Limits on the production of scalar leptoquarks from Z (0) decays at LEP

A search has been made for pairs and for single production of scalar leptoquarks of the first and second generations using a data sample of 392000 Z0 decays from the DELPHI detector at LEP 1. No signal was found and limits on the leptoquark mass, production cross section and branching ratio were set. A mass limit at 95% confidence level of 45.5 GeV/c2 was obtained for leptoquark pair production. The search for the production of a single leptoquark probed the mass region above this limit and its results exclude first and second generation leptoquarks D0 with masses below 65 GeV/c2 and 73 GeV/c2 respectively, at 95% confidence level, assuming that the D0lq Yukawa coupling alpha(lambda) is equal to the electromagnetic one. An upper limit is also given on the coupling alpha(lambda) as a function of the leptoquark mass m(D0)

First study of the interference between initial and final state radiation at the Z resonance

The interference between initial and final state radiation in the process \eemm\ at \sqs \approx \MZ has been studied by measuring the forward-backward asymmetry as a function of the acoplanarity angle between the final state muons. The interference is expected to be sensitive to the space-time separation of the initial and final state radiation. The measured asymmetry distribution has been compared to theoretical predictions using the \KORALZ\ generator, with and without Ördalph\ interference. The magnitude of the interference between initial and $\pm 0.06$ (syst.) \GeV} . \end{center} has been extracted. The interpretation of this result is discussed. There is an additional uncertainty in the estimate of \GZ\ from as yet uncalculated higher order interference terms. By assuming a value of \GZ\ consistent with the world average, the data were used to estimate the size of these uncalculated corrections. The interference between initial and final state radiation in the process \eemm\ at \sqs \approx \MZ has been studied by measuring the forward-backward asymmetry as a function of the acoplanarity angle between the final state muons. The interference is expected to be sensitive to the space-time separation of the initial and final state radiation. The measured asymmetry distribution has been compared to theoretical predictions using the \KORALZ\ generator, with and without \ordalph\ interference. The magnitude of the interference between initial and final state radiation was found to be of the order predicted and to follow the expected distribution. Using the theoretical predictions, a value of \begin{center} \mbox{ \GZ = 2.50 \pm 0.21 (stat.) \pm 0.06 (syst.) \GeV} . \end{center} has been extracted. The interpretation of this result is discussed. There is an additional uncertainty in the estimate of \GZ\ from as yet uncalculated higher order interference terms. By assuming a value of \GZ\ consistent with the world average, the data were used to estimate the size of these uncalculated corrections

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 Σ0\Sigma^{0} and Ω−\Omega^{-} in Z decays

Reconstructed \lam\ baryon decays and photon conversions in DELPHI are used to measure the \sig\ production rate from hadronic Z^0 decays at LEP. The number of \sig\ decays per hadronic Z decay is found to be: \begin{center} \sig + \sigb = \wffe{0.070}{0.010}{0.010 }{}. \end{center} \bigskip The \Omega^- production rate is similarly measured to be: \begin{center} \Omega^- + \overline{\Omega}^+ = \wffe{0.0014}{0.0002}{0.0004}

Search for exclusive decays of the Λb\Lambda_{b} baryon and measurement of its mass

A search for fully reconstructed \lb beauty baryons is performed using about 3 million Z decays collected with the DELPHI detector at LEP. The analysis relies on the combined use of the accurate tracking and of the hadron identification capabilities of DELPHI. A total of four events has been found, three in the \lc\ppm channel and one in the \lc\a1m channel over a small background. The \lb beauty baryon mass is measured to be (~ 5668 \pm 16~ ({\rm stat.}) \pm 8~({\rm syst.})~)~ \mv

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)

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