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8/16/2004ICHEP2004, Beijing Rare Kaon Decay Results From E949 At BNL: K + Shaomin CHEN TRIUMF, Canada

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Motivation is clear and simple (sin2 sin J/ Ks Use K + and K 0 to measure the CPV related elements To construct the K unitarity triangle and confirm the “golden” relation which is valid in the SM and MFV. A.J. Buras et.al hep-ph/0405132

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Experiment is challenging Weapons needed to suppress background by 10 10 in order to see the signal. ProcessEvents K 6343000000 K 2113000000 K 55000000 Beam bkg25000000 K n p, K l - 46000 K 1 Where can we find such a “WMD”? Standard Model: Br( K + 10 -10

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BNL-E949 detector -- Rejecting beam backgrounds e+e+ Beam backgrounds include pion scattering, kaon decay-in-flight, and charge-exchange reactions. cerenkov K cerenkov B4 K Top half of side view Charge exchange Beam 1 Beam 2 K cluster cluster K decay Target fibres

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BNL-E949 detector -- Powerful and redundant particle ID Resolutions: P/P ~ 1.1%; R/R~ 3.0%; Rejections for ~ for photon with 4 sr coverage. Top half of end view -ID from its decay chain. dE/dxR/P Photon veto E787 E949

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This is a team effort! 16 institutes and universities in 4 countries. …more than 60 physicists from 6 countries. BNL/FNAL/SBU/UNM, U.S.A IHEP/INR, Russia Fukui/KEK/Kyoto/NDA/Osaka, Japan TRIUMF/UA/UBC, Canada

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Many thanks to BNL-AGS people Before data taking After data taking Twice the instantaneous intensity Platinum target used in 2002

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Unique Strategy for data analysis Full data set 1/3 data2/3 data Beam bkg Background study Background estimate Step 1 Step 2 Step 3 Step 4 Open the box Step 5 E787/E949: we bought a one-way ticket,- no way back after opening the box! Cuts tuning E787: <0.1 event E949: <0.5 event

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Background suppression Cut Bkg Kinematics cuts(p/R/E) Particle ID Photon veto Timing cuts K K Beams K n p, K l -

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How to achieve a reliable background estimate? Blind analysis in tuning the cuts. Bifurcated analysis in estimating backgrounds. 1.Form two independent cuts. 2.Estimate the background. 3.Check the correlation.

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Backgrounds inside the box Items E949E787 Nk(10 12 )1.85.9 K 0.044 ± 0.0050.062 ± 0.045 K 0.216 ± 0.0230.034 ± 0.007 Beams0.024 ± 0.0100.025 ± 0.016 K n p, K l - 0.014 ± 0.0030.025 ± 0.008 Total bkg (evts)0.298 ± 0.0260.146 ± 0.049 Errors are statistical only.

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What could be our nightmares? Correlation between two bifurcated cuts. A 0.3 event background estimated, but more than 3 events observed! Appearance of pathology backgrounds. Underestimating background because… For example… It can happen if an observable is used in both CUT1 and CUT2 categories, spoiling the independence of the bifurcated analysis. It rarely happens but can appear thru a loophole. Hand-scanning the events outside the box is the only way to spot the problem.

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Example1: Correlation between two cuts Loosen PV x KIN Obs.Pred. 20 x 2031.8 20 x 5083.0 50 x 502211.1 Loosen PV x KIN Obs.Pred. 20 x 2044.9 20 x 50912.4 50 x 502231.1 A new geometrical cut that gained 10% more rejection again 0 was found to cause trouble in the 1/3 outside-the-box study: N(obs.)~2.0 N(pred.) retune PV Reject any event with a photon in opposite side. Reason: a new cut was developed that used the direction of the + to place a tighter requirement again photons Remedy: remove this cut & retune PV.

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Example 2: Pathology background K+K+ K+K+ B4 Target Pions scattered back into the target overlaying a non- decaying kaon can make a fake signal. x y z r Reason: track reconstruction only down to the kaon decay vertex. Remedy: check the hits in the opposite direction of the track. Readout fibre

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Backgrounds outside the box Motivations: 1)To check the correlations; 2)To estimate systematic errors. Bkg.C Prob. 0.85 0.17 1.15 0.67 1.06 0.40 Fit N obs. = c N pred. N pred. N obs. Deviations of c from unity give the systematic errors.

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Acceptance Br( P.D.G value is 0.211±0.001 Phase space and nuclear interaction effects from Monte Carlo. Acceptance loss due to PID, photon veto, timing and other kinematics cuts directly measured from data. E949 acceptance: 0.0022 ± 0.002 E787 acceptance: 0.0020 ± 0.002 Verification from B( measurements.

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Open the box and guess what? We see one new event! E949

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Branching Ratio Result To make best use of our knowledge of the signal and the background distributions inside the box, we define cells inside the box. E949E787 CandidateE949AE787AE787C S/b0.9507 W0.480.980.88 Br ( 1.47 ) 10 -10 B i : background of cell containing candidate. S i : Br A i N K. N K : Stopped K + ’s. A i : acceptance in a cell. Br: branching ratio. W: S/(S + b).

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Impact on CKM matrix More data needed for reducing the uncertainty. New neutral kaon experiments (for example KOPIO) expected for further justification. More data needed for reducing the uncertainty. New neutral kaon experiments (for example KOPIO) expected for further justification. Many thanks to A. Höcker & J. Ocariz et al The CKM fitter group, hep-ph/0406184

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Summary A successful run of E949 with a factor of two more beam intensity as in E787. A new candidate event observed for K + , leading to a combined result from E787/E949, Br( K + 10 -10 ~2 Br( K + SM, if (sin2 sin J/ Ks. Statistical fluctuation or evidence of new physics? hep-ex/0403036

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