Radiative Leptonic B Decays Edward Chen, Gregory Dubois-Felsmann, David Hitlin Caltech BaBar DOE Presentation Aug 10, 2005

Caltech BaBar DOE Presentation 2 The purely leptonic decay B +  l +  l provides a theoretically clean means of testing a QCD calculation of a simple process:  But these decays are difficult to observe: Intro b u l+l+ W+W+ Calculable in Lattice QCD  – The radiation of a photon relieves the helicity suppression, at the cost of an additional 30-50% theoretical uncertainty (at least for the time being). ~ ModeChallengeSM predictionCurrent upper limit (90% C.L.) B! B!  Helicity suppression (4 § 2)  <6.6 £ BaBar PRL 92, ‘04 B! B!  Multi- final state(9 § 4)  <2.6 £ BaBar hep-ex/

Aug 10, 2005 Caltech BaBar DOE Presentation 3 Radiative leptonic decay Theoretical predictions: B(B +  l +  ) ~ (1–4)×10 -6 R, a ratio of moments of the spectator quark momentum, also arises in the description of the nonfactorizable corrections to certain hadronic B decays. A measurement or even an upper limit on the branching fraction of B ! l  would allow us to derive non-trivial constraints on R. (Korchemsky, Pirjol, and Yan, PRD , ’00) Previous measurement: CLEO (PRD ) using 2.5 fb -1 : Simple inclusive neutrino reconstruction  B( B  e e  ) < 2.0  (90% C.L.) (unidentified backgrounds?)  B( B    ) < 5.2  (90% C.L.) Belle (hep-ex/ ) prelim using 140 fb -1 :  B( B  e e  ) < 2.2  (90% C.L.)  B( B    ) < 2.3  (90% C.L.)

Aug 10, 2005 Caltech BaBar DOE Presentation 4 Analysis scheme I Neutrino reconstruction is required  Recoil against fully reconstructed hadronic B decay: very clean reconstruction  Recoil against semileptonic B decay: more tagged events, poorer momentum constraint  Inclusive – sum up all missing E, p in event: high statistics, but high background Too few tags yet for recoil analyses – wait for at least ~400 fb -1

Aug 10, 2005 Caltech BaBar DOE Presentation 5 Analysis scheme II Use 210 fb -1 on-peak data (+ off-peak, MC) Blind analysis  Validate simulation with control samples before unblinding Event selection criteria:  Signal side: lepton, photon energies and angle  Recoil B side: total recoil energy and momentum  Continuum rejection: event shape variables  Two-photon rejection: longitudinal momentum, etc.  Neutrino reconstruction: missing E – missing |p| Iterative cut optimization procedure

Aug 10, 2005 Caltech BaBar DOE Presentation 6 Final signal extraction After all event selections:  Divide plane of recoil m ES and neutrino E-|p| into 1 signal and 3 sideband regions  Obtain signal, BB, and continuum (MC or off-res) templates  Perform maximum likelihood fit to extract event counts for each category Include the statistical uncertainties on the templates  Important for off-peak subtraction One or two degrees of freedom, depending on whether we fix overall normalization of continuum background  Region locations optimized and tested with toy Monte Carlo

Aug 10, 2005 Caltech BaBar DOE Presentation 7 Expected sensitivity Expected 90% upper limit for electron mode : 6.1 £  Assuming BF = 3 £  Statistical uncertainties only Studies under way for muon mode Electron mode signal MCElectron mode background MC Sideband regions Signal region

Aug 10, 2005 Caltech BaBar DOE Presentation 8 Systematic error studies underway Experimental systematics  Tracking efficiency  Neutral reconstruction efficiency  Signal lepton particle identification  Uncertainties in signal efficiency due to data vs Monte Carlo discrepancies Theoretical uncertainties  Sensitivity of our result to our signal model For example, form-factor dependence Changes to shape of selection variable distributions in signal Monte Carlo

Aug 10, 2005 Caltech BaBar DOE Presentation 9 Current status Completing systematic uncertainty analysis now Muon mode next Review committee / unblinding Fall 2005