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Roberto Covarelli (Università degli Studi di Perugia & INFN Frascati)

PARTIAL RECONSTRUCTION OF B 0 D*D s ( * ) AND EXTRACTION OF BR(D s  fp ) AT THE BaBar EXPERIMENT. Roberto Covarelli (Università degli Studi di Perugia & INFN Frascati) Frascati Spring School, 22/5/2003. Physical motivations …. …for measuring BR(B 0  D*D s ( * ) )

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Roberto Covarelli (Università degli Studi di Perugia & INFN Frascati)

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  1. PARTIAL RECONSTRUCTION OF B0D*Ds(*) AND EXTRACTION OF BR(Ds  fp) AT THE BaBar EXPERIMENT Roberto Covarelli (Universitàdegli Studi di Perugia & INFN Frascati) Frascati Spring School, 22/5/2003 ROBERTO COVARELLI

  2. Physical motivations … • …for measuring BR(B0  D*Ds(*)) • They provide a test of factorization at high q2 values (q2 = MDs(*)2) • They are quite poorly known: BR(B0  D*-Ds+) = (1.11±0.33)% (PDG 2002) BR(B0  D*-Ds*+) = (1.8±0.6)% • …for measuring BR(Ds  fp) • All Ds branching fractions are measured as relative toBR(Ds  fp) • Latest measurement (CLEO, 1995) shows a 25% relative uncertainty: BR(Ds+ fp+) = (3.6±0.9)% • Most b- or c-physics analyses where a Ds or a Ds* are reconstructed rely on this uncertain value ROBERTO COVARELLI

  3. Partial reconstruction • We refer to partial reconstruction of a B candidate when all but one of its decay products (the “missing” candidate) are fully reconstructed. For instance: • Disadvantage: no constraints can be applied on MB or EB in signal extraction  worse signal purity • Advantage: you don’t need to take into account the missing candidate reconstruction efficiency  statistics increased by a large factor (often ≈10) B0 D* + p The fast and slow pion are reconstructed (blue circle), while no attempt is made to reconstruct the D0 (red box). D0p ROBERTO COVARELLI

  4. The B0  D*Ds(*) decay • Partial reconstruction technique can be exploited in two ways: • Fully reconstructed Ds(*) • Slow pion from D* •  missing candidate = D0 • or • Fully reconstructed D* • Soft photon from Ds* •  missing candidate = Ds B0 D* + Ds(*) D0p (Dsg) B0 D* + Ds* D0p Dsg ROBERTO COVARELLI

  5. Signal extraction variable:the missing mass • Assume that the reconstructed candidates come from a B0  D*Ds(*) decay, the mass of the missing candidate, or missing mass, can be evaluated: • Signal extraction is achieved by requiring it to stay inside a M(D0) (or M(Ds)) mass window. Method I: Ds(*) – p Method II: D*– g ROBERTO COVARELLI

  6. Extraction of BR(B0  D*Ds(*)) • Let ND0 (NDs) be the signal yield from the missing mass calculation. BR(B0  D*Ds(*)) can be promptly extracted: • If we use Method II, BR(B0  D*Ds(*)) will no longer depend on Ds branching fractions, but on D0 ones, which are known with smaller uncertainties.  reduced systematics from BR’s Method I: Ds(*) – p Method II: D*– g ROBERTO COVARELLI

  7. Extraction of BR(Ds  fp) • Let’s now consider both partial reco methods applied on the same data sample. Two independent estimates of BR(B0  D*Ds*) are obtained, one assuming D0 decay BR’s, the other assuming Ds BR’s. • Dividing (2) by (1), BR(Ds  fp) can be extracted: (1) (2) Ri = BRi / BR(D0 Kp) ROBERTO COVARELLI

  8. 1. Ds(*) – p reconstruction: candidate selection 20.8 fb-1 data • Dsselection: • Ds  fp, f  KK • Ds  K0K, K0s  pp • Ds  K*0K, K*0 Kp • Cuts on kaon ID, invariant mass of intermediate states, helicity angles • M(Ds) is required to be within 3sof the signal distribution peak Mpeak (Ds) seen in the data • For Ds* candidates only: • Photon selection : cuts on Eg and Eg*, p0 veto • |DM – DMpeak| < 2.5sDM M(Ds) peak M(Ds) DM ROBERTO COVARELLI

  9. 1. Ds(*) – p reconstruction: signal extraction • Observe the missing mass distributions • The Mmiss distribution is fitted with a Gaussian + a threshold background function • fB= f (Mmiss – M0). The yield in 20.8 fb-1 data (≈22.7 x 106 BB pairs) is shown in figure 20.8 fb-1 data Dsp ND0 = 3700 ± 230 Ds* p ND0 = 1493 ± 95 Kinematical end point M0 = MD* - Mp ROBERTO COVARELLI

  10. 1. Ds(*) – p reconstruction: final results • Branching fractions: BR(B0  D*-Ds+) = (1.03±0.14stat.±0.13syst.±0.26)% BR(B0  D*-Ds*+) = (1.97±0.15stat.±0.30syst.±0.49)% • The longitudinal polarization fraction in B0  D*Ds* (pseudoscalar  vector + vector decay) can also be measured: GL/G = (51.9 ± 5.0stat.±2.8syst.)% 25% systematics from the uncertainty on BR(Ds  fp) ROBERTO COVARELLI

  11. 2. D* – g reconstruction: D* candidate selection • 4 D0 decay modes: • D0  Kp • D0  Kpp0 • D0  Kppp • D0  K0spp, K0s  pp • Cuts on: • vertex fit probabilities • |Q(D*) – Qpeak(D*)| < 2 MeV/c2 (Q(D*) = M(D*) – M(D0) – M(p)) • |M(D0) – Mpeak(D0)| < 2.5sM(D0) Q(D*) in MC M(D0) in MC ROBERTO COVARELLI

  12. 2. D* – g reconstruction: g candidate selection • Kinematic cuts and p0 veto are not enough to reject the substantial background from low energy particles. More stringent requirements needed: • cuts on shower shape variables • best photon to be chosen on the basis of a likelihood ratio that combines both kinematical and cluster shape variables Eg* in MC Signal (dashed) Bkgd (solid) ROBERTO COVARELLI

  13. 2. D* – g reconstruction: signal extraction (MonteCarlo) • Fit the missing mass distribution with a Gaussian + ARGUSbackground function (fit yields in MC shown in the figure) BR(B0  D*-Ds*+) = (1.5±0.1)% (in MC)  No bias (1.4% in MC Production) ROBERTO COVARELLI

  14. 2. D* – g reconstruction: signal extraction (data) • The Mmiss plot on data (20.25 fb-1) is shown • The background shape and signal resolution are well reproduced by MonteCarlo but • Before we apply the fitting procedure, we need • cross-checks in order to validate this result ROBERTO COVARELLI

  15. 2. D* – g reconstruction: cross - checks M(D0) MC (B0) MC (B±) MC (qq) data • Data/MC agreement  Kolmogorov test for binned distributions • Other cross-checks: “flipped” sample (the reconstructed D* flight direction is inverted to obtain a pure combinatoric background sample) Missing mass normal flipped • Peaking backgrounds, systematics from detector • inefficiency and other sources still under evaluation ROBERTO COVARELLI

  16. Expected error on BR(Ds  fp) • Let’s put ourselves in the most favourable conditions (full BaBar statistics, just one D0 and Ds reconstruction mode, so that the systematics from branching fractions totally cancel) • The relative uncertainty on BR(B0  D*-Ds*+) (Method I) is: 6.6%stat. 9.4%syst. = 11.5% • and on BR(B0  D*-Ds*+) (Method II): 6.7%stat. 10%syst. = 11.6% • we estimate a 16% maximum uncertainty on BR(Dsfp) (was 25% in the most recently published measurement) Statistics from MonteCarlo Rough estimate ROBERTO COVARELLI

  17. Summary • The branching fractions BR(B0  D*-Ds+) and BR(B0  D*-Ds*+), as well as the fraction of longitudinal polarization in B0  D*-Ds*+, have been measured using a Ds(*) – ppartial reco technique. The results are: BR(B0  D*-Ds+) = (1.03±0.14stat.±0.13syst.±0.26Ds  fp)% BR(B0  D*-Ds*+) = (1.97±0.15stat.±0.30syst.±0.49Ds  fp)% GL/G = (51.9±5.0stat.±2.8syst.)% • The complementary reconstruction, D* - g, is being performed, in order to obtain a second, independent measurement of BR(B0  D*-Ds*+) and extract BR(Dsfp) ROBERTO COVARELLI

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