Search for new physics via rare 𝑲𝑳 decays
Nobuhiro Shimizu for the KOTO collaboration
12Outline
 Introduction⚫ 𝐾𝐿 → 𝜋 0 𝜈𝜈ҧ CsI calorimeter upgrade
to reduce neutron background Charged kaon background Search for 𝑲𝑳𝟎→ 𝝅 𝜸 decay

3KOTOexperimentIntroduction

𝜋0+𝐾𝐿NP⚫ Search for New Physics via measurement of𝐾𝐿 → 𝜋 0 𝜈𝜈ҧ decay⚫

Very rare and theoretically clean decay:ℬSM 𝐾𝐿 → 𝜋 0 𝜈𝜈ҧ = (3.0 ± 0.3) × 10−11

JHEP 11 033 (2015)⚫ KOTO experiment

Online collaboration meeting in July4Experimental principle
Goldtarget

Veto counter
𝑲𝑳𝜸

𝜋0𝐸1𝐸2𝜸𝒛30 GeV
proton beam

CsI calorimeter𝜈 Signature of 𝐾𝐿 → 𝜋 0 𝜈𝜈ҧ decay⚫ Measure energy and hit position oftwo 𝛾’s by the CsI calorimeter⚫ Neutrinos are not measured→ Require no signal in the hermetic veto counters

𝜈ҧ𝟎

𝑷𝝅𝒕

Define signal region
𝟎In 𝒁𝒗𝒕𝒙 - 𝑷𝝅𝒕 plane

Signature of signal → 2𝛾 + 𝑛𝑜𝑡ℎ𝑖𝑛𝑔⚫ Calculate decay z-position assuming
2 = 2𝐸 𝐸 1 − cos𝜃𝑀𝜋20 = 𝑀𝛾𝛾
1 2𝛾𝛾𝒛𝒗𝒕𝒙

6CsI calorimeterupgradeNeutron backgroundneutronTo achieve SM sensitivity,
we need to suppress
neutrons by a factor of ten

7

Idea of the CsI calorimeter upgrade
upstreamCsI crystal

Previous

PMT

Attach MPPCs
6×6 𝐦𝐦𝟐

S13360-6050CS(HPK)

𝑿𝟎 ∼ 𝟐 cm𝜸

PMT𝐧𝐞𝐮𝐭𝐫𝐨𝐧MPPCs

interaction length 〜 40 cm

PMT

Measure the depth with the time difference 𝚫𝑻 ≡ 𝑻𝑴𝑷𝑷𝑪 − 𝑻𝑷𝑴𝑻

→ Small 𝚫𝑻 implies 𝜸8Bias circuit

Crystals MPPC readout# of MPPCs: 4080 (>#PMT=#CsI)
To reduce # of channels..
4 MPPCs are connectedLarge

 Bias connection

readout

2.5cm. . r . r 2r
. . .
. . .
2rrr
.. . .

5cm“Hybrid”

Small
5cm

10cm

“Hybrid” bias connection• adopted by MEG II upgrade• AC line: series, to read out signals• DC line: parallel, to apply bias voltage9Segmentation of readout
Hybrid x 4mixer(summing amp)

+HV
𝑽−𝒐𝒖𝒕𝑽+𝒐𝒖𝒕

4080 MPPCs
Integrate 4 MPPCs at bias (hybrid)1020
Sum 4 hybrids at amplifier

256 channels

16 crystals are finallysummed as 1 ch10

MPPC installation (2018 autumn)

QuartzplateBoardFrames to support
gluing jigs

1112
MPPC installation (2018 autumn)
Time lapse movie Glue MPPCs on two rows in a day Spent 45 days to finish all Installation finished as scheduled#MPPC4080

Progress0

1st Oct.

1020

30

40 days

1st Nov.

15th Nov.

𝚫𝑻 distribution of the control samples 13𝜸

neutronmax𝚫𝑻 ≡ 𝐦𝐚𝐱 𝚫𝑻𝟏 , 𝚫𝑻𝟐✓ Use the larger 𝚫𝑻 out of two clusters (max𝚫𝑻)✓ 𝐾𝐿 → 3𝜋 0 MC well reproduces the distribution of data

Retaining 90% of 𝜸 from 𝑲𝑳 → 𝝅𝟎 𝝂ഥ𝝂 decay,
neutron contribution can be suppressed down to 1/60 !

14

Analysis of
2016-2018 data andcharged kaon
background15

Post-unblind studies of 2016-2018 analysis
Data taken during
2016-2018
Presentedat Kaon2019

Signalregion

blind region

Expected # of BGs in the signal region

U: Updated from Kaon2019
N: New

 Adopted blind analysis technique and opened the blind region (2019).
1◆ SES == 7.1 × 10−10 or 0.04 SM events expected.
𝑁𝐾𝐿 𝜖𝑠𝑖𝑔

 We found four candidate events in the signal region and carefully checked ouranalysis.
◆ New concern: 𝐾 + background was not negligible, but uncertain remained.

Charged kaon?
KL𝑲𝑳

Sweeping
magnet 2ndcollimator 𝐾𝐿 interacts with the inner wall ofcollimator and produces 𝐾 ± Geant3-based beamline simulation
predicts 𝐾 ± /𝐾𝐿 ∼ 1.6 × 10−6 at theentrance of the decay volume
 𝐾 ± → 𝜋 0 𝑒 ± 𝜈 (BR=5%) can generate
a 𝜋 0 with large 𝑃𝑡# of BG from 𝐾 ± decays= (0.33 ±0.09) × uncertainty of simulation

16

Data collection to measure 𝐾 ± flux
We are finalizing
the analysis

PRL 122021802

Analysisongoing

PTEP 2017021C012020
May-June run 2020 May-June run→ Measure 𝐾 ± flux with 𝐾 ± → 𝜋 + 𝜋 0 decay✓ Develop a new trigger scheme✓ Install a prototype charged veto counter in the upstream (UCV)✓ Study selection criteria to purify 𝐾 ± → 𝜋 + 𝜋 0 events

We have successfully collected the data17

Measurement of 𝐾 ± → 𝜋 ± 𝜋 0 decay
cluster image

𝜸𝑲±𝝅𝟎𝜸𝒛𝝅±CsIcalorimeter Measure 𝐾 ± → 𝜋 ± 𝜋 0 decay (BR=20%)
 Trigger three cluster events in the calorimeter Reconstruction◆ For two neutral hits, impose 𝑀𝛾𝛾 = 𝑀𝜋0 → define 𝑲± decay vertex◆ For a charged hit, from the hit position and assumption of Pt balance of 𝜋 ± and 𝜋 0→ calculate the magnitude of the momentum→ reconstruct four vectors of all the particles◆ Calculate 𝑀𝜋𝜋018

Event selection of 𝐾 ± → 𝜋 ± 𝜋 0 decay

Rec. invariant massof 𝑲+ , 𝑴𝝅𝝅𝟎 (MeV)

MC simulation
Black 𝑲+ → 𝝅+ 𝝅𝟎 MC
Green 𝑲𝑳 → 𝝅+ 𝝅− 𝝅𝟎 MC

Selection criteria◆ determined by the MC study◆ Sufficiently large acceptance𝜖~4 × 10−4
Purity of 𝐾 + → 𝜋 + 𝜋 0 events◆ >> 90% by MC studyRec. vertex position 𝒁𝝅𝟎 (mm)Signal region

Backgrounds◆ 𝑲𝑳 → 𝝅+ 𝝅− 𝝅𝟎 (BR=13%)
populates in low 𝑀𝜋𝜋0 region

19

Measured 𝐾 ± flux
Projected mass distribution
Rec. 𝑴𝝅𝝅𝟎 (MeV)

Data
𝑲+ → 𝝅+ 𝝅𝟎 *𝑲+ → 𝝅𝟎 ℓ𝝂 *Data in 2020 run
Preliminary

Preliminary+20− 𝟎𝑲𝑳 → 𝝅 𝝅 𝝅𝑲𝑳 → 𝝅+ 𝒆− 𝜸𝝂Distribution of eventsin the signal region
Rec. momentum of 𝑲±

Preliminary The distribution of selected events are well reproduced by MC simulation of𝐾 ± decays.
 𝐾 ± flux ratio:ℛ𝐾± = 𝐹𝐾± /𝐹𝐾𝐿Comparison between simulation
𝑚𝑒𝑎𝑠.
𝑀𝐶→ ℛ𝐾/ℛ𝐾±± = 3.0 ± 0.1

Measured 𝐾 ± flux is
3 times larger than MC.
* 𝑲+ distribution is scaled by best fit

2016-2018 analysis BG table (updated, preliminary)Preliminary
NewNewNew# from 𝐾 ± decays= (0.33±0.08) × uncertainties of simulation
Prediction by
MC simulation

Uncertainty offlux → x 3.0

 BG table was updated based on the result of the 𝐾 ± flux.
 Preliminary total BG estimation → 1.1 ± 0.3→ The BG level is not negligible21

2016-2018 analysis BG table (updated, preliminary)PreliminaryNewNew

NewAfter wrapping upour# from 𝐾 decays= (0.33±0.08)we× uncertainties
of simulation
post-unblinded study,plantoPrediction bysimulation
submit a paper inMCthis
autumn.
±Uncertainty offlux → x 3.0

 BG table was updated based on the result of the 𝐾 ± flux.
 Preliminary total BG estimation → 1.1 ± 0.3→ The BG level is not negligible

22

23

Search for
𝟎𝑲𝑳 → 𝝅 𝜸 decay𝑲𝑳 → 𝝅𝟎 𝜸 decay?

24

 No measurements so far✔ 𝑠 → 𝑑𝛾 transition, forbidden by FCNC

spin𝝅𝑲𝑳𝜸⚫ Violates an angular conservation••

the spin of 𝜸 must be “compensated” by an orbital angularmomentum, but back-to-back configuration cannot produce 𝐿𝑧
IF v > “speed of light”, it’s allowed. (see PRD 59 116008)

Oppositely to say,good test of the Lorentz invariancein the realm of short distances

Reconstruction of 𝑲𝑳𝟎→𝝅 𝜸25clusters

𝑲𝑳 beam𝝅𝟎𝒛𝜸

1. Find events which have exactly 3 clusters
𝟎2. Reconstruct a 𝝅𝟎 : 𝑚𝜋2 0 = 2𝐸1 𝐸2 1 − cos𝜃𝛾𝛾 → 𝒛𝝅𝒗𝒕𝒙
3.

2Reconstruct a 𝑲𝑳 : 𝑚𝐾=𝐿𝑝𝛾1 + 𝑝𝛾2 + 𝑝𝛾32

Two types of vertex position, which should be close.
4.

𝑲𝑳→ 𝒛𝒗𝒕𝒙

𝑲𝑳𝝅𝟎Define 𝚫𝒛𝒗𝒕𝒙 = 𝒛𝒗𝒕𝒙 − 𝒛𝒗𝒕𝒙
to suppress various BGs

Event selection of 𝑲𝑳𝟎→𝝅 𝜸

Analyzed data collected between 2016-2018 runs𝚫𝒛𝒗𝒕𝒙
𝐃𝐚𝐭𝐚
𝟑𝝅𝟎𝟐𝝅𝟎𝝅𝟎 𝜸(scale is arbitrary)

𝜸𝑬𝒎𝒊𝒏
𝐃𝐚𝐭𝐚
𝟑𝝅𝟎𝟐𝝅𝟎𝝅𝟎 𝜸(scale is arbitrary)

The dominant background: 𝑲𝑳 → 𝟐𝝅𝟎𝜸
𝚫𝒛𝒗𝒕𝒙 and 𝑬𝒎𝒊𝒏 cuts suppress the contribution

26𝑍𝜋0 -𝑀𝜋0 𝛾 correlation plot𝑲𝑳 → 𝟐𝝅𝟎𝑲𝑳 → 𝟑𝝅𝟎𝑲𝑳 → 𝝅𝟎 𝜸 (MC)Backgrounds (MC)𝝅𝟎The signal region (SR) is defined in (𝒛𝒗𝒕𝒙 , 𝑴𝝅𝟎 𝜸 ) plane andmasked the signal region before opening the box.

27

Open signal box

𝟎. 𝟑𝟒 ± 𝟎. 𝟏𝟎

Single event sensitivity of signal:
𝟏= 𝟕. 𝟏 ± 𝟎. 𝟑𝐬𝐭𝐚𝐭 ± 𝟏. 𝟔𝒔𝒚𝒔𝒕 × 𝟏𝟎−𝟖𝟎
𝑵𝑲𝑳 𝝐 𝑲𝑳 →𝝅 𝜸28

Open signal boxℬ 𝐾𝐿 → 𝜋 0 𝛾 < 1.7 × 10−7 at 90% C.L.
Paper is now under the referee process29Summary30

 KOTO searches for New Physics
via very rare (𝐵𝑅 = 3 × 10−11 ) decay 𝐾𝐿 → 𝜋 0 𝜈 𝜈ҧ

 To reduce neutron background,
we attached >4000 MPPCs on the front surface of CsI
crystal and succeeded to reduce the background by 60.
 By opening the signal region of 2016-2018 data, wefound 4 candidate events.
⚫ After the post-unblind study, we found charged kaon incident
had non-negligible contribution.
⚫ Using 2020 data, we measured 𝐾 + flux to be 3 times
larger than simulation

 Forbidden decay 𝑲𝑳 → 𝝅𝟎 𝜸 was searched for using 2016-2018 data.
We found no candidate events in the signal region andset the first upper limit 𝐵𝑅 < 1.7 × 10−7 at 90% C.L

That’s allThankyou!

31

To veto 𝐾 ± (upstream charged veto, UCV)

UCVUCV A new detector installed before 2020 May-June run Purpose⚫ To confirm the existence of 𝐾 ±⚫ To veto 𝑲±

 Basic design⚫ 1-mm-thick plastic scintillation plate(composed of 1mm□ plastic scintillation fibers)⚫ Use 6mm□ MPPCs (Si-photo sensor) to detect
scintillation photons

32Veto functionality of the prototype UCV33
Distribution in the signal region

Rec. 𝑴𝝅𝝅𝟎 (MeV)Energy deposit in UCV(only on-time hit)
Hit decision
Data
𝑲+ → 𝝅+ 𝝅𝟎
𝑲+ → 𝝅𝟎 ℓ𝝂
𝑲𝑳 → 𝝅+ 𝒆− 𝜸𝝂Event with UCV hit
Event without hit

 The selected events have on-time and MIP likeenergy deposit in UCV.
 30% inefficiency exists but can be explained by① limited coverage of 𝑲+ halo
② limited sensitive region of scintillation fiber③ noise fluctuation

MeV

𝐾 ± production at UCV
w/o UCV

Preliminary

w/ UCV

Preliminary

34ℛ𝐾± = 𝐹𝐾± /𝐹𝐾𝐿 was measured withand without inserting UCV in beam.
◆ To confirm whether UCVproduces 𝐾 ± .
𝑀𝑒𝑎𝑠.
MCℛ𝐾/ℛ𝐾++ = 3.0 ± 0.1 w/UCV= 3.0 ± 0.3 w/o UCV✓ We did not observe 𝑅𝐾± difference
between w/UCV and w/o UCV.

Rec. 𝑴𝝅𝝅𝟎 (MeV)

Preliminary

Rec. 𝑴𝝅𝝅𝟎 (MeV)

Preliminary→ Level of UCV-induced 𝑲± was not
significant compared to beamline
originated 𝑲± .
Data
𝑲+ → 𝝅+ 𝝅𝟎
𝑲+ → 𝝅𝟎 ℓ𝝂

𝑲𝑳 → 𝝅+ 𝝅− 𝝅𝟎
𝑲𝑳 → 𝝅+ 𝒆− 𝜸𝝂Charged kaon BGKL2ndcollimator

Sweepingmagnet• 𝑲𝑳 interacts inner wall of the 2nd collimator and produces 𝑲±• Currently, the dominant contribution comes from 𝑲± decays
𝑲±𝒛

We need to block the incident of 𝑲± in front of KOTO detector

35Summary of inefficiency

36Source

Estimated inef.
of prototype(%)

Possible goalwith fiber
option (%)

Possible goalwith plate
option (%)

Limited coverage

800

Gap and insensitive region

70.30

Noise and photo-statistics

11(3.5+2)%>5%

Low light yield of black fiber

2――

Inactive interaction

0.5%

0.5%

Not yet considered

Masking

Included in noiseNegligible

Need to consider

Total286%For the estimation ofeach category, please see backup

Prototype upstream charged veto (UCV)
130 mm

S133606050CS

UCVRead 7 fibers
by one MPPC6㎜6㎜

84 fibers= 7 fibers×12 channels

 A new detector installed Dec. 2019.
 Purpose⚫ To confirm the existence of 𝐾 +⚫ To veto 𝑲+

 Basic design⚫ 1-mm-thick plastic scintillation plate(composed of 1mm□ plastic scintillation fibers)⚫ Use 6mm□ MPPCs to detect scintillation photons⚫ A unique front-end readout to accept
high hit rates and the severe irradiation environment†

37Study of the tilting angle
Simulate interaction of the charged
particles, generated based on
Nomura-san’s beamline simulation.
The particles were required to enter
NCC hole.

0° 45°

Energy depositvs tilt angle

38

Configuration and hit definition
0.5-mm-thick fibers
Gap due to cladding is 4%.
Plate is rotated around y axis by θ(0, 2, 5, …,45 degrees).
Hits defined by Edep > 40 KeV/cosθ
Inefficiency
vs tilt angle

Intrinsic
inefficiency

Edep (KeV)• (As is expected) the mean energy deposit increases as tilting angle increases.
• If plate is rotated by ~20 degrees, the inefficiency is reduced from 3.5% → 0.3%

→ Design target is > 20 degrees

39Mechanics (UCV design)Core UCV
0.5-mm-thick fiber
3mm MPPC×20

Halo catcher (two modules)
1-mm-thick fiber
6mm MPPC×8

14 fibers are read by a MPPC
→ 7mm x 20 = 140 mm

7 fibers are read by a MPPC
→ 7mm x 14 = 28 mm(per module)

250 mm

250 mm

140 mm20 MPPCs are
separately amplified

28 mm

2 MPPCs are summed atbias circuit. In total, 4channels are separatelyamplified.

Core region is read by thin 0.5 mm fiber with high granularity.
Halo region is read by thick 1 mm fiber with modest granularity.
In total, 24 (= 20 from core and 4 from halo) signals come out.
In the future upgrade, we may separately read 24 channels, but in the next beam time, we will reducethem by a factor of two at summing amplifiers placed outside of the vacuum chamber.

The important key of this two module scheme is that
we can adjust the tilting angle of core UCV (see next page)Mechanics (UCV entire view)
θ=45°case → large light yieldand maximum from theconstraint of z widthyz

40θ=20°case → the best in terms ofinefficiency and material budget

θ=20°

θ=45°Beam
99 mm

High rateregion

131 mm

99 mm~70 mmθ is important parameter, which determines material budget, inefficiency and light yield.
θ is not completely fixed yet and will be finally determined based the result of beam test.

CsI calorimeter of the KOTO detector
 CsI crystal⚫ undoped CsI (𝜆~300 nm)#crystal = 2716
2240 small (25×25 mm2 )476 large (50×50 mm2 )

411m

27𝑿𝟎

Performance tests (𝛾/𝑛 separation)
 Beam test at RCNP-Osaka cyclotron⚫ 𝛾/𝑛 beam from Li target
𝜸: continuous beam
up to 392 MeV

collimator

CsI

p392MeVLi target

𝒏: 392 MeV
MPPC𝜸/nupstream

PMT
downstream

Distribution
𝚫𝒕 ≡ 𝑻𝑴𝑷𝑷𝑪 − 𝑻𝑷𝑴𝑻

Retain 90% of 𝛾 while
suppressing 𝒏 to 34%42

Rejection of neutron BG

43

 Halo-neutron BG* Prog. Theor. Exp. Phys. (2017) 021C01

Result of 4 days run: ℬ 𝐾𝐿 → 𝜋 0 𝜈 𝜈ҧ < 5.1 × 10−8 (90% C.L.) *The largest contribution from BG⚫ We need 3 more magnitudes of suppression
two-dimensional shower envelope → 1/10 ✓done
Pulse shape likelihood→ 1/10 ✓done

measure shower development (in z)
in the calorimeter → O(1/10)

Quality assurance of MPPCs

44MPPCs

Individual test

Quartz gluing
Solderingtemperature test

I/V inspection
LED test

SummedMPPCs

Process 80 MPPCs/day

I/V curves#MPPC～500

Inspect all of MPPCs (#~4000) before installation→ Start gluing on CsI in this summer

Fabrication of MPPCs115
Put MPPCs into oven andwait 24 h (keeping 45 deg)

Insert MPPCon jig

2

3

Drop glue

Drop glue onquartz

45

26dispense
epoxy glue(araldite 2011)37

apply weight67

4

wait for curekeeping thequartz floated48
wait 24 h for cure

46

Setup

MPPC
sampleBe
10 cm

𝐃+D+ ~10 MeVDeuteronbeam

Neutron energy spectrum

OursampleLED attached with fiber
is under this sheet

Effect of the irradiation on the timing 47
resolution
timing resolution (ns)From the smoothed waveform, peak and
CFT were calculated.
The fitted sigma divided by 2 was used
to define the timing resolution of MPPC.
Neutron flux
(1MeV eq n/cm2)

For the innermost
layer of MPPC,
we are aroundhere.

Future study

Small MPPC w/ series resistor
Small MPPC w/ series resistor
Small MPPC
Small MPPC

Current of theinnermost layer is~25uA now

Neutron flux (1MeV eq n/cm2)

The timing resolution may be recovered by increasing the bias voltage.
This degradation may come from the decrease of gain.

Linearity of the dark current as a 48
function of dosetime

Smallw/ series
resistor

X axis is converted to the accumulated
neutron flux.

Current (uA)

Current (uA)

Neutron flux
(1MeV eq n/cm2)

Beam current is simultaneously
monitored and we can convert theabsolute neutron flux based on theprevious experiment.

timeNeutron flux (1MeV eq n/cm2)

Systematic uncertainty of S.E.Sℬ 𝐾𝐿 → 𝜋 0 𝛾 =𝑁𝑜𝑏𝑠(𝑆𝑅)𝑜𝑏𝑠(𝐶𝑅2)
𝑁 0
2𝜋49

𝜖 𝐶𝑅200⋅ 2𝜋𝑆𝑅 ⋅ ℬ 𝐾𝐿 → 2𝜋𝜖𝜋0 𝛾Source𝝈𝝐 /𝝐 (in %)Commentℬ 𝐾𝐿 → 2𝜋 00.6From PDG

Geometry

1.5

Estimated by varying beam E (+1%)
and position (x,y=1mm) for signal

Veto cuts

17By comparing data and MC in CR2 with 𝚫𝒁𝒗𝒕𝒙 cut

Online veto

6.4

From the detector bits in the minimum bias data

Kinematic cuts

122𝜋2𝜋100% error of |1-𝜖𝐷𝑎𝑡𝑎/𝜖𝑀𝐶|Clustering

1.0

Compare five and six cluster events

CSD cuts

1.5

Use five cluster events

Reconstruction

0.3

𝑜𝑏𝑠 6𝑐𝑙𝑠
𝑜𝑏𝑠 6𝑐𝑙𝑠
By comparing 𝑁𝑟𝑒𝑐
3𝜋0 and 𝑁𝑟𝑒𝑐 2𝜋0Trigger

1.8

Difference of CDT efficiency from unity

Statistics

4.4

Statistics of normalization

Total

2200

Syst. uncertainty due to 𝜖 𝑣𝑒𝑡𝑜
𝜖 𝑣𝑒𝑡𝑜 =𝑁𝑎𝑙𝑙𝑁𝑎𝑙𝑙 𝑤𝑜/ 𝑖𝑡ℎ 𝑣𝑒𝑡𝑜is compared𝑀𝐶𝐷𝑇
between data and MC: 𝑅 = 𝜖𝐶𝑅2/𝜖𝐶𝑅2

𝜎𝜖𝑣𝑒𝑡𝑜=𝜖 𝑣𝑒𝑡𝑜෍𝑅𝑖 − 1 2 = 17%𝑖:𝑎𝑙𝑙 𝑑𝑒𝑡𝑒𝑐𝑡𝑜𝑟𝑠

𝐃𝐚𝐭𝐚MC

Inefficiency of vetodetector is wellunderstood by MC.

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