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 (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 EM shower 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 –tan x and tan y <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 1234

Shower reconstruction Real data –Brick exposed to 6 GeV electron beam Electron 6 GeV (real data) in 20 emulsion films: ~ 3.3 X 0 XZ projection YZ projection

e/  separation and 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 measurement results PRELIMINARY Energy resolution: )( E % ~ GeV  

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 located in Salerno (brick 22530) PRELIMINARY Gamma shower pointing 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) )( E % ~ GeV  EE

Part II Analysis of the “NC” event located in Bern (I.Kreslo, C.Pistillo,L.Scotto Lavina)

Event NC Selected brick: W23,R39,C22 (35.0%) Other bricks W23,R39,C21 (22.6%) 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 = 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 1x1cm 2 area on 25 plates (34-57) Two vertices found

c) Angle (pβ = 0.27 GeV/c) (0.17/ %CL) Other 4 short tracks recontructed (low pβ) PRIMARY VERTEX a) angle (pβ > 8 GeV/c) b) angle (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 ~ 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 e CC possible? The CNGS beam has a e (+antineutrinos) contamination of ~1%

A shower is reconstructed Response of the NN: electron (100%) Estimated energy: 6  2 GeV Associated base-tracks: 80 gamma conversion? Application of the shower reconstruction algorithm to our event

e / e CC hypothesis  NC hypothesis Beam  /  CC Cut on momentum and interaction inside the brick P>8P>10P>12P>8P>10P> Total (10 -3 ) Not included: cross section for QE+RES,DIS the particle a) is an electron

P( e / e CC) / P(  NC) p > 8 GeV/c0.71 p > 10 GeV/c1.23 p > 12 GeV/c1.77

Event shows an interesting topology. The interpretation as  NC doesn’t seem to be the only possible. The probability of a e CC 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)