Search for B  t n with SemiExclusive reconstruction

1. Search for B tn with SemiExclusive reconstruction C.Cartaro, G. De Nardo, F. Fabozzi, L. Lista Università & INFN - Sezione di Napoli

2. Searches for B tn at BaBar • Two analyses • based on 1999-2002 data sample (81.9 fb-1) • First: reconstruct one B meson • Semi-Exclusively • In a DlnX mode (X = g, p0, nothing) • Then: the recoil is analyzed to search for a B tn decay • t decays are reconstructed in the following modes: SemiExclusive Tags • (e, ) (e, )  • (, 0, )  SemiLeptonic Tags •  (e, ) (e, )  • The samples of the two analyses are statistically independent and the results have been combined F.Fabozzi

3. B sample counting • In Semi-Exclusive analysis we fully reconstruct a B meson in a B  DXhad mode • For the BR determination we need the number of B+B- events with a fully reconstructed B • determined with a fit to mES • for the systematics we fit also with a Gaussian (4.5% less events) • Too conservative? We’ll try to fit with a double Gaussian • mES distribution fitted as Argus  Crystal ball • B sample counting: • NBB = (1.670.09)105 sideband peak F.Fabozzi

4. Events preselection mESdata, 1GTL & 00 • Preselections for the interesting channels • 1 GTL & 0 p0 • 1 GTL & 1 p0 • 3 GTL & 0 p0 • Then fit to the mES distributions • Crystal Ball + Argus • Fits after the preselection used for • data-MC comparison • expected background prediction • The shape of the Argus function after the preselection is in agreement with the shape after the full selection mESdata, 3GTL & 00 mESdata, 1GTL & 10 F.Fabozzi

5. Selections of tau decays 1-prong events • 1 track, 0 0 • No KS • Neutral Energy <110MeV • Neutral Bumps < 1 • Pmissing > 1.2 GeV/c • Kaon veto • Particle ID •  only • Lepton veto • pc.m.s. > 1.2 GeV/c • SemiExcl purity mode > 50% •  events • decay proceeds via two intermediate resonances, an a1 and a  • 3tracks and00 • Pmissing > 1.2 GeV/c • Neutral Energy < 100 MeV • Neutral Bumps < 1.5 • 600 MeV < m(0) < 950 MeV • 1.1 GeV < m(+) < 1.6 GeV • |p1 + p2 +p3 |> 1.6 GeV/c (c.m.s.) • Lepton & kaon veto • SemiExcl purity mode > 30%  0 events decay proceeds via an intermediate  • 1 track and 10 • pmissing> 1.4 GeV/c • Neutral Energy < 100MeV • 0.55 GeV < m(0) < 1 GeV • SemiExcl purity mode > 50% • Optimized for the best upper limit • Rejecting events with pmiss in the beam pipe? F.Fabozzi

6. Integrated purity cut • For each selection we apply a cut on the “nominal”integrated purity of the Breco side • Different from the integrated purity that we see • from the fit to mES distribution • Not relevant from the point of view of the analysis but generated confusion during review Charged B sample Nominal Int. Pur. = 50 % F.Fabozzi

7. e    0 00  0 e 22.9% 0 0.6% 0 0 0 0  0 7.4% 0.5% 0 0 0 0  0.1% 2.7% 21.6% 0.4% 1.2% 0 0.1%  0 0 0 6.8% 0 0 0 0 0 0.3% 1.0% 0.1% 6.6% 0.8% 0.6% Selection efficiencies • Total efficiency = 11.3% F.Fabozzi

8. Selected events B    Wrong sign control sample B    Unblinded data candidates F.Fabozzi

9. 1-prong lepton 1-prong pi rho-channel Hadronic Vcb Hadronic Vcb Semilept. Vub Semilept. Vcb Semilept. Vcb Semilept. Vcb Background from generic B+B- • Mostly from semileptonic Vcb • Can we fight this background looking at the charged tracks list? 3-prongs channel only 3 events selected 2 semilept. Vcb 1 hadronic charmless F.Fabozzi

10. Systematic error (I) • Uncertainty on NBB • Uncertainty on signal selection efficiencies • Better understanding of systematics on neutrals • Now: smearing procedure on the photon energy • Cut on neutral energy in the EMC • Sensitive to the Data-MC agreement in the low-energy region (ex. simulation of machine background in the EMC) • Developing a module to study t+t- events • Tag one t (t-3prongs) and look the neutral energy distribution associated to the other t decay • Data-MC comparison F.Fabozzi

11. Systematic error (II) • Uncertainty on the expected background (bi) estimate • Peaking background: from generic B+B- MC • More generic MC is available  will reduce main systematic error • Continuum+combinatorial background: from data sideband • scaling to signal region based on the fitted Argus shape • Possible dependencies of the fitted Argus shape on the variables used in the selection taken into account • mES fits in bins of a given variable • a correction factor for each variable • Total correction as the product of the individual corrections • assumption that variables are uncorrelated • the effect of correlation between variables is small (total error on bi: 4.9  5.0, since the statistical error on bi is dominant • We may think to remove highly correlated variables if they do not add in signal-to-background discrimination F.Fabozzi

12. Branching fraction extraction • -2logQmin gives the statistical significance of the signal • ~ 0.1s significance • Likelihood ratio estimator Q • Value of branching fraction  Minimum of -2logQ Unphysical region +3.8 Br(B-  t- n )= 1.1  10-4 -1.1 F.Fabozzi

13. Upper limit determination • Determination of C.L. and upper limit with a Toy MC • 10000 random experiments generated for 400 values of the branching fraction Br(B-  t -n )< 7.7  10-4 (90%C.L.) Uncertainty on bi included COMBINED with semi-lept analysis: Br(B-  t -n )< 4.1  10-4 (90%C.L.) F.Fabozzi

14. Expected sensitivity • For each channel we fluctuate the observed events according to a Poisson distribution F.Fabozzi

15. Conclusions • Conference paper will be made public soon • Some delay due to discussion on if/how to quote a central value • Another iteration of the analysis is starting • More signal and generic B+B- MC available • Better understanding of neutral systematics • Try to see if margins to improve background rejection • Semileptonic Vcb is the main source F.Fabozzi