new results of kl π νν from j-parc koto...kl flux from kl→2π0 analysis n decay kl :...
TRANSCRIPT
Kota Nakagiri (Kyoto Univ.)on behalf of the KOTO collaboration
1
New results of KL→π0νν from J-PARC KOTO
OK Tν
νs
d
16th Conference on Flavor Physics & CP Violation
FPCP 201814 -18 July 2018, Hyderabad, India
KL→π0νν Decay
2
⌫⌫̄
s dd̄
W�
t
d̄
Z0V ⇤tdVts
＜崩壊ダイアグラムの１つ＞
V ⇤ts Vtd
• direct CPV• highly suppressed in the SM : Br = 3×10-11• theoretically clean : ~2%➡ good probe for new physics that breaks CP symmetry 0
@d0
s0
b0
1
A =
0
@Vud Vus Vub
Vcd Vcs Vcb
Vtd Vts Vtb
1
A
0
@dsb
1
A
λ ~ 0.2λ2λ3
K system ~λ5 << Bd system ~λ3
1
⚫ SM⚫ SM + NP
M.Tanimoto, K.Yamamoto, PTEP2016,12,123B02
a model w/ O(10TeV) SUSY
indi
rect
limit
1σ from exp.
direct limit : 2.6×10-8 (90% C.L.)indirect limit (Grossman-Nir bound) Br(KL) < 4.4 × Br(K+) = 1.5×10-9
10-8
10-9
10-10
10-11
NP ?
s→d FCNC
a model w/ heavy Z’ (50 TeV)
A.J.Buras, et al. JHEP11(2014)121
Br(K+→π+νν)Br(K+→π+νν) [10-11]Br(
KL→π0νν
) [10
-11 ]
Br(
KL→π0νν
)
KOTO Experiment @ J-PARC
3
Japan Proton Accelerator Research Complex = J-PARC
KL beam
30GeV Proton Beam
16°
KOTO
K0 at TOkai
Tokai village
Au target
Experimental Method
4
CsI Calorimeter ( CSI )
KL
γ
ν ν
R~1m
(π0) γ
KL ! ⇡0 ⌫⌫̄π0→2γ on CSI calorimeter
: Signal detection
5
KL ! ⇡0 ⌫⌫̄
KL
Veto counters
ν ν
(π0)γ
γ
Experimental Methodπ0→2γ on CSI calorimeter
R~1m
CsI Calorimeter ( CSI )
missing : hermetic veto around the decay volume
: Signal detection
6
KL
Veto counters
(2π0)
γ
γγ
γ
Experimental Method
R~1m
CsI Calorimeter ( CSI )
KL→2π0 BG
CSI
Veto
: Background rejection
KL ! ⇡0⇡0 ! ����
7
Decay Z
π0 P
t Signal boxν ν
KL 2γ
( π0 )
KLZ
θ
γ (E1)
γ (E2)
cos ✓ = 1�M2
⇡0
2E1E2
ν ν
Experimental Method: π0 reconstruction from 2γ
missing Pt by two neutrinos = finite π0 Pt➡ signal region on Pt - Z plane
CSI
Dec2012
Dec2013
Dec2014
Jan2016
Dec2016
Dec2017
Acc
umul
ated
P.O
.T.
01020304050607080
1810×
Bea
m P
ower
(kW
)
0
20
40
60
80
History of the KOTO data taking
8
2015 run2013 run
Hadron facility renovation
first physics run
2016-2018 run data taking continues
First physics run in 2013
9
One candidate event observed consistent with BG expectation→ Br(KL→π0νν) < 5.1 ×10-8 (90%C.L.)
Single Event Sensitivity = 1.28 ×10-8
cf. KEK E391a final
: SES =1.11 ×10−8best upper limit so far
w/ only 100H data
What we learned from the 2013 run
10
①
③
②
What we learned from the 2013 run
11
1st Hit
2nd Hit
neutron①
① hadron cluster BG
dominant BG in the 2013 run
halo-neutron : our ENEMY !
“pencil” beam KL, n, γ
Beam profile(X[mm]) @ CSI surface
core
halo
hit detector ➡ BG sourcego through beam hole
What we learned from the 2013 run
12π± disappear @vacuum pipe
1st Hit
2nd Hit
①
③
②
③ KL→π+π-π0
π±
2γ
② upstream π0 2γ
π± →missing
① hadron cluster BG
dominant BG in the 2013 run
neutron
KL
neutron
13π± disappear @vacuum pipe
KL ③ KL→π+π-π0
π±
2γ
② upstream π0 2γ
π± →missing
Upgrades for the 2015 run
• thinner vacuum window 125μm→12.5μm
• beam profile monitor for better beam alignment
neutron
halo-neutron
➡ Nhalo ~1/2
KL→π+π-π0 BG• low-mass beam pipe
SUS → Al •new scintillation counter
➡ N ~1/20π+π-π0 BG
source of Hadron cluster BG and Upstream π0 BG
Other detector upgrades
14
new in-beam charged veto
Beam
in-beam charged veto
• wire chamber• insensitive to neutron
additional photon counters
in-beam photon veto
※ original one was 3-mm-thick plastic scintillator
lead/aerogel Cherenkov counters# modules : 12 -> 16
new lead/acrylic Cherenkov counter
More photon detection efficiency
High rate capability: downstream region
CSI
the 2015 run analysis
15
•Single Event Sensitivity
•KL-decay background
•Neutron induced background
➡ unblind the signal region
]2 mass [MeV/cLRec. K300 400 500 600
]2#
of e
vent
s/4.0
0 [M
eV/c
1
10
210
310
kmass
Data0π3→LK 0π2→LKγ2→LK
kmass
]2 mass [MeV/cLRec. K250 300 350 400 450 500 550 600 650
250 300 350 400 450 500 550 600 650
Dat
a/M
C
0
1
2
3
ndf = 95 = 3642.292χ
p-value = 0.000/ndf = 38.342χ
Z [mm]0πRec. 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Pt [
MeV
/c]
0 πR
ec.
0
50
100
150
200
250
300
350
400
450
500
0
5
10
15
20
25
30
35
40
45
Single Event Sensitivity (SES)
16
KL flux from KL→2π0 analysis
N Decay KL : 1.46×1011
Single Event Sensitivity = 1(KL yield) × (Signal acceptance)
= 1.3 ×10-9
×10 better sensitivity than that of the 2013 run
Signal Acceptance from MC
Acceptance : 0.52%
Rec. KL mass —Data —KL→3π0 MC —KL→2π0 MC
(3000 < Z[mm] < 4700)
c.f. GN bound : <1.5×10−9
KL→π0νν MC
Z [mm]0πRec. 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Pt [
MeV
/c]
0 πR
ec.
0
50
100
150
200
250
300
350
400
450
500
0
1
2
3
4
5
6
Z [mm]0πRec. 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Pt [
MeV
/c]
0 πR
ec.
0
50
100
150
200
250
300
350
400
450
500
0
1
2
3
4
5
6
N = 0.05 ± 0.02
KL→π+π-π0 background
17KL→π+π-π0 BG
π±
- low-mass beam pipe (SUS→Al)- ×2 reduction
- new scintillation counter- ×10 reduction
CSI veto ONveto OFF
33 130.4±0.64 1.13±0.12
0.57±0.09 0.05±0.021/10
Observed Estimated
N = 0.02 ± 0.02
KL→2π0 background
18
CSI Additional photon counters - lead/aerogel Cherenkov counters - lead/acrylic Cherenkov counters
KL→2π0 BG
Z [mm]0πRec. 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Pt [
MeV
/c]
0 πR
ec.
0
50
100
150
200
250
300
350
400
450
500
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0.02
0.022
Z [mm]0πRec. 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Pt [
MeV
/c]
0 πR
ec.
0
50
100
150
200
250
300
350
400
450
500
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0.02
0.022
veto OFF veto ON
0.09±0.05 0.02±0.021/4
KL→2π0 MC KL→2π0 MC
Neutron induced background
19
halo neutron ① Hit CSI
② Hit upstream detector
③ Hit CV
3 types of backgroundCharged Veto
( CV )
Z [mm]0πRec. 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Pt [
MeV
/c]
0 πR
ec.
0
50
100
150
200
250
300
350
400
450
500
0
10
20
30
40
50
60
70
Hadron cluster background
20
1st Hit
2nd Hit
neutron n
Al plate (t=10mm) as a scattering source
Special run to take control sample
Z [mm]0πRec. 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Pt [
MeV
/c]
0 πR
ec.
0
50
100
150
200
250
300
350
400
450
500
0
10
20
30
40
50
Physics data control sample
※Loose veto
Reduction of the Hadron cluster BG
21
KL→π0νν signal = 2 gammas on CSI + nothing Hadron cluster BG = 2 clusters on CSI + nothing
need n-γ discrimination w/ CSI information
Hadron-cluster (n)EM-cluster (γ)
discrimination methods•Cluster Shape Discrimination
•Pulse Shape Discrimination
22
: Cluster Shape Discrimination
• Shape neural net method
➡1/300 BG reduction & 80% signal acceptance
n-γ discrimination with CSI
• Shape χ2 method (used in 2013 run)
a.u.
NN output0 10.2 0.4 0.6 0.8
if data is consistent with γ MC➡ acceptCSI
1900 mm
1200 mm
25×25 mm2 crystal50×50 mm2 crystalc.f. RMoliere = 35.7mm
Energy, Time of crystals as input variables
shower shape can be seen !
1/10 BG reduction & 90% signal acceptance
neutron pulse has a longer tail
n-γ discrimination with CSI
waveform fitting
: Pulse Shape Discrimination
23
Likelihood ratio
Time [Clock = 8ns]
AD
C c
ount
• waveform of individual crystalsof CSI stored with 125MHz digitizer
• Template of neutron/gamma events to calculate likelihood
Z [mm]0πRec. 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Pt [
MeV
/c]
0 πR
ec.
0
50
100
150
200
250
300
350
400
450
500
0
2
4
6
8
10
12
24
0.24±0.17
0.21±0.15 0.05±0.04
0.19±0.13
0.02±0.02
Hadron cluster background
Preliminary
0.00±0.00
N = 0.24 ± 0.17Had. BG
Z [mm]0πRec. 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Pt [
MeV
/c]
0 πR
ec.
0
50
100
150
200
250
300
350
400
450
500
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
Upstream-π0 background
25
neutron γπ0 (+X)
N = 0.04 ± 0.03Upstream-π0 BG
Simulation
- smaller visible energy→ Zvtx shift to downstream
- “γ + n” event323.0±2.7
0.04±0.03
CV-Eta background : Mechanism
26Z=6148
Z=5842
CV ( Charged Veto )
neutron
η→2γ
rec. π0 Z
Ω
cos⌦ = 1�m2
⇡0
2E1E2Ω is calculated to be smaller because mη > mπ (mη ~ 4mπ).➡ rec. Z goes upstream.
CSI
Z [mm]0πRec. 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Pt [
MeV
/c]
0 πR
ec.
0
50
100
150
200
250
300
350
400
450
500
0
0.002
0.004
0.006
0.008
0.01
0.012
0.26
0.05
0.07
CV-Eta MC
New Cut for the CV-Eta BG
27
η-χ2 cut : • if the cluster shapes are consistent with the assumption that
they are derived from η→2γ decay at CV, the event is rejected• good consistency = small χ2 value
η-χ2 distribution
Preliminary
N = 0.03 ± 0.02CV-η BG
Z [mm]0πRec. 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Pt [
MeV
/c]
0 πR
ec.
0
50
100
150
200
250
300
350
400
450
500
0
0.002
0.004
0.006
0.008
0.01
0.012CV-Eta MC
0.02 ±0.01
0.03 ±0.02
Z [mm]0πRec. 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Pt [
MeV
/c]
0 πR
ec.
0
50
100
150
200
250
300
350
400
450
500
0
1
2
3
4
5
6
344
0 0
1
01
Background summary
28
0.27±0.15
1.40±0.15
0.54±0.16
0.11±0.06
Observed Estimated
Preliminary
0.40±0.18
BG source # of BG
KL→2π0 0.02 ± 0.02
KL→π+π-π0 0.05 ± 0.02
Hadron Cluster 0.24 ± 0.17
Upstream-π0 0.04 ± 0.03
CV-η 0.03 ± 0.02
others 0.02 ± 0.02
Total 0.40 ± 0.18
324.4±2.7
Results
29
Z [mm]0πRec. 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Pt [
MeV
/c]
0 πR
ec.
0
50
100
150
200
250
300
350
400
450
500
0
1
2
3
4
5
6
344
0 0
1
01324.4 ±2.7
0.27 ±0.150.11 ±0.06
0.54 ±0.16
1.40 ±0.15
0.40 ±0.18
S.E.S. : 1.3 ×10-9
Preliminary
Results
30
Z [mm]0πRec. 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Pt [
MeV
/c]
0 πR
ec.
0
50
100
150
200
250
300
350
400
450
500
0
1
2
3
4
5
6
344
0 0
1
01
No candidate event observed
324.4 ±2.7
0.27 ±0.150.11 ±0.06
0.54 ±0.16
1.40 ±0.15
S.E.S. : 1.3 ×10-9
Preliminary
Results
31
Z [mm]0πRec. 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
Pt [
MeV
/c]
0 πR
ec.
0
50
100
150
200
250
300
350
400
450
500
0
1
2
3
4
5
6
344
0 0
1
01324.4 ±2.7
0.27 ±0.150.11 ±0.06
0.54 ±0.16
1.40 ±0.15
S.E.S. : 1.3 ×10-9
※ systematic uncertainty is not taken into accountPreliminary
➡ Br(KL→π0νν) < 3.0×10−9 (90% C.L.)
No candidate event observed
32
➡ Br(KL→π0νν) < 3.0×10−9 (90% C.L.)
Results
upper limit by E391a : 2.6×10-8 (90% C.L.)
~ one order of magnitude improvement !
※ systematic uncertainty is not taken into accountPreliminary
Prospects : 2016-2018 run
33
•additional ×1.5 physics data ➡ SES ~ 5×10-10 (combined with 2015 run data)
• ×10 control samples for the hadron cluster BG ➡ improvement of the n-γ discrimination
additional photon counter(13.5X0+5X0) ➡ KL→2π0 BG ↓1/3
Prospects : Detector upgrades
34
during THIS summer shutdownCsI calorimeter upgrade 6
PMT
PMT
𝜸
𝐧
𝑿𝟎 ∼ 𝟐 cm
interaction length 〜 40 cm
Attach MPPCs in this summer 2018
S13360-6050CS (HPK)
CsI crystal
PMTNow
MPPCs
upstream
6×6 𝐦𝐦𝟐
locate interaction Z position
CSI upgrade to reduce the hadron cluster BG
50cm
Prospects : Detector upgrades
35
CsI calorimeter upgrade 6
PMT
PMT
𝜸
𝐧
𝑿𝟎 ∼ 𝟐 cm
interaction length 〜 40 cm
Attach MPPCs in this summer 2018
S13360-6050CS (HPK)
CsI crystal
PMTNow
MPPCs
upstream
6×6 𝐦𝐦𝟐
locate interaction Z position ➡ NBG ～ 1/10
CSI upgrade to reduce the hadron cluster BG
50cm
Simulation
during THIS summer shutdown
Prospects : Future run
36
• expect O(10-11) SES with the next3-year data- beam power 50→90kW and more
• aim to SM sensitivity- better beam condition by the power supply upgrade of the accelerator
10-8
10-9
10-10
10-11
SES
This results
2015 run
2016-2018 run
2019-2021 run
2022 ~
SM
Summary
37
•KL→π0νν search at KOTO
•No candidate event in the 2015 run data•Br(KL→π0νν) < 3.0×10-9 @90% C.L. (preliminary)•one order of magnitude improvement