Theoretical Motivation Analysis Procedure Systematics Results

1. Theoretical Motivation • Analysis Procedure • Systematics • Results David Doll, on behalf of the BaBar Collaboration APS 04/12/08

2. arXiv:0708.4089v2 [hep-ex], PRL 99, 221802 (2007) *535 M pairs at Belle • Highly suppressed Flavor Changing Neutral Current • Not well constrained experimentally • Several models enhance BF(Unparticle Model, MSSM at large tan β,…) BaBar’s previous best upper limit is 7.8x10-5 for semileptonic tags with 81.9 fb-1 Current analysis at 319 fb-1 Theoretical Motivation APS 04/12/08

3. Perform a ‘semileptonic’ tagged analysis • Fully reconstruct the ‘tag B’ in the decay • Look at the rest of the event for our signal Tag B Signal B B+ B- Analysis Procedure, Tagging APS 04/12/08

4. Use a multivariate analysis tool from StatPatternRecognition (arXiv:physics/0507143v1) • Sampling with replacement of both the training data and the input variables (bagging) • Optimize the ‘Punzi’ Figure of Merit • The important input variables: • number of charged tracks in the signal B (opposite the ‘tag B’) • the missing energy in the event • the signal Kaon candidate’s momentum • the unmatched neutral energy in the event Random Forest (RF) APS B->Knunu 04/12/08

5. Use sideband region in • Estimate the continuum data in the signal region from amount of data in sideband RF continuum est. sideband Final Predictions, Continuum APS B->Knunu 04/12/08

6. Peaking estimate from RF output, separated into sideband/signal regions • Subtract sideband from signal region in both Data and MC and take the ratio MC:Data • Extrapolate a line into the signal region signal region trendline Final Predictions, Peaking APS B->Knunu 04/12/08

7. Continuum systematic from difference between MC and data • Peaking background systematic from difference between the a trendline fit to all the MC:Data, vs. a trendline fit to just the peaking component (above) • We also take a systematic based on our MC weighting procedure. Background Systematics MC Background prediction Statistical Uncertainty Systematic uncertainty APS B->Knunu 04/12/08

8. Both Bs decay semileptonicly requiring: • no remaining charged tracks in the event • momentum of each lepton>1.24 GeV/c • Resolved differences between signal MC and double tag data: • particle substitutions • kinematic corrections • brute force variable redistribution. • Serves as control sample for evaluating systematics for the multivariate analysis. B- B+ Control Sample APS B->Knunu 04/12/08

9. Tagging Efficiency: Taken from ratio below in which both tags are • Kaon Momentum: Evaluated by comparing phase space theory with SM-predicted theory Signal Systematics APS B->Knunu 04/12/08

10. Correlations btwn. Variables: • 1-D distributions already resolved • Need to account for correlations in order use the control sample to evaluate signal box efficiency in signal MC Signal Systematics APS B->Knunu 04/12/08

11. Signal Box Eff.: • Retrain RF with double tag MC control sample substituted for signal MC • Evaluate systematic by comparing efficiency of the RF cut on double tag MC to double tag data • Ntrkleft=1: • The control sample identified with this cut, not present in signal MC • Evaluate systematic from separate rectangular cut based investigation Signal Systematics APS B->Knunu 04/12/08

12. Upper limit at the 90% confidence level • Expect 30.7 +/- 10.7 events, corresponding to an upper limit of 2.9 x 10-5 • Inside the RF box, we saw 38 events, which gives an upper limit: Results APS B->Knunu 04/12/08