Performance of a liquid Argon TPC exposed
to the WANF neutrino beam
for the ICARUS collaboration
Ewa Rondio, SINS, Warsaw, Poland
NuFact06, Irvine 25 August 2006
Exposure of 50 liter TPC
to high energy neutrino beam
• The detector was placed between CHORUS
and NOMAD experiments
(CERN West experimental area)
• Beam from West Area Neutrino Facility (WANF)
at CERN
• Goals of the test:
- observe neutrino interactions in LAr TPC
- check importance of nuclear effects
Measurement
jointly performed
by:
ICARUS
collaboration
and
groups from
INFN,
Milano Univrsity
with help
from
NOMAD
WANF neutrino beam
Neutrino momentum
distribution
- full spectrum
Neutrinos which
could hit the 50l
TPC chamber
νµ beam
with
7% νµ
1% νe contamination
20 GeV
150 GeV
Data collected in 1997
450 GeV protons extracted in 2 spills of 6ms
2.5 s apart every 14.4 s,
1.8 103 p.o.t. on a Be target
Mean ν energy 24.3 GeV
1.2·1019 p.o.t. during the exposure, registered about 105 triggers
Neutrino interaction in NOMAD detector
Reconstruction of muon in the spectrometer
Same spectrometer was used for muon
reconstruction of events in 50 liter detector
* good resolution
* only events with muon reconstructed
in NOMAD were used in the analysis
Detector at CERN neutrino beam
Trigger :
Veto from last CHORUS plane + additional scintillators in front of TPC
+ signal in local trigger counters just behind TPC + T1, T2 NOMAD scintillator planes
Efficiency 97%, dead time 3% from TPC and 15% from NOMAD
For calibration and alignment – trigger for passing muons
50 liter TPC
Liquid argon volume in the
detector 32x32x46.8 cm3
active volume 67kg of argon
doped with 3.5ppm TMG
Stainless steel container
(cylinder 90cm height, 35cm radius)
Constant electric field – 214V/cm
Anode planes: 2 at distance of 4mm
wires – stainless steel, 100µm,
distance 2.54mm
orientation perpendicular (planes)
Cathode – copper strips, 5mm thick,
10 mm distance
Schematic
view of
the installation
50 liter TPC
…. and one of the registered neutrino interaction
Active chamber volume
Visible: - minimum ionizing track – muon candidate
to be matched with Nomad spectrometer
reconstructed tracks
- stopping proton
- two photon conversions (pointing to
interaction vertex)
Condition to see tracks –
good Argon purity
obtained after several days of purification
Electron life time τe>10 ms
Drift velocity
vd=0.905+/-0.005 mm/µs
extracted from fits of muon
position in TPC with respect
to Nomad measurements
Relative position of TPC and Nomad determined from fits
of the through going muon tracks
…. and
for much more
complicated
event
Hit finding and fitting
B – baseline
A – amplitude
t0 – time with signal = A/2
τ1, τ2 – falling and rising characteristic
times
The fitted hit area is proportional to energy,
so provides good calorimetric information
How we define
Quasi- Elastic event??
Selection criteria:
* proton
* one muon
* primary vertex > 1cm
from TPC walls
* if other stopping particles
present their range must be
< 40 MeV proton
* no tracks other than µ
leaving TPC
* no photons with energy
above 10 MeV
matching with track
in the Nomad spectr.
projection on
wire plane>12 wires
with requirement that
it is fully contained in TPC
T>40 MeV
very clean topology
for scanning
Let’s look closer
at the
reconstruction of
one of
the golden
QE events
• only 2 tracks
• proton and muon
with correct dE/dx
• proton fully contained
in liquid Argon
• muon matching track
in Nomad spectrometer
• muon giving trigger
for the m.i.p. track
in LAr TPC
signal/noise =11
Matching muon track between TPC and NOMAD
Plots for QE „golden” sample
Muon momentum measurements, calibration
Mip signal derived from the sample
of 3000 crossing µ
momentum difference
as obtained from spectrometer
and from TPC
- dependence on #of hits
(from the QE event)
Taking all protons
from the „golden”
QE-sample
dQ/dx
as a function of
residual range
very good
proton-pion
separation
Statistics of QE events
• 1.2·1019 protons on target
• trigger efficiency 97% + dead time (TPC – 3% NOMAD – 15%)
gives effective lifetime of 75%
• 70k triggers collected,
• 20k have reconstructed µ in fid.vol. Æ CC candidates
• 50% of them have a vertex in fid.vol. (scanning)
gives 10k CC νµ interactions
Æ from which 86 QE „golden events” were selected
(scanning + checking of all the criteria)
Selected sample contains:
• Expected signal: pure Quasi Elastic interactions
• Background dominated by:
- resonance productions (followed by pion absorption)
- additional background from reactions with undetected
neutral particles (neutrons and gammas)
next step – comparison with Monte Carlo
simulation
•
•
Simulation by FLUKA, with DIS and resonances in NUX
For QE events axial mass 1.03 GeV
ν interactions as on free nucleon but with initial (Fermi motion)
and final (Pauli blocking, re-interactions) state effects
(PEANUT code for low/intermediate hadron energy)
model used for it:
Intra-nuclear cascade + pre-equilibrium + evaporation/fission or Fermi
break-up
nuclear density given by Saxon-Woods potential, p and n densities different
•
•
•
Fermi momentum depends on the local density, smearing with uncertainty principle
(δr2=2fm)
Average Fermi momentum depends on nucleus (for p and n)
Effect of Pauli principle is visible on the cross section (next slide)
•
Detector simulation and reconstruction as for the data
Influence of Pauli principle on cross section
– for ν CC interactions on Argon
P.Sala, NuInt02
In the simulation – look at number
of final state particles
with such Monte Carlo
compare reconstruction resolution
Muons are horizontal,
resolution depends on observed
track length (in the spectrometer)
at p=10 GeV,
L about 5 m, σp/p=2.2%
Proton reconstruction resolution
angular resolution
- depends on number of wires
with signal (N) (for 10 wires – 15 mrad)
general formula:
Now look at processes and event
rates in the simulation:
•
Sample consists of:
2.3% of QE events
91.5% of DIS events
6.2% of resonance events
•
Classification in the „golden
channel”:
16% of QE events classified in this
channel
0.14% of DIS and RES events also
get such classification
(105 ν interactions were used to estimate
this contamination)
Æ this leads to 20% contamination
of the „golden sample” with non-QE
events
•
from beam simulation – expected flux
2.37·10-7 νµCC/cm2/p.o.t.
• convoluting with σν
and scaling to fiducial mass gives
2.05·10-15νµCC/p.o.t.
• for total exposure with 75% lifetime
expected – 18450 events
• with muon in acceptance (p>8GeV and
θ>300mrad) expected:
- 400 - QE events
- 11 700 - DIS+RES (65%effic)
total – 12 100
to be compared with 10 000 observed
in visual scanning
finally 16% of QE is in „golden channel”
giving 64 events
and 0.14% from 11 700 = 16 events backgr.
Total 80 „golden” events
Rates for golden QE events
• from 400 QE – golden fraction 16%
• background – additional 20%
finally expected
80±9(stat.)±13(syst.)
Æ mainly QE fraction
and beam simul.
to be compared with 86 events observed
Very good consistency with expectations
MC describes rates and resolutions
Æ compare kinematic distributions
• Test of production mechanism simulations
• Test of description of nuclear effects which
have strong influence on some distributions
Proton kinematic variables –
measured only in TPC (T, Pt)
For this variable also reconstruction resolution
plays crucial role
smearing of muon
modifies the distribution,
proton reconstruction has
no effect (is very precise)
Variable
most sensitive
to nuclear
effects:
missing pT
on free nucleon
expectation =0.
inside nucleus
<fermi momentum>
tail at higher
momenta
due to nuclear
cascade …
250 MeV
Strong influence of nuclear
effects was also expected in
Acollinearity
(tail at high values)
but it appeared that
here the most important
are reconstruction effects
summary
•
The first exposure of a LAr TPC to the neutrino beam provides
important information about effectiveness of the technique and
strength of the 3-D reconstruction
•
Very good description of the data is obtained within MC model
including initial and final state nuclear effects (Fluka) reproducing:
- event rates
- experimental resolution
- shapes of several kinematic distributions
•
The results illustrate importance of nuclear effects even at such high
energies
•
The technique provides possibility to study details of the interactions
giving a chance for better understanding underlying physics and
separate models for nuclear effects
•
This step is necessary for future precision studies