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TRANSCRIPT
2019 8/2
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Search for ð²ð³ â ð ððž decay in the KOTO experiment
Nobuhiro Shimizu Sep. 9th
ð²ð³ â ð ððž decay?
⢠No measurements up to nowâ ð â ððŸ transition, forbidden by FCNC
â Violates CP
⢠neither for ð²ðº â ð ððžâ¢ Nice prove of New Physics??
⢠violates an angular conservation⢠ð² and ð = spin zeros: the spin of ðž must be âcompensatedâ by
an orbital angular momentum, but back-to-back configuration cannot produce ð¿ð§ (along the decay axis).
⢠IF v > âspeed of lightâ, itâs allowed. (see PRD 59 116008)
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Oppositely to say, good test of the Lorentz invariance in the realm of short distances
New Physics search via ð²ð³ â ð ððàŽ€ðâ«â¬SM ðŸð¿ â ð0ð Ò§ð = (3.0 ± 0.3) à 10â11
â«âkeyâïŒ not to miss any photonsHermetic Veto counters surrounding the decay volume
The KOTO experiment
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JHEP 1511 033 (2015)
ðž
ðž
àŽ¥ðð
â High intensity ðŸð¿ beamïŒ low inefficiency KOTO detector is ideal tool to search for ð²ð³ â ð ððž
KOTO detector 4
Measure energy and position of photons by undoped CsIcalorimeter
Decay volume: surrounded by 18 types of veto detectors.
Two barrel counters to veto photons: main barrel (MB) and inner barrel (IB)
Charged veto in front of CsI
Data taking
Data taken during 2016 â 2018: ⌠19 à 1018 Protons On Target (POT)
Trigger: 3-cluster w/ energy deposit at CsI calorimeter
â Data corresponding to ð. ð Ã ðððð POT was analyzed.
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Inner barrel installation(2016, June)
Inner barrel
Main barrel
Reconstruction of ð²ð³ â ð ððž
1. Find events which have exactly 3 clusters
2. Reconstruct a ð ð : ðð02 = 2ðž1ðž2 1 â cosððŸðŸ â ðððð
ð ð
3. Reconstruct a ð²ð³: ððŸð¿2 = ððŸ1 + ððŸ2 + ððŸ3
2â ðððð
ð²ð³
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ð ðð²ð³ beamðž
Two types of vertex position, which should be close.
4. Define ð«ðððð = ððððð ð â ðððð
ð²ð³ to suppress various BGs
clusters
ð
Event selections 9
1. Online trigger (veto and cluster counting)
2. Requirement of ðµððð == ð offline
3. Vetoes
4. Cluster shape discrimination (CSD)
5. Kinetic cuts
âð«ðððð = ððððð ð â ðððð
ð²ð³
â ð¬ðžððð
6. Signal box1. (ðððð
ð ð , ðŽð ððž)
Possible background sources
⢠ðŸð¿ â 2ð0
1. a photon missed by some way
2. two photons fuse
⢠ðŸð¿ â 3ð0
mechanisms similar to 2ð0
⢠Neutron induced BGð0 produced from n and a single acc. cluster
⢠ðŸð¿ â 2ðŸ
combines with neutrons
⢠ðŸð¿ â ð+ðâð0
combines with neutrons
⢠ðŸð¿ â âðð (â = ð or ð)
e.g., ð+ðdetectorâ â 2ðŸ and ðâ missed by CV
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ðž ðž ðž ðžÃðž ðž ðž ðž
ðž ðž ðž ðž ðž ðžÃ
ðž ðž ð
ðž ðž ð ð +ð âÃÃ
Large
1
<1
<0.01
<0.01
<0.001
<0.01
Veto counters
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ðð ð
ðððð
Main barrel veto energy
ðð ð
ðððð
Inner barrel veto energy
Main barrel
Inner barrel
Two barrel veto counters play crucial roles to suppress ðŸð¿ â 2ð0 and ðŸð¿ â 3ð0 decay modes.
Cluster shape discrimination (CSD) 12
â neutron clusterâ¡ fused cluster
minimum CSD (the smallest one out of 3)
ðð ððð ððððð
ððž
ð ððž(scale is arbitrary)
ðž likenot ðž like
Using neural network, the shower shape development in the CsI calorimeter is evaluated.
â Separate clean photon clusters from those of BG
Energy deposition of photon cluster
MC Data
ð«ðððð distribution 13
Under vetoes, CSD
âððð < ð«ðððð/mm < ððð
One of the important cut to reduce 2ð0 decay
ð§ð£ð¡ð¥ð0ð§ð£ð¡ð¥
ðŸð¿ðŸ
ðŸ
ðŸð¿
ð«ðððð
ðž
ð0
ðð ððð ððððð
ð ððž(scale is arbitrary)
In the signal case ð«ðððð~ð
ðŽð ððž and ððððð ð distributions 14
ðð ððð ððððð
ð ððž(scale is arbitrary)
Under vetoes, CSD, ð«ðððð cuts
ðð ððð ððððð
ððž
ð ððž(scale is arbitrary)
ðŽð ððž ððððð ð
These variables are used for defining the signal box.
ððð < ðŽð ððž/ððð < ððð, ðððð < ððððð ð /mm < ðððð
ðžððððŸ
distribution 15
ðð ððð ððððð
ððž
ð ððž(scale is arbitrary)
Under vetoes, CSD, ðŽð²ð³, ð«ðððð cuts
ð¬ððððž
Under ðŽð²ð³cut,
ð¬ððððž
is useful
variable to remove ðð ð.
To measure ðð ð and
ðð ð a loose ð¬ððððž
cut is
also used.
loose
nominal
ðžððððŸ
: minimum photon energy
Signal box
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Signal MC
Signal region
Control region 2(CR 2)ðð ð dominant
Control region 1 (CR 1)3ð ð and 2ð ð
â â¬(ðŸð¿ â ð0ðŸ) is obtained as a ratio towards â¬(ðŸð¿ â 2ð0)â CR2 is used to normalize statistics/estimate ðð ð BGâ Signal region (SR) is blinded before determining the selection critaria to avoid
human bias.
Under the condition of 1. vetoes, 2. CSD, 3. ð«ðððð,
4. ð¬ððððž
cuts
Signal box, defined in the 2D plane:
(ððððð ð , ðŽð ððž)
Summary of the event selection 17Uncertainties are statistical only.
Step (after)
Signal ACC
(ððâð)ðð ð ACC(ððâð)
Nobs (Data)
Ncls==3 and online veto*
219 31 1150585
Shape ðð 204 24 233568
Fiducial cut 189 22 214043
Veto 71 2.0 15732
Cluster shape 51 1.3 9483
ððððð 49 0.41 2544
(loose) ð¬ððððž (24.4) 5.0 (0.1) 0.0041 (635) 36
loose CR2 ð. ðððð ± ð. ððð 528
SR ð. ðð ± ð. ðð ð. ðð ± ð. ðð à ððâð ?
Single event sensitivity of signal: ð
ðµð²ð³ð ð²ð³âð ððž
= ð. ð Ã ððâð
Z-ðð0ðŸ correlation plot under loose cut 18
MCDataAfter applying veto, CSD, ð«ðððð, loose minð¬ðž cut ð²ð³ â ðð ð
ð²ð³ â ðð ð
ð²ð³ â ð ððžðž
normalize
We use CR2 region with loose minðžðŸselection to measure ðŸð¿ â 2ð0
contribution of data.
Control region 2 (CR 2)
ðð ððð ððððð
ððž
ð ððž(scale is arbitrary)
loose
After applying veto, CSD, ð«ðððð, cut
minð¬ðž
Z-ðð0ðŸ correlation plot of under tight cut19
The expected number of events in the signal region is 0.34 ± 0.10
MCDatað²ð³ â ðð ðð²ð³ â ðð ð
ð²ð³ â ð ððžðž
BG contribution (summary) 20
Sources Expected in signal box
Comment
ð²ð³ â ðð ð ð. ðð ± ð. ðð Evaluated by MC
ð²ð³ â ðð ð <0.5 (68% C.L) Data driven estimation
ð²ð³ â ððž << 0.06 (68% C.L) Evaluated by full MC
Neutron induced BG <<0.02 (68% C.L) Evaluated by dedicated MC
Other ðŸð¿ decays <0.02
Sum 0.85 (<1.0 at 68% C.L.)
Dominant background: the ð²ð³ â ðð ð decay due to inefficiency of the barrel veto
ð²ð³ â ðð ð decay can be BG due to â production of the overlapped ðŸ clusters (fusion)â¡ wrong paring of ð ð â ððž conbinations.Production of the sufficient amount of MC sample is difficult,but data driven approach gives an upper limit of 0.5 at 68% C.L.
Other decays â negligible.
Systematic uncertainty of S.E.S 21
⬠ðŸð¿ â ð0ðŸ =ðððð (ðð )
ð2ð0ððð (ð¶ð 2) â
ð2ð0ð¶ð 2
ðð0ðŸðð â ⬠ðŸð¿ â 2ð0
Source ðð/ð (in %) Comment⬠ðŸð¿ â 2ð0 0.6 From PDG
Geometry 1.5 Estimated by varying beam E (+1%) and position (x,y=1mm) for signal
Veto cuts 17 By comparing data and MC in CR2 with ð«ðððð cut
Online veto 6.4 From the detector bits in the minimum bias data
Kinematic cuts 12 100% error of |1-ðð·ðð¡ð2ð0 /ððð¶
2ð0|
Clustering 1.0 Compare five and six cluster events
CSD cuts 1.5 Use five cluster events
Reconstruction 0.3 By comparing ðððð 3ð0ððð 6ððð and ðððð 2ð0
ððð 6ððð
Trigger 1.8 Difference of CDT efficiency from unity
Statistics 4.4 Statistics of normalization
Total 22
Syst. uncertainty due to ðð£ðð¡ð 22
ðð£ðð¡ð =ðððð
ðððð ð€ð/ ðð¡â ð£ðð¡ðis compared
between data and MC: ð = ðð¶ð 2ð·ð /ðð¶ð 2
ðð¶
ððð£ðð¡ð
ðð£ðð¡ð=
ð:ððð ððð¡ððð¡ððð
ð ð â 1 2 = 17%
MC
ðððð
Inefficiency of veto detector is well understood by MC.
Open signal box 23
Single event sensitivity of signal:ð
ðµð²ð³ð ð²ð³âð ððž
= ð. ð ± ð. ðð¬ððð ± ð. ððððð à ððâð
Open signal box 24
Single event sensitivity of signal:ð
ðµð²ð³ð ð²ð³âð ððž
= ð. ð ± ð. ðð¬ððð ± ð. ððððð à ððâð
Open signal box 25
⬠ðŸð¿ â ð0ðŸ < 1.7 à 10â7 at 90% C.L.
Taking into account the systematic uncertainty, the obtained upper limit is
Summary Neither ð²ð³ â ð ððž nor ð²ðº â ð ððž decay have been measured yet.
These decays violate the Lorentz invariance and are forbidden by the SMâ Lorentz invariance should be tested also in the high energy regime.
Studying backgrounds, we fixed the selection criteria withS.E.S = 6.8 Ã 10â8. The expected number of background is 0.3.
We opened the signal box and could not find any signal candidate.
The upper limit of the branching fraction is obtained for the first time!
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⬠ðŸð¿ â ð0ðŸ < 1.7 à 10â7 at 90% C.L.