EM shower reconstruction and located neutrino event analysis
Ciro Pistillo (Bern LHEP)
on behalf of the Swiss OPERA groups
Summary
• Part I:
EM shower reconstruction, e/π 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)
Part I
EM shower reconstruction, e/π separation and energy measurement,
gamma conversion search
(F.Juget, F Meisel)
Algorithm for EMshower reconstruction
• Principle • Collect base-tracks inside a defined cone
(20 mrad opening angle, maximum radius 800 m)
• Used connection criteria to select base-tracks– Slope and position
• To remove background: only keep BT with– tanx and tany <0.4
• Proceed in « back-propagation » method
• Propagate back each BT contained in the cone, keep only the one satisfying the following criteria:– r < 150 microns– < 150 mrad
(criteria optimised to have ~ 5% background)
Back-propagation
Primary track
1 2 3 4
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
e/ separationand energy measurement
• Using the reconstructed shower– 2 algorithms have been developed based on a
Neural Network method to perform:• e/ 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 r distribution– Mean and RMS of distribution
Neural Network Input variables
• Red: Electron • Blue: Pion
• Red: Electron • Blue: Pion
• Red: Electron • Blue: Pion
e/ separation results
• 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.
(published in JINST 2 P02001 february 2007)
Shower energy measurementresults
PRELIMINARY
Energy resolution:
)(E40-50 %
~GeV
Vertex and showerAn 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)
PRELIMINARY
Gamma showerpointing to the vertex
Some gamma activity was found at ~ 1.7 cm from
a CC event with 6 prongs.
Gamma should com from decay produced at the vertex
Vertex and shower
PRELIMINARY
Impact parameters:
- All tracks have good IP
- Gamma Shower: Dist. From vertex 1.7 cm (13 films)
IP = 42.5 m
The gamma shower is well attached to the vertex
• 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…)
Conclusion (I)
)(E40-50 %
~ GeVE
Part IIAnalysis of the “NC”
event 183545620located in Bern
(I.Kreslo, C.Pistillo,L.Scotto Lavina)
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%)
Predictions from SS
6 double base-tracks located in the CS at the Gran Sasso scanning station
VT1: 2 prong
IP = 2.9 m
VT2: 5 prong
<IP> = 52.1 m
The scan-back of the 6 candidates stops immediately after a few plates (low momentum)
DZ= 12.5 mm
Online scanning
GS of a 1x1cm2 area
on 25 plates (34-57)
Two vertices found
c) Angle 0.040 -0.014 (pβ = 0.27 GeV/c) (0.17/0.69 90%CL)
Other 4 short tracks recontructed (low pβ)
PRIMARY VERTEX
a) angle 0.031 0.017 (pβ > 8 GeV/c)
b) angle 0.009 0.006 (low pβ)
a)
b)
c)
momentum measured with the last release of M.Besnier’s algorithm
Event analysis
reconstrucred tracks
RMS X = 2.2 mrad and Y = 2.1 mrad comparable with the microscope resolution (1.8 mrad) the MCS routine says p > 8 GeV/c
Identification of track a)
Residual of base-track angles wrt the track angle
We then consider a sample of 1000 NC DIS simulated events
P > 8 Gev/c
Identification of track a)
P
slope
The number of events where one or more particles with p > 8 GeV/c are present is ~12%
Focusing on p>8 Gev/c and subdividing the samples according to the particle type
According to MC particle a) is likely a pion
In 1000 NC simulated DIS events we found only 28 times a
particle with momentum > 8 GeV interacting inside the brick
Adding the condition that particle a) interacts inside the brick only 28/1000 events survive (29 particles)
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 (p>8 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 <IP> ~ 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%
A shower is reconstructed
Response of the NN: electron (100%)
Estimated energy: 62 GeV
Associated base-tracks: 80
gamma conversion?
Application of the shower reconstruction algorithm to our event
e/e CC hypothesis NC hypothesis
Beam 0.0087 1
/ CC 1 0.31
Cut on momentum and interaction inside the brick
P>8 P>10 P>12 P>8 P>10 P>12
0.71 0.66 0.63 0.028 0.015 0.010
Total (10-3) 6.2 5.7 5.5 8.7 4.7 3.1
Not included: cross section for QE+RES,DIS
the particle a) is an electron
P(e/eCC) / P(NC)
p > 8 GeV/c 0.71
p > 10 GeV/c 1.23
p > 12 GeV/c 1.77
Event 183545620 shows an interesting topology. The interpretation as NC doesn’t 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.
Conclusion (II)
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