New measurement of the  + →  + branching ratio _ Zhe Wang (E949 collaboration) Brookhaven National Laboratory 2008 at Cornell, U. Wisconsin and Fermi Lab

_ Zhe Wang (BNL)  + →  + 2

_ Zhe Wang (BNL)  + →  + 3  + →  + instandard model (SM)  + →  + in standard model (SM) _ FCNC is permitted through loop diagrams, however it is highly suppressed. Quark contents Mass (MeV/c 2 ) Lifetime (ns) K+K+ us ++ ud ≈0≈0∞ - -

_ Zhe Wang (BNL)  + →  + 4 Branching ratio prediction Short-distance interaction dominant Relevant hadronic operator matrix element can be extracted from  + →  0 e + Relevant hadronic operator matrix element can be extracted from  + →  0 e + Effective Hamiltonian: X: Wilson coefficients M.K. Gailard and Benjamin W. Lee 1974

_ Zhe Wang (BNL)  + →  + 5 Probe SM at quantum level, thereby allowing an indirect test of high-energy scales through a low-energy process (sensitive to new physics) (sensitive to new physics) top quark contribution ~70% (sensitive to V td ) (sensitive to V td ) Precise theory prediction: (0.85±0.07)× (50% of the error is from CKM parameters) Rare process BR~ with small theory uncertainty (8%) arXiv: hep-ph/ Andrzej J. Buras, et. al.

_ Zhe Wang (BNL)  + →  + 6 Searching for the holy grail “Just look for a decay of a short-lived particle with 10 9 background, with a poor signature and no kinematic closure.” The theorist The experimentalist Marco S. Sozzi at CKM2008

_ Zhe Wang (BNL)  + →  + 7 Proton beam Pt target K + beam Detector High intensity K + beam is supplied by AGS K + :  + ~ 3 : 1 Low energy separated beamline 6×10 12 protons/2 sec-long spill/ 5 sec 21 GeV/c 3.5×10 6 incident K + /spill Alternating gradient synchrotron

_ Zhe Wang (BNL)  + →  + 8 Data taking 1.7×10 12 stopped K + (this study)   + trigger 142×10 6 on tape 142×10 6 on tape  K      monitor  K    monitor  beam   scattering  Charge exchange ( K  n  K 0 p, K 0 S     ) _

_ Zhe Wang (BNL)  + →  + 9 Big picture for searching  + →  + Big picture for searching  + →  + 1. K + stop in detector target and decay at rest 2. Measure and identify  + 3. Veto on any other activity in the detector _ K  goes in   comes out    No additional particles 

_ Zhe Wang (BNL)  + →  + 10 K + beam 710 MeV/c

_ Zhe Wang (BNL)  + →  + 11 Beam detectors  Cerenkov counter  B4 counter Top half of side view  Wire chamber  Active degrader

_ Zhe Wang (BNL)  + →  + 12  Target (ADC, TDC and CCD)  I counter  V counter E949 Target End view of target  Top half of side view

_ Zhe Wang (BNL)  + →  + 13 P E R momentum, energy and range of  + of K + →  +  o  Drift chamber  Range stack Momentum, energy and range measurement range measurement  MeV  MeV  cm Top half of end view Drift chamber, range stack 

_ Zhe Wang (BNL)  + →  + 14 Range stack  Range stack 500 MHz waveform digitizer Pion, Muon ID Pion, Muon ID  Range stack straw chamber chain  →  → e chain

_ Zhe Wang (BNL)  + →  + 15  Active Degrader  Target fibers  Endcap veto  Barrel veto  Barrel veto liner …… Photon veto detector E787 E949 4π sr coverage

_ Zhe Wang (BNL)  + →  + 16 Signal box and background overview pnn2 pnn1

_ Zhe Wang (BNL)  + →  + 17 After the trigger P vs R of  + pnn2 pnn1

_ Zhe Wang (BNL)  + →  + 18 Cut CutBkg Kinematics cuts (P/R/E) Particle ID (K/  /  ) Photon veto Target pattern Timing cuts K      scattering  K        Beam muon K      e + K      e +  Charge exchange K  n  K 0 p  Important background list

_ Zhe Wang (BNL)  + →  + 19 Strategy 1: avoid bias in cut tuning Full data set 1/3 data 2/3 data...  Beam bkg Backgroundstudy Background estimate estimate Open the box Freeze cuts

_ Zhe Wang (BNL)  + →  + 20 Strategy 2: blind analysis - bifurcation Step 1: Background isolation Step 2: Suppress each background with two independent cuts

_ Zhe Wang (BNL)  + →  + 21 Strategy 2: blind analysis - bifurcation Step 3: Check the correlation

_ Zhe Wang (BNL)  + →  + 22 K       target scattering background Typical pattern in target:

_ Zhe Wang (BNL)  + →  + 23 Transversal Scattering PNN2

_ Zhe Wang (BNL)  + →  + 24 Longitudinal scattering CCD pulse cut

_ Zhe Wang (BNL)  + →  + 25 Measure K +  +  0 background Photon tagged CCD pulse Target cut B  scat tagged C+D Photon cut C

_ Zhe Wang (BNL)  + →  + 26 Beam background Single beam: particle ID, timing Single beam: particle ID, timing Double beam: Double beam: redundant particle ID along beam line redundant particle ID along beam line Cerenkov Cerenkov Wire chamber Wire chamber Target Target B4 B4 AD

_ Zhe Wang (BNL)  + →  + 27 Muon background Range momentum Range momentum dE/dx range stack dE/dx range stack chain  →  → e chain K++K++K++K++K++K++K++K++

_ Zhe Wang (BNL)  + →  + 28 Ke4 (K +  +  - e + ) background Ke4 MC event K+K+K+K+ ++++ ---- e+e+e+e+ A Ke4 candidate from data Their energy could be very low signal region

_ Zhe Wang (BNL)  + →  + 29 Ke4 (K +  +  - e + ) background A Ke4-rich sample is tagged in data by selecting events with extra energy in target. A Ke4-rich sample is tagged in data by selecting events with extra energy in target. Use MC to evaluate the rejection of cuts Use MC to evaluate the rejection of cuts The  - annihilation The  - annihilation energy spectrum is from our experimentalmeasurement

_ Zhe Wang (BNL)  + →  + 30 Charge exchange ( K  n  K 0 p ) background A CEX rich sample is tagged in data by selecting events with a gap between K + and  + A CEX rich sample is tagged in data by selecting events with a gap between K + and  + Model K L momentum from K S monitor sample Model K L momentum from K S monitor sample Use MC to evaluate the rejection Use MC to evaluate the rejection Character: a gap between K + and  + a gap between K + and  + z info of  + is not consistent with K + track z info of  + is not consistent with K + track

_ Zhe Wang (BNL)  + →  + 31 Single/double cut(s) failure study -- look for a loophole Usually introduce a cut or a group of cuts aiming at a background Usually introduce a cut or a group of cuts aiming at a background By inverting the cut(s) the background is selected By inverting the cut(s) the background is selected The number of events and their characteristic can be predicted The number of events and their characteristic can be predicted If observation is not consistent with prediction a new background is found or your cuts are not complete If observation is not consistent with prediction a new background is found or your cuts are not complete

_ Zhe Wang (BNL)  + →  + 32 Acceptance measurement Accidental hits and noise will bias acceptance measurement, so data (monitor sample) is used. Accidental hits and noise will bias acceptance measurement, so data (monitor sample) is used. example: example: Acc loss of photon veto cut on K +  Acc loss of photon veto cut on K +   No photon in the final state of  + →  +  No photon in the final state of  + →  + Acc of photon veto is measured with K +  Acc of photon veto is measured with K +  MC is only used to measure the acceptance of kinematics box cuts and detector solid angle MC is only used to measure the acceptance of kinematics box cuts and detector solid angle BR(K +  +  0 ) is measured to validate acceptance measurement BR(K +  +  0 ) is measured to validate acceptance measurement

_ Zhe Wang (BNL)  + →  + 33 Total background and sensitivity SES is the branching ratio for a single event observed w/o background For E787+E949 pnn1 SES=0.63×10 -10

_ Zhe Wang (BNL)  + →  + 34 Outside box study Keep signal region blinded Keep signal region blinded Relax photon veto or ccd pulse cut Relax photon veto or ccd pulse cut Check the predicted events and observed events in the extended region A’ Check the predicted events and observed events in the extended region A’

_ Zhe Wang (BNL)  + →  + 35 Division of signal region – We know more about cuts Acceptance and background is not uniformly distributed in the signal region Acceptance and background is not uniformly distributed in the signal region A candidate in a cleaner region is less likely to be background A candidate in a cleaner region is less likely to be background Momentum (MeV/c) of: Signal K +  +  - e + K +  +  - e + Lower box of p (MeV/c): p>140 p>140 Tight p box: p>165 p>165

_ Zhe Wang (BNL)  + →  + 36 Cell information – 9 cells Four cuts are tightened to define 9 cells Kinematics box Kinematics box Photon veto Photon veto  →  →e  →  →e timing (delay coincidence) timing (delay coincidence)

_ Zhe Wang (BNL)  + →  + 37 Surprise me!

_ Zhe Wang (BNL)  + →  + 38 Three candidates are found

_ Zhe Wang (BNL)  + →  + 39 Measured  + →  + BR of this analysis _ BR= (7.89± )× BR= (7.89± )× The probability of all 3 events to be due to background alone is The probability of all 3 events to be due to background alone is due to signal of SM prediction and background is due to signal of SM prediction and background is SM prediction: SM prediction: BR=(0.85±0.07)×

_ Zhe Wang (BNL)  + →  + 40 Combined with all E787/E949 result BR= (1.73± )× BR= (1.73± )× The probability of all 7 events to be due to background alone is The probability of all 7 events to be due to background alone is due to signal of SM prediction and background is 0.06 due to signal of SM prediction and background is 0.06 SM prediction: SM prediction: BR=(0.85±0.07)×

_ Zhe Wang (BNL)  + →  + 41 Conclusion BR(  + →  + ) = (1.73± )× BR(  + →  + ) = (1.73± )× Our measurement is twice of standard model prediction, however consistent with it within uncertainty Our measurement is twice of standard model prediction, however consistent with it within uncertainty E787/E949 made tremendous progress on rare K decay study E787/E949 made tremendous progress on rare K decay study 25 years ago only a limit <1.4× years ago only a limit <1.4×10 -7 Found 3 candidates in this study Found 3 candidates in this study (7 candidates totally) (7 candidates totally) _ arXiv:hep-ex/ has been accepted by PRL for publication.

_ Zhe Wang (BNL)  + →  + 42 Your questions are welcome!

_ Zhe Wang (BNL)  + →  + 43 BACKUP

_ Zhe Wang (BNL)  + →  + 44 E787/E949 K +   +  E787/E949 E391a K 0   0  E391a An independent measurement for CKM unitary triangle in Kaon sector BR and 

_ Zhe Wang (BNL)  + →  + 45 Cerenkov counter, B4  K

_ Zhe Wang (BNL)  + →  + 46 Expecting: Next: rare K + decay experiment NA62 Next: rare K + decay experiment NA62 Next: rare K 0 L decay experiment E391a Next: rare K 0 L decay experiment E391a

_ Zhe Wang (BNL)  + →  + 47 Before isospin breaking correction: X is a monotonically increasing function with