Measurement of the B-d(0) oscillation frequency using kaons, leptons and jet charge

A measurement of the mass difference, Delta m(d), be tween the two physical B-d(0) states has been obtained from the analysis of the impact parameter distribution of a lepton emitted at large transverse momentum (p(t)) relative to the jet axis and from the analysis of the flight distance distribution of secondary vertices tagged by either a high p(t) lepton or an identified kaon. In the opposite hemisphere of the event, the charge of the initial quark has been evaluated using a high p(t) lepton, a charged kaon or the mean jet charge, With 1.7 million hadronic Z(0) decays recorded by DELPHI between 1991 and 1993, Delta m(d) is found to be: Delta m(d) = 0.531 (+0.050)(-0.046) (stat.) +/- 0.078 (syst.) ps(-1)

Measurement of the forward-backward asymmetry of e(+)e(-)-]Z-]B(B)over-bar using prompt leptons and a lifetime tag

The forward-backward asymmetry of the process e(+)e(-) --> Z --> b ($) over bar b has been measured using events collected ee by the DELPHI experiment during the 1991 and 1992 LEP runs. This data sample corresponded to 884 000 hadronic Z decays at a centre-of-mass energy root s similar to M(Z). The tagging of b-quark events was performed using two approaches; the first was based on the semileptonic decay channels b --> X + mu and b --> X + e, the second used a lifetime tag with jet-charge reconstruction. The results of these two methods were combined to give A(FB)(b ($) over bar b) = 0.107 +/- 0.011(stat. + syst. + mixing). With the semileptonic sample, the forward-backward asymmetry of the process e(+)e(-) --> Z --> c ($) over bar c was also measured to be A(FB)(c ($) over bar c) = 0.083 +/- 0.022(stat.) +/- 0.016(syst.). The effective value of the Weinberg mixing angle derived from these measurements was sin(2) theta(eff)(lep) = 0.2294 +/- 0.0021

Determination of vertical bar V-cb vertical bar from the semileptonic decay B-0->D*(-)l(+)nu

Semileptonic decays B --> D*(-)l(+)nu X were selected from a sample of 3.1 million hadronic Z decays collected by the DELPHI detector at LEP. A topological search for semileptonic B decays to resonant and non-resonant D*(-)pi(+) states was performed and the ratio of the branching fractions: Br(B --> D*(-)l(+)nu X)/Br(B --> D*(-)l(+)nu X) + Br(B-0 --> D*(-)l(+)nu) = 0.19 +/- 0.10(stat) +/- 0.06(syst) was determined. Taking into account this contribution, the differential decay width of B-0 --> D*(-)l(+)nu was measured as a function of the momentum transfer from the B to the D*(-) in two separate analyses, using exclusive and inclusive methods of D*(-) reconstruction. The distributions were fitted over the full momentum transfer range to extract the product of /V-cb/ times the normalization of the decay form factor F(q(max)(2)): F(q(max)(2))/V-cb/ = (35.4 +/- 1.9(stat) +/- 2.4(syst)) . 10(-3). The value of /V-cb/ was computed using theoretical calculations of F(q(max?2), giving: /V-cb/ = (38.9 +/- 2.0(stat) +/- 2.6(syst) +/- 1.7(theory)) . 10(-3). The total branching fraction Br(B-0 --> D*(-)l(-)nu) was determined to be: Br(B-0 --> D*(-)l(+)nu) = (5.52 +/- 0.17(stat) +/- 0.68(syst))%

Production of Strange B-baryons Decaying Into Xi-/+-l-/+ Pairs At Lep

An excess of events containing, in a jet, a same-sign Xi(-/+) - l(-/+) pair as compared to those with an opposite-sign Xi(-/+) - l(-/+) pair has been observed in an analysis of 1.7 million hadronic Z(o) decays collected by the DELPHI detector at LEP between 1991 and 1993 inclusive. The probability for this signal to come from non B-baryon decays is less than 5 x 10(-4). The measured production fraction corresponds to: P(b --> B - baryon) x BR(B - baryon --> Xi(-) l(-) X) = (5.9 +/- 2.1 +/- 1.0) x 10(-4), per lepton species, averaged for electrons and muons and assuming the two channels have an equal contribution. Semileptonic decays of Lambda(b) baryons can account for less than 10% of these events and the major part of the signal has to originate from Xi(b) semileptonic decays, Using the subsample of these events where the Xi(-/+) trajectory has been measured in the Vertex Detector, the lifetime of B-baryons producing a Xi(-/+) in their semileptonic decay final state is found to be: tau(B - baryon --> Xi(-) l(-) X) = 1.5(-0.4)(+0.7) +/- 0.3 ps

Measurement of the partial decay width R(b)(0)=Gamma(b(b)over-bar)/Gamma(had) of the Z with the DELPHI detector at LEP

The partial decay width of the Z to quark pairs has been measured by the DELPHI detector at LEP. B-hadrons, containing b-quarks, were tagged by several methods using tracks with large impact parameters to the primary vertex complemented sometimes by event shape variables or using leptons with high transverse momentum relative to the hadron. In order to reduce the systematic uncertainties, in all methods the b-tagging efficiency has been extracted directly from the data. Combining all methods, the value: Gamma()/Gamma(had) = 0.2216 +/- 0.0016(stat.) +/- 0.0021(syst.) was found, where the production fraction was fixed to its Standard Model value

Determination of |Vcb_{cb}| from the semileptonic decay B0^{0} --> D∗−*^{-}l+ν^{+}\nu

Semileptonic decays B \rightarrow D^{*-} \ell^+ \nu X were selected from a sample of 3.1 million hadronic Z decays collected by the DELPHI detector at LEP. A topological search for semileptonic B decays to resonant %(D^{**}) and non-resonant D^{*-} \pi^{+} states was performed and the ratio of the branching fractions: \frac {Br ({\mathrm{B} \rightarrow {\mathrm D}^{*-}} \ell^+ \nu X)} {Br({\mathrm{B} \rightarrow {\mathrm D}^{*-}} \ell^+ \nu X) + Br( {\mathrm{B}^0 \rightarrow \mathrm{D}^{*-}} \ell^+ \nu)} = 0.19 \pm 0.10({\mathrm{stat}}) \pm 0.06({\mathrm{syst}}) was determined. Taking into account this contribution, %DB the differential production fraction of the decay the differential decay width of {\mathrm{B}^{0} \rightarrow \mathrm{D}^{*-}} \ell^+ \nu was measured as a function of the momentum transfer from the B to the D^{*-} in two separate analyses, using exclusive and inclusive methods of D^{*-} reconstruction. The distributions were fitted %DB with a linear function over the full momentum transfer range to extract the product of \mathrm{|V_{cb}|} times the normalization of the decay form factor F(q^2_{max}): F(q^2_{max})\mathrm{|V_{cb}|} = (35.0 \pm 1.9({\mathrm{stat}}) \pm 2.3({\mathrm{syst}}) )~\cdot~10^{-3}. The value of \mathrm|V_{cb}| was computed using theoretical calculations of F(q^2_{max}), giving: {\mathrm{|V_{cb}|}} = (38.5 \pm 2.1({\mathrm{stat}}) \pm 2.5({\mathrm{syst}}) \pm 1.7({\mathrm{theory}}))~\cdot~10^{-3}. The total branching fraction Br(\mathrm{B}^{0} \rightarrow D^{*-} \ell^+ \nu) was determined to be: Br ({\mathrm{B}^0 \rightarrow \mathrm{D}^{*-}} \ell^+ \nu) = (5.47 \pm 0.16({\mathrm{stat}}) \pm 0.67({\mathrm{syst}})) \%

Determination of the average lifetime of b-baryons

The average lifetime of b-baryons has been studied using 3 × 106 hadronic Z0 decays collected by the DEL-PHI detector at LEP. Three methods have been used, based on the measurement of different observables: the proper decay time distribution of 206 vertices reconstructed with a Λ, a lepton and an oppositely charged pion; the impact parameter distribution of 441 muons with high transverse momentum accompanied by a Λ in the same jet; and the proper decay time distribution of 125 Λc-lepton decay vertices with the Λc exclusively reconstructed through its pKπ, pK0 and Λ3π decay modes. The combined result is: τ(b-baryon) = (1.254-0.109+0.121(stat) ±0.04(syst)-0.05+0.03(syst)) ps where the first systematic error is due to experimental uncertainties and the second to the uncertainties in the modelling of the b-baryon production and semi-leptonic decay. Including the measurement recently published by DELPHI based on a sample of proton-muon vertices, the average b-baryon lifetime is : τ(b-baryon) = (1.255-0.102+0.115(stat) ±0.05) ps. © Springer-Verlag 1996

Measurement of inclusive pi0 production in hadronic Z0 decays

An analysis is presented of inclusive \pi^0 production in Z^0 decays measured with the DELPHI detector. At low energies, \pi^0 decays are reconstructed by \linebreak using pairs of converted photons and combinations of converted photons and photons reconstructed in the barrel electromagnetic calorimeter (HPC). At high energies (up to x_p = 2 \cdot p_{\pi}/\sqrt{s} = 0.75) the excellent granularity of the HPC is exploited to search for two-photon substructures in single showers. The inclusive differential cross section is measured as a function of energy for {q\overline q} and {b \bar b} events. The number of \pi^0's per hadronic Z^0 event is N(\pi^0)/ Z_{had}^0 = 9.2 \pm 0.2 \mbox{(stat)} \pm 1.0 \mbox{(syst)} and for {b \bar b}~events the number of \pi^0's is {\mathrm N(\pi^0)/ b \overline b} = 10.1 \pm 0.4 \mbox{(stat)} \pm 1.1 \mbox{(syst)} . The ratio of the number of \pi^0's in b \overline b events to hadronic Z^0 events is less affected by the systematic errors and is found to be 1.09 \pm 0.05 \pm 0.01. The measured \pi^0 cross sections are compared with the predictions of different parton shower models. For hadronic events, the peak position in the \mathrm \xi_p = \ln(1/x_p) distribution is \xi_p^{\star} = 3.90^{+0.24}_{-0.14}. The average number of \pi^0's from the decay of primary \mathrm B hadrons is found to be {\mathrm N} (B \rightarrow \pi^0 \, X)/\mbox{B hadron} = 2.78 \pm 0.15 \mbox{(stat)} \pm 0.60 \mbox{(syst)}