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

Theoretical Motivation Standard Model BF Experimental Limit on BF (90% CL)* 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 APS 04/12/08

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

Random Forest (RF) 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 APS B->Knunu 04/12/08

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

Final Predictions, Peaking 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 APS B->Knunu 04/12/08

Background Systematics 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. MC Background prediction Statistical Uncertainty Systematic uncertainty APS B->Knunu 04/12/08

Control Sample B- B+ 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- APS B->Knunu 04/12/08

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

Signal Systematics 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 APS B->Knunu 04/12/08

Signal Systematics Signal Box Eff.: Ntrkleft=1: 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 APS B->Knunu 04/12/08

Results 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: APS B->Knunu 04/12/08