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The upgrade of the KOTO CsI calorimeter for the separation of 𝒏 and 𝜸
Nobuhiro Shimizu for the KOTO collaboration1
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2
Introduction
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⚫ 𝐾𝐿 → 𝜋0𝜈 ҧ𝜈⚫
⚫Detector
Signal: 2𝛾 + nothingTwo electromagnetic showers in CsI and no signal in the hermetic veto counters
KOTO experiment
3
JHEP 11 033 (2015)
𝜸
𝜸
ҧ𝜈𝜈
+NP
𝐾𝐿 𝜋0
Very rare decay:ℬSM 𝐾𝐿 → 𝜋0𝜈 ҧ𝜈 = (3.0 ± 0.3) × 10−11
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CsI calorimeter of the KOTO detector
CsI crystal⚫undoped CsI (𝜆~300 nm)
#crystal = 27162240 small (25×25 mm2)
476 large (50×50 mm2)
Excellent energy resolution⚫ 𝜎𝐸/𝐸 = 0.99%⨁1.74%/ 𝐸[GeV]
PMT signals are digitized by ADC⚫14 bit 125 MHz sampling with
Gaussian filter
⚫512 ns timing window (64 samples)
⚫ Timing resolution 𝜎𝑡 ∼ 1ns
4
1m
JPS Conf. Proc. 8, 024007 (2015)
27𝑿𝟎
ADC
0 63
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Neutron background 5
To achieve SM sensitivity, we need to suppress neutrons by a factor of ten
shallow distribution uniform distribution
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Idea of the CsI calorimeter upgrade 6
PMT
PMT
𝜸
𝐧𝐞𝐮𝐭𝐫𝐨𝐧
𝑿𝟎 ∼ 𝟐 cm
interaction length 〜 40 cm
Attach MPPCs
S13360-6050CS (HPK)
CsI crystal
Previous
MPPCs
upstream
6×6 𝐦𝐦𝟐
PMT
Measure the depth with the time difference 𝚫𝑻 ≡ 𝑻𝑴𝑷𝑷𝑪 − 𝑻𝑷𝑴𝑻
→ Small 𝚫𝑻 implies 𝜸
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7
Readout
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New front-end readout
MPPC readout# of MPPCs: 4080 (>#PMT=#CsI)
Bias connection
8
To reduce # of channels..4 MPPCs are connected
“Hybrid” bias connection• adopted by MEG II upgrade• AC line: series, to read out signals• DC line: parallel, to apply bias voltagereadout
2r
r r 2r
r r
“Hybrid”
...
.
. ..
.
.
.
.
..
Crystals
5cm
5cm
Large
2.5
cm
10cm
Small
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Development of front-end: amplifier 9
4 MPPCs are connected ×1/4
4 readout is summed × 1/4
→Manageable number of channels
4080 MPPCs
1020
256 channels
𝑽𝒐𝒖𝒕−
𝑽𝒐𝒖𝒕+
mixer (sum amp)+HVHybrid
H
H
H
32 hybridsare connected to one board
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Segmentation 10
Read 10 cm x 10 cm region as a single channel
~typical size of EM shower
Small crystalsLarge crystals
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11
Installation of MPPCs
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MPPC on the CsI surface
Two problems to be addressed① Concave shape of MPPC
② Bubbles appear at low temperature
12
𝑻 ∼ 𝟓℃
Quartz
silicone
MPPC
UV-transparentGlueCsI
Quartz plate to make sure the flatness and transparency in advance
weight
Keep positive pressure inside the glue
Wait for curing strong glue
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MPPC installation 13
Frames to support gluing jigs
Quartzplate
Board
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MPPC instrumentation 14
Glue MPPCs on two rows in a day Start from 1st Oct. and 45 days to
finish all Installation finished as scheduled
1st Oct. 15th Nov.1st Nov.
4080
#MP
PC Progress
Time lapse movie
days20 30 40100
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New outer wear of the calorimeter 15
-2018 2019-
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2019 run
16
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Data correction in 2019 runs 17
All physics runs
Good physics runs after selection
Neutron control sample
Collected good quality data with∼ 𝟐𝟎 × 𝟏𝟎𝟏𝟖 POT
(almost same amount as 2015 data)
Neutron control sample∼ 𝟑 × 𝟏𝟎𝟏𝟖 POT
In spite of the limited time schedule, we could collect physics data with new detectors!
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MPPC irradiation 18D
ark
curr
en
to
f M
PP
C (
uA
)
1st2nd3rd
1st layer
2nd layer
Increase of dark current of MPPC due to irradiation→ 20 times larger than what we had expected
Dark current after 2019 runs
Separately evaluated by another beam test at Kobe Tandem facility
0
75userbeamstart
Run81end
Run82start
Run82end
Layer of MPPCs
Date
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MPPC irradiation 19D
ark
curr
en
to
f M
PP
C (
uA
)
1st2nd3rd
Increase of dark current of MPPC due to irradiation→ 20 times larger than what we had expected
Dark current after 2019 runs
Separately evaluated by another beam test at Kobe Tandem facility
0
75userbeamstart
Run81end
Run82start
Run82end
Even with >50 times the irradiation in 2019, the timing resolution will be within our specification
Layer of MPPCs
1st layer
2nd layer
Date
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Analysis
20
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Control sample 21
𝒎𝟑𝝅𝟎 distribution
Control of sample of 𝜸
𝐾𝐿 → 3𝜋0 decay• Clean and abundant control
sample of 𝛾
Control sample of neutron
Insert Al plate in the upstream of detector.Scattered neutrons well emulate the BG of physics data
Gamma energy (MeV)
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𝚫𝑻 distribution of the control samples 22
max𝚫𝑻 ≡ 𝐦𝐚𝐱 𝚫𝑻𝟏, 𝚫𝑻𝟐
✓Use the larger 𝚫𝑻 out of two clusters (max𝚫𝑻)✓𝐾𝐿 → 3𝜋0 MC well reproduces the distribution of data
𝜸
neutron
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Emulated distribution of 𝑲𝑳 → 𝝅𝟎𝝂ത𝝂 23
✓ The spectra of 𝐾𝐿 → 𝜋0𝜈 ҧ𝜈 is harder than 𝐾𝐿 → 3𝜋0 decay
✓ Weight based on 𝑤 𝐸 = 𝑃𝐷𝐹 𝜋0𝜈ഥ𝜈 (𝐸)/𝑃𝐷𝐹 3𝜋0 (𝐸)
Emulated 𝚫𝑻 distribution weighted spectrum of 𝐾𝐿 → 3𝜋0 with 𝑤 𝐸
Retaining 90% of 𝜸 from 𝑲𝑳 → 𝝅𝟎𝝂ഥ𝝂 decay, neutron contribution can be suppressed down to 1/45 !Achieve much better performance than the goal of design 1/10
𝑬𝜸 spectrum
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Summary
KOTO collaboration aims to search for New Physics via very rare decay 𝐾𝐿 → 𝜋0𝜈 ҧ𝜈, ℬSM = (3.0 ± 0.3) × 10−11.
In the last autumn, we attached >4000 MPPCs on the front surface of CsI crystal to improve 𝜸/n separation power:
In 2019 run, we successfully collected physics data and confirmed⚫ irradiation of MPPCs was acceptable for future data collection⚫performance of the neutron rejection was to be 1/45
(for 90% 𝜀 of signal), which was much better than our goal of design
Detector paper is now under preparation
24
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25
Thank you!
That’s all
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Degradation of neutron rejection 26
Effi
cien
cy o
f n
eutr
on
s w
ith
ret
ain
ing
90
% e
ffic
ien
cy o
f𝛾
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Correlation b.t.w. CSDDL vs 𝚫𝑻
high E clus
low E clusLet us separate regions into “front” and “rear” region and evaluate the reduction by applying CSDDL value>0.9.
low E clusfront
high E clusfront
high E clusrear
low E clusrear287/15304
=1.9%
599/35176=1.7%
358/39080=0.92%
85/11400=0.75%
We cannot see strong degradation of the performance of neutron rejection by CSDDL.
(Rather, E dependence can be observed.)
Q. Does the CSD see the depth of clusters?
Correlation does not look large.
27
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Correlation b.t.w. PSLH vs 𝚫𝑻
Let us separate regions into “front” and “rear” region and evaluate the reduction by applying PSLH value>0.1. (pulse shape template is run74,75)
low E clus front
high E clus front high E clus rear
low E clus rear
For high energy cluster, we can observe small correlation between PSLH and 𝚫𝑻.
Nevertheless, the degradation of performance is by a factor of 1.5.
Q. Does the PSLH see the depth of clusters?
Correlation does not look serious.
6873/15304 =44.9%±0.4%
16926/35716 =48.1%±0.2%
5839/39080 =14.9%±0.2%
2683/11400=23.4%±0.4%
high E clus
low E clus
28
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Effect of the irradiation
Dark current⚫ increases ×100 for
⚫Prepare irradiated sample of MPPCs
Instability of bias voltage
29
∼ 1 × 109 1MeV- 𝑛/cm2 (3-years operation)
readout
Series
.
𝑹 (cm)
Position dependence of the neutron fluence
Position dependence of different 𝐼 𝑉causes instability of bias for the series connection.
→ Solved by adopting the Hybrid connection
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Beam test at RCNP-Osaka cyclotron⚫𝛾/𝑛 beam from Li target
Performance tests (𝛾/𝑛 separation) 30
Distribution𝚫𝒕 ≡ 𝑻𝑴𝑷𝑷𝑪 − 𝑻𝑷𝑴𝑻
PMT𝜸/nMPPC
p392MeV
Li target
collimatorCsI
𝜸: continuous beamup to 392 MeV
𝒏: 392 MeV
upstream downstream
Retain 90% of 𝛾 whilesuppressing 𝒏 to 34%
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Performance tests (𝜎Δ𝑡) Beam test at the ELPH (Tohoku, Japan)
electron synchrotron
⚫evaluate 𝜎Δ𝑡 (as a func. of E)• Monochromatic 200, 400, 600, 800
MeV 𝑒+ beams
⚫Used setup as realistic as possible
⚫Confirmed MPPC functionality after dose→Irradiated MPPCs were used
31
◉Beam
summed100×100 𝐦𝐦𝟐
region
✓ Irradiated MPPCs worked enough✓ Readout worked well
𝝈𝜟𝒕
(ns)
𝑬𝒃𝒆𝒂𝒎
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𝐾 → 𝜋𝜈 ҧ𝜈 decay 32
Suppressed by FCNC in the SM
Small QCD uncertainty⚫useful prove to the New Physics
Two compatible processes⚫𝐾+ → 𝜋+𝜈 ҧ𝜈: 𝒜 ∝ 𝑉𝑡𝑑⚫𝐾𝐿 → 𝜋0𝜈 ҧ𝜈 : 𝒜 ∝ Im𝑉𝑡𝑑
ℬ(𝐾+ → 𝜋+𝜈 ҧ𝜈) = 17.3−10.5+11.5 × 10−9 E949
ℬ(𝐾𝐿 → 𝜋0𝜈 ҧ𝜈) < 2.6 × 10−8 (90% C.L.) E391a
𝓑(𝑲+ → 𝝅+𝝂ഥ𝝂)
𝓑(𝑲
𝑳→𝝅𝟎𝝂ഥ 𝝂)
JHEP11 033 (2015).
EXP
SM prediction
ℬ(𝐾+ → 𝜋+𝜈 ҧ𝜈) = (9.11 ± 0.72) × 10−11ℬ(𝐾𝐿 → 𝜋0𝜈 ҧ𝜈) = (3.0 ± 0.3) × 10−11
JHEP11 033 (2015).
+NP
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Rejection of neutron BG
Halo-neutron BGResult of 4 days run: ℬ 𝐾𝐿 → 𝜋0𝜈 ҧ𝜈 < 5.1 × 10−8 (90% C.L.)
⚫We need 3 more magnitudes of suppression two-dimensional shower envelope → 1/10 ✓done
Pulse shape likelihood → 1/10 ✓done
33
* Prog. Theor. Exp. Phys. (2017) 021C01
*
The largest contribution from BG
measure shower development (in z)
in the calorimeter →O(1/10)
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Performance tests (𝛾/𝑛 separation) 34
→ suppresses halo neutron BG to 10%while retaining 90% efficiency of two 𝜸signal events!
Taking into account…
MC evaluation of performance for halo neutron events,based on the result of beam test
① Correlation of two cluster position:• the second cluster is deeper
② Other neutron cuts
𝚫𝒕𝒎𝒊𝒏
𝚫𝒕𝒎𝒂𝒙
𝒏 neutron case
𝜸 gamma case
𝚫𝒕𝒎𝒊𝒏
𝚫𝒕𝒎𝒂𝒙
The larger one → 𝚫𝒕𝒎𝒂𝒙
The smaller one → 𝚫𝒕𝒎𝒊𝒏
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Quality assurance of MPPCs 35
Quartz gluingSolderingtemperature test
MPPCs
I/V inspectionLED test
Process 80 MPPCs/day
I/V curves#MPPC~500
Summed MPPCs
Individual test
Inspect all of MPPCs (#~4000) before installation → Start gluing on CsI in this summer
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Development of front-end: monitor 36
0.1uF
51Ω0.1uF
510Ω
510Ω130kΩ
10kΩ
+
-
to AMP
Signal readout
to ADC
Current monitor+HV
𝑰 𝑽 =𝜶 𝑽 − 𝑽𝟎
𝟐
𝟏 − 𝜷 𝑽 − 𝑽𝟎𝟐 + 𝜸
DC dark current is continuously monitored to confirm the functionality and level of radiation damage.
Operation current increases by a factor of 100 in three snowmass year:𝑰𝒐𝒑 = 0.5𝜇A→ 50𝜇A
AC DC
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Development of front-end
MPPC readout
Bias connection
37
readout
Series
.
readout
2r
r r 2r
r r
“Hybrid”
...
.
. ..
.
.
.
.
..
Cross section of CsI
1mTwo types of crystals
4 MPPCs are simultaneously read out
S13360-6050CS (HPK)
6×6 mm2 sensitive regionSi-window
☺ small time constant (~200ns) high bias voltage (220V) unstable individual operation voltage toward irradiation
large time constant (~0.5us)☺ low bias voltage (55V)☺ stable breakdown voltage
toward irradiation
“Hybrid”-connection• adopted by Meg2 upgrade• AC line: series• DC line: parallel• have both pros
readout
Parallel
adopted!
2716 crystals
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Fabrication of MPPCs 38
Drop glue
1 1
Insert MPPC on jig
2
3
4
Drop glue onquartz
2
3
4 wait for cure keeping the quartz floated
5
dispense epoxy glue (araldite 2011)
6
apply weight
6
7
7
Put MPPCs into oven andwait 24 h (keeping 45 deg)
8
wait 24 h for cure
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39
Summed MPPCs
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PIC microcontroller
I/V inspection of MPPCs 40
16ch MUX
16ch OPAMPs (8×2)
20 cm
10
cm
+HV
to MUX
⋯
ADC
OPAMP→FET input(high impedance)Gain 100
16×3
I/V inspection front end
I/V conversion
Basic design• 16ch are chosen by MUX • DC voltage is buffered by voltage follower
after the MUX
MUX
register
4bit
Bottom view
Buffer
AMP