Transcript Test

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
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Contents
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Introduction
Detector Outline
HPD – Hybrid Photo Diode
Simulation & Cut without Geant4
Simulation & Cut with Geant4
Higher Neutrino Beam momentum
Conclusion
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Introduction
• 1998: First evidence for Neutrino
Oscillation
• Super Kamiokande Experiment saw
missing nm from atmospheric data
Explanation
nm  nt
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What is n oscillation?
3 “flavor eigenstates”
ne nm nt
3 “mass eigenstates”
n1 n2 n3
| nl = Sm Ulm | nm
, l = e, m, t
i.e. nl is with probability | Ul1 |2 a n1 a.s.o.
…
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nm have different masses
different speed
different phases after propagation
At L = 0 nm
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CERN to Gran Sasso Neutrino Beam
(CNGS)
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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 … ?
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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
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Detector Outline
•rd
•rm
•rd
•a
= 0.67 rm
= 150 cm
= 100 cm
= 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
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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
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Hexagonal Pattern
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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
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 mm thick, 50 mm diam., 2048 pads, 1 x 1 mm
electronics
16 IDEAS VA chips, ENC ~ 350 e
max. HV
ca. 20 kV
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Signal/Noise ratio
~ 15 at 20 kV, using VA3 chip
HPD-Hybrid Photo Diode
•Quantum Efficiency is about 20%
•rd is about 10cm (Q 10 times smaller)
•2.3 times demagnification
•High position resolution
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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
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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
the particles track length
the transmission of the media
reflectivity of the mirror
Quantum Efficiency
(sin2qc is a function of p)
(L)
( T = 1)
(R = 0.95)
(Q = see curve)
N = (a/hc)L∫QTR sin2qc dE
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Simulation – without G4
An example; the t
•The average of t momentum is
about 11.6 GeV/c
•The average of t track length is
0.05 cm
a
•The average of
number Cherenkov
photons emitted by
the t is 7
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Cut – without G4
• A reconstruction program gives the emition
angles given emition and hit point
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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
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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
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Cut – without G4
• It also works for more complicated events
a
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Cut – without G4
• It also works when pixalisation is introduced

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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
…
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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
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Simulation – with G4
• To easily view the event whit the Cherenkov
process the Cherenkov photons’ hits on the HPD
surface are displayed
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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
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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
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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
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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
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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
Average values
from 50 events
Nbr
photons
from t
Nbr
photons
from e
CNGS beam
energy
4
161
CNGS beam
energy * 10
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212
Average values
from 50 events
Angle
Angle
between between
n and t n and m
CNGS beam
energy
0.1 rad 0.2 rad
CNGS beam
energy * 10
0.1 rad 0.1 rad
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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
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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
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