Limitations in the use of RICH counters to detect tau-neutrino appearance

1. Limitations in the use of RICH counters to detect tau-neutrino appearance Tord Ekelöf /Uppsala University Roger Forty /CERN Christian Hansen This talk can be found at http://chansen.home.cern.ch/chansen/WORK/talks.html

2. Contents • Introduction • Detector Outline • HPD – Hybrid Photo Diode • Simulation & Cut without Geant4 • Simulation & Cut with Geant4 • Higher Neutrino Beam momentum • Conclusion

3. Introduction • 1998: First evidence for Neutrino Oscillation • Super Kamiokande Experiment saw missing nmfrom atmospheric data Explanation nm  nt

4. What is n oscillation? 3 “flavor eigenstates” nenmnt 3 “mass eigenstates” n1n2n3 | nl = SmUlm | nm, l = e, m, t i.e. nlis with probability | Ul1 |2 a n1 a.s.o. …

5. nm have different masses different speed different phases after propagation At L = 0 nm

6. CERN to Gran Sasso Neutrino Beam(CNGS)

7. Detection of nt appearance at Gran Sasso Opera http://operaweb.web.cern.ch/operaweb/index.shtml Icarus http://pcnometh4.cern.ch/ But would it be possible in a third way … ?

8. A new concept of nt appearance detection nt interacts with a large target volume and via weak interaction a t is produced • Use RICH-technique to discern Cherenkov light from t from Cherenkov light from background particles

9. Detector Outline • rd = 0.67 rm • rm = 150 cm • rd = 100 cm • a = 34 degrees Note For gaseous Cherenkov detectors, where qc is very small, rd = 0.5 rm. Here we focus Cherenkov light emitted in liquid, i.e. large qc, so then rd = 0.67 rm

10. Detector Outline • Target volume within one module:0.45m3 • Suggested total mass for target: 1 kiloton • Density for target (C6F14): 1.67g/cm3 a 1300 modules

11. Hexagonal Pattern

12. HPD-Hybrid Photo Diode • HPD – an ongoing project in CERN • Now existing HPDs is about 10 times smaller than the HPDs wanted for the tau neutrino appearance detector

13. HPD-Hybrid Photo Diode • Quantum Efficiency is about 20% • rd is about 10cm (Q 10 times smaller) • 2.3 times demagnification • High position resolution

14. Simulation (with and without G4) • Used Neutrino Scattering Event Generator “JETTA” from CHORUS(also used by Opera) • JETTA takes as input • Neutrino beam momentum (e.g. CERN Gran Sasso neutrino beam momentum spectrum; <Pn>=17GeV) • JETTA gives as output • Particles from scattering vertex • Momentum of the particles

15. Simulation – without G4 • JETTA also gives • tau track length • secondary particles from t decay vertex • To calculate number detectable Cherenkov photons a particle emits, use: • the particles momentum (sin2qc is a function of p) • the particles track length (L) • the transmission of the media (T = 1) • reflectivity of the mirror (R = 0.95) • Quantum Efficiency (Q = see curve) N = (a/hc)L∫QTR sin2qc dE

16. Simulation – without G4 • The average of t momentum is about 11.6 GeV/c • The average of t track length is 0.05 cm An example; the t a • The average of number Cherenkov photons emitted by the t is 7

17. Cut – without G4 • A reconstruction program gives the emition angles given emition and hit point

18. Cut – without G4 For each track in the event that hit the tracking station histogram q for each hit in this event assuming the emition point was in the middle of this track, here qtrue1=0.54rad and qtrue2=0.65rad for the proton and muon respectively. a

19. Cut – without G4 • For each hit reconstruct q for each point along a track to find qmin and qmax for this track • Cut away this point if qmin < qtrue < qmax • Do this for each track in this event a

20. Cut – without G4 • It also works for more complicated events a

21. Cut – without G4 • It also works when pixalisation is introduced ↴

22. Simulation – with G4 • To introduce particles interaction with media a GEANT4 version of the simulation was written • The G4 simulation takes as input • momentum of the tau and it’s starting point and other particles from the first vertex (from the JETTA event generator) • The G4 simulation takes care off • tau decay • particle interaction (e.g. multiple scattering) • Secondary particle production (e.g. delta electrons) • cherenkov light emition • light reflection on the mirror • cherenkov light detection • …

23. Simulation – with G4 • The G4 simulation shows allot of delta electrons • The delta electrons then produce background cherenkov photons that the cut algorithm cannot handle (see later) • In the picture • the tau decays to a muon • delta electrons are produced when the muon traverses the media • one high momentum electron goes out of the module • others scatters and transforms into gammas • green are neutral tracks and red are charged tracks

24. Simulation – with G4 • To easily view the event whit the Cherenkov process the Cherenkov photons’ hits on the HPD surface are displayed

25. Cut – with G4 • The same cut algorithm (described earlier) are used on the events from the G4 simulation version • The photon hits from delta electrons cannot be cut

26. Cut – with G4 • The cut algorithm handles all Cherenkov rings • Again, photon hits from delta electrons cannot be cut • All signal photons in this event are also cut

27. Cut – with G4 • Photons from particles with large angles might hit the HPD without being focused by the mirror • Here a pion produced a “comet” that are not touched by the cut algorithm

28. Cut – with G4 • This is the best true event I’ve found • And even here it would be impossible to distinguish the tau ring from remaining delta electron background photons

29. Higher n beam energy • Would we get around the problem with delta electron background by having higher energy for the n beam? • Number Cherenkov photons from tau would increase more than from electrons • But the kink angle between the tau and muon would be smaller

30. An event with 100 times the CNGS energy for the n beam Many more t photons but they are all in the m ring and are cut away

31. Conclusions • We have investigated the limitations in the use of RICH counters to detect tau-neutrino appearance • Delta electrons give a too disordered background and make the developed cut algorithm unfeasible • At higher energies than CNGS n beam energy the tau Cherenkov ring aligns with a ring from a tau decay product • No further work is needed to complete this investigation and this project is about to end. This talk can be found at http://chansen.home.cern.ch/chansen/WORK/talks.html