KOTO: Status and Future Prospects
Tadashi Nomura (KEK/J-PARC)-

Introduction and basics of the KOTO experiment
Latest results from the 2021 data analysis
Prospects

HQL2023 @ TIFR, Mumbai, India on November 28-December 2, 2023

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Physics in rare Kaon decays: K→πνν•

s-d transition via loop diagrams,
Flavor Changing Neutral Current(FCNC) process ~ A2λ5
~ 10-4 suppression in SM•

0

KL → π νν••••Top quark dominates
K 0 − K 0 superposition extracts
imaginary part of the amplitude
CP violating++

K → π νν•BR∝Im(As→dZ*)2

BR∝|As→dZ*|2

Top and charm contributes
2KL→ 0

BRSM(KL → π 0νν) = (2.94 ± 0.15) × 10−11
CKM uncertainties are dominant while intrinsic one ~2%.

K+→ +BRSM(K + → π +νν) = (8.60 ± 0.42) × 10−11
BRSM are quoted from Acta Phys. Pol. B 53, 6-A1 (2022)

𝜈

𝜈

𝜈

𝜈

𝜋

Small theoretical uncertainty, suppressed in SM
Good probe to search for New Physics beyond SM
𝜋

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K→πνν in the Standard Model (SM)3￼

New physics can appear in the loop
Illustration of correlations in
Z’ models with LH and RH couplings
the BR(K+→π+νν) and BR(KL→π0νν) plane

JHEP12(2020) 097+
Figure 10. The ratios for K →and enhancements
KL → πν ν̄ definedfromin (4.1)
R0π,Rν+:ν̄ BR
SMare plotte
region inside the curves is allowed) and LH-EW
JHEP 1511 (2015)shown
166 in green (where the wholefor neutral and charged modes!
+

and red with εK ∈ [−0.2, 0.2] and [−0.5, 0.5] respectively for a Z of 3 TeV. T
satisfies0]. be O(10-10).
∆MK ∈ [−1.0, to
BR(KL→π0νν) canbeRenhanced•

Even in case that BR(K+→π+νν) is consistent with the SM prediction.
pattern of simultaneously enhancing KL

4→ π 0 ν ν̄ and suppressing K +￼KL→π0νν branching ratio
KOTO
sensitivity

KOTO experiment
KL→π0νν study
at J-PARC

2015
2016-18

10-8

KOTO 2015(<3×10-9)

10-9Grossman-Nirbound (<6.4×10-10)

10-10
Search down
to <10-10,
approachingSMprediction

SM (3×10-11)
10-11
Grossman-Nir bound: indirect limit
from relation to BR(K+→π+νν);Calc’d from NA62 results (2021)
with 1σ region5￼

KOTO collaboration
KOTO stands forK0 at Tokai.
Zoom

KEK Tokai campus
Photo @ collaboration meetingon June 30 - July 2, 2023

KEK, NDA, Osaka, Saga, Yamagata,Jeonbuk, Korea, NTU, Arizona State, Chicago, Michigan
6￼J-PARCIn Tokai-village, Ibaraki, Japan

LINAC
400MeV

3GeV Rapid Cycle
Synchrotron (RCS)

Neutrino beam to Kamioka

Material and Life
science Facility (MLF)

30GeV Main Ring
(MR)

KOTO in HEF

Hadron Experimental Facility (HEF)
7

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Experimental principle0

KL → π ( → 2γ) νν (nothing)
KL beam line
Two stages of collimator anda sweeping magnet

KOTO detector area
Calorimeter
VETO

Photon absorberγ

30 GeV/cprotonγ

66mm-longgold target
16 degree
production

•

𝐿

•

VETO

ν

ν

2γ in the calorimeter in the end-cap
No extra particle, and thus no hit in veto detectors
8

200
K
L pencil beam"
崩壊位置 Zvtx が求まる。
1502γ+
nothing0得られた Zvtx を使うと
1
1 2 つの γ の運動量ベクトルを得ることができ、その和が π の運動量ベ
FB

NCC

MB

CV

CC04

CC05 CC06 BHCV

BHPV

400

102

pi0nunu/pi0nunu/pi0nunuann05_

400

Pt

pi0nunu/pi0nunu/pi0nunuann00_100.root
Pt

pi0nunu/pi0nunu/pi0nunuann08_100.root
Pt••

CsI calorimeterという関係が得られる。E1 、E2 は入射 γ のエネルギーである。Eqs.(2.3-2.6)を用いると、π 0 の

102•Reconstructionof event kinematics
!

400

100
!
Calorimeter+
Hermetic
vetoKL0
クトルになる。したがって π のビーム軸に垂直な運動量成分 P も求めることが出来る。この 2
350

350

300

350

t

300

50

300

Decay region
つのパラメータ Zvtx と Pt をシグナルとバックグラウンドの識別に利用する。この部分について
250

250

10

250

10

200
0200
10001500200025003000350040004500500055006000"
は参考文献[21] に詳しく記述されている。またこの1502 つのパラメータ平面上でシグナルイベント
Vacuum chamber
150
150
BCV
LCV
OEV
CC03
HINEMOSZ Vertex
Z
Vertex112000πCalculatethedecayvertexandtransverse
momentum(P
T)
Signalreconstruction
は Fig.2.3 のように分布する。図の赤枠で囲まれる部分を signal box と呼ぶ。
0
Assume2
gammas
come
from
π
•
with 2γ energies and positions
measured by the calorimeter
unu/pi0nunuann08_100.root
pi0nunu/pi0nunu/pi0nunuann00_100.root

00200025003000350040004500500055006000

100

50

Saturday, April 20, 201350

010001500200025003000350040004500500055006000
2 Z Vertex

M⇡20

102

2E1 E2 10
10

250
200

1

1

100
50

!0

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10001500200025003000350040004500500055006000

350
350

pi0nunu/pi0nunu/pi0nunuann05_
Calorimeter

102
100

VETO

γ

300
300
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10

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γ

250

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150

11

200

100
100
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5050

VETO

0

10-1
0.1

50

ν

10-1

0
[mm]
Z Vertex
10001500200025003000350040004500500055006000
Reconstructedvertex[mm]

Z Vertexν010001500200025003000350040

KAON13 @ Univ. of Michigan Ann ArborPT

Z Vertex

Fig. 2.2 KL0 → π 0 νν からの π 0 の再構成。

𝐿fi

Z Vertex [mm]Fig. 2.3 KL0 → π 0 νν 分布。赤枠で囲まれた部分を信号事象と同定する。横軸の 0
は Front Barrel( 2.6.1) の上流端。

The signal region is de ned
on PT vs Z plane￼¯ signal box

10001500200025003000350040004500500055006000
1000
3500
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𝐾
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00200025003000350040004500500055006000


KL ! ⇥

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KO T

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Z Vertex

250

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0
pi0nu
10001500200025003000350040

010001500200025003000350040004500500055006000

350

Pt [MeV/c]
Reconstructed Pt [MeV/c]

Pt

cos 350= 1

500pi0nunu/pi0nunu/pi0nunuann00_100.root

pi0nunu/pi0nunu/pi0nunuann08_100.root
400

100

Pt

Require large transverse momentum
400
10z vertex- Ptcalculation
distribution
Vertex

10m

Pt

•
•

100

Pt [MeV/c]•

Z vertex9￼

KOTO detector
The core part of thedetector, surrounding the KL
decay region, are locatedinside the vacuum tank.

[m]
2

FBHinemos NCC MB BCV
IB  CV LCVCC03OEV CC04 CC05 CC06 BHCV

BHPV

BHTS
BHGC

1

0

KL beam

Decay volume(Vacuum)

Downstream
beam pipeMembrane

Calorimeter
(CSI)
0

2

4

6

8

10

y

Concrete/iron shield
12

14

x

z

16

[m]
10

Inner Barrel (IB)KOTO detector

KOTO detector
Main Barrel (MB)Front Barrel (FB)

Charged Veto (CV)Detector

NCC

meter + Herme2c veto system

￼CC)

Main barrel (MB)[m]2Charged veto (CV)Calorimeter

FBHinemos NCC MB BCV
BHPV (CSI)
BHTS
BHGC
IB  CV LCVCC03OEV CC04 CC05 CC06 BHCVCalorimeter10

KL beam

Decay volume(Vacuum)

Downstream
beam pipeMembrane

Calorimeter
(CSI)
0

2

4

6

8

10

y

Concrete/iron shield
12

14

x

z

16

[m]
11

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KOTO latest results- From KOTO 2021 Data Analysis

12￼

P.O.T. = Protons On Target

Accumulated Protons On Target

KOTO data accumulation history
Beam power: 64.5kW at highest in 2021

2016-18 data
2015 data

2019-21 data
In analysis(Phys. Rev. Lett.
126, 121801)(Phys. Rev. Lett.
122, 021802)

2013 data

Scopeof thelatestanalysis(PTEP 2017,
021C01)2013 2014 2015 2016 2017 2018 2019 2020 2021 2022
Detector upgrades

Additional barreltype veto (IB)Calorimeter upgrade
for n/γ separation

K± veto: UCV
1mmt

0.5mmt0.2mmt13

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Review of the previous analysis (2016-18 data)

Black: observed
Single Event Sensitivity=
PHYSICALREVIEW LET
Final PT vs Z plot Red: expected BG
PHYSICALREVIEW
126,
121801(7.20± (2021)0.05stat ± 0.66syst) × 10-10
Contour:signalLETTERS
MC
00.53 ±0.13

00Source350KL

41.97 ±0.35300including signal region

250
200

3
1.22 ±0.26

150

00.14 ±0.06

100
50

0
0.20

±0.09

0
0

0
1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000

Zvtx (mm)N(=3)

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

103

400

Number of events

102

K L → 3π 0
K L → 2γ (beam halo)
Other K L decays

10

1
K!
NeutronData/MCP t (MeV/c)

450

439
436.79 ±3.83

Events /(20.0 MeV/c2)

500

TABLE II. Summary of the numbers of background events with
Data
aBackgroundcentral
104 value estimate.
MC
Sample
:
table

Total

Hadron cluster
CV η
Upstream π 0

2
1
0

300

a

400

500

600

0.01 ! 0.01
0.26 ! 0.07a
0.005 ! 0.005
0.87 ! 0.25a
0.017 ! 0.002
0.03 ! 0.01
0.03 ! 0.03
1.22 ! 0.26

700

800

Background sources
studiedReconstructed
Kaftermass looking(MeV/c2) inside the blindNewlyevaluatedbackgrounds
region.
±FIG. 4. Reconstructed K ! mass (M K! ) distribution after imposing the K !BG→ π ! π 0 selection criteria except for the requirement on Mto
The1.22bottomthe ratioof central
data and MC
estimated! panel0.26 showsby addingthevalues of
K ! .beeventsfor Better
each histogrambin. Notebut
sensitivity
limit
each
backgroundsource.
thatworse
the backgrounds
from

Statistically consistent with N(=1.22±0.26)

FIG. 3. observed
Reconstructed π 0 transverse momentum (Pt ) versus π 0
decay vertex position (Zvtx ) plot of the events after imposing theK L → π 0 νν̄ selection criteria. The region surrounded by dotted-9 observedlines is the signal region.
L The0 black dots represent!
K
and beam-halo
2γfromdecaysweredata
not known whenthan theKresult
2015
0
L →
events, and the shaded contour indicates the K L → π νν̄ districollected data with theπ ! π 0 trigger in this magnet-off
first looked inside
the blindandobserved
they werebution from the MC simulation. The black italic (red regular) we
normalize
the K !region,
yield. We
27 added± configuration to
L theregion
numbers indicate the number of observed (background) events after
performing
studies
in tothistheLetter.
eventsin the signal
afterdescribedimposing laterthe cuts
140
for different regions. In particular, 1.22 ! 0.26ð1.97 ! 0.35Þ is
The K sample.
→ 3π Thisbackgroundcontrol
number arises
agreed fromwith photon
26.0 ! detection
3.2

BR(K →π νν)<4.9×10 (90% C.L.)

Must reduce K and halo K backgrounds
LK ± → π 0e ±ν

Halo KL → 2γ

The interaction of KL with the 2nd collimator
produces K±.

The interaction of KL with the 2nd collimator
produces halo KL.

PT vs Z
plots
Signalregionin 2016-18
analysis
15
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K± and halo KL→2γ backgrounds

UCV - Upstream Charged Veto•

For K± detection in the beam at theentrance of the KOTO detector•

A plane of square scintillation bers,
read by MPPC

0.5mm-square bersfi

fi

Tilted 25 degree to reduce inef ciency
due to bers’ inactive region (clad)
fi

•

fi

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New in 2021 run:16

KOTO 2021 data analysis:

Evaluation of K± ux and rejection by UCV

M(π±π0) (MeV/c2)•

K± ux was evaluated by using control data which were
simultaneously taken in physics run• 3-cluster trigger, collecting 2γ (from π0)+1π±
RUN87(2021)
64.5kWK± events

K±→π±π0reconstructed events
KL→π+π-π0Reconstructed Z (mm)flfl

K± flux: R(K±/KL)=3.3×10-5

UCV detection inefficiency=7.8%
Corresponding to K± BG rejection by a factor of 12

Analysis method to reject halo KL → 2γ
Shower shape likelihood

Kinematic MVA

Consistency between shower shapeand reconstructed angle•

Multi-variable analysis by using
kinematical variables•

CSI calorimeter

Signalπ 0 → 2γ

KL

CSI calorimeter
Distributions of input variables
θγ66
KL → 2γ
第 7 Halo
章 運動学的変数の違いを用いた新たな削減手法の開発
Rec. Z
RCOE
1

Blue:signal
Red:Halo KL→2γθγ2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

Validation by data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。

Performance

Background rejection versus Signal efficiency

0.9
0.8

2.5

Signal (test sample)

Signal (training sample)

Background (test sample)

Background (training sample)

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

2

1.5
0.6

Signal1

0.5

BG

MVA Method:
Fisher

0.5

BDT

0.3

0

1
Likelihood Ratio

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

0.7

0.4

0.2

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 7.7

Validation by data

TMVA overtraining check for classifier: Fisher

1(1/N) dN / dx

SignalBG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

Background rejection

Performance

信号事象と KL → 2γ 背景事象のテスト

0

−0.6

-0.6

図 7.8

Fisher response
0
0.6
Fisher response
−0.4

−0.2

0

0.2

0.4

0.6

Fisher response

信号事象と KL → 2γ 背景事象の訓練

サンプルでの、FD (黒)、BDT (緑)、BDTG (赤)サンプル (ヒストグラム) およびテストサンプル

での信号事象感度と点線 に対する()
FD
Reduce halo KL→2γ Kby→ 2γa factor of 8,
while
signal ef ciency = 94%
K → 2γ
背景事象の削減能値。青が信号事象を、赤が背景事象を示す。

L

fi

L

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力の相関。

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New in 2021 data analysis:18￼

KOTO 2021 data analysis:

PT vs Z plot after applying all the cuts
Single Event Sensitivity = 8.7 × 10-10

y
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a
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Black: Observed
Red: BG estimation

286.1±2.3

P

KOTO 2021 Data

0.02±0.006

0blind region
0.490±0.103

Signal region

00.255±0.058

00.195±0.083

19

CV η
VETO

Background evaluations in detail:KL→2π0 background
•

•

VETO

π 0 → 2γ

Halo neutron

n η

CSI

n

Halo neutron

ZVT

CV
VETO

VETO

A halo neutron interacts with CV and
A halo neutron interacts
produces π 0
produces η →wrong ver
KL→2π0 background

Inef ciency of the photon detection is criticalfor KL→2π0 BG estimation.
• We relied on the simulation in the past
analysis but different versions of GEANT4
gave us different results.
• Data-driven evaluations (and corrections)
are needed.
We use KL→3π0 events with 5γ in thecalorimeter as evaluation samples.
••

Calculate energy (E6) and direction of remaining
1γ by using kinematic constraints (vertex from
2π0 reconstruction, transverse momentum
balance, KL mass)
Check the energy deposit in the detector ofdestination

KL → π 0π 0 → 4γ
VETO

γγ
𝐾𝐿

γ

CSI

γ

VETO

2 of 4γ hit CSI and
2 are lost due to detection inefficiency

Inef ciency evaluation by using KL→3π0

6th γ
KL
5γ in CSI

fi

20
fi

￼

CV π

Inef ciency evaluation with 5γ data
Category: Barrel (IB or MB), high E

valuate Systematic error

orReconstructed
for high energy
photonE6: >200MeV
Destination: IB/MB region

Energy deposit (Edep) in the barrel detectors
Black: data
Red: KL→3π0 MC (GEANT4 v10.6)
Blue: KL→3π0 MC (GEANT4 v9.5)

1 in data (uncertainty is 100%)
Corresponding inef ciency= (4.8±4.8)×10-5
1.3±0.5 in MC - G4 v10.6••

A × σMC)2

MC-to-data correction factor:
1/1.3= 0.77ssian with
mean=0and sigma=1
Uncertainty: 100%
A within -1<Diﬀ<1
fi

fi

fi

red as σdet

fi

Limited by statistics of control data

Data/MC(cf. 0.4±0.3 in G4 v9.5 MC)

Corresponding inef ciency-5
A × Ineffi=MC)(6.2±2.5)×10

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EventsInef ciency event ≡ N(Edep<1MeV)<1MeV)/All

Max(Edep(IB), Edep(MB))(MeV)
Max(CBAR,
IB)
Inefficiency
21

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Background evaluations in detail:KL→2π0 background with correction factors•

Applied the weight to BG events in MC, according toenergies and destinations of missing photons in each event
Category

Correction factor on
inefficiency

Uncertainty(Stat error of 5γ data)

Barrel, high E

0.77+85% / -100%

Barrel, low E

1.10

±9.9%

FB (upstream)

1.42±12.6%

BHPV (downstream)

1.5+42% / -51%

Destination categories“Barrel”
MB/IB

γ

“FB”

γ
𝐿

MB/IB

“BHPV”

γ

γ

BHPV

22

Background evaluations in detail:KL→2π0 background on PT vs Z plane

The number of KL→2π0 BG
MC with correction

0.060± 0.022(stat) +0.051−0.060 (sys)cf. :N(KL→2π0)
before SF corre-0.049 0.018(s0.023± 0.013(stat) +0.016−0.019 (sys)0.034± 0.016(stat) +0.034−0.040 (sys)14(

NBG=0.060
NBG, w/o correction=0.049)

Upstream π0 background•

π0 can be produced by the interaction
between halo neutrons and the
upstream detector (NCC).
• The reconstructed vertex must be
around the NCC position, which isoutside of the signal region.
• If the measured photon energy iswrong (Emeasured<Etrue), the resultant
reconstructed vertex can be inside
the signal region.
This happens due to photonuclear interactions in thecalorimeter.
➔ Need data-driven evaluations

Upstream π 0
VETO

Halo neutron

π

0

γ
γ

CSI

Wrong vertex
reconstruction
VETO

24
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Background evaluations in detail:γ

Photo nuclear

interactioneData-driven evaluation of photo-nuclear
Key: interaction
Probability of mis-energy
measurement in CsIin the CsI calorimeterγ

Halo neutron

Upstream π-0interaction

Evaluate the probability of mis-energy measurement in CSI

γπ
0KL→3π samplesγ
Halo neutron0

Neutrons

Photo nuclear

Wrong E measurement due tophoto-nuclear interaction in CSI
VETO

KL→3π0

CSI

γ

KL π0

with large Center of Energy (COE) in CsI
M6γWrong vertex
MKLreconstruction
VETO

21

6γ events with large COE

The MC reproducibility of the photonuclear interaction (PN) in the CsI
calorimeter can be evaluated by using 6γ
sample (KL→3π0).
• M(6γ) ≠ MKL• Large center of energies radius (RCOE)
This discrepancy was taken into
account as the correction factor.
￼

￼

￼

￼

￼

γ

Data

MC

Core KL (PN)
Core KL (non PN)
Halo KL

Scattered KL
Discrepancy in this region
could be due to photonuclear interaction.

0

200

400

RCOE(mm)25

0 background events
0f upstreamπUpstream π background on PT vs Z plane
Background evaluations in detail:MC with correction

0.16± 0.08(stat) ± 0.01(sys)286.1± 2.3(stat)0.0018± 0.0018(stat)Nobs = 215

0.064± 0.050(stat) ± 0.006(sys)ence between data/MC

m an imperfectcf. :N(upstream π0) BG

bility of π0ʼs kinematics

before correction-(0.035 0.025(stat)

25% discrepancy comesfrom an imperfect
reproducibility of π0’skinematics in MC.

0.14± 0.08(stat) ± 0.01(sys)18(

NBG=0.064
NBG, w/o correction=0.035)

￼

KOTO 2021 data analysis:

Summary of backgrounds
SourceEstimated value

Upstream π00.064 ± 0.050 (stat.) ± 0.006 (syst.)

y
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e
r

KL→2π00.060 ± 0.022 (stat.) +0.051(syst.)−0.0600.043 ± 0.015 (stat.) +0.004(syst.)−0.030K±
Scattered andhalo KL(→2γ)

P

0.022 ± 0.005 (stat.) ± 0.004 (syst.)
0.018 ± 0.007 (stat.) ± 0.004 (syst.)

Hadron cluster BG

0.024 ± 0.004 (stat.) ± 0.006 (syst.)

η production in CV

0.023 ± 0.010 (stat.) ± 0.006 (syst.)

Sum

0.255 ± 0.058 (stat.) +0.053(syst.)−0.06827

￼

KOTO 2021 data analysis:

Ready for opening the box

KOTO 2021 Data
Black: Observed
Red: BG estimationyraSingle Event Sensitivity
n
i
= 8.7
× 10
m
i
l
e
r
P

y
r
a
n
i
m
i
l
e
r
286.1±2.3-10P

0.02±0.006

0blind region
0.490±0.103

Signal region

00.255±0.058

00.195±0.083

28

Results•

Unblinded the hidden region➡ No signal candidates
were observed in thesignal region.
•

KOTO 2021 Data

y
r
a
n
i
m
i
l
e
r

Set the upper limit to beBR(KL→π0νν) < 2.0×10-9
at 90% con dence level.
-

P

Corresponding to SES×2.3
based on Poisson statistics.

We are preparing the paper of this result.
fi￼

KOTO 2021 data analysis:29￼Prospects

30

UCV upgrade

Less material and better ef ciency••

DAQ upgrade
To prepare for a higher beam power,
and to accommodate more control
data simultaneously in physics run
Capable to handle ~×2 more trigger
rate

•

•

•

Beam line upgrade
50-cm-long, 0.9-T permanent magnet
has been installed at the end of thebeam line to sweep out K±.

•

Introduction : UCV u

The MR accelerator
was shutdown
in 2021-22
Next generationof UCV: Film UCV
for the magnet power
supply upgrade.
0.2-mm-thickplastic scintilla
New UCVAim to get 1%
Optical box 160 mm(Al mylar)
160 mm

PMT array

0.2mm-thick
Scintillatorlm scintillator>99.9%
ef ciencythe
was light
achieved
Collectesca
in a short run in June 2023
Collaboration meeting

We are ready for next run.
31
fi

•

fi

fi

￼

Improvement after 2021 run￼Protons on Target (POT) / year

KOTO expected sensitivity in the near future
The accumulated POT will be ×10
larger than 2021 in 4-5 years.
Assumption• The beam power increases
as 80→90→100kW.
• 60 days beam time / year.

SES:8.7×10-10

Expected➡ SES will reach the level better
than 10-10.
- The achievable sensitivity willbe (5-8)×10-11.

Thanks to the MR power supply upgrade in 2021-22,• A higher repetition (5.2s→4.2s cycle) can be adopted ➔ Higher beam power• 65kW in 2021 with 5.2s repetition → 80kW with 4.2s repetition• A smoother time-structure beam is expected ➔ Reduction of accidental loss

32

And to go further, …
KOTO II in Extended Hadron Experimental Facility

F
E
tH

n
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ur

n
o
i
s x)
n
e
xt F-e
e
E
F
H
E
(
H
t
c
e
j
o
Dump
r
p

C

T2

•
•

5 degreeproduction angle
Larger andlonger detector
than KOTO

KOTO
step-244m
from T2 0m

HEF-ex is on top of the
KEK-PIP 2022•

KL2

Expecting that programsin the new hall will belaunched in 2030's

New physics search with
• # of signal ~35 (in SM) / # of BG ~30
0• ~5σ discovery of KL → π νν (in SM)• 25% measurement of the BR (in SM)• Sensitive to new physics thatenhances the BR by >40%

T1

￼

￼

Next generation KL→π0νν experiment

3m

12m

3m

15m

20m

33

￼

Summary•

KOTO concluded the 2021 data analysis• The single event sensitivity = 8.7×10-10,
the expected number of backgrounds = 0.255• After opening the signal box, no candidate events wereobserved inside the signal box.
• New upper limit: BR(KL→π0νν)<2.0×10-9 (90% C.L.) Preliminary•

KOTO continues taking data and will reach the sensitivity level
better than 10-10 in 4-5 years.

•

Next generation experiment “KOTO II” is being discussed.
• Aiming to observe >30 SM events with S/N ratio of 134