200 research outputs found

    Observation of orbitally excited B mesons

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    Experimental evidence for the existence of orbitally excited B meson states is presented in an analysis of the Bπ and B*π distribution of Q = m(B**) - m(B(*)) - m(π) using Z0 decay data taken with the DELPHI detector at LEP. The mean Q-value of the decays B** → B(*)π is measured to be 284 ± 5 (stat.) ± 15 (syst.) MeV/c2, and the Gaussian width of the signal is 79 ± 5 (stat.) ± 8 (syst.) MeV/c2. This signal can be described as a single resonance of mass m = 5732 ± 5 (stat.) ± 20 (syst.) MeV/c2 and full width Γ = 145 ± 28 MeV/c2. The observed shape is also consistent with the production of several broad and narrow states as predicted by the quark model and partly observed in the D-meson sector. The production rate of B** per b-jet is found to be 0.27 ± 0.02 (stat.) ± 0.06 (syst.).0info:eu-repo/semantics/publishe

    A Measurement of the mean lifetimes of charged and neutral B hadrons

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    The decays of B-hadrons have been reconstructed using the charged particles recorded in the DELPHI silicon microstrip detector. The sum of the charges of the secondaries determines the charge of the B-hadron parent. Some 232 114 multihadronic Z0 decays recorded during the 1991 run of LEP at centre-of-mass energies between 88.2 GeV and 94.2 GeV yield 253 B-hadron candidates with well-measured charge. From these the mean lifetimes of neutral and charged B-hadrons are found to be 1.44 ± 0.21 (stat.) ± 0.14(syst.) ps and 1.56 ± 0.19(stat.) ± 0.13(syst.) ps respectively. The ratio of their lifetimes is 1.09+0.28 -0.23(stat.) ± 0.11 (syst.). Under some assumptions on the abundance and lifetime of the Λ0 b and B0 s states, the B0 and B+ lifetimes are inferred.0SCOPUS: ar.jinfo:eu-repo/semantics/publishe

    Determination of alpha-s for b quarks at the Z0 resonance

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    The strong coupling constant for b quarks has been determined, and its flavour independence, as predicted by QCD, investigated. The analysis involved events with lepton candidates selected from approximately 356 000 hadronic decays of the Z0, collected by the DELPHI detector at LEP in 1990 and 1991 A method based on a direct comparison of the three-jet fraction in a b enriched sample, selected by requiring leptons with large momenta and transverse momenta, to that of the entire hadronic sample, illustrated the significant effect of the b quark mass on the multi-jet cross section, and verified the flavour independence of the strong coupling constant to an accuracy of ±6%. A second procedure based on a fit to the momentum and transverse momentum spectra of the lepton candidates in both two-jet and three (or more)-jet event samples simultaneously determined the b content in each, and, using second order QCD calculations, gave an absolute measurement of αs for b quarks of 0.118 ± 0.004 (stat.) ± 0.003 (syst.) ± 0.008 (scale). A comparison with αs for all quark flavours, as measured from the three-jet fraction in all hadronic events, further allowed the coupling strength for b quarks to be expressed in terms relative to that for udsc quarks, thereby cancelling certain common systematic uncertainties, and yielded αsb/αsudsc = 1.00 ± 0.04 (stat.) ± 0.03 (syst.)

    A Study of B0 - anti-B0 mixing using semileptonic decays of B hadrons produced from Z0

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    The B0-B̄0 mixing was studied by using about 250000 hadronic decays of the Z0,_collected with the DELPHI detector at LEP. With 1665 dilepton events, the probability for a b quark to become a b̄ before decaying was found to be χ = 0.121-0.040 0.044 ± 0.017. The semileptonic branching ratio of the b was measured from the dilepton and single lepton events and found to be Br(b → l) = (10.0 ± 0.7 ± 0.7)%.0SCOPUS: ar.jinfo:eu-repo/semantics/publishe

    Production of charged particles, K0(s), K+-, p and Lambda in Z --> b anti-b events and in the decay of b hadrons

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    A sample of events enriched in bb̄ quark pairs was selected in the data recorded by the DELPHI experiment at LEP during 1992 and 1993, by the presence of secondary decay vertices from short-lived particles. Using this sample, the average multiplicities of KS0, K±, p(p̄), Λ(Λ̄) and of charged particles in bb̄ events have been measured, distinguishing the component from fragmentation and the component coming from the decay of b-hadrons. The measurement of the average charge multiplicity in bb̄ events was used to compute the mean fractional beam energy carried by the primary b-hadron, and the difference in charged particle multiplicity between bb̄ events and light quark (uū, dd̄, ss̄) events

    Measurement of lambda(b) production and lifetime in Z0 hadronic decays

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    Evidence for the production of b-flavoured baryons in Z0 decays is reported from the analysis of 365 000 hadronic events collected by the DELPHI detector. Assuming that most of the weakly decaying b flavoured baryons produced by the Z0 are Λb, the observed signal of (Λ-+Λl+) pairs, where l is a high transverse momentum lepton in the same jet as the Λ, leads to a production rate: f(b→Λb)×Br(Λb→ Λlν X) = (0.41±0.13±0.09)%. Using a sample of 18 reconstructed Λb decay vertices, the lifetime is determined to be τ(Λb) = (1.04+0.48-0.38±0.10) ps.0info:eu-repo/semantics/publishe

    Evidence for B(s)0 meson production in Z0 decays

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    Seven unambiguous events out of a sample of 270 000 Z0 decays, contain in the same jet a Ds meson and a muon at large transverse momentum relative to the jet axis. These events are direct evidence for Bs 0 meson production in hadronic Z0 decays. The production rate of these events, relative to all hadronic Z0 decays is (18 ± 8) × 10-5, this number including the relevant branching fractions of the Bs 0 and Ds. The value of the Bs 0 meson lifetime relative to the average B meson lifetime is measured to be 0.8 ± 0.4.0SCOPUS: ar.jinfo:eu-repo/semantics/publishe

    Observation of B0Bˉ0B^0 - \bar B^0 Mixing

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    Using the ARGUS detector at the DORIS II storage ring we have searched in three different ways for B 0 - B 0 mixing in ϒ (4S) decays. One explicitly mixed event, a decay ϒ (4S)→B 0 B 0 , has been completely reconstructed. Furthermore, we observe a 4.0 standard deviation signal of 24.8 events with like-sign lepton pairs and a 3.0 standard deviation signal of 4.1 events containing one reconstructed B 0 ( B 0 ) and an additional fast ℓ + (ℓ) − . This leads to the conclusion that B 0 - B 0 mixing is substantial. For the mixing parameter we obtain r =0.21±0.08

    Measurement of the Z0 branching fraction to b quark pairs using the boosted sphericity product

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    From a sample of about 120 000 hadronic Z0 decays, using a technique based on a separation of the different event categories in the boosted sphericity product, the fraction of bb̄ decays has been measured to be 0.219 ± 0.014 (stat) ⋯ 0.019 (syst). Using the DELPHI determination of the hadronic Z0 width, this corresponds to a partial width Γbb- = 378 ± 42 MeV (in good agreement with the standard model prediction of ≃ 380 MeV). Combining this measurement with the determinations based on events with high pt leptons gives an estimate for the branching ratio of b into leptons at LEP of (11.2 ± 1.2)%, consistent with previous determinations.0SCOPUS: ar.jinfo:eu-repo/semantics/publishe

    Measurement of Gamma (b anti-b) / Gamma (hadrons) using impact parameter measurements and lepton identification

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    The partial decay width of the Z to {Mathematical expression} quark pairs has been measured by the DELPHI detector at LEP. b-hadrons, containing b-quarks, were tagged by leptons with high transverse momentum relative to the hadron or by tracks with large impact parameters to the primary vertex. The ratio of the numbers of events with a single such tag to those with two tags was used to estimate the efficiency of the method and to reduce the systematic uncertainty. Combining all methods, the value: {Mathematical expression} was found, where the third error corresponds to a±8% uncertainty on the {Mathematical expression} production width. A maximum likelihood in to the single and di-lepton distributions gave the branching fraction of the decays of b-quarks to leptons as: {Mathematical expression} © 1995 Springer-Verlag.0SCOPUS: ar.jinfo:eu-repo/semantics/publishe
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