Theoretical MotivationAnalysis ProcedureSystematicsResults

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APS 04/12/08

David Doll, on behalf of the

BaBar Collaboration

ννKB

ννKB

Theoretical Motivation

• Highly suppressed Flavor Changing Neutral Current• Not well constrained experimentally• Several models enhance BF(Unparticle Model, MSSM at large tan β,…)

Standard Model BF Experimental Limit on BF (90% CL)*

62.16.0 108.3

5104.1

2

*535 M pairs at Belle

arXiv:0708.4089v2 [hep-ex],PRL 99, 221802 (2007)

BaBar’s previous best upper limit is 7.8x10-5 for semileptonic tags with 81.9 fb-1

Current analysis at 319 fb-1

BB

APS 04/12/08 ννKB

Analysis Procedure, TaggingPerform a ‘semileptonic’ tagged analysis

◦Fully reconstruct the ‘tag B’ in the decay◦Look at the rest of the event for our signal

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νlDB 0

}πππ,ππ,{πKD 00

B+

0D

l

B-

KTag B Signal B

APS 04/12/08 ννKB

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

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BNS

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APS B->Knunu 04/12/08

Final Predictions, Continuum Use sideband region in

Estimate the continuum data in the signal region from amount of data in sideband

RF continuum est.

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0Dm

πKfor m -D0

sideband

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

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APS B->Knunu 04/12/08

trendline0Dm signal

region

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.

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Statistical Uncertainty

Systematic uncertainty

APS B->Knunu 04/12/08

MC Background prediction

Control Sample• 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.

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APS B->Knunu 04/12/08

B-

0D

l

B+

0D

l

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

) tagsingl tagdouble(

) tagsingle tagdouble(

MC

Data

R

R

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APS B->Knunu 04/12/08

πKD0

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

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APS B->Knunu 04/12/08

Signal Systematics 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

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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:

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C.L. 90% @104.2)ννKBF(B 5

APS B->Knunu 04/12/08