572 research outputs found

    Measurement of the forward - backward asymmetry of e+ e- ---> Z ---> b anti-b using prompt leptons and a lifetime tag

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    The forward-backward asymmetry of the process e+e-→Z→b {Mathematical expression} has been measured using events collected 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 {Mathematical expression}. The tagging of b-quark events was performed using two approaches; the first was based on the semileptonic decay channels b→X+μ and b→X+e, the second used a lifetime tag with jet-charge reconstruction. The results of these two methods were combined to give {Mathematical expression} With the semileptonic sample, the forward-backward asymmetry of the process e+e-→Z→ {Mathematical expression} was also measured to be {Mathematical expression} The effective value of the Weinberg mixing angle derived from these measurements was {Mathematical expression} © 1995 Springer-Verlag

    Hadronization properties of b quarks compared to light quarks in e(+)e(-)→ q(q)over-bar from 183 to 200 GeV

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    The DELPHI detector at LEP has collected 54 pb(-1) of data at a centre-of-mass energy around 183 GeV during 1997, 158 pb(-1) around 189 GeV during 1998, and 187 pb(-1) between 192 and 200 GeV during 1999. These data were used to measure the average charged particle multiplicity in e(+)e(-)→b (b) over bar events: (n)(b (b) over bar), and the difference delta(hl) between (n)(b (b) over bar) and the multiplicity, (n)(l (l) over bar), in generic light quark (u,d,s) events: delta(hl)(183 GeV) = 4.55 +/- 1.31(stat) +/- 0.73(syst), delta(hl)(189 GeV) = 4.43 +/- 0.85(stat) +/- 0.61(syst), delta(hl)(200 GeV) = 3.39 +/- 0.89(stat) +/- 1.01 (syst). This result is consistent with QCD predictions, while it is inconsistent with calculations assuming that the multiplicity accompanying the decay of a heavy quark is independent of the: mass of the quark itself

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

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    The forward-backward asymmetry of the process e(+)e(-) --> Z --> b ()overbarbhasbeenmeasuredusingeventscollectedeebytheDELPHIexperimentduringthe1991and1992LEPruns.Thisdatasamplecorrespondedto884000hadronicZdecaysatacentreofmassenergyrootssimilartoM(Z).Thetaggingofbquarkeventswasperformedusingtwoapproaches;thefirstwasbasedonthesemileptonicdecaychannelsb>X+muandb>X+e,thesecondusedalifetimetagwithjetchargereconstruction.TheresultsofthesetwomethodswerecombinedtogiveA(FB)(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 ()overbarcwasalsomeasuredtobeA(FB)(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

    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}

    Lambda(b) polarization in Z0 decays at LEP

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    The longitudinal polarization of the Λb baryon is measured at the LEP e+e- collider by DELPHI. It is determined from b the charged lepton and neutrino energy spectra in 249±19 Λb semileptonic decays reconstructed in ≈3.5 million hadronic b Z0 decays using Λ0-lepton correlations. The measured polarization is: PΛb = -49q±0.32 -30(stat)±0.17(syst.)0info:eu-repo/semantics/publishe

    Upper limits on the branching ratios tau --> mu gamma and tau --> e gamma

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    The DELPHI collaboration has searched for lepton flavour violating decays \tau \rightarrow \mu \gamma and \tau \rightarrow e \gamma using a data sample of about 70~pb^{-1} of integrated luminosity corresponding to 81 000 produced \tau^+ \tau^- events. No candidates were found for either of the two modes. This yields branching ratio upper limits of \rm{B}( \tau \rightarrow e \gamma ) < 1.1 \times 10^{-4} and \rm{B} ( \tau \rightarrow \mu \gamma) < 6.2 \times 10^{-5} at 90\% confidence level

    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

    A Precise measurement of the partial decay width ratio R(b)**0 = Gamma(b anti-b) / Gamma(had)

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    The partial decay width of the Z to b (b) over bar quark pairs has been measured by the DELPHI detector at LEP using data taken in the years 1992 to 1995. Decays of b-hadrons were tagged by several methods using tracks with large impact parameters and/or reconstructed secondary vertices; complemented by event shape variables. Combining these methods in a multivariate analysis the value Gamma(Z --> b (b) over bar)/Gamma(Z --> had) = 0.21634 +/- 0.00067(stat) +/- 0.00060(syst) was obtained, where the c (c) over bar production fraction was fixed to its Standard Model value. RI De Angelis, Alessandro/B-5372-2009; Ragazzi, Stefano/D-2463-2009; Gonzalez Caballero, Isidro/E-7354-2010; Krammer, Manfred/A-6508-2010; Muresan, Raluca-Anca/C-3725-2011; Katsanevas, Stavros/A-4297-2011; Ruiz, Alberto/E-4473-2011; Marti-Garcia, Salvador/F-3085-2011; Verzi, Valerio/B-1149-2012; branchini, paolo/A-4857-2011; Shellard, Ronald/G-4825-2012; Monge, Maria Roberta/G-9127-2012; Petrolini, Alessandro/H-3782-2011; Fruhwirth, Rudolf/H-2529-201

    B-asterisk production in Z-Decays

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    The decay B* --&gt; B gamma has been observed with the DELPHI detector at LEP, where the B* meson is produced in Z boson decays, The combination of inclusively reconstructed B mesons with well-measured converted photons yields a measurement of the flavour-averaged B* - B mass difference of 45.5 +/- 0.3 (stat.) +/- 0.8 (syst.) MeV/c(2). 95% confidence level upper limits at 6 MeV/c(2) are placed on both the isospin (i.e, B+-B-0) and the B-s-B-ud splitting of the mass difference, The production ratio of B* to B mesons in Z decays is measured to be 0.72 +/- 0.03 (stat.) +/- 0.06 (syst.). Limits on the production cross-section of other hypothetical excited B hadron states decaying radiatively are established. The differential B* cross section has been measured to be in good agreement with the average b fragmentation, yielding an average fractional B* energy of [x(E)] = 0.695 +/- 0.009 (stat.) +/- 0.013 (syst.). From the decay angular distribution the relative contribution of longitudinal B* polarisation states is measured to be sigma(L)/(sigma(L) + sigma(T)) = 0.32 +/- 0.04 (stat.) +/- 0.03 (syst.)
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