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

2. 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)

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

4. 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

5. 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

6. 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

7. 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

8. Neural Network Input variables Red: Electron Blue: Pion Red: Electron Blue: Pion Red: Electron Blue: Pion

9. 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)

10. Shower energy measurement results PRELIMINARY Energy resolution: )( E 40-50 % ~ GeV  

11. 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) 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

12. 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

13. 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 40-50 % ~ GeV  EE

14. Part II Analysis of the “NC” event 183545620 located in Bern (I.Kreslo, C.Pistillo,L.Scotto Lavina)

15. 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%)

16. Predictions from SS 6 double base-tracks located in the CS at the Gran Sasso scanning station

17. 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

18. 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

19. 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

20. 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%

21. Focusing on p>8 Gev/c and subdividing the samples according to the particle type According to MC particle a) is likely a pion

22. 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)

23. 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%

24. 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

25. e / e CC hypothesis  NC hypothesis Beam0.00871  /  CC 10.31 Cut on momentum and interaction inside the brick P>8P>10P>12P>8P>10P>12 0.710.660.630.0280.0150.010 Total (10 -3 )6.25.75.58.74.73.1 Not included: cross section for QE+RES,DIS the particle a) is an electron

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

27. Event 183545620 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)