Search for rare kaon decays
at the J-PARC KOTO experiment

Koji Shiomi (KEK) for the KOTO collaboration
20th LOMONOSOV CONFERENCE 2021/8/21

10
Physics on KL→π ννKLd

CP even０π＝

CP oddStandard Model

CP oddNew
Physics

CP odd• Rare, Theoretical clean, CP violation
Branching ratio(BR) = (3.0 0.3) 10-11
The uncertainties mainly come from the CKM parameter errors,
The theoretical uncertainties are only 2% .

1→Λ:O(100)
TeV
NP
2
ΛNP

0
Experimental search for KL→π νν
0BR(KL → π νν)
−8

10

−9
B
<
3.0
×
10
(90 % CL)
Direct limit (KOTO 2015) KL→π0νν

−9

−10

10

Indirect limit BKL→π0νν < 6.4 × 10
1011
)

−11

0

10

+

SM

A

Glimpserecentatanalysis

theofd’OeuvreNew

Zeptouniverse

tree

level

flavourby oit n r
m pi
i
L s
r
i Iso
N y
a n eca
m d
s
s KL
o
r
G →
NA62 1𝜎
y
a
c
de

Reach

changing

K ! ⇡⌫ ⌫¯ decays sensitive toscales up to 2000 TeV if left- andright-handed FV couplings arepresent(fine-tuned)in K 0K̄ 0Physics

BR(KL

Hors

i
t
ta

+

K

of

Flavour

Z 0:

Physics

Buras

6

al.

(2014)

cancellation of e↵ects
mixing required

new physics reach of B
decayslower by an order of magnitude(⇠ 100 TeV!)‚(2021)etBNL E787+E949 Limit

10!7pinn stuffs
on

BNL E787+E949 Limit

New Physics?

−10(68 % CL)

+
+
11
BR(K
)
10
+ Buras 2014high precision in rare K
and B decays
is crucial!
3StandardModel3
http://www.lnf.infn.it/wg/vus/content/Krare.html
M. Blanke

FlavourPhysics

BeyondtheStandard

Mo del3 GeV !
Synchrotron

Hadron hall

KOTO experiment•

J-PARC /3$ 

KOTO area

J-PARC Laboratory
0
J-PARC Laboratory
Study of KLLinac→π νν@J-PARC
30GeV
Main Ring.
̅

30•GeV! (30 GeV protons)
Main ringG
;
Main RingG

30 GeV
Main Ring



Linac

Extraction!
in 16 degree

3 GeV

Primary proton !
30 GeV/c;H A'$;H
3
Tokai,
Ibaraki,Japan

Synchrotron

Target (Ni, Pt, Au)

3
Hadronhall            
   !" !# $#Collaboration  %" meetingwith Zoom(July 2021)

4Experimental principle
KL→π0νν̅ decaytargetNeutral beam line

γ
0
π
θ
KL
γ

E1
E2

“2γ+Nothing+Pt”
Assuming 2γ from π0,
Calculate z vertex.

Rec. Pt

protonM2(π0)=2E1E2(1-cosθ)Calculate π0 transverse momentumν ν̅
SignalBox
Rec. Z
5

KOTO detector

dding 5 X0 another inside the MB. According to the Monte-Carlo (MC)nt of KL0 → 2π 0 will be suppressed by a factor of threes a sampling calorimeter as shown in Fig. 2. It consists of 25 layers of
ors and 24 layers of 1-mm-thick lead plates, corresponding to 5 X0 . The
de in a trapezoidal shape and formed as a cylindrical detector. The volume
he beam direction, and inner and outer diameters are 1.5 m and 1.9 m,
ation light is read out by a photomultiplier (Hamamatsu R329-EGP or
Neutroncollar counter
Main barrel (MB)nds via Wave Length
Shifting(WLS) (NCC)
Fibers (BCF92).

Detector

CsI calorimeter + Herme2c veto system

KOTO detector

Front Barrel (FB)

Inner Barrel (IB)Main Barrel (MB)Charged veto (CV)Calorimeter

Charged Veto (CV)Calorimeter

Front barrel (FB)c cross-sectional view of
The main backgroundalso displayed. The newblue color.

NCC

FBHinemos NCC MB BCV
IB  CV LCVCC03OEV CC04 CC05 CC06 BHCV

[m] Figure2

2. Top Left) The WLS fibers areattached in the scintillators. Bottom Left)
One module consists of 25 layers of 5 mm
1
thick scintillators and 24 layers of 1 mm leadplates. Right) Formation as cylindrical shape.
0

KL beam

K

L
Decay volume(Vacuum)

ν

BHPV

BHGC
BHTS

γ
γ

ν

tion, construction and insertion to the existing KOTO detector
WLS fibers to all 800 scintillators with UV adhesive. After the fibers weree cracks in the scintillators caused by uncured adhesive behind fibers. We
tillators with fibers for those who have large cracks and also annealed to
0
2
4
6
◦
80 C for 3 hours to increase chemical resistance based on the result of

Downstream
beam pipeMembrane

Calorimeter
6 8

8

10

y

Concrete/iron shield
12

14

x

z

16

[m]

Data Accumulation History
P. O. T = Proton On Target

2016-18 data
Published(Phys. Rev. Lett. 126, 121801)2015 data(Phys. Rev. Lett. 122,
021802)

2013 data(PTEP 2017,
021C01)

7

2019-21 data
In analysis

2016-18 data analysis-

Preliminary results at Kaon conference in
September 2019-

Post-unblind analysis-

Final results8Preliminary results at Kaon 2019
-Determined selection criteria and opened signal box in Aug. 2019.
-Observed 4 candidate events inside the signal box-Reported @ Kaon2019
500
450
400
350
300
250
200
150
100
50
0

Pi0Pt

Rec. π0 PT (MeV/c)&&MinCSDDLVal>0.985&&MaxThetaChisq<4.5&&MinWFourierLH>0.5&&EtaCSD>0.91&&KLpipipi0DLVal>0.922

7

S.E.S:6.9 10-10x1.9 better than
2015 analysis

6
5
4
3
2
1

2000 3000 4000 5000 6000
Rec. π0 vertex(mm)Pi0RecZ

0#Bkg estimation table
before opening signal box#BG

KL→2π0KL→π+π-π0
KL→3π0+ accid.
Ke3 + accid.
KL→2γ
9Upstream π0
CV-π0
CV-η
Hadron cluster
Total

<0.18
<0.02
<0.04
<0.09
0.00 0.00
0.00 0.00<0.1
0.03 0.01
0.02 0.00
0.05 0.02

Post-unblind analysis• No change in cuts
• Found an error in timing parameters.
4→3 events by ﬁxing it.

• Found two new background sources, andupdated background estimation.
10

Halo KL→2γ

0.26 inside the signal region
2021/03/26

14

KOTO @Moriond 2021

Backgrounds found in post-unblind analysis

Beam-halo !! → 2' background

KL beam line

KOTO detector area

CSI calorimeter

sweeping magnet

Beam-halo 0$ → 32 %ZVTX(→ 66)
Considering only areas RCOE > 200mm (Signal requirement) 3$%& =VETO

POT Normalization
To increase
statistics...
Considering only areas RCOE > 200mm (Signal requirement)
CSI
Only below cuts were
CSI event displayskim.root 44
1000
0
!)
imposedData(K→!"• Halo KL flux was evaluated by using L 800
sampleL
POT
Normalization
To increase statistics...
2016-2018 MC
600
Only
cuts were(Kine)belowCSI
calorimeter400
imposedΔt, Etot, Emin, Emax,
Data (KL →!"! )VETO
200
Fiducial, ClusDistance,
2016-2018
3$%& MC0
KL → 3π0Halo KL MaxShpeChisq(<7.0)(Kine)

atio Ratio #Event#EventY(mm)K → 3π

z

100
90
80
70
60
50

< 200
40
Halo KLΔt, Etot, Emin, Emax,< 400
30
Fiducial,(Veto) ClusDistance,< 600
20
MaxShpeChisq(<7.0)
NCC,CV,IB,CBAR,< 800
10CC03,CC04,CC05,CC06,LC
VETO
RCOE
< 10000
V,OEV,IBCVCH27,IBCH55(Veto)<1000 <800 <600 <400 <200 0
200 400 600 800 1000
NCC,CV,IB,CBAR,
0
100
200
300
400 X(mm)50011RCOE=Radius ofCC03,CC04,CC05,CC06,LC
Of Energy on CSI calorimeter+Center
CDTNum==6
RCOE [mm]∑ 5' 6(∑ 6(

Nevents/40mm

Au target• Beam-halo $< flux : evaluated by using $<𝐾𝐿 → 2𝛾
Halo KL

Comparison of COE
Comparison of COE

𝐾𝐿

30 GeV/cproton

VETO

Energy(MeV)

Photon absorber

2nd collimator

Data/MC

1st collimator102=→ 3' sample(COE: Center of Energy)+ Data+ Beam-halo&" → 3# ! MC

10
1
10
5
0
200 250 300 350 400 450 500 550 600
RCOE (mm)HaloKL&" flux: Data/MC
Halo
flux:
〜 × 7=~7of MC0.81 inside the signal region

K in the beam
Backgrounds found in post-unblind analysis
KL beam line

KOTO detector area
CSI calorimeter

sweeping magnet
1st collimator

2nd collimator

VETO

Photon absorbere±𝐾𝐿

30 GeV/cproton𝐾

Au target

K ± → π 0e ±ν±2γ
PHYSICAL REVIEW LETTERS 126, 121801 (2021)

VETO

400
• K± yield was evaluated by a special run± 0→ π π in June 2020.
P t (MeV/c)

to collect K±

CSI calorimeter
VETO

±

± 0

K →π π

𝐾

±

VETO

2γ
π±

350
300

0
0.53 ±0.13
41.97 ±0.35

including signal region

250
200
150

0
0

3
1.22 ±0.26

00.20

Events /(20.0 MeV/c2)

450

439
436.79 ±3.83

data

4

±0.09

10

103
102
10
1

± 0

π π

3⇡
<latexit sha1_base64="ZcRc/13iJhtaDsCz4g7kVS9VD2E=">AAAB63icdVDLSgMxFM3UV62vqks3wSK4GjJ92LorunFZwT6gHUomzbShSWZIMkIZ+gtuXCji1h9y59+YaSuo6IELh3Pu5d57gpgzbRD6cHJr6xubW/ntws7u3v5B8fCoo6NEEdomEY9UL8CaciZp2zDDaS9WFIuA024wvc787j1VmkXyzsxi6gs8lixkBJtMqgxiNiyWkIsuql7Zg8itIa9RRktyWa9Az0ULlMAKrWHxfTCKSCKoNIRjrfseio2fYmUY4XReGCSaxphM8Zj2LZVYUO2ni1vn8MwqIxhGypY0cKF+n0ix0HomAtspsJno314m/uX1ExM2/JTJODFUkuWiMOHQRDB7HI6YosTwmSWYKGZvhWSCFSbGxlOwIXx9Cv8nnbLr1Vx0Wy01r1Zx5MEJOAXnwAN10AQ3oAXagIAJeABP4NkRzqPz4rwuW3POauYY/IDz9gkTI45F</latexit>

Data
MC Sample :K± → π ± π 0
K± → π0l ±ν (l =e,µ)KL → π+π-π0
Other KL decays

π 0l ±νothersData/MC500

23 clusters on CSI
0
0
100
0
1
0.14 ±0.06
0• π 50vertex reconstruction
0
±
300
400
500
600
700
800
• π 0 reconstruction
Reconstructed K± mass (MeV/ c2)1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Zvtx (mm)assuming transverse-5!
• FIG.
K±/K
ratio
=
4. L
Reconstructed
K2.6×10
mass (M K ) ,distribution after im!
! 00posingtheK→ππ selection criteria except for the requireFIG.momentum
3. Reconstructedbalance
π 0 transverse
momentum(P)
versus
π
t
12

•

!

decay vertex position (Zvtx ) plot of the events after imposing the~3 times larger than MC expectation

ment on M K! . The bottom panel shows the ratio of data and MC

The Final results of 2016-2018 data analysis
PHYSICAL REVIEW LETTERS 126, 121801 (2021)
Editors' Suggestion

Single Event Sensitivity = (7.20 ± 0.05stat ± 0.66syst) × 10-10

Study of the K L → π0 νν̄ Decay at the J-PARC KOTO ExperimentJ. K. Ahn,1 B. Beckford,2 M. Campbell,2 S. H. Chen,3 J. Comfort,4 K. Dona,2 M. S. Farrington,5 K. Hanai,6 N. Hara,6
H. Haraguchi,6 Y. B. Hsiung,3 M. Hutcheson,2 T. Inagaki,7 M. Isoe,6 I. Kamiji,8 T. Kato,6 E. J. Kim,9 J. L. Kim,9H. M. Kim,9 T. K. Komatsubara,7,10 K. Kotera,6 S. K. Lee,9 J. W. Lee,6,* G. Y. Lim,7,10 Q. S. Lin,5 C. Lin,3 Y. Luo,5 T. Mari,6T. Masuda,11 T. Matsumura,12 D. Mcfarland,4 N. McNeal,2 K. Miyazaki,6 R. Murayama,6,† K. Nakagiri,8,‡ H. Nanjo,8,§
H. Nishimiya,6 Y. Noichi,6 T. Nomura,7,10 T. Nunes,6 M. Ohsugi,6 H. Okuno,7 J. C. Redeker,5 J. Sanchez,2 M. Sasaki,13
N. Sasao,11 T. Sato,7 K. Sato,6,∥ Y. Sato,6 N. Shimizu,6 T. Shimogawa,14,¶ T. Shinkawa,12 S. Shinohara ,8,§ K. Shiomi,7,10
R. Shiraishi,6 S. Su,2 Y. Sugiyama,6,¶ S. Suzuki,14 Y. Tajima,13 M. Taylor,2 M. Tecchio,2 M. Togawa,6,¶ T. Toyoda,6
13
Y.-C. Tung,5,** Q. H. Vuong,6 Y. W. Wah,5 H. Watanabe,7,10 T. Yamanaka,6 H. Y. Yoshida,
and L. Zaidenberg2
Data

Final PT vs Z plot

PHYSICALREVIEW
126, 121801 (2021)
Black: observed,Red: expected
BG,Contour:
signalLETTERS
MC

00.53 ±0.13

0
0

0
0.20

Events /(20.0 MeV/c2)

450

439
436.79 ±3.83

00Data/MC

500

P t (MeV/c)

300

41.97 ±0.35

including signal region

250
200
150
100
50

103

MC Sample :K± → π ± π 01±0 ±Department of Physics, Korea University, Seoul 02841, Republic of KoreaK→πl ν (l =e,µ)2
Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA
+ - 0
3
K
→
π
ππ
Department of Physics, National Taiwan University, Taipei, Taiwan 10617, Republic
of
China
L
4
Department of Physics, Arizona State University, Tempe, Arizona 85287, USA
Other KL decays
5
Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA(KOTO Collaboration)

Background Table

400
350

104

3
1.22 ±0.26
00.14 ±0.06

±0.09

27

6

Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan
10 Institute of Particle and Nuclear
Studies, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
8

Department of Physics, Kyoto University, Kyoto 606-8502, Japan
Division of Science Education, Jeonbuk National University, Jeonju 54896, Republic of Korea
10
J-PARC Center, Tokai, Ibaraki 319-1195, Japan
11
Research Institute for Interdisciplinary Science, Okayama University, Okayama 700-8530, Japan
12
Department of Applied Physics, National Defense Academy, Kanagawa 239-8686, Japan
13
Department of Physics, Yamagata University, Yamagata 990-8560, Japan
14
Department of Physics, Saga University, Saga 840-8502, Japan
9

10
1

13

2
1

(Received 15 December 2020; accepted 18 February 2021; published 24 March 2021)
The rare decay K L → π 0 νν̄ was studied with the dataset taken at the J-PARC KOTO experiment in 2016,2017, and 2018. With a single event sensitivity of ð7.20 " 0.05stat " 0.66syst Þ × 10−10 , three candidate
events were observed in the signal region. After unveiling them, contaminations from K " and scattered K L
decays were studied, and the total number of background events was estimated to be 1.22 " 0.26. We
conclude that the number of observed events is statistically consistent with the background expectation. For
this dataset, we set an upper limit of 4.9 × 10−9 on the branching fraction of K L → π 0 νν̄ at the
90% confidence level.
DOI: 10.1103/PhysRevLett.126.121801

0 Introduction.—The rare kaon decay K → π νν̄ directly 90% confidence level (C.L.) [6] set by the KOTO experibreaks CP 300
symmetry [1,2] and 400
has a highly suppressed
ment [7,8] at600
the Japan Proton 700
Accelerator Research800
500
branching fraction predicted to be ð3.00 " 0.30Þ × 10 inComplex (J-PARC) [9] with the dataset taken in 2015.
L

01000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000

Zvtx (mm)0

Total± indirect upper limit, called the2Grossman-Nir boundthe standard model (SM) [3]. The accurate prediction of theAn
ReconstructedK
mass (MeV/ c )branching fraction makes this decay sensitive to new[10], of 7.8 × 10 is set using the K → π νν̄ decay [11].
−11

−10

þ

þ

Nobs (=3) is statistically consistent with NBG (=1.22±0.26).
physics beyond the SM (e.g., [4,5]). The current best
upper limit on the branching fraction is 3.0 × 10−9 at the

The KOTO experiment is dedicated to studying theK L → π 0 νν̄ decay. We presented preliminary findings on02019-2021 data analysis-

Detector upgrade-

Further Background rejection

14

Detector upgrade•

Downstream charged veto(DCV)against KL→π+π-π0 backgroundIn 2019

KL

γ

ππ+
CC05

•

Upstream charged Veto(UCV)
against K± background• Installed a prototype in 2020.
• Upgraded in 2021•

Calorimeter’s both-end readout
against neutron backgroundIn 201915

CC06

Calorimeterʼs both read out
Calorimeter upgrade
to suppress the hadronic background

Reduction of Hadron-Cluster B
Max ΔT distribution
Energy Weighted
ΔT = TMPPC− TPMT (energy
weighted for MPPC energy > 10 MeV)
0
deltaTMax

Arbitrary Units

π :90%

10 mm AlKL → 3π 0 (Data)

0.06(w/kinematic cut)
cut at = 31.05 ns

0.05

neutron back ground(2.1 ±0.1)%
hadron-cluster
control
sample (Data)

0.04

NeutronMax ΔT (downstream(w/kinematiccut,
w/tight veto)gamma efficiency= 89.9

0.03

v

0.02

0.01

Conﬁrmed good separation ability
with 2019 data

0
26

28

30

32

34

36

38

40

-2
neutronbkg:
2.1x10
ΔT range : ΔT : 25 ns < ΔT < 31.05 ns

42

44
Δ T (ns)

16

Downstream charged veto (DCV)

Pt vs Zto suppress KL→π+π-π0 background

CSI

CC04

Diﬀerence from 2016-2018 analysis-w/o FPSD, KL->pi+pi-pi0 DL, FTT cutγ-Veto timing for CV,MB,IB are diﬀerence from 2016-2018 analysis

KLπ(Use original value IB,MB:60ns, CV:80ns)-w/ DCV veto (5MeV threshold) π+
Run82
Run81 After run30680
CC05
w/o DCV cut(9.4e18 P.O.T)
SES=1.9e-9

w/o DCV cuts

Run82 all run

CC06(8.28e18 P.O.T)

w/ DCV cuts

SES=2.1e-9

Low PT eventsare eﬀectively removed

!8

17

e

Upstream charged veto(UCV)

UCV

γ

K

To veto K in beam
• Prototype (installed in 2020)• Plate with 1-mm square scintillating ﬁbers• New UCV (Updated in 2021)• Plate with 0.5-mm square scintillating ﬁbers readout by MPPC

read out by MPPC

• 30% ineﬃciency due to a limited coverage,ν• Fully cover beam, tilt detector, and put MPPCfar from beam.

insensitive region, and irradiation eﬀect.

18

The performance of new UCV±Energy in UCV(K± event)± 0• Can correct enough K → π π samples toevaluate eﬃciency in a short time

• With turning oﬀ the sweeping magnet in theThreshold forhit

beam line

1MIP equiv.

• Achieved 95% eﬃciency with 5% accidental loss
19

0.1

0.2

0.3

Energy (MeV)8.1 ２つのカットの相関

図 8.1 に、FD 値によるカットをかけた後の、MC で生成した信号事象と KL → 2γ 背景事象

る Likelihood Ratio の分布を示す。ここで、 Likelihood Ratio の閾値を 0.764 とし、それ以上を選択した場合、信号事象の感度を 90% に保ち、背景事象の 92% を削減できる。これは、多変Reduction against halo KL→2γのカットをかけていない MC に対する Likelihood Ratio の KL → 2γ 背景事象削減能力であるを上回る。したがって、２つのカットは相関が小さく、それぞれ独立した KL → 2γ 背景事象をきる。

Halo KLCSI calorimeter

Fisher Output

CSI calorimeter

Signal

第 7 章 運動学的変数の違いを用いた新たな削減手法の開発

KL

KL → 2γ0

π → 2γ66

に対する性能は向上するが、それ以外のサンプルに対しては性能が悪化するため、訓練サンプルとテス
トサンプルの出力の分布が異なる。

Reconstructed Zvtx

図 7.8 に、信号事象と KL → 2γ 背景事象の訓練サンプルおよびテストサンプルに対する、FD 値の分布を示す。信号事象と KL → 2γ 背景事象どちらにおいても、訓練サンプルとテストサンプルの分布

Kinematic
MVA
TMVA
overtraining checkfor classifier: Fisher

MVA using the reconstructedkinematic variables
0.8

2.5

Signal (training sample)

Background (test sample)

Background (training sample)

Kolmogorov-Smirnov test: signal (background) probability = 0.783 (0.918)

2

8.2 ２つのカットを組み合わせたときの KL → 2γ 背景事象削減能

２つのカットを組み合わせたときの KL → 2γ 背景事象削減能力を調べた。図 8.2 に、KL →

1.5
0.6

Reduce halo KL→2γ by a factor of ~15,
0.4
while signal efficiency = 90%

Signal1

BG

MVA Method:

Fisher0.2

景事象および信号事象の MC において、信号事象感度が 90% に対応する 2 つのカットの閾値の
わせそれぞれにおける KL → 2γ 背景事象削減能力を示す。

0.5

BDT

0.3

KL → 2γ 背景事象の MC における、 Likelihood Ratio と多変数解析の出力の値に対して90% の信号事象感度を満たす組み合わせでの KL → 2γ 背景事象削減能力。２本の赤い線は、それぞれ最適化された２変数の閾値を示す。影をつけた領域を棄却した場合、信号感度 90% で KL → 2γ
背景事象を 94% 削減できる。

Signal (test sample)

0.7

0.5

BDTG

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Signal efficiency

Likelihood Ratio

図 8.2

U/O-flow (S,B): (0.0, 0.0)% / (0.0, 0.0)%

0.9(1/N) dN / dx•

Shower shape consistency
Background rejection versus Signal efficiency- Likelihood Ratio 1
Background rejection•に大きな乖離はない。したがって、過学習が起こっていないと判断した。

0

−0.6

20

−0.4

−0.2

0

0.2

0.4

0.6

Fisher response

Prospect
Data Accumulation
2016-2018
Run81

W/O UCV

Run82

Data taken in
2019-2021

Run85

Prototype UCV

Run86

New UCV

DCV,
Both-readout on CsI

Run87
0

7.5

15

22.5

30

P.O.T
-10
Rough
estimationofthesingleevent
sensitivity
for
Run81-87
is5
10•• Physics data taking will be resumed from fall (winter) of 2022 with a higher
intensity beam (〜100 KW) after the MR power supply upgrade.
21Summary0νν decay
The
KOTO
experimentstudies
the
K
→π
L
•

• Results of the 2016-2018 analysis has been published-10
Thesingleevent
sensitivity
is
7.2
10
•

• 3 observed events is consistent with the estimated 1.22 0.26
background events• KOTO will continue to take data and improve sensitivity by reducing
background events with new detectors and improved analysis methods.
22