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