572 research outputs found

    Measurement of the rate of b anti-b b anti-b events in hadronic Z decays and the extraction of the gluon splitting into b anti-b

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    The rate ZbbbbZ \rightarrow b\overline{b}b\overline{b} was measured using about 2x1062 x 10^6 hadronic decays collected by the DELPHI experiment in 1994 and 1995. Events were forced into 3-jets with ymin>0.06y_{min} > 0.06 and a b-tag was required for every jet. The rate was measured to be: R4b=BR(Zbbbb)/BR(Zhadrons)=(6.0+1.9(stat)+1.4(syst.))x104R_{4b} = BR(Z \to b\overline{b}b\overline{b}) / BR(Z \to hadrons) = (6.0 +- 1.9(stat) +- 1.4(syst.)) x 10^{-4} where the invariant mass of every bbb\overline{b} system is above twice the b quark mass. Using the value of R4bR_{4b} the probability of secondary production of a bbb\overline{b} pair from a gluon per hadronic ZZ decay, gbbg_{bb}, was extracted and found to be: gbb=(3.3+1.0(stat.)+0.8(syst.))x103g_{bb} = (3.3+-1.0(stat.) +- 0.8(syst.)) x 10^{-3}

    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

    m(b) at M(Z)

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    The value of the bb quark mass at the MZM_Z scale defined in the MS\overline{MS} renormalization scheme, mb(MZ)m_b(M_Z), is determined using 2.8 million hadronic Z decays collected during 1992-1994 by the DELPHI detector to be, mb(MZ)=2.67±0.25 (stat.)±0.34 (frag.)±0.27 (theo.) GeV/c2. m_b(M_Z)=2.67 \pm 0.25\ ({\rm stat.}) \pm 0.34\ ({\rm frag.}) \pm 0.27\ ({\rm theo.}) \ {\rm GeV/c^2}. This analysis considers NLO corrections to the three-jet production rate including mass effects, and the result obtained agrees with the QCD prediction of having a running bb-quark mass at an energy scale equal to MZM_Z. This is the first time that such a measurement has been performed far above the bbb\overline{b} productio

    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

    Search for exclusive decays of the Lambda(b) baryon and measurement of its mass

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    A search for fully reconstructed Lambda(b)(0) 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 Lambda(c)(+)pi(-) channel and one in the Lambda(c)(+)a(1)(-) channel over a small background. The Lambda(b)(0) beauty baryon mass is measured to be (5668 +/- 16 (stat.) +/-8 (syst.)) MeV/c(2)

    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.)

    SEARCH FOR THE t AND b-prime QUARKS IN HADRONIC DECAYS OF THE Z0 BOSON

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    We present a search for the third generation up type quark t and a possible fourth down type quark b' in hadronic Z0 decays observed in DELPHI at the LEP collider. For any scenario with a decay through the charged current or into a charged Higgs with a mass at least 6 GeV/c2 below the t and 3 GeVc2 below the b' mass, we set a lower limit for the t quark mass at 44.0 GeV/c2 and for the b' mass at 44.5 GeV/c2. For specific scenarios the mass limits are slightly higher, e.g. for charged current decays the limits are 44.5 and 45.0 GeV/c2 respectively, where all limits are given at a 95% confidence level. © 1990.0SCOPUS: ar.jinfo:eu-repo/semantics/publishe

    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

    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

    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
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