New measurement of the + → + branching ratio _ Zhe Wang (E949 collaboration) Brookhaven National...
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New measurement of the New measurement of the ++→→++ branching ratio branching ratio
_
Zhe Wang (E949 collaboration)Zhe Wang (E949 collaboration)Brookhaven National LaboratoryBrookhaven National Laboratory
2008 at Cornell, U. Wisconsin and Fermi Lab2008 at Cornell, U. Wisconsin and Fermi Lab
_Zhe Wang (BNL) + → + 3
++→→++ in in standard model (SM)standard model (SM)
K
K
K
_
FCNCFCNC is permitted through is permitted through looploop diagrams, diagrams,however it is highly suppressed.however it is highly suppressed.
Quark content
s
Mass(MeV/
c2)
Lifetime
(ns)
K+ us 493.7 12.4
+ ud 139.6 26.0
- ≈0 ∞
--
_Zhe Wang (BNL) + → + 4
Branching ratio predictionBranching ratio predictionK
Short-distance interaction Short-distance interaction dominantdominantRelevant hadronic operator matrix element cRelevant hadronic operator matrix element can be extracted from an be extracted from ++→→00ee++
Effective Hamiltonian:
X: Wilson coefficientsK
M.K. Gailard and Benjamin W. Lee 1974
_Zhe Wang (BNL) + → + 5
Probe SM at quantum level, Probe SM at quantum level, thereby allowing an indirect test thereby allowing an indirect test of high-energy scales through a of high-energy scales through a low-energy processlow-energy process (sensitive to new physics)(sensitive to new physics)
top quark contribution ~70top quark contribution ~70%% (sensitive to V(sensitive to Vtdtd))
Precise theory prediction:Precise theory prediction: (0.85±0.07)×10(0.85±0.07)×10-10-10
(50% of the error is from CKM parameters)(50% of the error is from CKM parameters)
Rare processRare process BR~10BR~10-10-10 with small with small theory uncertainty (8%)theory uncertainty (8%)
arXiv: hep-ph/0405132 Andrzej J. Buras, et. al.
_Zhe Wang (BNL) + → + 6
Searching for the holy grailSearching for the holy grail
“Just look for a decay of a short-lived particle with 109 background, with a poor signature and no kinematic closure.”
The theoristThe experimentalist
Marco S. Sozzi at CKM2008
_Zhe Wang (BNL) + → + 7
Proton Proton beambeam
Pt Pt targettarget
KK++ beambeam
DetectDetectoror
High intensity KHigh intensity K++ beam is supplied by AGS beam is supplied by AGS
KK++ : : ++ ~ 3 : 1 ~ 3 : 1Low energy separated beamline
6×1012 protons/2 sec-long spill/ 5 sec 21 GeV/c
3.5×106 incident K+/spill
Alternating gradient synchrotron
_Zhe Wang (BNL) + → + 8
Data taking Data taking 1.7×101.7×101212 stopped Kstopped K++ (this study)(this study)
++ trigger trigger 142×10142×106 6 on tapeon tape KK monitor monitor KK monitor monitor beam beam scatterin scatteringg Charge exchange Charge exchange ((KKn n KK00p,p, K K00
S S ) )
_
_Zhe Wang (BNL) + → + 9
Big picture for searchingBig picture for searching ++→→++ 1. K1. K++ stop in stop in detector target and detector target and decay at restdecay at rest
2. Measure and 2. Measure and identify identify ++
3. Veto on any 3. Veto on any other activity in other activity in the detectorthe detector
_
K goes in
comes out
No additional particles
_Zhe Wang (BNL) + → + 11
Beam detectorsBeam detectors Cerenkov counterCerenkov counter
B4 B4 countercounter
Top half of side view
Wire Wire chamberchamber
Active degraderActive degrader
_Zhe Wang (BNL) + → + 12
Target Target
(ADC, TDC and (ADC, TDC and CCD)CCD) I counter I counter V counter V counter
E949 TargetE949 Target
End view of target
Top half of side view
_Zhe Wang (BNL) + → + 13
PP EE RR
momentum, energy and range of + of K+ → +o
Drift chamberDrift chamber Range stackRange stack
Momentum, energy anMomentum, energy andd range measurementrange measurement
MeV MeV cm
Top half of end viewDrift chamber, range stackDrift chamber, range stack
_Zhe Wang (BNL) + → + 14
Range stackRange stack Range stackRange stack 500 MHz waveform digitizer500 MHz waveform digitizer Pion, Muon IDPion, Muon ID Range stack straw chambeRange stack straw chamberr
→→e chainchain
_Zhe Wang (BNL) + → + 15
Active DegraderActive Degrader Target fibersTarget fibers Endcap vetoEndcap veto Barrel vetoBarrel veto Barrel veto liner Barrel veto liner
…………
Photon veto detectorPhoton veto detector
E787
E94944 ππ sr coverage sr coverage
_Zhe Wang (BNL) + → + 16
Signal box and background Signal box and background overviewoverview
pnnpnn22
pnnpnn11
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After the triggerAfter the trigger
P vs R of P vs R of ++
pnnpnn22
pnn1pnn1
_Zhe Wang (BNL) + → + 18
CutCut
BkgBkg
KinematiKinematics cuts cs cuts (P/R/E)(P/R/E)
ParticlParticle ID e ID (K/(K///))
PhotoPhoton veton veto
Target Target patternpattern
TiminTiming cutsg cuts
KK scatteringscattering
KK BeamBeam muonmuon KKee++ Charge exchCharge exchangeange
KKn n KK00pp
Important background listImportant background list
_Zhe Wang (BNL) + → + 19
Strategy 1: Strategy 1: avoid bias in cut avoid bias in cut tuningtuning
Full data setFull data set
1/3 data1/3 data 2/3 data2/3 data
...... Beam bkgBeam bkg
Background Background studystudy
BackgroundBackground estimateestimate
Open the boxOpen the box
Freeze cutsFreeze cutsFreeze cutsFreeze cuts
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Strategy 2: Strategy 2: blind analysis - bifurcationblind analysis - bifurcation
Step 1: Step 1: Background Background isolationisolation
Step 2: Suppress Step 2: Suppress each background each background with two with two independent independent cutscuts
_Zhe Wang (BNL) + → + 21
Strategy 2: Strategy 2: blind analysis - bifurcationblind analysis - bifurcation
Step 3: Step 3: Check the Check the correlationcorrelation
_Zhe Wang (BNL) + → + 22
KKtarget scattering backgroundtarget scattering background
Typical pattern in target:Typical pattern in target:
_Zhe Wang (BNL) + → + 25
Measure Measure KK++++00 backgroundbackground
Photon taggedPhoton tagged
CCD pulseCCD pulse
Target cutTarget cut
BB
scatscat tagged tagged
C+DC+D
Photon cutPhoton cut
CC
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Beam backgroundBeam background Single beam: particle ID, Single beam: particle ID,
timingtiming
Double beam: Double beam:
redundant particle ID along beam lineredundant particle ID along beam line
CerenkovCerenkov Wire chamWire cham
berber TargetTarget B4B4 ADAD
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Muon backgroundMuon background
Range Range momentummomentum
dE/dx range dE/dx range stack stack
→→e chainchain
KK++++KK++++
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Ke4 (Ke4 (KK++++--ee++) background) background
Ke4 MC Ke4 MC eventevent
KK++
++ --
ee++
A Ke4 candidate A Ke4 candidate from datafrom data
Their energy could be very Their energy could be very lowlow
signal region
_Zhe Wang (BNL) + → + 29
Ke4 (Ke4 (KK++++--ee++) background) background
A Ke4-rich sample is tagged in data by A Ke4-rich sample is tagged in data by selecting events with extra energy in tarselecting events with extra energy in target.get.
Use MC to evaluate the rejection of cutUse MC to evaluate the rejection of cutss
The The -- annihilation annihilationenergy spectrumenergy spectrumis from our is from our experimentalexperimentalmeasurementmeasurement
_Zhe Wang (BNL) + → + 30
Charge exchange (Charge exchange (KKn n KK00pp) background) background
A CEX rich sample is tagged in A CEX rich sample is tagged in data by selecting events with a data by selecting events with a gap between Kgap between K++ and and ++
Model KModel KLL momentum from K momentum from KSS monitor sample monitor sample Use MC to evaluate the rejectionUse MC to evaluate the rejection
lKpKnK L00 , lKpKnK L
00 ,
Character:Character: a gap betweena gap between K K++ and and ++
z info of z info of ++ is not consistent w is not consistent with Kith K+ + tracktrack
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Single/double cut(s) failure study Single/double cut(s) failure study -- look for a loophole-- look for a loophole
Usually introduce a cut or a group of cuts aiUsually introduce a cut or a group of cuts aiming at a backgroundming at a background
By inverting the cut(s) the background is selBy inverting the cut(s) the background is selectedected
The number of events and their characteristThe number of events and their characteristic can be predictedic can be predicted
If observation is not consistent with predictIf observation is not consistent with prediction a new background is found or your cuts arion a new background is found or your cuts are not completee not complete
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Acceptance measurementAcceptance measurement Accidental hits and noise will bias Accidental hits and noise will bias
acceptance measurement, so data acceptance measurement, so data (monitor sample) is used.(monitor sample) is used.
example:example:
Acc loss ofAcc loss of photon veto cut onphoton veto cut on K K++No photon in the final state ofNo photon in the final state of++→→++ Acc of photon veto is measured withAcc of photon veto is measured with KK++
MC is only used to measure the MC is only used to measure the acceptance of kinematics box cuts and acceptance of kinematics box cuts and detector solid angledetector solid angle
BR(BR(KK++++00 ) is measured to validate accept) is measured to validate acceptance measurementance measurement
_Zhe Wang (BNL) + → + 33
Total background and sensitivityTotal background and sensitivity
SES is the branching ratio for a single event SES is the branching ratio for a single event observed w/o backgroundobserved w/o background
For E787+E949 pnn1 SES=0.63×10For E787+E949 pnn1 SES=0.63×10-10-10
_Zhe Wang (BNL) + → + 34
Outside box studyOutside box study Keep signal region blindedKeep signal region blinded Relax photon veto or ccd pulse cutRelax photon veto or ccd pulse cut
Check the predicted events and Check the predicted events and observed events in the extended region observed events in the extended region A’A’
_Zhe Wang (BNL) + → + 35
Division of signal region – We Division of signal region – We know more about cutsknow more about cuts Acceptance and background is not uniformly Acceptance and background is not uniformly
distributed in the signal regiondistributed in the signal region A candidate in a cleaner region is less likely to A candidate in a cleaner region is less likely to
be backgroundbe backgroundMomentum (MeV/c) oMomentum (MeV/c) o
f:f:SignalSignal KK++++--ee++ Lower box of Lower box of p (MeV/c):p (MeV/c): p>140p>140Tight p box:Tight p box: p>165p>165
_Zhe Wang (BNL) + → + 36
Cell information – 9 Cell information – 9 cellscells
Four cuts are tighteFour cuts are tightened to define 9 cellsned to define 9 cells• Kinematics boxKinematics box• Photon vetoPhoton veto• →→→→ee• timing (delay ctiming (delay coincidence)oincidence)
_Zhe Wang (BNL) + → + 39
Measured Measured ++→→++ BR of this BR of this analysisanalysis
_
BR=BR=(7.89± )×10(7.89± )×10-10-10
The probability of aThe probability of all ll 33 events to be due t events to be due to background alone io background alone is s 0.0370.037
due to signal of SM due to signal of SM prediction and backgprediction and background is round is 0.0560.056
SM prediction:SM prediction:BR=(0.85±0.07)×10BR=(0.85±0.07)×10-10-10
9.269.265.105.10
_Zhe Wang (BNL) + → + 40
Combined with all Combined with all E787/E949E787/E949 resultresult BR=BR=(1.73± )×10(1.73± )×10-10-10
The probability of aThe probability of all ll 77 events to be due t events to be due to background alone io background alone is s 0.0010.001
due to signal of SM due to signal of SM prediction and backgprediction and background is round is 0.060.06
SM prediction:SM prediction:BR=(0.85±0.07)×10BR=(0.85±0.07)×10-10-10
1.151.151.051.05
_Zhe Wang (BNL) + → + 41
ConclusionConclusion BR(BR(++→→++) ) ==(1.73± )×10(1.73± )×10-10-10
Our measurement is twice of standard Our measurement is twice of standard model prediction, however consistent wmodel prediction, however consistent with it within uncertaintyith it within uncertainty
E787/E949 made tremendous progress E787/E949 made tremendous progress on rare K decay studyon rare K decay study 25 years ago only a limit <1.4×1025 years ago only a limit <1.4×10-7-7
Found 3 candidates in this study Found 3 candidates in this study (7 candidates totally)(7 candidates totally)
1.151.151.051.05
_
arXiv:hep-ex/0808.2459arXiv:hep-ex/0808.2459 has been accepted has been accepted by PRL for publication.by PRL for publication.
_Zhe Wang (BNL) + → + 44
K++E787/E94E787/E9499K00E391aE391a
An independent measurement for CKM An independent measurement for CKM unitary triangle in Kaon sectorunitary triangle in Kaon sector
BR and BR and
_Zhe Wang (BNL) + → + 46
Expecting: Expecting: • Next: rare KNext: rare K++ decay experiment decay experiment NA62NA62
• Next: rare KNext: rare K00LL decay experiment decay experiment
E391aE391a