Report on the CHORUS analyses

1
Report on the CHORUS analyses
Alessia Satta
Università degli studi di Roma La
Sapienza and INFN
For the
Collaboration
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)
APS meeting
2
Contents

• Chorus design
• Experimental layout
• Results from first phase of the analysis
• Status of the second phase of the analysis

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The experiment proposal

• designed to search for nm ?nt oscillation
• proposal exclusion plot
• aim explore with high sensitivity the Dm2 region
  -suggested by the hypothesis of a large
  contribution of neutrinos to dark matter-

4
Conceptual design of the experiment

• Search for the appearance of nt in an
  pure nm beam
• the nt is detected through CC interactions
• oscillation signature identify t as one of
  the products of a n interaction
• the t identification is possible exploiting its
  short decay path ( 1.5 mm at CHORUS energy)
• use of emulsion (-1 mm spatial resolution)
• as neutrino target and t detector electronic
  detector to help distinguishing t from background
  

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CHORUS apparatus
Hadron spectrometer
m spectrometer
calorimeter
Emulsion target scintillating tracker
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Analysis chain

• Use electronic detector information.
• Select events compatible with the t decay modes
• m- nt nm (BR 18 ) h- nt npo
  (BR 50 )
• Select the events with a t daughter candidate
• reduce the scanning load
• daughter 1) m with Plt 30 GeV/c
• candidates 2) h with 1ltPhlt20 GeV/c

Preselection

• Starting from the reconstructed events in the
• electronic detector the t daughter candidate
• tracks are followed back through emulsion
• to the neutrino vertex searching for a kink
• topology along their path

Scanning
Post-scanning

• Validation of the nt hypothesis for the events
• with a kink topology, i.e. background reduction

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Automatic Emulsion Scanning(developed in Nagoya)
year views/s
1994 0.008
1996 0.25
1999 3
2001 10
The major part of the scanning is fully
automated an hardware video processor (Track
Selector) digitizes images (focused at 16
different depths). A track is found searching
for grains coincidence in adjacent layers
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Background sources

• SOURCE COMMON TO ALL APPEARANCE EXPERIMENT
• The nt prompt component of the beam gives
  negligible background, less than 0.1 events is
  expected at the end of the analysis
• SOURCE TYPICAL OF CHORUS EXPERIMENT
• The kink signature may originate from
• Short-lifetime particles decay mainly charmed
  hadrons produced in neutrino or antineutrino
  interactions where the primary lepton is not
  identified
• Hadrons-emulsion interactions if no nuclear
  recoil or break-up signs are detected (known as
  White Star Kink)

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Status of the scanning
1994 1995 1996 1997 Total
Protons on target/1019 0.81 1.20 1.38 1.67 5.06
Emulsion trigger/103 422 547 617 719 2305
0m sample 0m sample 0m sample 0m sample 0m sample 0m sample
Events reconstructed and vertex predicted in emulsion 56111 77443 95751 106093 335398
Events with at least 1 selected negative track 19846 29350 37143 36073 122412
Events scanned so far 13047 17859 29773 24532 85211
Vertex located 3024 4424 8704 7054 23206
1 m sample 1 m sample 1 m sample 1 m sample 1 m sample 1 m sample
Events reconstructed and vertex predicted in emulsion 96995 168668 209136 238552 713351
Events with at least 1 negative muon 66309 112943 139077 159296 477625
Events scanned so far 50025 62568 114923 127879 355395
Vertex located 20400 21610 44867 56865 143742
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Results of phase I analysis

• Lets look at the data !
• NO CANDIDATE
• HAS BEEN FOUND

0m 1m
expected 1.1? 0.3 0.65 ?0.11
found 0 0
Where 2.34 takes into account the systematic
uncertainty (15)
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Chorus phase II

• One year ago a second phase of the analysis has
  begun
• New algorithms for reconstruction in electronic
  detector
• New scanning method for secondary vertices
  detection (Net scan)
• This second phase is focused both on oscillation
  and charm physics

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CHORUS Phase II net scan
All track segments (? lt 0.4 rad) in Fiducial
volume 1.5 x 1.5 mm2 x 8 plates Offline
analysis of emulsion data Increase in efficiency
2 , higher purity
Reconstruct full vertex topology
At least 2-segment connected tracks
Track segments from 8 plates overlapped
Eliminate passing-through tracks
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Charm physics

• Charm hadrons similar lifetime to t so possible
  exploring also charm
• physics direct detection of decay point and
  topology
• (unique feature in neutrino experiment)
• The purity of the automatic scanning analysis is
  so high that almost no manual scan is needed !
• D0 production rate
• Already published on a subsample
• ? (D0) / s (CC) 1.99 ? 0.13 ? 0.17
• Inclusive charm studies
• Expected 4,000 neutrino-induced charm events
• Fragmentation fractions D0 D Ds ?c
• B(c??), Vcd, s(x), ...
• Associated charm production
• Exclusive channels
• ?c absolute BR - QE ?c production

E531
CHORUS

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Outlook

• First phase analysis is finished
• A new MORE efficient phase is started using
• the power automatic scanning technique Net scan
• new off-line analysis algorithms
• Goals of this Phase II
• reach the proposal sensitivity in oscillation
  search
• Charm physics exploiting the chorus unique
  feature to be able to detect the decay topology
  event by event

15
WSK background evaluation

• Few measurements available
• Difficult to use because the whiteness depends
  from the emulsion composition and from the manual
  scanning rules
• Internal normalization
• Definition of a signal free reference region
• (Ldecay gt3plate 2800 mm )
• Measurement of the WSK effective cross-section
  using CHORUS data in the reference region
• Extrapolation in the signal region with the help
  of full MonteCarlo simulation
• Automatic inclusion of the CHORUS efficiency

16
White Star Kink simulation

• The hadron-emulsion interaction is simulated by
  FLUKA (A. Ferrari).
• The simulation of the emulsion response to the
  hadron interactions is a critical point
• It uses empirical criteria (b and range) to
  decide if a particle, a nucleus or a nuclear
  fragment outgoing from the interaction can be
  detected during the scanning

Check with KeK data good agreement
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Comparison of WSK simulation and CHORUS data

• Using data in the reference region (26 events)
  check Pt distribution

WSK simulation DATA
t ? h- nt npo
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Comparison of WSK simulation and CHORUS data (2)

• Using data in the reference region (26 events)
  check qkink distribution

t ? h- nt npo
WSK simulation DATA
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Automatic scanning phase I
One emulsion stack 36 plates piled-up along the
beam direction
Vertex plate localization
Kink finding
Vertex plate where the track
deseappeares
Explore part of the vertex plate searching for a
small I.P. track with the scan back one ( I.P.lt
9 mm)
The kink signature by I.P. is checked manually
It contains the neutrino interaction or the
decay point
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Background at the 0m sample

• Charm production and missed muon
• from antineutrino
• bg 3.6 x 10 6 / N located
• from neutrino
• if the hadron produced in charm
• decay is wrongly reconstructed as negative
• bg 3.4 x 10 5 / N located
• Hadronic interaction in NC

missed
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Background at the 1m sample

• Charm production and missed muon
• from antineutrino
• from neutrino
• if the muon produced in charm
• decay is wrongly reconstructed as negative
• CC interactions when the primary muon is wrongly
  associated to an hadron that undergoes WSK
  interaction or to an hadron from charm decay