1

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

appearanceTord Ekelöf /Uppsala University

Roger Forty /CERN

Christian HansenThis 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 from atmospheric data

Explanation

4

What is oscillation?

3 “flavor eigenstates”

e

3 “mass eigenstates”

1

| l = mUlm | m, l = e,

i.e. l is with probability | Ul1 |2 a 1 a.s.o. …

5

m have different masses different speed

different phases after propagation

At L = 0

6

CERN to Gran Sasso Neutrino Beam(CNGS)

7

Detection of appearance at Gran Sasso

Operahttp://operaweb.web.cern.ch/operaweb/index.shtml

Icarushttp://pcnometh4.cern.ch/

But would it be possible in a third way … ?

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

Detector Outline

•rd = 0.67 rm

•rm = 150 cm•rd = 100 cm• = 34 degrees

Note For gaseous Cherenkov detectors, where c is very small, rd = 0.5 rm. Here we focus Cherenkov light emitted in liquid, i.e. large c, 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

1300 modules

11

Hexagonal Pattern

12

HPD-Hybrid Photo Diode

active diameter 114 mm

entrance window borosilicate glass, cut off < 250 nm

field configuration fountain shape, defined by 4 ring electrodes

demagnification 2.3

Photo cathode bialkali (K2CsSb), semi-transparent

silicon sensor 300 m thick, 50 mm diam., 2048 pads, 1 x 1 mm

electronics 16 IDEAS VA chips, ENC ~ 350 e

max. HV ca. 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

HPD-Hybrid Photo Diode

•Quantum Efficiency is about 20%•rd is about 10cm ( 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; <P>=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 decay vertex

• To calculate number detectable Cherenkov photons a particle emits, use:– the particles momentum (sin2c 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 sin2c dE

16

Simulation – without G4An 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

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 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

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

Cut – without G4

• It also works for more complicated events

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 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

4 161

CNGS beam energy * 10

33 212

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 0.1 rad

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

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