1. Liquid Argon technique: a pizero identification analysis for T2K beam Paweł Przewłocki, A. Soltan Institute for Nuclear Studies, Warsaw Warsaw Neutrino Group, 2005

2. Liquid Argon Technique LAr technique: –Precisely reconstructable topology of events –High granularity: 3x3x0.6mm 3 –Particle ID capability –Very good calorimetry Applications: –ICARUS (Gran Sasso, Italy) –Proposed small LAr detector in the intermediate station of T2K experiment (Tokai, Japan) More on it in Anselmo Meregaglias talk!

3. The outline of my presentation Pi identification studies overview Energy loss pattern study Vertex visibility study - my analysis –Three wires visibility criterion –Visibility in lower energies Conclusions

4. What would we like to do? Separate e events from other, topologically similar It is essential in experiments looking for e appearance (like T2K) Main background: NC events with single 0 production ( 0 decays into two gammas) ν μ NC 0 ν e CC

5. Three methods –Looking for two showers pointing at one location (supposedly a vertex) (still remains to be done) –Examining dE/dx energy loss pattern if we see one shower only (ICARUS analysis) –Or looking for some particle (proton or charged pion) which marks the primary vertex of interaction, which is at some distance (gap) from visible shower (the analysis which I present below)... And how?

6. Energy loss analysis Performed by scientists from ICARUS Collaboration Based on dE/dx measurement (single mip signal of an electron vs. double signal of e + e - pair) First wires of a track (before showering starts) are studied

7. Energy loss analysis continued Main source of misidentification: compton scattering of gammas coming from pizero decay make the shower look like electron (single electron instead of a pair) Compton

8. The energy loss analysis shows that: With this method 96.3% pions are rejected for 90% electron efficiency (for 1GeV pions) For 250MeV pions (these will be most frequent in T2K) 93.5% are rejected The quality of rejection increases with energy – different from Cherenkov detectors (overlapping of two gamma induced rings) Misidentified pizeros (%) 1GeV

9. The third method Pizero can be visually recognized if the distance between the vertex and the point of gamma conversion is more than 1cm Conversion length of a photon in LAr is 18cm; hence, we get 5.4% of pions confused with electrons If we combine this method with the previous one we can get 0.2% for 1GeV (0.35% for 250MeV) pion misidentification, but… We have to know where the vertex is, and this information is only available when some other particle (apart from showering gammas) is visible at the vertex

10. My analysis My analysis is based on the gap method, its aim is to evaluate the efficiency of finding pizero events this way It is done with the T2K-LAr detector in mind (the detectors geometry, T2K neutrino energy spectrum) Events generated using Nuance 3.006 simulation software Nuance was originally created (by Dave Casper) for water Cherenkov detectors, but recent version (3.006) allows the user to specify any medium needed (Argon in particular)

11. My analysis Two stages of the study –First, a conservative approach: lets see how many pi zero events we can spot, when we assume that a particle in the vertex is visible if it lit up at least three wires –Then we can try to look at the events which were recognized as invisible and see if its really true – we have to scan the events

12. How to see a particle (in LAr) We assume that for a particle to be seen we have to observe a signal on three consecutive wires (the conservative assumption) We take into account every charged particle track (protons, charged pions, muons, electrons, charged kaons), and calculate length of its track using known energy-range dependence for each type of particle We project it onto the wire plane We project it again, this time onto 2 directions perpendicular to the wires directions (45deg, -45deg) If greater of these exceeds 9mm ( 3 wires times 3mm [wire pitch]) we conclude that the track is visible z y Exclusion of y coordinate Rotation by 45deg. x and z give us needed lengths x

13. General info Data simulated by Nuance 3.006 T2K beam @ 2km Two samples: w/o and with FSI (final state interactions [intranuclear cascade] in Ar nucleus) Muon neutrinos, ca 25.000 NC events generated No detector simulation Simple C++ code used instead, to see which tracks are visible

14. Protons and pions – with and without FSI (momentum) Dashed line – w/o FSI Solid line – with FSI application of FSI gives us many low momentum protons MeV/c

15. Can we trust FSI model in Nuance? NUX+Fluka simulation as a cross-check A thoroughly tested simulation for nuclear and particle physics Resonant channels, energy of neutrinos @ 1750MeV With and without rescattering in nucleus A number of particles of each type per event and average momentum are given Nuance no rescattering (Ar) Nuance with rescattering (Ar) NUX+Fluka with rescattering (Ar) pi 0 0.36/evt 0.43GeV 0.31/evt 0.35GeV 0.32/evt 0.38GeV pi plus 0.75/evt 0.44GeV 0.52/evt 0.41GeV 0.54/evt 0.41GeV pi minus 0.06/evt 0.36GeV 0.10/evt 0.26GeV 0.06/evt 0.32GeV n 0.31/evt 0.81GeV 2.27/evt 0.30GeV 3.13/evt 0.21GeV p 0.84/evt 0.79GeV 2.95/evt 0.36GeV 2.34/evt 0.39GeV There is a rough agreement between both generators when it comes to number of nucleons

16. Protons and pions - visibility Black – all, red – invisible MeV/c

17. Visible protons and pions – with and without FSI (momentum) Dashed line – w/o FSI Solid line – with FSI MeV/c

18. Visibility - summary particle w/o FSIwith FSI visibleinvisiblevisibleinvisible protons 10472 79.8% 2646 20.2% 17199 33.4% 34301 66.6% charged п 6973 99.8% 16 0.2% 6237 98.8% 78 1.2% charged K 119 99.2% 1 0.8% 103 97.2% 3 2.8%

19. Vertex visibility and pizeros NC evts w/o FSIwith FSI #evts2588925636 #evts with 1 or more pizeros 6334 (24%) 4301 (17%) #pizero events with 1 or more visible track 3325 (13% of all, 53% of all pizero evts) 2762 (11% of all, 64% of all pizero evts) #unidentifiable pizeros 3009 (48% of all pizero events) 1539 (36% of all pizero events) FSI: Less pizeros, but more of them identifiable!

20. Relaxing the cuts... Are those low energy protons really invisible? Might it be that we can see them even if the signal is on two or even a single wire? To investigate it, one has to make use of detector simulation and visually scan the events Such analysis has been carried out. Several events with low-energy protons from Geant4 T2K-LAr simulation were scanned The analysis is in progress, the results presented here should be considered preliminary, but they can serve as a hint

21. Test sample of low energy protons 60 NC events At least one pi zero produced One proton with kinetic energy less than 20MeV (corresponds to ~200MeV/c in momentum) Any number of neutrons GeV/c Proton momentum

22. Sample events Two weak cascades Induction Collection

23. Sample events Two showers overlapping

24. Sample events Neutron noise

25. Sample events Total chaos:-)

26. Identifiability criteria Vertex is considered visible, if: –Two gammas point at one location and there is a visible hit there –Two gammas point at a distant location and there can be more than one hit around this point –There is at least one particle, giving signal on at least two wires and there is only one cascade visible and pointing at it (no other visible hits around)

27. Visibility in the test sample Red histogram shows protons from events that cannot be identified as pizero events We can see that many low energy protons are in fact visible, which allows us to identify pizero events Black – all, red – invisible GeV/c

28. Conclusions Analysis still in progress Gap study: –Electronic noise has to be considered –More events, more detailed scanning Independent analysis needed for two visible showers pointing at one location case Further study neccesary, including electron events (visual scanning) Combining dE/dx with my results in conservative case gives 1.5% (1GeV pions) and 2.6% (0.25GeV) pion misidentification Eventually one should have the possibility to combine the three different analyses

29. Backup