new measurement of the + → + branching ratio

New measurement of the New measurement of the ++→→++ branching ratio branching ratio

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Zhe Wang (E949 collaboration)Zhe Wang (E949 collaboration)Brookhaven National LaboratoryBrookhaven National Laboratory

March 2009 at University of MichiganMarch 2009 at University of Michigan

_Zhe Wang (BNL) + → + 2


++→→++ in in standard model (SM)standard model (SM)

K

K

K

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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 ∞

--


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


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.


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


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


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 ) )

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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

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K goes in

comes out

No additional particles


KK++ beam beam710 MeV/c710 MeV/c


Beam detectorsBeam detectors Cerenkov counterCerenkov counter

B4 B4 countercounter

Top half of side view

Wire Wire chamberchamber

Active degraderActive degrader


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


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


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


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


Signal box and background Signal box and background overviewoverview

pnnpnn22

pnnpnn11


After the triggerAfter the trigger

P vs R of P vs R of ++

pnnpnn22

pnn1pnn1


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


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


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


Strategy 2: Strategy 2: blind analysis - bifurcationblind analysis - bifurcation

Step 3: Step 3: Check the Check the correlationcorrelation


KKtarget scattering backgroundtarget scattering background

Typical pattern in target:Typical pattern in target:


Transversal Transversal ScatteringScattering

PNN2


Longitudinal scatteringLongitudinal scatteringCCD pulse cutCCD pulse cut


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


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


Muon backgroundMuon background

Range Range momentummomentum

dE/dx range dE/dx range stack stack

→→e chainchain

KK++++KK++++


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


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


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


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


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


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


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’


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


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)


Surprise Surprise me!me!


ThreeThree candidates are candidates are foundfound


Measured Measured ++→→++ BR of this BR of this analysisanalysis

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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 Sdue to signal of SM prediction + backgM prediction + 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


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 Sdue to signal of SM prediction + backgM prediction + 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


BR of BR of ScalarScalar and and TensorTensor form form factorsfactors

Trigger simulatio

n

BR (×10BR (×10--

1010):):


Limit on the BR of Limit on the BR of ++→→++XX

The mass ofThe mass of X X is is unknown.unknown.

XX might might have some have some limited limited lifetimelifetime

We assume We assume the the detection detection efficiency of efficiency of XX’s ’s daughter daughter particle is particle is 100% if 100% if decay decay within within detectordetector


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

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www.phy.bnl.gov/e949www.phy.bnl.gov/e949PRL 101, 191802 (2008) and arXiv:0903.0030 PRL 101, 191802 (2008) and arXiv:0903.0030


Your Your questionquestion

s are s are welcomewelcome

!!


BACKUPBACKUP


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


Cerenkov counter, B4

K


Expecting: Expecting: • Next: rare KNext: rare K++ decay experiment decay experiment NA62NA62

• Next: rare KNext: rare K00LL decay experiment decay experiment

E391aE391a


Before isospin breaking correction:Before isospin breaking correction:

X is a monotonically increasing function with

new measurement of the + → + branching ratio

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