Event reconstruction and energy calibration using
cosmic ray muons for the T2K pizero detector
Trung Le
Stony Brook University
Thesis defense, Dec. 10, 2009
Introduction
• From atmospheric (Super-Kamiokande) and accelerator (K2K, MINOS)
neutrino oscillation experiments
� ∆m232 = 2.5 x 10-3 eV2 and θ23 ~ 450
• From solar (Super-Kamiokande, SNO) and reactor (KamLAND) neutrino
oscillation experiments
� ∆m2 = 7.6 x 10-5 eV2 and θ ~ 370� ∆m221 = 7.6 x 10-5 eV2 and θ12 ~ 370
� Neutrinos are massive and there are at least 3 distinct masses
� Large mixing angles
• CHOOZ reactor experiment (with short baseline ~ 1 km)
� sin22θ13 < 0.1
2
3-flavor neutrino oscillations
• Neutrino mixing νf = U νm, with νf = (νe, νµ, ντ), νm = (ν1 ,ν2, ν3)
• Mixing matrix: three angles and one phase
• Oscillation probability
• νµ � νe oscillation channel (α ≡ ∆m221/ ∆m2
32 << 1)
3
Oscillation amplitude is proportional to sin22θ13
Oscillation phase
Tokai-to-Kamioka (T2K) experiment
TokaiTokai
KamiokaKamioka
JJ--PARCPARC
4
TokaiTokai
T2K physics program
• Precision measurement of atmospheric oscillation parameters using
νµ disappearance
� θ23 � ≈ 1%, , ∆m232 � ≈ 2%
• νe appearance search to measure θ13 or improve the sensitivity on
sin22θ13 by an order of magnitude.
From T2K letter of intent (OA = 20, 5x1021 POT):From T2K letter of intent (OA = 2 , 5x10 POT):
5
Reconstructed neutrino energy of expected
signal + BG. Assuming 5 yr exposure (5×1021
POT), ∆m232 = 0.003 eV2, sin22θ13 = 0.1
Off-axis angle 20 ννννµµµµ CC ννννµµµµ NC 1ππππ0 Beam ννννe Signal ννννe
Generated in F.V. 10713.6 4080.3 292.1 301.6
1R e-like 14.3 247.1 68.4 203.7
e/π0 3.5 23.0 21.9 152.2
0.4 < Erec < 1.2 GeV 1.8 9.3 11.1 123.2
θ13 sensitivity (T2K official plots)
5×1021 POT at 50 GeV, 22.5kt, δCP=0, normal hierarchy
T2K neutrino beamline
11
0 m
• Off-axis neutrino beam, off-axis
angle 2-2.5 deg., Eν ~ 0.6 GeV at
2.5 deg.
• The peak energy corresponds to
the first oscillation maximum given
the 295 km baseline and ∆m232.
• About 0.5% of νe at peak energy
Neutrino beam monitor
• Muon monitor
� Detect µ+ from π+� µ+ νµ decay
� Real time status monitor:
� Proton beam position on target
� Horn performance 10 m
• On-axis neutrino detector
� Monitor ν beam direction and
profile on a day-by-day basis.
� Measure ν beam direction
within 1 mrad. accuracy.
8
(INGRID)
10 m
Near detector at 280 m
• Measure the properties of the neutrino beam before oscillation
• P0D (π0 detector):� Scintillator sampling detector
� Measure NC1π0 cross section
• Tracker:� Two Fine-Grained Detectors + 3 Time
Projection Chambers (TPC)
� One FGD filled with water
B = 0.18~ 0.2 T
� One FGD filled with water
� High-resolution tracking chambers with Micromegas readout
� Measure νµ, νe spectra for ν oscillations.
• ECAL (Electromagnetic calorimeter):� Around the tracker + downstream ECAL
� Lead + rectangular scintillator with double-end readout.
• SMRD (Side Muon Range Detector):� Scintillator detectors inserted in iron yokes
of the magnet.
� Muon range detector
� Cosmic muon trigger + veto
π0 detector (P0D)
WATER TARGET
Scintillator sampling detector: tracking + calorimetry
Dimensions: 2200 (X) x 2340 (Y) x 2404 (Z) mm
Masses: 17,585 kg (full), 14,668 kg (empty)
ECAL: Scintillator (34 mm) + Lead (5 mm)
TARGET: Scintillator (34 mm) + water ( 30 mm)
Total channels: 10400
ν2.4-2.8 mm
17.0mm
±0.5
10
8.50mm
±0.25
±0.5
16.50mm
±0.25
33.0mm ±0.5
� Since SK is a water Cherenkov detector
� π0 production cross section on water.
� statistical subtraction of events with
water out from water in to determine cross
section on water
Photo-sensor: Multi-Pixel Photon Counter• Array of avalanche photodiodes
• Developed by Hamamatsu & customized for T2K
• About 11000 MPPCs installed in the P0D
• MPPC characteristics:� Active area : 1.3 x 1.3 mm
� Num. of pixels : 667 (50 x 50 µm2 each)
� Operation voltage : 70 V
� PDE at 515 nm : > 25 %
� Dark count : < 1.35 MHz @ 25C (Gain = 7.5 x 105)� Dark count : < 1.35 MHz @ 25C (Gain = 7.5 x 105)
� Operational in magnetic field (0.2 T) 1.3 mm
68.5 69.0 69.5 70.0 70.5 71.0 71.50.0
2.0
4.0
6.0
8.0
10.0
12.0
Bias Voltage / V
Gain
/10^5
15℃
20℃
25℃
LED amplitude spectrum
Event reconstruction
Signal and background events
signal eventThe ππππ0 detector is designed for:
� Measure neutral-current single π0
� Detect and reject charged-current ν interactions
Reconstruction steps:
� Separate neutrino interactions using time
ν
νµ + N � νµ + N + π0
600 MeV/c π0
13
background event
170 MeV/c muon
P0D
P0D
450 MeV/c π0
� Separate neutrino interactions using time
� Detect ν interactions (µ-)
� Line detection in two dimensions
� Track matching and fitting
� Particle id (p, µ-)
� Reconstruct neutral-current single π0
� π0 vertex finder
� π0 energy momentum reconstruction
ν
600 MeV/c π
νµ + n � µ- + p + π0
Beam spill structure and electronics readout.
4.2µs 4.2µs 4.2µs2-3.3s 2-3.3sSpill structure
in spill after spill inter spill
neutrino interactions delayed signals switch to cosmic/
TIME
14
neutrino interactions delayed signals switch to cosmic/calibration mode
Bunches
58 ns 58 ns 58 ns
241 ns 241 ns
Trip-t timing structure
Integration Reset Readout
2.3µs
Charged particle track finding:
line detection
µ- track is relatively straight in the P0D � Line detection algorithm
Hough transform: r = x cosθ + y sinθ, detected lines given by peak in (r,θ)
475 MeV π0
1500 MeV µ-
νµ + n � µ- + p + π0
15Muon track detected by Hough transform
θ
r
Detected 2D tracks are matched to form 3D tracks.
Track fitting
�Track fitting is implemented using Kalman filter.
�Trajectory model: straight line with random, small perturbation to particle direction (multiple scattering).
� Extrapolation using the prediction step during the Kalman filtering.
� Use forward-backward smoothing to obtain track parameters
1 GeV µµµµ- MC
16
Deviation of reconstructed trajectory from true trajectory.
x residual (mm)
σσσσ ~ 2.5 mm σσσσ ~ 2.5 mm
Gaussian fit
1 GeV µµµµ- MC
y residual (mm)
Particle identification (p, µ-)
� Proton/muon separation
� For low-energy heavy particle (≥ mµ), mean energy loss ~ 1/β2
�Make a likelihood classifier using energy deposits in the 10 scintillator planes [from the stopping point]
muon-likeproton-like
286 MeV/c muon
800 MeV/c protonLikelihood generated using
17True µµµµµµµµ-- momentum (MeV/c) True p momentum (MeV/c)
mis-identified
Classifier test Likelihood
Muon decay tagging
Muon lifetime: 2.1µs � make delayed hits
TIME
CC ν interaction, µ-
not reconstructed later bunches after-spill
18
Decay hits found, connected to the cluster
ν
�The event is tagged as charged-current interaction if ≥ 3 delayed hits are found
�The tagging efficiency is ∼ 50%, including muon capture, for single µ- (using 15-
bunch spill)
Not decay hits since not connected to the cluster
Extrapolation of tracks from the TPC04cm
gate
µ- tracks obscured by showers or too short to be tracked by the P0D itself.
Extrapolate using the Kalman filter:µµµµ- track reconstructed in the TPC
P0D TPC
TPC track
projected track
19
4cm
gate
• Reverse the TPC track momentum to go
backward, use this to project the track point to
the P0D most downstream plane.
• Hits within the gate are used as new position
measurement. Measurement update the filter.
• If the gate is empty, stop extrapolating.
• Note: 3D extrapolation, alternate x,y
scintillator planes.
P0D last scintillator plane
A TPC track successfully
extrapolated into the P0D
TPC track
π0 reconstruction
475 MeV π0
1500 MeV µ-
π0 hits
20
Reconstructed vertex
fitted track
π0 hits
Hits that are not associated with
charged track are considered π0 hits.
π0 vertex finder
),,(log iii zyxl
Do a grid search to maximize the log likelihood:
i runs over the whole P0D
Likelihood variables:
For each (x,y,z), do a density-based clustering to find showers
21
Likelihood variables:
� Fraction of shower charge/total charge
� Number of showers
� Shower width (~ Moliere radius)
� Distance from vertex to the closest shower (~ γ mean free path)
This work was originally done by K. Kobayashi, later re-written by C. McGrew
σ ~ 3cm σ ~ 4cmσ ~ 0.1
Single π0 reconstruction
Use 110- 516 MeV/c π0, direction along z
Deviation from
true vertex
σ ~ 5 cm σ ~ 5 cm
22
Energy resolution, ∆E/E
∆x (mm) ∆y (mm)
Fully contained π0
σ ~ 0.12
νµ interaction reconstruction
� Test reconstruction software using 100,000 νµ with T2K spectrum
� Generated by NEUT event generator, vertex distribution using weight density, gone through detector, electronics simulations.
� Neutral-current single π0 reconstruction efficiency ~ 18%
� Charged-current νµ rejection efficiency 99.8%.
23
ππππ0 sample after all ππππ0 selection cuts
ππππ0 reconstructed energy (MeV)
47%
ππππ0 selection cuts:
� No muon-like track
� No muon decay
� 3 ≤ showers ≤ 5
� 80 MeV/c2 ≤ mγγ ≤ 190 MeV/c2
21%
32%
Energy calibration using cosmic muons
Introduction
• Hardware and software validation
� Make sure all channels are connected and working properly.
� Test full readout chain, from photosensors to DAQ computer
� Test full software chain, from DAQ to reconstruction software
� Gain experience with the new photosensors.
• Energy calibration goals:
Any two channels should give the same energy given the same amount of � Any two channels should give the same energy given the same amount of
energy deposit.
� The same channel should give the same energy at two different running
conditions given the same amount of energy deposit.
• Outline:
� Correct for gain change at different temperature.
� Measure light attenuation in WLS fibers.
� Correct for channel response.
25
Cabling & readout Upstream ECAL, side view
Readout Module
DAQ PC
50 cm coaxial cable
26
Fibers and MPPC
64 channels
Trip-t Front-end Board
Cosmic trigger
Divide TFBs in 4 groups
Requires each group has one
TFB with at least 4 TDC hits to
trigger
Hit map for 1 p0dule, x side
27
Hit map for 1 p0dule, x side
TFB plates can be seen clearly
Bar position (mm) ECAL
Upstream ECAL photo
A cosmic µ- in the ECAL
TFBs
28
ECAL water target
Data summary• Take cosmic data for each detector module separately while they await to be installed.
• Use cosmic data from the upstream ECAL, ~ 200 cosmic runs.
• Each 1 hr cosmic run is preceded by a 2 min calibration (random trigger) run.
• Cosmic trigger at about 30 Hz
• MPPCs are biased at 70.9 V (corresponds to gain ∼10 ADC, next slide).
• Temperature and humidity was recorded every 30 seconds by a single sensor mounted
by the side of the detector.
29
Maximum ∆T during runs
by the side of the detector.
Jul. 24Jul. 17
One photoelectron calibration
pedestal
1 p.e. by dark noise
ADC spectrum from one channel
30
ONE CHANNEL
TFB response function
Fitted line
Gain ≡ 1 p.e. - pedestal
TFB response
peQ
pedestalQadcQ
1
)()( −Number of p.e. =
� Q(x) converts adc to charge
� Q1pe charge corresponds to 1 p.e.
� Q(x) is measured using the onboard charge
injection circuit for each channel.
Gain vs. run number
31
Use to test
energy
calibration
Use to study light yield
change as a function of
gain
Dark noise and correlated noise
• Dark noise and correlated noise of
each channel can be measured using
the calibration runs.
• Dark noise is random and caused by
thermal electrons in the depletion
region of the MPPCs. Mean dark noise
All channels & runs
region of the MPPCs. Mean dark noise
rate is about 600 KHz.
• Correlated noise (crosstalk, after-
pulsing) is when secondary avalanches
follow an original avalanche.
Characterized by correlated noise
probability: (N1pred – N1expt)/N1pred
32
All channels
Muon track reconstruction
• µ- track is reconstructed using the P0D event reconstruction software:
� Select the timeslice with the most hits (dark noise hits + muon hits)
� Apply the track pattern recognition, matching, and fitting on the selected timeslice
� The output is a 3D muon track.
X bars Y bars
33
33
X bars Y bars
σ ~ 2.8 mm σ ~ 3.2 mm
MC: σ ~ 2.5 mm
x residual (mm) y residual (mm)
MC: σ ~ 2.5 mm
Light yield from all channels
• Calculate the light yield per cm by
using the light yield in each plane,
correct for the angle and divide by
the plane thickness (1.7 cm).
A muon barely passes the second bar,
creating no hit or hit below threshold.
Light yield per cm by MIP from all channels
34
Doublet Singlet
creating no hit or hit below threshold.
Use doublet in the following slides.
Light yield vs. run
Only runs with temp. data
35
Mean light yield from doublets vs. run number
Light yield vs. gain
runs with ∆T < 0.2 0C
runs with ∆T > 0.2 0C
36
� Light yield change due to correlated noise and PDE
� For cosmic runs with large temperature variation, the corresponding
calibration runs become less relevant.
Correction for gain change
Y = k G
Y = k G + b
Assume that the light yield is a linear function of gain, fit the light yield
vs. gain plot to the linear function: Y = k G + b. The parameters (k,b)
are determined from the fit.
37
Parameters from the Y = k G + b fit: k = 2.2, b = 3.9
Empirical correction
Apply the empirical correction on the MEAN
light yield, not individual channel.
Use the correction factor 1/(1 + ∆G/G0(1 – b/(k G
0))) = 1/(1 + 0.85∆G/G
0)
38
Bar light yield
• Use the 1/(1 + ∆G/G0) correction for
this analysis.
• Combine 90 hours of cosmic data,
about 10,000 to 15,000 µ-/bar.
• Calculate muon path length in each
bar using reconstructed 3D track and
detector geometry.
ONE BAR
Light yield per cm by MIP
detector geometry.
39
Mean light yield from channels on a
P0Dule (x bars)
Light attenuation calculation
• Divide each bar into 20-cm segments
• Calculate the mean light yield for
each segment and normalize to the
light yield of the middle segment.
• The plot shows the distributions of the
means over all bars at two segments,
- 60 cm
use both x and y bars
+60 cmmeans over all bars at two segments,
+60 cm and -60 cm.
40
mirror MPPC
20 cm
+-
0
60 cm
Light attenuation curve
ONE BAR
Mean yield along bar after attenuation correction
� Obtain the means from all segments to
� Parameterize the attenuation curve using a
double-exponential function
41
+4 +3 +2 +1 0 -1 -2 -3 -4 (× 20 cm)
r = 0.9
Energy calibration
� Use the second data set to test the
energy calibration.
�Correct for gain change and light
attenuation.
� Calibrated energy: Ei(MIP) = Yi/Y0i, Y0i
is the mean light yield of channel i.
Mean energy, one P0Dule (x bar)
42
all upstream
ECAL channels
Use to test
energy
calibration
Used to derive
calibration constants
Slide 31, reproduced here
SUMMARY
• A complete event reconstruction chain for
the P0D:
� Track pattern recognition, matching, fitting, and
muon decay tagging.
� CC rejection efficiency > 99.8%
� NC π0 efficiency 18%, purity 47%.
• Energy calibration using cosmic muons:
� Measure dark noise, correlated noise
� Correct for gain change, light yields are uniform
within ± 0.4 p.e. using the empirical parameters
� Measure light attenuation curve, a single curve
for all bars.
� Correct for channel response difference.
� Calibrated energy from all bars is uniform, σ ~
0.01
43
A cosmic muon event in ND280
44
Backup slides
45
Track matching
xz
46
� Track ends ±1 layer
� If two tracks in one projection match to the same track in the other projection, select the one with the smaller charge difference.
one layeryz
Photon Detection Efficiency
∆Tcosmic ~ 5C � ∆V ~ .25V
Not corrected for PDE in this analysis
47Plot taken from “Characterization of T2K photo-sensors, nd280.org/photosensors
Corrected light yield (1)
Remove runs with ∆T > 0.2 0C
Since the light yield is approximately linear to the gain: Y = k G, k a
proportional constant, assuming zero intercept for now.
� Correction factor 1/(1 + ∆G/G0), G
0is a reference gain, taken 10 ADC
� Correction factor does not depend on k. Can apply correction per
channel even before muon track reconstruction.
48
Corrected mean light yield
Corrected light yield (2)
+0.04
over-
correcting
Yield deviation vs. gain
49
-0.04
The y range is the light yield
spread before correction.
Efficiency loss by different cuts
• ===========================================
• Particle ππππ0 µµµµ-
• ===========================================
• All 1067 69420
• Muon-like cut 1029 33038
• Unmatched 993 26291
• Muon decay cut 906 17606
• Low hits cut 775 8393
• Shower cut 496 3787
CUTS
• Shower cut 496 3787
• Invariant cut 286 1385
• Fiducial cut 198 135
• ===========================================
• counts % ππππ0 loss by cuts
• ===========================================
• 38 3.56
• 36 3.37
• 87 8.15
• 131 12.28
• 279 26.15
• 210 19.68
• 88 8.25
• ===========================================
• 198 18.56
50
15%
pizero-like cut
e- selection
• Apply cuts similar to the π0 selection cuts.
• Cuts before the invariant mass cut:1. All 1636
2. Muonlike cut 1580
3. Unmatched 1457
4. Muon decay cut(≥3) 1345
5. Shower cut (2-5) 12375. Shower cut (2-5) 1237
• π0 mγγ cut 611
• CCQE νe efficiency: 36%
51
e-like π0-like
Invariant mass after all other cuts
Same cut as π0 selection
ννννe reconstruction
• Use ~ 1500 files of νe from MCD0, full-
power and P0D with water, about .75x1021
POT
• νe with vertex inside the P0D: ~20,300
• ν with vertex inside the P0D fiducial (xy +/- CC
Attempt to reconstruct beam νe (~ .5% at peak energy)
• νe with vertex inside the P0D fiducial (xy +/-
25cm, z +/-6cm): 7,250
• CCQE νe with vertex inside the P0D fiducial:
1,644
• Energy spectrum of e- from CC νe and CCQE
νe
• Run the detector simulation (1νe/spill),
electronics simulation and reconstruction
52
CC
CCQE
π0 momentum
i
hits
iQrkp ∑=γ
γ
assume pT small
Understand the approximationApproximate γγγγ(e-) momentum:
When ri = 1 , becomes exact
53
hitsππππ0 momentum: vertex
Notes:
� Don’t have to separate hits into two gamma’s
210 γγπppp +=
e- energy and energy leak
Energy resolution of selected e-Energy resolution after removing e-
with ‘true’ energy leak
54
Substantial energy leaking
µµµµ ~ -2%
σσσσ ~ 10%
The efficiency reduces from 36% to 25%
‘true’ energy leak: daughter trajectories stop
outside the P0D.
Fitting results
55
Angle (degrees) Delta z(mm)
56
π0 reconstruction challenge