New results from the CHORUS Neutrino Oscillation Experiment

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New results from the CHORUS Neutrino Oscillation
Experiment
XXIX International Conference on High Energy
Physics UBC, Vancouver, B. C., Canada July 23-29
1998

• Pasquale Migliozzi
• CERN

Belgium CERN Germany Korea Japan Israel Italy
Turkey The Netherlands Russia
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Outline

• The CHORUS experiment
• Data
• Present results
• Outlook
• Conclusions

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The CHORUS experiment

• Search for using the WBB at CERN
• at the Dark Matter scale (Dm21 eV2)
• Philosophy
• direct detection of t decay (kink)

EMULSION AS ACTIVE TARGET
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The neutrino beam
The mean energy of the nm beam at the CHORUS
emulsion surface is
The anti-neutrino contamination of the beam is
The prompt nt contamination
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n target and target tracker

• Target
• 800 kg nuclear emulsion
• (lt1mm space resolution)
• Observe the t decay topology
• (kink) in emulsion

• Purpose of electronic detector
• predict tracks back into emulsion
• kinematical cuts
• suppress background, e.g. D decays (particle id,
  kinematics)

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Data (data taking finished in 1997)
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Microscope event view
150 mm
Emulsion to the beam
Good tracks appear as dots (grains)
120 mm
Tracking implies connection of dots in different
layers
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Automatic scanning Track Selector(developed in
Nagoya)
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1m channel
Signal definition (BR18)
Background

• Identify the muon track and require 0gtPmgt-30
  GeV/c
• no other leptons at the primary vertex
• require a kink along the muon track with PTgt250
  MeV
• Flight length cut (llt5plates)

• Antineutrino charm production (D-) with the
  primary lepton not identified
• 1.6x10-6xNCC

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0m channel
Signal definition (BR68)
Background

• Identify a non-muon track and require
  -1gtPhgt-20 GeV/c
• no other leptons at the primary vertex
• require a kink along the hadron track with PTgt250
  MeV
• Flight length cut (llt3plates)

• Antineutrino charm production (D-) with the
  primary lepton not identified
• 2.3x10-6xNCC
• Neutral current events with a p- kink-like
  interaction (white kink)
• 2x10-5xNCC

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Analysis steps

• Event reconstruction by electronic detectors
• track predictions in emulsion
• Automatic scanning in interface emulsion sheets
• Automatic scanning in bulk emulsion vertex
  location and rejection of not-t candidates by
• search for short decays (decay inside the vertex
  plate)
• search for long decays (decay outside the vertex
  plate)
• Semi-automatic eye scan and remeasuring of
  retained events
• Event topology and kinematics (emulsionelectronic
  detector)
• t- candidate or identified background

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Vertex location
scan-back track
Location efficiency
Vertex location procedure
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Vertex validation
Impact parameter measurement
scan-back track
Small IP normal nm event Big IP interesting
event (nt?)
Semi-automatic eye check
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t- kink detection (parent search)

• Offline selection
• small impact parameter between
• parent and daughter
• kink point is in the vertex plate
• Semi-automatic eye check
• the events selected automatically
• are checked by eye for the final
• judgement

Principle Parent track (t) can be detected
by wider view and general angle scanning at the
vertex plate
scan-back track
Impact parameter
m or h
t
general scanning area
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Experimental check of the t detection efficiency

• Computed using Montecarlo simulation
• Checked using events with 2 muons in the final
  state (2 of the total)
• locate the muon tracks in the emulsion
• scan the emulsion searching for a kink
• charm dimuon candidates (muonic D decays)
• Yield in agreement with Montecarlo assuming
  scharm/sCC 5 (measured)

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A special event
CERN-EP/98-87
Parallel session 5 July 25th talk given by O.
Melzer
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Results
New data 31974N1m 362N0m events have been
analysed
No nt candidate has been found!
_at_ 90 C.L.
where
if i4 the m is not identified
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CHORUS efficiencies
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The exclusion plot (_at_ 90 C.L.)
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Outlook

• Phase I (the present till the end of the year)
• we will just complete the scanning of the whole
  statistics
• we gain a factor 3 in the 1m sample
• we gain a factor 6.7 in the 0m sample
• Pmt2x10-4
• Phase II (towards the final result)
• improvements on the reconstruction algorithms are
  going on
• because of the increasing scanning power the kink
  finding efficiency will improve by about a factor
  2
• if no t candidate will be found, the proposal
  sensitivity will be reached
• Pmt1x10-4

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Conclusions

• Neutrino data taking finished in 1997
• Emulsion data taking in progress
• CHORUS has shown that automatic scanning
  procedures are reliable and fast
• No t candidate has been found so far
• The proposal sensitivity can be reached