Diapositive 1

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EM shower reconstruction and located neutrino
event analysis
Ciro Pistillo (Bern LHEP) on behalf of the Swiss
OPERA groups
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Summary

• Part I
• EM shower reconstruction, e/p separation and
  energy measurement, gamma conversion search
  (F.Juget, F Meisel)
• Part II
• Analysis of the NC event 183545620
  (I.Kreslo, C.Pistillo, L Scotto Lavina)

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• Part I
• EM shower reconstruction, e/p separation and
  energy measurement, gamma conversion search
• (F.Juget, F Meisel)

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Algorithm for EMshower reconstruction

• Principle
• Collect base-tracks inside a defined cone
• (20 mrad opening angle, maximum radius 800 mm)
• Used connection criteria to select base-tracks
• Slope and position

• To remove background only keep BT with
• tanx and tany lt0.4
• Proceed in  back-propagation  method

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Back-propagation

• Propagate back each BT contained in the cone,
  keep only the one satisfying the following
  criteria
• Dr lt 150 microns
• DQ lt 150 mrad
• (criteria optimised to have 5 background)

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Primary track
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Shower reconstruction

• Real data
• Brick exposed to 6 GeV electron beam

Electron 6 GeV (real data) in 20 emulsion films
3.3 X0
XZ projection
YZ projection
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e/p separationand energy measurement

• Using the reconstructed shower
• 2 algorithms have been developed based on a
  Neural Network method to perform
• e/p separation
• shower energy measurement
• The Neural Net. is fed with shower information
• Several variables from Longitudinal Profile
• 1 variable Number of basetracks
• 4 input variables from criteria connections
• Mean and RMS of Dr distribution
• Mean and RMS of Dq distribution

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Neural Network Input variables

• Red Electron
• Blue Pion

• Red Electron
• Blue Pion

• Red Electron
• Blue Pion

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e/p separation results
(published in JINST 2 P02001 february 2007)

• 90 efficiency is obtained at E 2 GeV with
  ??cont. less 1
• 80 efficiency is obtained at 1 GeV for 1
  ??cont.
• 95 efficiency at E 2 GeV gives 1-2 of ?
  cont.
• 90 efficiency at 1 GeV gives 5 ??cont.

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Shower energy measurementresults

• PRELIMINARY

Energy resolution
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Vertex and shower
An algorithm has been developed in order to
perform gamma conversion search in a large
scanned volume.A blind search was performed using
data from the event 178969961 located in Salerno
(brick 22530)
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Vertex and shower
The gamma shower is well attached to the ? vertex
PRELIMINARY
Impact parameters - All tracks have good
IP - Gamma Shower Dist. From vertex 1.7
cm (13 films) IP 42.5 mm
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Conclusion (I)

• Good e/? separation is obtained (up to 95)
• Good energy resolution can be obtained
• Data analysis from last OPERA run is under study
  (gamma conversion search, vertex assignment)

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Part II Analysis of the NC event
183545620 located in Bern (I.Kreslo,
C.Pistillo,L.Scotto Lavina)
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Event 183545620 NC
Selected brick 47344 W23,R39,C22 (35.0) Other
bricks 47340 W23,R39,C21 (22.6)
47316 W23,R40,C22 (22.1)
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Predictions from SS
6 double base-tracks located in the CS at the
Gran Sasso scanning station
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Online scanning
The scan-back of the 6 candidates stops
immediately after a few plates (low momentum)
VT1 2 prong IP 2.9 ?m
GS of a 1x1cm2 area on 25 plates (34-57) Two
vertices found
DZ 12.5 mm
VT2 5 prong ltIPgt 52.1 ?m
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Event analysis
PRIMARY VERTEX a) angle 0.031
0.017 (pß gt 8 GeV/c) b) angle
0.009 0.006 (low pß)
reconstrucred tracks
a)
c)
b)
c) Angle 0.040 -0.014 (pß 0.27 GeV/c)
(0.17/0.69 90CL) Other 4 short
tracks recontructed (low pß)
momentum measured with the last release of
M.Besniers algorithm
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Identification of track a)
Residual of base-track angles wrt the track angle
RMS X 2.2 mrad and Y 2.1 mrad comparable
with the microscope resolution (1.8 mrad) ? the
MCS routine says p gt 8 GeV/c
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Identification of track a)
We then consider a sample of 1000 ??NC DIS
simulated events
P
P gt 8 Gev/c
slope
The number of events where one or more particles
with p gt 8 GeV/c are present is 12
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Focusing on pgt8 Gev/c and subdividing the samples
according to the particle type
According to MC particle a) is likely a pion
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Adding the condition that particle a) interacts
inside the brick only 28/1000 events survive (29
particles)
In 1000 ??NC simulated DIS events we found only
28 times a particle with momentum gt 8 GeV
interacting inside the brick
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Summarizing our doubts

• We see a high momentum track from the neutrino
  interaction vertex interacting inside the brick
  12 mm downstream. Within 1000 ?? NC DIS events
  such a behavior (pgt8 GeV secondary interaction)
  occurs only 28 times (3).
• According to MC, particle a) is a hadron, most
  likely a pion but... The secondary
  vertex is reconstructed with ltIPgt 52 ?m. It
  shows a small opening angle and the 5 daughter
  tracks have low momentum.
• To our best knowledge track a) resembles to an
  electron showering after a few plates could be
  then the interpretation of the event as ?eCC
  possible?
• The CNGS beam has a ?e (antineutrinos)
  contamination of 1

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Application of the shower reconstruction
algorithm to our event
A shower is reconstructed Response of the NN
electron (100) Estimated energy 6?2
GeV Associated base-tracks 80
gamma conversion?
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Not included cross section for QERES,DIS
the particle a) is an electron
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Conclusion (II)

• Event 183545620 shows an interesting topology.
  The interpretation as ??NC doesnt seem to be the
  only possible. The probability of a ?eCC
  interaction is indeed of the same order of
  magnitude. We are going to perform a more
  analitic study. Further investigation of the
  primary vertex will be exploited as well.

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