[PPT] - Status of the ! " $ % & & search at J-PARC KOTO PowerPoint Presentation

SLIDE 1

Status of the !" → $%& ̅ & search at J-PARC KOTO experiment

Satoshi Shinohara 2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

2019/12/23

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Kuno and Yamanaka Groups End-of-the Year Presentation

KAON2019 @ Perugia Univ. Italy

SLIDE 2

KOTO

!" → $%& ̅

&

highly suppressed :

BR(SM) : 3.0×10-11

small theoretical uncertainty（~2%）

→ good probe for new physics search

signal : (π0→２γ)+ Nothing
2γ→CsI calorimeter
Nothing → Hermitic veto detectors

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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J-PARC signal region on the Pt vs. Z plane (blinded analysis)

SLIDE 3

Dec 2012 Dec 2013 Dec 2014 Jan 2016 Dec 2016 Dec 2017 Dec 2018 Accumulated P.O.T. 10 20 30 40 50 60 70 80 90 100

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10 × Beam Power (kW) 20 40 60 80 100

History of data taking

2019/12/23

2013 data

(PTEP 2017 021C01)

Operation

f Hadron

facility was stopped.

2015 data

(Phys. Rev.

Lett. 122,

021802)

2016, 2017, 2018 data

Kuno and Yamanaka Groups End-of-the Year Presentation

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2019 data

Barrel detector upgrade (2016 Mar.)

SLIDE 4

Single event sensitivity (S.E.S)

S.E.S = 1/((KL yield)×(signal acceptance))

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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reconstructed KL mass distribution from !" → 2%& analysis

KL yield (@ beam exit) : 7.1×1012 (×~1.5 from 2015)

Preliminary

signal acceptance : 2.0×10-4 (×~1.3 improved from 2015)

!" → %&' ̅ ' MC

Signal box : optimized better for S/N S.E.S : 6.9×10-10

Preliminary

(↔ S.E.S2015 : 1.3×10-9)

signal region blind region

SLIDE 5

Hadron cluster BG

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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n n n Al target

(10 mm thickness)

Physics run Special run Physics data Control sample n n preliminary preliminary

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Hadron cluster BG in 2015 analysis

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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BG source

No. events

!" → $%$&$' 0.05 ± 0.02 !" → 2$' 0.02 ± 0.02 Other !" decays 0.03 ± 0.01 Hadron cluster 0.24 ± 0.17 Upstream $' 0.04 ± 0.03 CV ) 0.04 ± 0.02 Total 0.42 ± 0.18

Neutron related BG KL BG

2015 run background summary table

neutron

Hadron cluster BG

CSI

Phys. Rev. Lett. 122, 021802

SLIDE 7

Hadron cluster BG update from 2015 (cut)

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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1. Cluster shape cut with

deep learning

S/N :×~2

from 2015

2. Pulse shape

discrimination with Fourier transformation

S/N :×~1.8

from 2015

cluster

SLIDE 8

Hadron cluster BG estimation

Found contamination in the control sample
Developed more reliable estimation method
based on single cluster reduction
reduction : ×27(±19) from w/o concerning scatter

KL→2γ

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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(10 mm thickness) (10 mm thickness)

SLIDE 9

Hadron cluster BG

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

9 0.02 ±0.00 0.00 ±0.00 0.00 ±0.00 0.01 ±0.00 0.00 ±0.00 0.01 ±0.00

Preliminary

SLIDE 10

Overlapped pulse BG

Overlapped pulse shifts measured time for veto detectors
Narrower veto window
recover signal acceptance
increase BG from overlapped pulse

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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clock [8ns] example of overlapped waveform (MC) pulse height [a.u.] veto window (125MHz waveform digitizer)

SLIDE 11

Overlapped pulse BG

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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Overlapped pulse discriminator
Pulse shape information : Fourier transformation
χ2 : compared with single hit template

Preliminary

w/o new method w/ new method χ2 distribution of upstream detector

SLIDE 12

Overlapped pulse BG

Narrower veto window + overlapped pulse discriminator

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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Overlapped pulse BG from !" → 3%& MC analysis Overlapped pulse BG from !" → %'( MC analysis

Narrower veto window : Recovered 10% signal acceptance No remaining event due to overlapped pulse

Preliminary Preliminary

<0.04 <0.09

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BG summary

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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1 2 3 4 5 6 7

Pi0RecZ 2000 3000 4000 5000 6000 Pi0Pt 50 100 150 200 250 300 350 400 450 500

462 473.1 ± 4.2 0.08 ± 0.06 0.00 ± 0.00 0.18 ± 0.09 0.00 ± 0.00 0.07 ± 0.05 0.05 ± 0.02

#BG !" → 2%& <0.18 !" → %'%(%& <0.02 !" → 3%& (overlapped pulse) <0.04 Ke3 (overlapped pulse) <0.09 !" → 2* 0.001±0.001 Upstream π0 0.001±0.001 Hadron cluster 0.02 ±0.00 CV-pi0 <0.10 CV-eta 0.03±0.01 Total 0.05±0.02

S.E.S : 6.9×10-10

bserved

expectation

Preliminary Preliminary

0.33 ± 0.11 (including signal region)

SLIDE 14

Unblinded

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1 2 3 4 5 6 7

Pi0RecZ 2000 3000 4000 5000 6000 Pi0Pt 50 100 150 200 250 300 350 400 450 500

Unblinded

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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Preliminary

unblinded in the end of Aug. 2019

SLIDE 16

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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1 2 3 4 5 6 7

Pi0RecZ 2000 3000 4000 5000 6000 Pi0Pt 50 100 150 200 250 300 350 400 450 500 upstream detector On-timing peak is shifted by large pulse

n-time

waveform of the event measured time 0.08 ± 0.06 0.18 ± 0.09 473.1 ± 4.2 0.00 ± 0.00 0.07 ± 0.05 0.05±0.02 expectation

Preliminary

P r e l i m i n a r y

clock [8ns] ADC [counts] 0.33±0.11 (125MHz waveform digitizer)

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2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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1 2 3 4 5 6 7

Pi0RecZ 2000 3000 4000 5000 6000 Pi0Pt 50 100 150 200 250 300 350 400 450 500

#BG KL3pi0 (overlapped pulse) <0.04 Ke3 (overlapped pulse) <0.09

BG estimation related

verlapped pulse
Underestimated the BG

from overlapped pulse?

Checking the properties
f the other candidates
Did we miss other

background sources?

planning to reevaluate
ther BG sources

0.08 ± 0.06 0.18 ± 0.09 0.00 ± 0.00 0.07 ± 0.05 expectation

P r e l i m i n a r y Preliminary

0.05±0.02 0.33±0.11 473.1 ± 4.2

SLIDE 18

Summary

!" → $%& ̅

& at J-PARC KOTO experiment

S.E.S : 6.9×10-10
BG estimation : 0.05 ± 0.02
Opened the box

→ 4 candidate events in the signal region Event properties: One event : overlapped waveform Checking other candidates carefully

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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END

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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Detector and DAQ upgrade in 2016-2018

Barrel detector was upgraded in 2016.4
13.5X0 → 18.5X0
!" → 2%& BG : ×1/3
DAQ upgrade (covered in poster session)
Online cluster counting (2017~)
Higher DAQ live time ratio
~80% (2015 @ 42kW beam)

→ ~99% (2018 @ 51kW beam)

2019/12/23

!" → 2%&

Inner barrel (IB)

nline

clustering map Energy deposit map of CsI

Kuno and Yamanaka Groups End-of-the Year Presentation

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cluster

SLIDE 21

Result of 2015 physics run

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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bserved

expectation contour : !" → $%& ̅ & (MC)

Phys. Rev. Lett. 122, 021802
Single event sensitivity :

1.30 ± 0.01-./. ± 0.14-1-. ×1034

No event in the signal region
Upper limit (90% C.L.) :

Br(56 → 789: 9) < 3.0×10-9 ×10 improvement from previous limit (KEK E391a)

SLIDE 22

Toward 2016-2018 data analysis

Higher beam power

→ larger signal loss due to the accidental hit (76% loss in 2015 analysis)

To achieve better sensitivity

→ Narrower veto window

Need BG study due to the narrower veto window

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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study from 2015 data 2018 June, 51kW

SLIDE 23

Single event sensitivity (S.E.S)

S.E.S = 1/((KL yield)×(signal acceptance))
Signal box : determined by S/N
S.E.S : 6.9×10-10

(S.E.S2015 : 1.3×10-9)

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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signal acceptance : 2.0×10-4

!" → $%& ̅ & MC blind region signal region 2015 signal box 2016 ~ 2018 signal box signal acceptance : + 6% (from 2015 signal box)

Preliminary

SLIDE 24

Upstream-π0 BG

Halo-neutron hits upstream

detector

γ + γ
small visible energy shifted

reconstructed Z position

n + γ
various reconstructed Z position
Shrink signal box of upstream Z
#BGupstream-π0 : 0.001 ± 0.001

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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n π0->2γ

CsI

Preliminary

SLIDE 25

CV-η BG

Neutron hits Charged Veto (CV)

and generates η

reconstruction :

assumed π0 mass

mη ~ 4mπ0

→ reconstructed Z position : shifted in the signal region

#BGη = 0.03 ± 0.01

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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η->2γ n CV (front) Z = 5862 [mm] CSI Z = 6168 [mm]

CsI

Preliminary

rec. Z [mm]
rec. Pt [MeV/c]

SLIDE 26

!" → $%$&$' BG

Neural net with deep leaning for !" → $%$&$' BG
Similar behavior as $' Pt and Z cut
S/N : improved even after applying Pt and Z cut which is
ptimized !" → $%$&$' BG

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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!" → $'( ̅ ( efficiency !" → $%$&$' reduction

Preliminary Preliminary

Rec. π0 Z [mm]
Rec. π0 Z [mm]
Rec. π0 Pt [MeV]
Rec. π0 Pt [MeV]

SLIDE 27

!" → $%$&$' BG

Applying neural net with deep leaning cut
#BG!

" → $%$&$ ' : 0.02 → <0.02 (90% C.L.)

signal acceptance : 90%

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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2.05 ±0.14 0.02 ±0.01 0.65 ±0.08 (including blind region) 0.05 ±0.02 0.01 ±0.01 <0.02

before Neural net cut after Neural net cut

bserved

expectation of !" → $%$&$' BG

Preliminary Preliminary

SLIDE 28

!" → 2%&, !" → 2' BG

2019/12/23 Kuno and Yamanaka Groups End-of-the Year Presentation

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Result from !" → 2%& MC study Result from !" → 2' MC study

Preliminary Preliminary