1. 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. 2 Contents Introduction Detector Outline HPD – Hybrid Photo Diode Simulation & Cut without Geant4 Simulation & Cut with Geant4 Higher Neutrino Beam momentum Conclusion

3. 3 Introduction 1998: First evidence for Neutrino Oscillation Super Kamiokande Experiment saw missing  from atmospheric data Explanation    

4. 4 What is oscillation? 3 “flavor eigenstates” e   3 “mass eigenstates” 1   | l =  m  U lm | m , l = e,  i.e. l is with probability | U l1 | 2 a 1 a.s.o. …

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

6. 6 CERN to Gran Sasso Neutrino Beam (CNGS)

7. 7 Detection of  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. 8 A new concept of  appearance detection  interacts with a large target volume and via weak interaction a  is produced Use RICH-technique to discern Cherenkov light from  from Cherenkov light from background particles

9. 9 Detector Outline r d = 0.67 r m r m = 150 cm r d = 100 cm  = 34 degrees Note For gaseous Cherenkov detectors, where  c is very small, r d = 0.5 r m. Here we focus Cherenkov light emitted in liquid, i.e. large  c, so then r d = 0.67 r m

10. 10 Detector Outline Target volume within one module:0.45m 3 Suggested total mass for target: 1 kiloton Density for target (C 6 F 14 ): 1.67g/cm 3  1300 modules

11. 11 Hexagonal Pattern

12. 12 HPD- Hybrid Photo Diode active diameter114 mm entrance windowborosilicate glass, cut off < 250 nm field configurationfountain shape, defined by 4 ring electrodes demagnification2.3 Photo cathodebialkali (K2CsSb), semi-transparent silicon sensor 300  m thick, 50 mm diam., 2048 pads, 1 x 1 mm electronics16 IDEAS VA chips, ENC ~ 350 e max. HVca. 20 kV Signal/Noise ratio~ 15 at 20 kV, using VA3 chip 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. 13 HPD- Hybrid Photo Diode Quantum Efficiency is about 20% r d is about 10cm (  10 times smaller) 2.3 times demagnification High position resolution

14. 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; =17GeV) JETTA gives as output –Particles from scattering vertex –Momentum of the particles

15. 15 Simulation – without G4 JETTA also gives –tau track length –secondary particles from  decay vertex To calculate number detectable Cherenkov photons a particle emits, use: –the particles momentum ( sin 2  c 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 = (  /  c)L ∫ QTR sin 2  c dE

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

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

18. 18 Cut – without G4 For each track in the event that hit the tracking station histogram  for each hit in this event assuming the emition point was in the middle of this track, here  true1 =0.54rad and  true2 =0.65rad for the proton and muon respectively. 

19. 19 Cut – without G4 For each hit reconstruct  for each point along a track to find  min and  max for this track Cut away this point if  min <  true <  max Do this for each track in this event 

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

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

22. 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. 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. 24 Simulation – with G4 To easily view the event whit the Cherenkov process the Cherenkov photons’ hits on the HPD surface are displayed

25. 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. 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. 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. 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. 29 Higher beam energy Would we get around the problem with delta electron background by having higher energy for the beam? Number Cherenkov photons from tau would increase more than from electrons But the kink angle between the tau and muon would be smaller Average values from 50 events Nbr photons from  Nbr photons from e CNGS beam energy 4161 CNGS beam energy * 10 33212 Average values from 50 events Angle between and  Angle between and  CNGS beam energy 0.1 rad 0.2 rad CNGS beam energy * 10 0.1 rad

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

31. 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 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