KL→π0νν at KOTO

Koji SHIOMI for the KOTO collaboration(Osaka University)

CKM 2014, Vienna, Austria2014/09/11

1

14年9月11日木曜日

KL→π0νν decay• Brakes CP symmetry directly• Highly suppressed in the SM• Small theoretical uncertainty (2～3%)• Sensitive to New Physics.

2

Standard Model predictionBR(KL→π0νν)=2.4×10-11

Experimental limitBR (KL→π0νν) <2.6×10-8@90% C.L. by KEK E391a

14年9月11日木曜日

KOTO experiment• Study of KL→π0νν@J-PARC 30GeV Main Ring.• Successor to the E391a experiment• Goal is to observe few SM events.J-PARC Laboratory

•Main ring (30 GeV protons)J-PARC Laboratory

KOTO = KL0 at TOkai

3

������� �

������/3�$��� �

������������ �������� �����������������������������������������

����������������� ��!����"���!#�����$�#���������%�"����

����G��������;������������G������ �

��;��H�A'$�;�H���3�

Primary proton !30 GeV/c Target (Ni, Pt, Au)

KOTO area

Hadron hall

Linac

3 GeV !Synchrotron

30 GeV !Main Ring Extraction!

in 16 degree

Tokai, Ibaraki, Japan

J-PARC

30 GeVMain Ring

Hadron hall

3 GeVSynchrotron

LinacJ-PARC

3

14年9月11日木曜日

Experimental principle

proton

targetNeutral beam line

4

θγγ

νν

Rec. Z

Rec

. Pt “2γ+Nothing+Pt”

Assuming 2γ from π0,Calculate z vertex.

Calculate π0 transverse momentum

M2(π0)=E1E2(1-cosθ)

E1E2

KL→π0νν decay

Signal Box

π0KL

14年9月11日木曜日

KOTO detector FB NCC MB CV

CsI calorimeter

CC03OEV

CC04 CC05 CC06 BHCV BHPV

LCVBCVHINEMOS

Saturday, April 20, 2013

γγ

KL

ν ν

CsI  calorimeter  +  Herme1c  veto  system

OK T�

�s

d

KAON13 @ Univ. of Michigan Ann Arbor

CsI calorimeter• The main detector of KOTO experiment• 2m diameter 500mm length full active pure CsI• Fine granularity • 25%25mm : 2240 crystals + 50%50mm : 476 crystals•

10

2014/7/30 5

KOTO detector“Two γ from π0 and nothing else” CsI calorimeter and Hermetic veto

π0 with high PT discriminate K

L→2γ

Other K decays have charged or γ more than two

“Veto almost everything” experiment

We installed all detectors by May 2013.

514年9月11日木曜日

KOTO first physics run• The first physics run: May 17-23, 2013–Beam power:24kW( 10% of design intensity)–Unfortunately, the data taking was terminated after100 hours due to an accident in J-PARC Hadron hall.

• We have been analyzing the KL→π0νν mode withoutlooking inside the signal box.

• Today, “inside the signal box” is presented for the first time.

6

14年9月11日木曜日

]2#

of e

vent

s/2.0

[MeV

/c

1

10

210

310

410

kmass

Data

MC0/3AK

]2 mass [MeV/cLRec. K400 450 500 550 600

400 450 500 550 600

Dat

a/M

C

0.51

1.52

# of

eve

nts/1

00.0

[mm

]

1

10

210

310

410

510

hrecz

Data

MC0/3AK

pos [mm]LRec. K2000 4000 6000

2000 4000 6000

Dat

a/M

C

0.51

1.52

CsI calorimeter performance• 6 cluster samples • BR(KL→3π0)=20%• High statistics

7

ZvtxM6γ

γ

14年9月11日木曜日

]2#

of e

vent

s/10.

0 [M

eV/c

1

10

210

310

410

510

kmass

Data MC0/0/AK

MC0/3AK

]2 mass [MeV/cLRec. K200 300 400 500 600 700

200 300 400 500 600 700

Dat

a/M

C

0.51

1.52

]2#

of e

vent

s/10.

0 [M

eV/c

1

10

210

310

410

510

kmass

Data MC0/0/AK

MC0/3AK

]2 mass [MeV/cLRec. K200 300 400 500 600 700

200 300 400 500 600 700

Dat

a/M

C

0.51

1.52

Veto detector performance• 4 cluster samples

w/o veto cut

8

M4γ M4γ

γγBR(KL→3π0)

=20%

w/ veto cut

BR(KL→2π0)=8.6×10-4

14年9月11日木曜日

Rec. z [mm]2000 3000 4000 5000 6000

[MeV

/c]

tP

050

100150200250300350400450500

00.511.522.533.54

FB NCC MB CVCsI calorimeter

CC03OEV

CC04 CC05 CC06 BHCV BHPV

LCVBCVHINEMOS

Saturday, April 20, 2013

Events outside the signal box

9

3

9

0

1

0

2.0±1.1

7.64±0.53

0.72±0.40

1.70±0.42

0.002±0.002

ObservedExpected

2.16±1.06We understand the events outside the signal box well.

108106±12

signal region

Pt vs Z distribution after applying loose selection cuts

14年9月11日木曜日

Rec. z [mm]2000 3000 4000 5000 6000

[MeV

/c]

tP

050

100150200250300350400450500

00.511.522.533.54

Upstream eventsFB NCC MB CV

CsI calorimeter

CC03OEV

CC04 CC05 CC06 BHCV BHPV

LCVBCVHINEMOS

Saturday, April 20, 2013

Halo neutron π0 γγ

nA→π0X

10

• The number of upstream events expected in the signal box

• less than 0.1

14年9月11日木曜日

Rec. z [mm]2000 3000 4000 5000 6000

[MeV

/c]

tP

050

100150200250300350400450500

00.511.522.533.54

Low Pt eventsFB NCC MB CV

CsI calorimeter

CC03OEV

CC04 CC05 CC06 BHCV BHPV

LCVBCVHINEMOS

Saturday, April 20, 2013

γ

γπ0 π+

π-

KL→π+π-π0

KL

CC05 CC06

π-π+

Vacuumpipe

-π+/π- interacted in the vacuum pipe.-Cannot enter the signal box due to the kinematical limit. (133MeV/c)

1114年9月11日木曜日

Rec. z [mm]2000 3000 4000 5000 6000

[MeV

/c]

tP

050

100150200250300350400450500

00.511.522.533.54

High Pt and downstream events

• Hadron interaction events• Events spread into thethe signal box

• A possible mechanism• Single halo neutron make two clusters.

n

12

1.9±1.1

Halo neutron

14年9月11日木曜日

Special data to study hadron interaction events

nAl target

13

-Al target was inserted into the beam

-Core neutrons scattered at the Al target

-Collect many hadron interaction events

nCore neutron

14年9月11日木曜日

Neural net cuts to reduce hadron interaction events

R [mm]0 200 400 600 800 10000

50

100

150

200

250

300h1

[mm]1aRMS0 50 100 1500

500

1000

1500

2000

2500

3000h1

NN Output-0.5 0 0.5 1 1.50

100200300400500600700800

h1

NN Output-0.5 0 0.5 1 1.50

1000

2000

3000

4000

5000

h1

Black:KL→π0νν MCRed: Al target data

Accept

Accept

Hit position

Energy spread in a cluster

14

Kinematical neural net(5 variables)

Shape neural net(12 variables)

#BG reducedto 10%

14年9月11日木曜日

Rec. z [mm]2000 3000 4000 5000 6000

[MeV

/c]

tP0

50100150200250300350400450500

00.511.522.533.54

KL→π0νν plot after imposing allselection cuts

15

0.17±0.12

7.24±0.52

0.033±0.027

0.026±0.025

0.0018±0.0016

1

9

0

0

0

8787

BG source #BGHadron interaction events 0.18±0.15Kaon decay events 0.11±0.04Upstream events 0.06±0.06

Sum 0.36± 0.16

Summary of #BG inside the signal box

S.E.S. of the first physics run1.29×10-8 (preliminary)

KOTO achieved nearly the same sensitivity as KEK E391a experiment with only 100 hours of data

S.E.S. of KEK E391a:1.11×10-8

0.36± 0.16

ObservedExpected

Apply all selection cuts (Loose + Neural net cuts)

Preliminary Prelim

inary

14年9月11日木曜日

Opened the signal box

16 Rec. z [mm]2000 3000 4000 5000 6000

[MeV

/c]

tP

050

100150200250300350400450500

00.511.522.533.54

14年9月11日木曜日

Opened the signal box

17

• One event was observed inside the signal box.

Rec. z [mm]2000 3000 4000 5000 6000

[MeV

/c]

tP

050

100150200250300350400450500

00.511.522.533.54

14年9月11日木曜日

Opened the signal box

18 Rec. z [mm]2000 3000 4000 5000 6000

[MeV

/c]

tP

050

100150200250300350400450500

00.511.522.533.54

0.36± 0.161

• One event was observed inside the signal box.• Nobs is consistent with Nexp statistically

Observed Expected

Preliminary

00.14±0.11

14年9月11日木曜日

Rec. z [mm]2000 3000 4000 5000 6000

[MeV

/c]

tP0

50100150200250300350400450500

00.511.522.533.54

Check the validity of the BG estimation

• Applying loose selection cuts• Nobs are consistent with Nexp inside the signal box.

3

2

9

0

1

0

2.0±1.1

7.63±0.54

0.72±0.40

1.70±0.42

0.002±0.002

19

Observed Expected

108106±12

2.11±1.06

12.37±1.24

→The number of BG events inside the signal box is estimated properly.

14年9月11日木曜日

Prospect for the next run

• We expect to start the next run in early 2015.• We want to reach the sensitivity 20 times higher than this first run.

• We are preparing new detectors to reduce BG events• We are improving analysis methods to reject hadron interaction events

• We are planning special run to increase the statistics of hadron interaction events

2014年9月11日木曜日

Summary• The KOTO experiment took the first physics data in May 2013.

• Even the run was only 100h long, we achieved nearly the same sensitivity as KEK E391a.

• After opening the signal box, we observed one event• This is consistent with the BG estimation statistically.

• Preparing to suppress the background and to improve the experimental sensitivity for the next run.

2114年9月11日木曜日

• Back up

2214年9月11日木曜日

The event inside the signal box

ener

gy d

epos

it [M

eV]

20

40

60

80

100

120

140

160

180

x position [mm]-1000 -800 -600 -400 -200 0 200 400 600 800 1000

y po

sitio

n [m

m]

-1000

-800

-600

-400

-200

0

200

400

600

800

1000

event display - CsI energy (run00016887_node014_file6_g2ana.root node14 DstEntryID : 10974)

recconstructed valuesrec vertex : z=4437.6mm, 211.0ns

= 170.9 MeV/cT

p0/

vertex time difference = 0.0 ns°projection angle = 81.2

(534.3, -57.4) mm, 519.4MeV = 3.5, 13, 35.0mm ; 02r217.0ns,

(11.5, -244.3) mm, 357.4MeV = 3.5, 9, 16.3mm ; 02r216.8ns,

CsI veto info. (on timing hits)2.6MeV, 331.5mm, 5.8ns, 4195.8MeV, 139.5mm, 0.5ns, 4582.1MeV, 41.3mm, 0.1ns, 9612.3MeV, 45.0mm, 5.8ns, 1082

gnana outputfile

pos.arec.

not in cluster

event display - CsI energy (run00016887_node014_file6_g2ana.root node14 DstEntryID : 10974)

2314年9月11日木曜日

S.E.S.• # of decay KL’s in the signal region(NKL)obtained from KL→2π0 samples

• 7.61×109

• Geometrical Acceptance(Ageo)

• 0.285

• Cut efficiency(Acut)

• 0.0359

• S.E.S.

• 1/(NKL× Ageo×Acut)=1.29×10-8

24

Rec. z [mm]2000 3000 4000 5000 6000

[MeV

/c]

tP

050

100150200250300350400450500

020406080100120140160

KL→π0νν MC

S.E.S. of KEK E391a:1.11×10-8

KOTO achieved nearly the same sensitivity as the KEK E391a experiment.

14年9月11日木曜日

KL flux

• # of decay KL’s in the signal box is calculated from 4 cluster samples

• N(KL)= 7.61×109

• Check consistency by using KL→3π0, 2γ modes.

• the result agree within ±4%.

]2#

of e

vent

s/10.

0 [M

eV/c

1

10

210

310

410

510

kmass

Data0/0/AK

0/3AKOther K decays

]2 mass [MeV/cLRec. K200 300 400 500 600 700

200 300 400 500 600 700

Dat

a/M

C

0.51

1.52

# of

eve

nts/1

00.0

[MeV

/c]

1

10

210

hmome

Data0/0/AK

0/3AKOther K decays

momentum [MeV/c]LRec. K0 1000 2000 3000 4000 5000

0 1000 2000 3000 4000 5000

Dat

a/M

C

0.51

1.52

# of

eve

nts/2

00.0

[mm

]

1

10

210

310

hrecz

Data0/0/AK

0/3AKOther K decays

z pos. [mm]LRec. K2000 4000 6000

2000 4000 6000D

ata/

MC

0.51

1.52

14年9月11日木曜日

To reduce Hadron interaction events

FB NCC MB CVCsI calorimeter

CC03OEV

CC04 CC05 CC06 BHCV BHPV

LCVBCVHINEMOS

Saturday, April 20, 2013

-A part of core neutrons are scattered at the beam window. -> Replace the beam window with a thinner one.

Core neutron

beam windowReduce the number of neutrons hitting the CsI calorimeter

2614年9月11日木曜日

• Angle reconstruction • use cluster shape information as neural net inputs

Rec. z [mm]2000 3000 4000 5000 6000

[MeV

/c]

tP

050

100150200250300350400450500

00.511.522.533.54

Results in One Figure

Black dots are the Z vertex from neural net. For better display, Z set

to zero if the value < 0.

Red Stars show the original Rec. Z vertex.

Event ID

0 1 2 3 4 5 6 7

Z v

ert

ex

(cm

)

0

100

200

300

400

500

600

700

Yu-Chen Tung September 4, 2014 11 / 13

Results of Angle Reconstruction

Rec. angle resolution � = 3.2� with Gaussian fit at peak. I haven’t

calculated the resolution along di↵erent incident angles. Despite of

the non-Gaussian tail, now just set � = 3.5� for later calculation.

And, I haven’t studied the tail events, will try to improve the result of

those events if possible.

NNTheta - GenTheta (deg)

-50 -40 -30 -20 -10 0 10 20 30 40 500

50

100

150

200

250

NNTheta (deg)

0 10 20 30 40 50 60 70 80

GenT

heta

(deg)

0

10

20

30

40

50

60

70

80

Yu-Chen Tung September 4, 2014 3 / 13θnn-θtrue(degree)-50 50Loose cut condition

single γ MC

To reduce Hadron interaction events

0

700Rec. Z (mm)

Event ID0 7

300

500

Black:from θnnRed: from θrec27

14年9月11日木曜日

To reduce KL→π+π-π0 BG

-Replace the vacuum pipe with the thinner one-Install new charged detectors outside the beam pipe

FB NCC MB CVCsI calorimeter

CC03OEV

CC04 CC05 CC06 BHCV BHPV

LCVBCVHINEMOS

Saturday, April 20, 2013

γ

γπ0 π+

π-KL

CC05 CC06

π-

Vacuumpipe

π+

New charged detectors

To increase the detection efficiency for π+/π-

2814年9月11日木曜日

Detector upgrade for the next run

FB NCC MB CVCsI calorimeter

CC03OEV

CC04 CC05 CC06 BHCV BHPV

LCVBCVHINEMOS

Saturday, April 20, 2013

Replace the beam window witha thinner one

-Replace the vacuum pipe with a thinner one -Install charged veto counters outside the vacuum pipe

-increase the number of modules-Install new counters around the beam hole

-Install gas chambers

2914年9月11日木曜日