The OPERA emulsion detector for a long-baseline neutrino oscillation experiment

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The OPERA emulsion detector for a long-baseline
neutrino oscillation experiment

• H.Shibuya
• Toho Univ., Japan
• K.Hoshino, M.Komatsu, K.Niwa
• Nagoya Univ., Japan
• S.Buontempo, A.Ereditato, G.Fiorillo,
  P.Migliozzi, P.Strolin
• Naples Univ. and INFN, Italy
• G.Romano
• Salerno Univ. and INFN, Italy
• Y.Sato
• Utsunomiya Univ., Japan
• LNGS-LOI 8/97 and SPSC 97-24/I218

Presented by A.Ereditato, INFN Naples
M.Komatsu, Nagoya Univ.
Gran Sasso Laboratory, 6/2/1998
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Physics process neutrino oscillation
nm lt-gt nt t- X CC

• nt appearance esperiment direct
  t-decay detection
• Posc sin22qmt sin2 (1.27 x Dm2(eV2) x
  L(km)/E(GeV))
• Dm2min (Posc/2) x 1/1.27 x E/L

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OPERA
L.o.I study the atmospheric
neutrino anomaly, as indicated by Kamiokande
large mixing and Dm2 10-2 eV2

• New results from Super Kamiokande and CHOOZ
• importance of nm-nt oscillation
  search

nm-ne
nm-nt
Kamiokande
SuperK.
CHOOZ
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Is there room for improvements ?

• Explore the possibility of a higher sensitivity
  search
• exploit high intensity
  of the beam under study
• increase of detector mass (modularity)
• Perform the exercise with reference options
• optimization will be needed
• beam, detector design
• Assess the feasibility of the experiment tests
• background reduction,
  emulsion handling, technical issues

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Automatic emulsion scanning

• Pioneered by the Nagoya group Track Selector
• speed about x 100 w.r.t. semi-automatic
  systems
• New Track Selector routinely scanning in Japan
  and
  starting-up in Napoli
• speed about x 10 w.r.t. Track Selector
• Other CHORUS laboratories actively scanning
• RD going on at CERN, Nagoya, Napoli and Salerno

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Microscope event view
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Track selector
100 mm
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Aim, target mass and
experimental technique

• Atmospheric neutrinos (SK) Dm2
  sensitivity 10-2 -10-3 eV2
• CERN-Gran Sasso beam M O
  (1000) ton
• Impossible with pure emulsion target
• (CHORUS 0.8 ton , TOSCA 2.5 ton)
• New technique required
• iron (lead)-emulsion sandwich
• passive target material, emulsion for
  tracking
• Starting point the Emulsion Cloud Chamber (ECC)

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Rethinking the ECC technique

• Charm decays and hadron reinteractions in the
  passive material unacceptable backgrounds using
  impact parameter
• Hence, no impact parameter, no decays in Fe (Pb)
• The OPERA detector concept

- select t-decays in gaps between metal plates -
minimal plate thickness (et) , 2 emulsion
sheets - measure decay kink in space, by
emulsion tracking
) Oscillation Project with Emulsion tRacking
Apparatus ) A. Ereditato, K. Niwa, P. Strolin,
INFN/AE 97/06
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Fraction of ts decaying inL (lead), E
(emulsion layer), B (base), G (gap), L (long
kinks)
For Dm2 2 x 10-3 eV2 and 1 mm lead, 3 mm gap
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The detector

• Lead-emulsion target
• - element 1 mm Pb, ES, 3 mm gap, ES
• - brick stack of 30 elements ( 13 cm thick,
  15 x 15 cm2 X-sect.)
• - module 18 x 18 bricks ( 2.8 x 2.8 m2 )
• - electronic detector planes following each
  module ( 5 cm thick)
• - 300 modules 750 ton, subdivided into 10
  identical supermodules
• - overall target dimensions 3.5 x 3.5 x 40
  m3 (x 2)
• Muon detection
• - tracking in the target (electronic
  detectors)
• - magnetised iron m-spectrometer downstream
  sign of charge (momentum)
• Calorimetry
• - in the target Pb (each module 5 X0 )
  electronic det. (RPC, straws,...)
• Dp/p 10-20 at 1-30 GeV/c from multiple
  scattering in emulsion

Preliminary design
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element brick
3 mm
150 mm
1mm
150 mm
135 mm
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front view
5 m
12.5 m
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apparatus
5m
45 m
3.5m
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Emulsion

• No target (bulk) emulsion, but still 13 m3
  of emulsion layers
• Diluted emulsion AgBr content 1/2-1/3 w.r.t.
  short baseline experiments cost scales down
• (lower grain density allowed by automatic
  scanning and b.g. level)
• Industrial production time schedule, lower cost
• Alternative similar emulsion as for X-ray films
• RD on emulsion tests on prototype ES and bricks
  going on in Nagoya and Fuji company

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Electronic detectors

• Moderate position resolution (shower center)
  s few mm
  (low background tracks)
• Standard large-surface trackers can be used
• Resistive Plate Chambers,
• Honeycomb chambers,
• Streamer tubes.....
• Need reconstruction behind each emulsion module
• (i.e. using RPCs)
  7000 m2 total detector surface
• Similar detectors may be used for the muon
  spectrometers

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Data and event reconstruction

• Study t e-, m- , h- , (possibly 3p)
• Track localization by electronic detectors
• Start scanning from ES upstream of event in
  electronic detector
• General scanning and scan back in ES
• Find vertex plate (Pb) and neutrino vertex
• Follow down tracks from vertex
• Kink search (in gaps between Pb)
• Kinematics of candidate events
• (few of total)

Start scanning here
n
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nt interactions

• Scale reference option 5 x 1019
  pot/a , 75 efficiency, 220 days run
• assume 2.5 x 1020 pot/4 years
• Data 810 CC
  n interactions/kton x 1019 pot (Gran Sasso)
• 
  15000 CC in 4 years (750
  ton detector)
• 25 nt interacting in OPERA (Dm2 2 x 10-3
  eV2)
• 150 (Dm2 5 x 10-3 eV2)

possible improvements by design optimization
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t detection efficiency

• Decays outside Pb (1 mm) egap
  0.50
• (egap depends on beam features)
• 
  0.87 (t m)
• Kink finding efficiency ekink 0.84 (t
  e)
• 0.89 (t h)
• determined by the angular cuts
• (resolution) 20 lt qkink lt
  500 mrad (scanning bg rejection)
• BR t m, e, h
  0.174 , 0.178 , 0.498
• Fiducial cuts alignment
  egeom 0.93

Total efficiency for the 1-prong channels
0.36 (3p channel under study)
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Charm induced background

• (sign of daughter only measured if muon)

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Charm b.g. to t- h-, m-, e- (before
vertex kinematics of candidate events)

• 0.056 charm / CC
• x 0.37 D production
  probability
• x 0.306 BR (D h
  neutrals)
• x 0.47 D decay
  outside Pb
• x 0.86 e kink
• x 0.93 fiducial cuts
  alignment
• x 0.05 m- CC not identified
• x 14900 CC events
  1.8 events (h-)

Nbg(h-)
BR (charged D l neutrals) 0.075 m
charge measured by the downstream spectrometer
(1-e 0.3)
0.2 events (m-)
0.4 events (e-)
Total 2.4 events from charm
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Other backgrounds

• Prompt nt in the beam negligible
  (10-6 level)
• Hadron reinteractions a few kinks in the
  spacer are rejected by the kink angle cut
  (20 mrad) and by the detection of heavy
  fragments
• p , K decays
• (CC and NC) 0.6 events (further reduced
  by possible
• momentum cut)
• NC associated
• charm production double decay topology
  0.4 events before the vertex kinematics

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B.G. reduction by vertex kinematics

• Before kinematical analysis of candidate events
• Nbg(h-) 2 events Nbg(m-) Nbg(e-)
  0.5 events
• Nbg(associated charm) 0.4 events
• Vertex kinematics aim Nbg Nbg / 5 (to
  be studied)
• 
  Nbg (charm) lt 1 event
• 
  

Important vertex kinematics require track
before decay possible
only with emulsion granularity
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Sensitivity and discovery potential

• nm CC st/sm et x BR evert
• sin2 2qmt ( large Dm2 ) lt 2 x 2.3 / (14930 x
  0.48 x 0.36 x 0.90)
• lt 2 x 10-3
• Dm2 (full mixing) lt 10-3 eV2
• (90 CL)
• If oscillation occurs
• Dm2 2 x 10-3 eV2
  10 detected events
• Dm2 5 x 10-3 eV2
  50 detected events

NO OBSERVED EVENTS
total b.g. 1 event
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Emulsion scanning (1)

• 20000 events nm NCCC to be scanned
• (achievable with fast automatic microscopes)
• rougher event localization w.r.t. short baseline
  exp.
• (allowed by low track density)
• fast general scanning (downstream ES) over 1
  cm2
• scan back of all found segments up to the vertex
• scanning more elaborate, special care for
  candidates
• exploit on-going progress and equipment for
  CHORUS

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Emulsion scanning (2)

• 20000 events/4 years
• 5000 /100000 bricks removed per year
• aim emulsion developed and
• quasi on-line scanning
• replace bricks (?)
• fading regenerates the emulsion left in place

Prompt physics analysis Emulsion experiment with
a long-life
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Feasibility studies, optimization and RD (1)

• Emulsion diluted emulsion quality vs.
  cost
• procurement handling
• ES manufacturing
• dedicated pouring machine (industry?), X-ray
  films
• controlled fading
• Bricks passive material Pb vs.
  Fe, radioactivity
• spacers (plastic, honeycomb, ....)
• 
  low density, rigid
• vacuum vs. mechanical packing (both ?)
• optimize dimensions Montecarlo prototype
  tests
• Electronic trackers define
  requirements space time resolution
  optimize performance vs. cost
• industrial production
• tests on prototypes track association to
  emulsion

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Feasibility studies, optimization and RD (2)

• Apparatus design optimize module
  (supermodule) dimensions
• and layout
• temperature and humidity control
• detector mass and cost
• spectrometer design performance
  requirements
• Tests prototype bricks
  mechanics structure
• install bricks in the Gran Sasso
  Laboratory
• 
  ambient radioactivity,
  alignment by cosmics,
• hit density, optimize layer
  thickness
• beam tests
• kink efficiency, angular
  resolution,
• vertex finding
• RD emulsion collaboration with
  industry
• pouring machines
• dedicated scanning systems fast general
  scanning
• Beam optimize beam design intensity, spectrum,
  ltEgt

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A possible schedule for OPERA

• 1997 LoI studies, conceptual design
• 1998 Tests, feasibility, design,
  proposal
• 1999 Approval, prototypes, tests
• 1999-2002 Construction
• 2002 Start neutrino data taking
• 2003 Early physics results

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at Gran Sasso

• Possible design 750 ton , 2.5
  x 1020 pot (4 years)
• 20000 nm CCNC
  events
• Discovery potential small bg, a few events
  are meaningful_at_ Super K. (Dm2 5 x 10-3 eV2)
  50 events (1 b.g.)
• Negative search Dm2 lt 10-3 eV2
  sin2 2qmt lt 2 x 10-3
• covers natm (Super Kamiokande)
• Modular structure detector staging is possible
• 
  
• High sensitivity nm-nt search
• explore the atmospheric neutrino signal

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Conclusions

• Promising technique to detect nm-nt oscillation
  with a
• Long Baseline Experiment at the Gran Sasso
• Further studies, tests and RD needed to assess
  the feasibility of the experiment
• Explore the parameter region Dm2 gt 10-3 eV2
• to determine the source of the atmospheric
  neutrino signal