systematic error of 17 is included in the
evaluation of Nt 2.4
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nm nt exclusion plot
Pmt lt 3.4 ? 10-4 _at_ 90 CL1
For large Dm2 sin22qmt lt 6.8 ? 10-4
Dm2 eV2
Using a different approach2 to CL definition
We obtain Pmt lt 2.2 ? 10-4
sin2(2q)
0.6 eV2
1 T.Junk, NIM A434 (1999) 435 2 G.J.Feldman
and R.D.Cousins, Phys.Rev. D57 (1998) 3873
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ne nt exclusion plot
Neutrino beam contains ne 0.9
Limits on the ne nt oscillation can be set with
the same analysis for nm nt oscillation
ne?nt
nm?nt
Dm2 eV2
Pet lt 2.6 ? 10-2 _at_ 90 CL
For large Dm2 sin22q?t lt 5.2 ? 10-2
sin2(2q)
7.5 eV2
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CHORUS phase II

• New Emulsion data-taking and analysis
• Improvement for oscillation search to reach the
  proposal sensitivity
• New predictions and locations
• Full event analysis in the vertex region by new
  scanning technique (NetScan)
• Charm physics ( 5K events)
• Vcd, Lc branching ratios, Ds branching ratios,
  etc.

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NetScan

• Pick up all track segments (q lt 0.4 rad) in a
  fiducial volume around scan-back track (1.5 x 1.5
  mm2 ? 8 plates)
• Offline analysis of emulsion data

Reconstruct full vertex topology
At least 2-segment connected tracks
Track segments from 8 plates overlapped
Eliminate passing-through tracks
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Conclusions

• The data collected by CHORUS (1994-1997) have
  been analysed searching for the 1-prong decay of
  the t.
• No evidence for nt interactions has been found.

• For large Dm2

nm nt mixing excluded to sin22qmt lt 6.8 ?
10-4 (90 C.L.)
ne nt mixing excluded to sin22q?t lt 5.2 ?
10-2 (90 C.L.)

• Phase II emulsion data-taking has started.
• Oscillation search to reach the design
  sensitivity
• New results on charm physics
• Improved emulsion scanning technologies and
  techniques