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New measurement of the + → + branching ratio. _. Zhe Wang (E949 collaboration) Brookhaven National Laboratory. March 2009 at University of Michigan. _. + → + in standard model (SM). -. -. FCNC is permitted through loop diagrams, however it is highly suppressed.
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New measurement of the +→+ branching ratio _ Zhe Wang (E949 collaboration) Brookhaven National Laboratory March 2009 at University of Michigan
_ +→+ instandard model (SM) - - FCNC is permitted through loop diagrams, however it is highly suppressed. + → +
Branching ratio prediction Effective Hamiltonian: X: Wilson coefficients Short-distance interaction dominant Relevant hadronic operator matrix element can be extracted from +→0e+ M.K. Gailard and Benjamin W. Lee 1974 + → +
Rare processBR~10-10with small theory uncertainty (8%) Precise theory prediction:(0.85±0.07)×10-10 (50% of the error is from CKM parameters) arXiv: hep-ph/0405132 Andrzej J. Buras, et. al. top quark contribution ~70% (sensitive to Vtd) Probe SM at quantum level, thereby allowing an indirect test of high-energy scales through a low-energy process (sensitive to new physics) + → +
Searching for the holy grail The experimentalist The theorist “Just look for a decay of a short-lived particle with 109 background, with a poor signature and no kinematic closure.” Marco S. Sozzi at CKM2008 + → +
High intensity K+ beam is supplied by AGS Alternating gradient synchrotron 6×1012 protons/2 sec-long spill/ 5 sec 21 GeV/c 3.5×106 incident K+/spill Detector K+ beam Pt target Proton beam Low energy separated beamline K+ : p+ ~ 3 : 1 + → +
Data taking 1.7×1012stopped K+(this study) _ • + trigger 142×106 on tape • K+ p+p0 monitor • K+ m+n monitor • beam p+ scattering • Charge exchange (K+n K0p, K0S p+p-) + → +
_ Big picture for searching+→+ 1. K+ stop in detector target and decay at rest 2. Measure and identify p+ 3. Veto on any other activity in the detector comes out K goes in No additional particles + → +
K+ beam 710 MeV/c + → +
Beam detectors • Cerenkov counter Top half of side view • Wire chamber • Active degrader • B4 counter + → +
E949 Target Top half of side view • Target (ADC, TDC and CCD) • I counter • V counter p+ End view of target n n + → +
Drift chamber, range stack Top half of end view • Drift chamber • Range stack Momentum, energy and range measurement p+ s=2.3MeV s=3.0MeV s=0.9cm P R E momentum, energy and range of p+ of K+ →p+po + → +
Range stack • Range stack 500 MHz waveform digitizer Pion, Muon ID • Range stack straw chamber p→m→e chain + → +
Photon veto detector • Active Degrader • Target fibers • Endcap veto • Barrel veto • Barrel veto liner …… 4π sr coverage E949 E787 + → +
Signal box and background overview pnn1 pnn2 + → +
After the trigger pnn1 pnn2 P vs R of p+ + → +
Important background list + → +
Strategy 1: avoid bias in cut tuning Full data set 1/3 data 2/3 data Beam bkg p+p0 ... Background estimate Background study Freeze cuts Open the box + → +
Strategy 2: blind analysis - bifurcation Step 1: Background isolation Step 2: Suppress each background with two independent cuts + → +
Strategy 2: blind analysis - bifurcation Step 3: Check the correlation + → +
K+p+p0target scattering background Typical pattern in target: + → +
Transversal Scattering PNN2 + → +
Longitudinal scattering CCD pulse cut + → +
Measure K+p+p0background Photon tagged pscat tagged C+D Target cut CCD pulse Photon cut B C + → +
Beam background Single beam: particle ID, timing Double beam: redundant particle ID along beam line Cerenkov Wire chamber Target B4 AD + → +
Muon background K+m+ng K+m+np0 • Range momentum • dE/dx range stack • p→m→echain + → +
Ke4 (K+p+p-e+n) background Their energy could be very low A Ke4 candidate from data Ke4 MC event p- signal region p+ K+ e+ + → +
Ke4 (K+p+p-e+n) background • A Ke4-rich sample is tagged in data by selecting events with extra energy in target. • Use MC to evaluate the rejection of cuts • The p- annihilation • energy spectrum • is from our • experimental • measurement + → +
Charge exchange (K+n K0p) background • Character: • a gap between K+ andp+ • z info of p+ is not consistent with K+ track • A CEX rich sample is tagged in data by selecting events with a gap between K+ and p+ • Model KL momentum from KS monitor sample • Use MC to evaluate the rejection + → +
Single/double cut(s) failure study -- look for a loophole Usually introduce a cut or a group of cuts aiming at a background By inverting the cut(s) the background is selected 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 + → +
Acceptance measurement • Accidental hits and noise will bias acceptance measurement, so data (monitor sample) is used. • example: • Acc loss ofphoton veto cut onK+mn • No photon in the final state of+→+ • Acc of photon veto is measured withK+mn • MC is only used to measure the acceptance of kinematics box cuts and detector solid angle • BR(K+p+p0) is measured to validate acceptance measurement + → +
Total background and sensitivity For E787+E949 pnn1 SES=0.63×10-10 SES is the branching ratio for a single event observed w/o background + → +
Outside box study • Keep signal region blinded • Relax photon veto or ccd pulse cut • Check the predicted events and observed events in the extended region A’ + → +
Division of signal region – We know more about cuts • Acceptance and background is not uniformly distributed in the signal region • A candidate in a cleaner region is less likely to be background Momentum (MeV/c) of: K+p+p-e+n Signal Lower box of p (MeV/c): p>140 Tight p box: p>165 + → +
Cell information – 9 cells • Four cuts are tightened to define 9 cells • Kinematics box • Photon veto • p→m→e • timing (delay coincidence) + → +
Surprise me! + → +
Three candidates are found + → +
_ Measured +→+ BR of this analysis 9.26 • BR=(7.89± )×10-10 • The probability of all 3 events to be due to background alone is 0.037 • …due to signal of SM prediction + background is 0.056 • SM prediction: • BR=(0.85±0.07)×10-10 5.10 + → +
Combined with all E787/E949 result 1.15 • BR=(1.73± )×10-10 • The probability of all 7 events to be due to background alone is 0.001 • …due to signal of SM prediction + background is 0.06 • SM prediction: • BR=(0.85±0.07)×10-10 1.05 + → +
BR of Scalar and Tensor form factors Trigger simulation BR (×10-10): + → +
Limit on the BR of +→+X The mass of X is unknown. X might have some limited lifetime We assume the detection efficiency of X’s daughter particle is 100% if decay within detector + → +
Conclusion _ 1.15 • BR(+→+) =(1.73± )×10-10 • Our measurement is twice of standard model prediction, however consistent with it within uncertainty • E787/E949 made tremendous progress on rare K decay study • 25 years ago only a limit <1.4×10-7 • Found 3 candidates in this study • (7 candidates totally) 1.05 www.phy.bnl.gov/e949 PRL 101, 191802 (2008) and arXiv:0903.0030 + → +
Your questions are welcome! + → +
BACKUP + → +
BR and r h K+p+nn E787/E949 K0p0nn E391a An independent measurement for CKM unitary triangle in Kaon sector + → +
Cerenkov counter, B4 K + → +
Expecting: • Next: rare K+ decay experiment NA62 • Next: rare K0L decay experiment E391a + → +
Before isospin breaking correction: X is a monotonically increasing function with + → +
