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Rare Kaon Decay Results From E949 At BNL: K +  p + n n

Rare Kaon Decay Results From E949 At BNL: K +  p + n n. Shaomin CHEN TRIUMF, Canada. Motivation is clear and simple. To construct the K unitarity triangle and confirm the “golden” relation which is valid in the SM and MFV.

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Rare Kaon Decay Results From E949 At BNL: K +  p + n n

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  1. Rare Kaon Decay Results From E949 At BNL: K+p+ n n Shaomin CHEN TRIUMF, Canada ICHEP2004, Beijing

  2. Motivation is clear and simple • To construct the K unitarity triangle and confirm the “golden” relation which is valid in the SM and MFV. • Use K+p+ n nandK0p0 n n to measure the CPV related elements (sin2b)p n n = (sin2b)J/Ks A.J. Buras et.al hep-ph/0405132

  3. Experiment is challenging Standard Model: Br(K+p+ n n) = (0.790.12)  10-10 • Weapons needed to suppress background by 1010 in order to see the signal. • Where can we find such a “WMD”?

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

  5. BNL-E949 detector -- Powerful and redundant particle ID Top half of end view E949 R/P dE/dx E787 Photon veto -ID from its decay chain. Resolutions: DP/P ~ 1.1%; DR/R~ 3.0%; DE/E ~ 1.0%/E. Rejections:~105 for m;~106 for photon with 4p sr coverage.

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

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

  8. Unique Strategy for data analysis Full data set Step 1 1/3 data 2/3 data Step 2 m+nm(g) p+p0 Beam bkg Cuts tuning Step 3 E787: <0.1 event E949: <0.5 event Background study Background estimate Step 4 E787/E949: we bought a one-way ticket,-no way back after opening the box! Open the box Step 5

  9. Background suppression

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

  11. Backgrounds inside the box • Errors are statistical only.

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

  13. Example1:Correlation between two cuts A new geometrical cut that gained 10% more rejection again 0was found to g1 g2 p+ p0 Reject any event with a photon in opposite side.  retune PV cause trouble in the 1/3 outside-the-box study: N(obs.)~2.0 N(pred.) 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.

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

  15. Backgrounds outside the box • Motivations: • To check the correlations; • To estimate systematic errors. Npred. Fit Nobs. = c Npred. Nobs. +0.12 -0.11 +0.25 -0.21 +0.35 -0.29 Deviations of cfrom unity give the systematic errors.

  16. Acceptance • 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(K+p+p0) measurements. Br(K+p+p0)=0.2190.005 P.D.G value is 0.211±0.001

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

  18. 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. Bi: background of cell containing candidate. Si :Br  Ai NK. NK: Stopped K+’s. Ai: acceptance in a cell. Br: branching ratio. W:S/(S + b). +1.30 -0.89

  19. Impact on CKM matrix • 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

  20. Summary • A successful run of E949 with a factor of two more beam intensity as in E787. • A new candidate event observed for K+p+ n n, leading to a combined result from E787/E949, Br(K+p+ n n) = (1.47 )  10-10 +1.30 -0.89 hep-ex/0403036 ~2Br(K+p+ n n)SM, if(sin2b)p n n=(sin2b)J/Ks. Statistical fluctuation or evidence of new physics?

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