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Radiative Leptonic B Decays Edward Chen, Gregory Dubois-Felsmann, David Hitlin Caltech BaBar DOE Presentation Aug 10, 2005
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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) 10 -7 <6.6 £ 10 -6 BaBar PRL 92, 221803 ‘04 B! B! Multi- final state(9 § 4) 10 -5 <2.6 £ 10 -4 BaBar hep-ex/0507069
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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 61 114510, ’00) Previous measurement: CLEO (PRD 1997 56 11) using 2.5 fb -1 : Simple inclusive neutrino reconstruction B( B e e ) < 2.0 10 -4 (90% C.L.) (unidentified backgrounds?) B( B ) < 5.2 10 -5 (90% C.L.) Belle (hep-ex/0408132) prelim using 140 fb -1 : B( B e e ) < 2.2 10 -5 (90% C.L.) B( B ) < 2.3 10 -5 (90% C.L.)
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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
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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
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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
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Aug 10, 2005 Caltech BaBar DOE Presentation 7 Expected sensitivity Expected 90% upper limit for electron mode : 6.1 £ 10 -6 Assuming BF = 3 £ 10 -6 Statistical uncertainties only Studies under way for muon mode Electron mode signal MCElectron mode background MC Sideband regions Signal region
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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
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Aug 10, 2005 Caltech BaBar DOE Presentation 9 Current status Completing systematic uncertainty analysis now Muon mode next Review committee / unblinding Fall 2005
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