Search

The average lifetime of b-baryons has been studied using 3×10 hadronic Z decays collected by the DELPHI 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 Λ -lepton decay vertices with the Λ exclusively reconstructed through its pKπ, pK and Λ3 π decay modes. The combined result is: 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:. [ABSTRACT FROM AUTHOR]

This paper presents an update of the measurement of the mean lifetime of the B meson. Combining D −ℓ, D − h, φ−ℓ and inclusive D final states from the 3.2 million hadronic Z decays collected by DELPHI between 1991 and 1994, the B mean lifetime was measured to be: τ( B )=1.67±0.14 ps. [ABSTRACT FROM AUTHOR]

From a sample of 1.5 million Z → qq decays collected by the DELPHI detector during 1991, 1992 and 1993, the production cross section for isolated final state photons is measured and is compared with the O( α, α ) matrix element calculations implemented in the EEPRAD and GNJETS Monte Carlo generators. The observed photon yield is used to derive the electroweak couplings of charge 2/3 (u-type) and charge 1/3 (d-type) quarks to the Z boson. The measured values c =0.91 and c =1.62 are compatible with the Standard Model prediction, c =1.145 and c =1.477. [ABSTRACT FROM AUTHOR]

The hadronic photon structure function F has been measured in the Q range from 4 to 30 GeV/ c and down to x values of order 0.001, using data taken with the DELPHI detector at LEP between 1991 and 1993. A comparison is made with several F parameterizations with special emphasis on their low x behaviour. A result on the Q evolution of F is presented. [ABSTRACT FROM AUTHOR]

An analysis is presented of inclusive π production in Z decays measured with the DELPHI detector. At low energies, π decays are reconstructed by 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 qq and bb events. The number of π 's per hadronic Z event is N( π )/ Z =9.2±0.2(stat)±1.0(syst) and for bb events the number of π 's is N( π )/bb=10.1±0.4(stat)±1.1(syst). The ratio of the number of π 's in bb events to hadronic Z events is less affected by the systematic errors and is found to be 1.09±0.05±0.01. The measured π cross sections are compared with the predictions of different parton shower models. For hadronic events, the peak position in the ξ =ln(1/x) distribution is ξ*=3.90. The average number of π 's from the decay of primary B hadrons is found to be N( B→ π X)/B hadron=2.78±0.15(stat)±0.60(syst). [ABSTRACT FROM AUTHOR]

A search has been made for direct production of heavy quarkonium states in more than 3 million hadronic Z decays in the 1991-1994 DELPHI data. Prompt J/ψ, ψ(2 S) and Γ candidates have been searched for through their leptonic decay modes using criteria based on the kinematics and decay vertex positions. New upper limits are set at the 90% confidence level for Br( Z →( Qq) X)/ Br( Z → hadrons) for various strong production mechanisms of J/ψ and Γ; these range down to 0.9×10. The limits are set in the presence of a small excess (∼1% statistical probability of a background fluctuation) in the sum of candidates from prompt J/ψ, ψ(2 S), γ(1 S), γ(2 S) and γ(3 S) relative to the estimated background. [ABSTRACT FROM AUTHOR]

The decay B→ B 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. 95% confidence level upper limits at 6 MeV/c are placed on both the isospin (i.e. B- B) and the B- B 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〉=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 σ/(σ + σ) = 0.32 ± 0.04 (stat.) ±0.03 (syst.). [ABSTRACT FROM AUTHOR]

The production and decay of beauty baryons ( b-baryons) have been studied using 1.7×10 Z hadronic decays collected by the DELPHI detector at LEP. Three different techniques were used to identify the b-baryons. The first method used pairs of a Λ and a lepton to tag the b-baryon decay. The second method associated fully reconstructed Λ baryons with leptons. The third analysis reconstructed the b-baryon decay points by forming secondary vertices from identified protons and muons of opposite sign. Using these methods the following production rates were measured: The average b-baryon lifetime was determined to be: [ABSTRACT FROM AUTHOR]

An excess of events containing, in a jet, a samesign Ξ−ℓ pair as compared to those with an oppositesign Ξ−ℓ pair has been observed in an analysis of 1.7 million hadronic Z 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 5×10. The measured production fraction corresponds to: per lepton species, averaged for electrons and muons and assuming that the two channels have an equal contribution. Semileptonic decays of Λ baryons can account for less than 10% of these events and the major part of the signal has to originate from Ξ semileptonic decays. Using the subsample of these events where the Ξ trajectory has been measured in the Vertex Detector, the lifetime of B-baryons producing a Ξ in their semileptonic decay final state is found to be: [ABSTRACT FROM AUTHOR]

The lifetimes of charged and neutral B hadrons have been measured using data collected by the DELPHI experiment at LEP between 1991 and 1993. B hadrons are tagged as jets with a secondary vertex and the charge of the B candidate is taken to be the sum of the charges of the particles in the secondary vertex. Approximately 1,434,000 multihadronic Z decays yielded 1817 B hadron candidates. The B purity was estimated to be around 99.1±0.3%, and 83% (70%) of the events measured as neutral (charged) came from neutral (charged) B's. The mean lifetimes of charged and neutral B hadrons were found to be 1.72±0.08 (stat.) ±0.06 (syst.) ps and 1.58±0.11 (stat.)±0.09 (syst.) ps respectively. The ratio of their lifertimes, τ/τ, was 1.09 (stat.)±0.08 (syst.). By making assumptions about the B and Λ states, the B and B meson lifetimes were determined to be τ = 1.72 ± 0.08 (stat.) ±0.06 (syst.) ps and τ = 1.63 ± 0.14 (stat.)±0.13 (syst.) ps and the ratio of their lifetimes was: τ/τ = 1.06 ±0.10. The mean B lifetime was also deduced to be < τ > = 1.64 ±0.06 (stat.)±0.04 (syst.) ps. [ABSTRACT FROM AUTHOR]

A measurement of B meson lifetimes is presented using data collected from 1991 to 1993 by the DELPHI detector at the LEP collider. Samples of events with a D meson and a lepton in the same jet are selected where $$\bar D^{^0 } \ell ^ + $$ and Dℓ events originate mainly from the semileptonic decays of B and B mesons, respectively. From the reconstructed B decay length and the estimated B momentum, taking into account the dilution due to B decays into $$\bar D^{^{**} } \ell ^ + v$$ , the following B meson lifetimes and lifetime ratio are measured: and an average lifetime of B and B mesons is obtained: [ABSTRACT FROM AUTHOR]

A measurement of the strange quark forward-backward asymmetry at the Z peak was performed using 718,000 multihadronic Z decays collected by the DELPHI detector at LEP in 1992. The s-quark was tagged by the presence of high momentum charged kaons identified by the Ring Imaging Cherenkov detector and by Λ;s decaying into pπ. The s-quark purity obtained was estimated for the two hadrons to be 43%. The average s-quark asymmetry was found to be 0.131±0.035 ( stat.) ±0.013 ( syst.). The forward-backward asymmetry was measured for unresolved d-and s-quarks, tagged by the detection of a high energy neutron or neutral kaon in the Hadron Calorimeter. The combined d-and s-quark purity was 69% and their asymmetry was found to be 0.112±0.031 ( stat.) ±0.054 ( syst.). [ABSTRACT FROM AUTHOR]

A study of the production of strange octet and decuplet baryons in hadronic decays of the Z recorded by the DELPHI detector at LEP is presented. This includes the first measurement of the ∑ average multiplicity. The total and differential cross sections, the event topology and the baryon-antibaryon correlations are compared with current hadronization models. [ABSTRACT FROM AUTHOR]

A sample of Z→ττ events observed in the DELPHI detector at LEP in 1991 and 1992 is analysed to measure the τ polarisation in the exclusive decay channels $$ev\bar v$$ , $$\mu v\bar v$$ , πν, ρν and aν. The τ polarisation is also measured with an inclusive hadronic analysis which benefits from a higher efficiency and a better systematic precision than the use of the exclusive decay modes. The results have been combined with those published on the 1990 data. A measurement of the τ polarisation as a function of production angle yields the values for the mean τ polarisation 〈 P〉=−0.148±0.022 and for the Z polarisation P=−0.136±0.027. These results are used to determine the ratio of vector to axial-vector effective couplings for taus $$\bar v_\tau /\bar a_\tau = 0.074 \pm 0.011$$ , and for electrons $$\bar v_e /\bar a_e = 0.068 \pm 0.014$$ , compatible with e-τ universality. With the assumption of lepton universality, the ratio of vector to axial-vector effective couplings for leptons $$\bar v_l /\bar a_l = 0.072 \pm 0.008$$ is obtained, implying a value of the effective weak mixing angle sinθ=0.2320±0.0021. [ABSTRACT FROM AUTHOR]

A search for pair production of neutral heavy Higgs bosons decaying into $$b\bar b$$ has been carried out in a study of hadronic decays of the Z boson into four jet final states using data taken by DELPHI in 1991 and 1992. The two production mechanisms present in the two Higgs doublets scheme, bremsstrahlung production of hZ and associated production of hA, may lead to four beauty jets well recognizable using the precise microvertex detector measurements. No evidence for a signal was found, leading to limits on BR(Z→hA→4b) from 3.5 to 5.5×10 at 95% contidence level, depending on the mass of the ligthest Higgs. When combined with the results of the recent DELPHI standard Higgs search, this result allows the kinematical limit to be reached for the masses of h and A in the minimal supersymmetric extension of the Standard Model (MSSM) scheme. It also allows the tanβ≤1 domain to be explored, and a region above the kinematic limit for direct hA production is constrained by considering virtual hA production. Results are also given in the general two-doublet scheme. [ABSTRACT FROM AUTHOR]

The partial decay width of the Z to $$b\bar b$$ 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: was found, where the third error corresponds to a±8% uncertainty on the $$c\bar c$$ 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: [ABSTRACT FROM AUTHOR]

The forward-backward asymmetries for the processes $$e^ + e^ - \to c\bar c$$ and $$e^ + e^ - \to b\bar b$$ at the Z resonance are measured using identified D mesons. In 905,000 selected hadronic events, taken in 1991 and 1992 with the DEL-PHI detector at LEP, 4757 D→ Dπ decays are reconstructed. The c and b quark forward-backward asymmetries are determined to be: Constraining the b asymmetry to the value measured by DELPHI using independent analyses, the charm asymmetry is determined to be:. This result corresponds to an effective electroweak mixing angle measured using charm quark events of: [ABSTRACT FROM AUTHOR]

The forward-backward asymmetry of the process e e→ Z→ b $$b\bar b$$ 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 $$\sqrt s \sim M_Z $$ . 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 With the semileptonic sample, the forward-backward asymmetry of the process e e→ Z→ $$c\bar c$$ was also measured to be The effective value of the Weinberg mixing angle derived from these measurements was [ABSTRACT FROM AUTHOR]

An analysis of inclusive production of K and the meson resonances K(892), ρ(770), f(975) and f(1270) in hadronic decays of the Z is presented, based on about 973,000 multihadronic events collected by the DELPHI detector at LEP during 1991 and 1992. Overall multiplicities have been determined as 1.962±0.060 K mesons, 0.712±0.067 K(892) and 1.21±0.15ρ(770) per hadronic Z decay. The average multiplicities of f(975) for scaled momentum, x, in the range 0.05≤ x≤0.6 and of f(1270) for 0.05≤ x≤1.0 are 0.098±0.016 and 0.170±0.043 respectively. The f(975) and ρ(770) x-spectra have similar shapes. The f(1270)/ρ(770) ratio increases with x. The average multiplicities and the differential cross sections are compared with the JETSET Parton Shower model. The model with default parameters fails to reproduce the experimental K momentum spectrum at low momentum, describes the K(892) and ρ(770) x-spectrum shapes, but significantly overestimates their production rates. [ABSTRACT FROM AUTHOR]

Two measurements of $$\Gamma _{b\bar b} /\Gamma _{had} $$ are presented. Both measurements use 250000 Z decays taken with the DELPHI detector in 1991 and rely mainly on the precision of the microvertex detector. One tagging method is as simple as possible so that background rates can be reliably predicted by simulation. The other one uses a more involved tagging technique and reduces the dependence on simulation as much as possible. Combining both results, $$\Gamma _{b\bar b} /\Gamma _{had} $$ is found to be 0.2209±0.0041( stat.)±0.0042( syst.)±0.0018 $$\Gamma _{c\bar c} $$ . [ABSTRACT FROM AUTHOR]

An analysis is reported on the channel e e→μμ(nγ), n=1,2..., using data taken with the DELPHI detector at LEP from 1990 to 1992. Differential cross sections of the radiative photons as a function of photon energy and of the angle between the photon and the muon are presented. No significant deviations from expectations are observed. The data are also used to extract the muon-pair cross section and asymmetry below the Z peak by using those events with relatively hard initial state radiative photon(s). The measured cross section and asymmetry show no significant deviation from the Standard Model expectations. These results together with the DELPHI cross section and asymmetry measurements at the LEP energies from the 1990 to 1992 running periods are used to determine limits on the Z-Z′ gauge boson mixing angle θ and on the Z′ mass. There is no indication of the existence of a Z′; the limits obtained on the mixing angle substantially improve upon existing limits. The 95% confidence level allowed ranges of θ in various models are: [ABSTRACT FROM AUTHOR]

A search for charged Higgs bosons decaying into quarks is described, based on statistics of around 1.7 million hadronic Z decays detected in DELPHI. Despite the very high background from standard hadronic decays of the Z, masses in the range up to 43.5 GeV/ c are excluded at the 95% confidence level. After combination with a search for leptonic decays, this mass limit is extended to cover all branching ratios. A similar analysis sets new limits on the possible production of any pair-produced heavy scalar decaying into a pair of jets, such as neutral Higgs bosons in a two doublet scheme and diquarks. [ABSTRACT FROM AUTHOR]

The average lifetime of B hadrons was measured using data collected with the DELPHI detector at the LEP collider during 1991 and 1992. The measurement was performed using two different anayses. The first method was an improvement on a previous technique, which used charged particle impact parameter distributions. This analysis measured an average lifetime for B hadrons of The second method was based on a new technique, which used inclusively reconstructed secondary vertices to measure Taking into account both the statistical and systematic corelations, the results of these two methods were combined to give an average lifetime for B hadrons of [ABSTRACT FROM AUTHOR]

Two-particle correlations in invariant mass are studied separately for like-sign and unlike-sign pions. Strong correlations are observed only at small masses. The correlations decrease very rapidly for like-sign pions and slowly for unlike-sign pions, manifesting different particle dynamics. The like-sign correlations exhibit a power-law behaviour as a function of mass, compatible with the previously observed intermittency effect. The predictions of the parton shower model (JETSET 7.3) deviate significantly from the data. However, good agreement is obtained with the mass correlations, both for the unlike-sign and for the like-sign pairs if the production of the η′ and ρ mesons is reduced and Bose-Einstein correlations are included in the model. The value of the Bose-Einstein correlation strength is consistent with unity for pions which are produced directly or in the decays of short-lived resonances. [ABSTRACT FROM AUTHOR]

The production of multihadronic states in γγ collisions at LEP has been studied with the DELPHI detector. The analyzed data correspond to an integrated luminosity of about 32pb, collected in the LEP runs of 1990-1992. Minimum bias data and a sample of events with jets at high p have been selected under the requirement that no scattered electron or positron is observed. The two data sets have been compared to Monte Carlo predictions. The non-perturbative contribution described by the vector meson dominance Model and direct $$q\bar q$$ production from pointlike photons described by the quark parton model were found to be insufficient to reproduce the data. It has been necessary to include a third interaction component, which is due to perburbative hard scattering of the partonic constituents of the photon. Several parametrisations of the quark and gluon densities of the photon have been tested. The interplay with the cut in jet transverse momentum, which is necessary for the separation of the perturbative and non-perturbative regions, is discussed. The data favour parametrisations with rather soft partonic content of the photon. [ABSTRACT FROM AUTHOR]

The study of the properties of inclusive production of D mesons and of events in which a ϕ and a muon are present in the same jet provides two independent measurements of the probability, f, for a heavy quark to hadronize into a strange B or D meson. The data sample analysed corresponds to 243,000 hadronic Z decays. The combined value of these measurements is f=0.19±0.06±0.08. From the flight distance distributions of D and of (ϕ-lepton) secondary vertices, with the lepton emitted at high transverse momentum relative to the jet axis, two values are obtained for the B meson lifetime. Combining these measurements with a previous result based on the study of D-μ events, the B meson lifetime is measured to be: 0.96±0.37 ps. [ABSTRACT FROM AUTHOR]

A study of the fragmentation properties of charm and bottom quarks into D mesons is presented. From 263 700 Z hadronic decays collected in 1991 with the DELPHI detector at the LEP collider, D, D and D are reconstructed in the modes K π, K π K and D π followed by D→ K π, respectively. The fractional decay widths $$\Gamma {{(Z^0 \to {D \mathord{\left/ {\vphantom {D {\bar D}}} \right. \kern-\nulldelimiterspace} {\bar D}}X)} \mathord{\left/ {\vphantom {{(Z^0 \to {D \mathord{\left/ {\vphantom {D {\bar D}}} \right. \kern-\nulldelimiterspace} {\bar D}}X)} {\Gamma _h }}} \right. \kern-\nulldelimiterspace} {\Gamma _h }}$$ are determined, and first results are presented for the production of D mesons from $$c\bar c$$ and $$b\bar b$$ events separately. The average energy fraction of D in charm quark fragmentation is found to be 〈 X( D)〉=0.487±0.015 (stat)±0.005 (sys.). Assuming that the fraction of D and charm-baryons produced at LEP is similar to that around 10 GeV, the Z partial width into charm quark pairs is determined to be Γ/ Γ=0.187±0.031 (stat)±0.023 (sys). The probability for a b quark to fragment into $$\bar B_s $$ or b-baryons is inferred to be 0.268±0.094 (stat)±0.100 (sys) from the measured probability that it fragments into a $$\bar B^0 $$ or B. [ABSTRACT FROM AUTHOR]

From the combined data of 1990 and 1991 of the DELPHI experiment at LEP, 13057 4-jet events are obtained and used for determining the contribution of the triple-gluon vertex. The relevant variables are the generalized Nachtmann Reiter angle θ and the opening angle of the two least energetic jets. A fit to their two-dimensional distribution yields where C/ C is the ratio of the coupling strength of the triple-gluon vertex to that of gluon bremsstrahlung from quarks, and N/ N, the ratio of the number of quark colours to the number of gluons. This constitutes a convincing model-independent proof of the existence of the triple-gluon vertex, since its contribution is directly proportional to C/ C. The results are in agreement with the values expected from QCD: C/ C=2.25, and N/ N=3/8. [ABSTRACT FROM AUTHOR]

A new measurement of α is obtained from the distributions in thrust, heavy jet mass, energy-energy correlation and two recently introduced jet broadening variables following a method proposed by Catani, Trentadue, Turnock and Webber. This method includes the full calculation of O() terms and leading and next-to-leading logarithms resummed to all orders of α. The analysis is based on data taken with the DELPHI detector at LEP during 1991. I its found that the inclusion of the resummed leading and next-to-leading logarithms reduces the scale dependence of α and allows an extension of the fit range towards the infrared limit of the kinematical range. The combined value for α obtained at the scale μ= M is: [ABSTRACT FROM AUTHOR]

A study of B meson decays into Dl X final states is presented. In these events, neutral and charged D mesons originate predominantly from B and B decays, respectively. The dilution of this correlation due to D production has been taken into account. From 263 700 hadronic Z decays collected in 1991 with the DELPHI detector at the LEP collider, 92 D→ Kπ, 35 D→ Kππ and 61 D→ Dπ followed by D→ Kπ or D→ Kπππ, are found with an associated lepton of the same charge as the kaon. From the D l and D l, the probability f that a b quark hadronizes into a B (or $$\bar B^0 $$ ) meson is found to be 0.44 ±0.08±0.09, corresponding to a total ( B+ Λ) hadronization fraction of 0.12. By reconstructing the energy of each B meson, the b quark fragmentation is directly measured for the first time. The mean value of the B meson energy fraction is: Reconstructing D-lepton vertices, the following B lifetimes are measured: and an average τ( B)=1.23(stat.)±0.15(syst.) ps is found. Allowing for decays into $$D**\ell ^ - \bar v$$ , the B and B lifetimes are: [ABSTRACT FROM AUTHOR]

The spectra of prompt electrons and muons from the semi-leptonic decays of heavy hadrons produced in Z decays have been used to measure the coupling of the Z to b quarks weighted by the B hadrons mean semi-leptonic branching fraction, giving a value: $$BR_{sl}^b *{{\Gamma _{b\bar b} } \mathord{\left/ {\vphantom {{\Gamma _{b\bar b} } {\Gamma _H }}} \right. \kern-\nulldelimiterspace} {\Gamma _H }} = 0.0221 \pm 0.0015$$ . The data has also been used to measure the value of the fragmentation parameter, defined in the context of the LUND PS Model, version 7.2, giving: $$\varepsilon (b) = \left( {8\begin{array}{*{20}c} { + 5} \\ { - 3} \\ \end{array} \pm 2} \right)10^{ - 3} $$ . The corresponding value of the mean fraction of the beam energy taken by a B hadron in the fragmentation of a b quark is: $$\overline {X_E^b } = 0.69\begin{array}{*{20}c} { + 0.02} \\ { - 0.03} \\ \end{array} \pm 0.01$$ . If the values of $$\Gamma _{b\bar b} $$ and Γ are taken from the Standard Model, the following value is obtained for the mean semi-leptonic braching fraction of B hadrons: BR=(10.1±0.7)%. Taking the value of $${{\Gamma _{b\bar b} } \mathord{\left/ {\vphantom {{\Gamma _{b\bar b} } {\Gamma _{\rm H} }}} \right. \kern-\nulldelimiterspace} {\Gamma _{\rm H} }}$$ from an independent analysis of DELPHI data based on the use of the boosted sphericity product, a value: BR=(10.1±1.3)% is obtained. [ABSTRACT FROM AUTHOR]

The multiplicity distributions of charged particles in full phase space and in restricted rapidity intervals for events with a fixed number of jets measured by the DELPHI detector are presented. The data are well reproduced by the Lund Parton Shower model and can also be well described by fitted negative binomial distributions. The properties of these distributions in terms of the clan model are discussed. In symmetric 3-jet events the candidate gluon jet is found not to be significantly different in average multiplicity than the mean of the other two jets, thus supporting previous results of the HRS and OPAL experiments. Similar results hold for events generated according to the LUND PS and to the HERWIG models, when the jets are defined by the JADE jet finding algorithm. The method seems to be insensitive for measuring the color charge ratio between gluons and quarks. [ABSTRACT FROM AUTHOR]

From the analysis of a data sample corresponding to an integrated luminosity of 4.63 pb taken during the 1990 run of LEP at centre of mass energies between 88.2 GeV an 94.2 GeV, the tau decays $$\tau ^ - \to e^ - \bar v_e v_\tau ,\tau ^ - \to \mu ^ - \bar v_\mu v_\tau ,\tau ^ - \to \pi ^ - (K^ - )v_\tau ,\tau ^ - \to \rho {}^ - v_\tau $$ and their charge conjugates have been studied. The following branching ratios have been measured; $$BR(\tau ^ - \to e^ - \bar v_e v_\tau ) = 18.6 \pm 0.8(stat.) \pm 0.6(sys.)\% ,$$ , $$BR\left( {\tau ^ - \to \mu ^ - \bar v_\mu v_\tau } \right) = 17.4 \pm 0.7 \pm 0.6\% ,$$ , Br(τ → π (K)v)=11.9±0.7±0.7%, BR (τ → ρ v)= 22.4±0.8±1.3%, in good agreement with world averages. The measured electronic and muonic branching ratios lead to a measurement of the strong coupling constant, α (m) = 0.26. Extrapolating the α value from m to m yields α (m) = 0.109. The average polarization P of taus produced in Z → τ τ decays has also been measured using the above decay modes. The weighted mean of the polarizations obtained from the four decay modes is P=−0.24±0.07. This value of P gives, in the improved Born approximation, a ratio between the axial and vector coupling constants of the tau of υ/a = 0.12 ± 0.04, and hence a value of the effective electroweak mixing parameter sin θ(m). [ABSTRACT FROM AUTHOR]

Distributions of event shape variables obtained from 120600 hadronic Z decays measured with the DELPHI detector are compared to the predictions of QCD based event generators. Values of the strong coupling constant α are derived as a function of the renormalization scale from a quantitative analysis of eight hadronic distributions. The final result, α( M), is based on second order perturbation theory and uses two hadronization corrections, one computed with a parton shower model and the other with a QCD matrix element model. [ABSTRACT FROM AUTHOR]

The average lifetime of B hadrons produced in hadronic Z decays has been measured with the DELPHI detector at LEP. The measurement is based on the analysis of the impact parameter distributions of high p muons and hadrons. The resulting mean B lifetimes are τ=(1.30±0.10±0.08)ps and τ=(1.27±0.04±0.12)ps respectively, giving a combined value of τ=(1.28±0.10)ps. The hadronic sample was also used to measure the partial Z width $${{\Gamma _{b\bar b} } \mathord{\left/ {\vphantom {{\Gamma _{b\bar b} } {\Gamma _h }}} \right. \kern-\nulldelimiterspace} {\Gamma _h }}$$ and gave a value of 0.222±0.017. [ABSTRACT FROM AUTHOR]

The differential cross section for final state radiation from primary quarks is obtained from a study of isolated energetic photons produced in the reaction e e→ Z→hadrons+γ, as measured in the DELPHI detector at the CERN LEP collider. When combined with the measurement of the total hadronic width of the Z, the observed rate determines the electroweak coupling constants of up and down type quarks, i.e., No evidence is seen for additional photon production from anomalous decays of the Z or from decays of new particles. This measurement leads to upper limits on the production cross section times branching fraction of (a) the Higgs boson in the reaction e e→ Z→ H +γ, H→hadrons, (b) an excited quark, q→ q+γ, and (c) the contribution of an anomalous decay of the Z into a photon and hadrons. These limits, all at the 95% confidence level, vary from 3 to 10pb as the mass of the intermediate state ( H, q or Z) varies from 10 GeV/c to 80 GeV/c. [ABSTRACT FROM AUTHOR]

Using a data sample of 115000 hadronic Z decays, a search was performed for the production of excited charged leptons decaying into an ordinary lepton and one photon. Pair production of excited leptons of mass below 45 GeV/c is excluded. From an analysis of the lepton-photon mass spectra this experiment is also able to exclude single production of these particles for masses that reach up to 85 GeV/c, if the compositeness scale is below 1 TeV. [ABSTRACT FROM AUTHOR]

The multiplicity distributions of charged particles in restricted rapidity intervals in Z hadronic decays measured by the DELPHI detector are presented. The data reveal a shoulder structure, best visible for intervals of intermediate size, i.e. for rapidity limits around ±1.5. The whole set of distributions including the shoulder structure is reproduced by the Lund Parton Shower model. The structure is found to be due to important contributions from 3-and 4-jet events with a hard gluon jet. A different model, based on the concept of independently produced groups of particles, 'clans', fluctuating both in number per event and particle content per clan, has also been used to analyse the present data. The results show that for each interval of rapidity the average number of clans per event is approximately the same as at lower energies. [ABSTRACT FROM AUTHOR]

A search for light Higgs bosons was performed using the data sample collected in 1990 by the DELPHI detector at LEP, at centre of mass energies between 88.2 and 94.2 GeV. Using the process ee→ H+ Z, Z→ ff, it is possible to exclude the existence of the standard model Higgs particle with a mass between 0 and 210 MeV/c at the 99% confidence level. Extending this analysis to the minimal supersymmetric standard model restricts the lightest neutral Higgs boson to masses above 28 GeV/c irrespective of the value of the mixing angle. [ABSTRACT FROM AUTHOR]

This paper presents an analysis of the multiplicity distributions of charged particles produced in Z hadronic decays in the DELPHI detector. It is based on a sample of 25364 events. The average multiplicity is < n>=20.71±0.04(stat)±0.77(syst) and the dispersion D=6.28±0.03(stat)±0.43(syst). The data are compared with the results at lower energies and with the predictions of phenomenological models. The Lund parton shower model describes the data reasonably well. The multiplicity distributions show approximate KNO-scaling. They also show positive forward-backward correlations that are strongest in the central region of rapidity and for particles of opposite charge. [ABSTRACT FROM AUTHOR]

A determination of sin θ based on measurements of elastic scattering of muon-neutrinos and muon-anti-neutrinos on atomic electrons is described. These purely leptonic processes were studied using the CHARM calorimeter exposed to neutrino and antineutrino wide-band beams at the CERN super proton synchrotron. A total of 83±16 neutrino-electron and 112±21 antineutrino-electron events have been detected. From the measurement of the ratio of muon-neutrino and muon-antineutrino cross-sections a value of sin θ=0.211±0.037 was obtained. [ABSTRACT FROM AUTHOR]

A new beam-dump experiment has been performed at the CERN Super Proton Synchrotron using the CHARM neutrino detector. The instrumentation and the statistics have been significantly improved with respect to earlier experiments. For a neutrino energy above 20 GeV the asymmetry of the prompt muon-neutrino and electron-neutrino fluxes $$[(v_\mu + \bar v_\mu ) - (v_e + \bar v_e )]/[(v_\mu + \bar v_\mu ) + (v_e + \bar v_e )]$$ is found to be 0.20±0.10 (stat.)±0.05 (syst.), and the asymmetry of prompt antineutrino and neutrino fluxes for muonneutrinos $$(v_\mu - \bar v_\mu )/(v_\mu + \bar v_\mu )$$ is 0.02±0.16 (stat.)±0.02 (syst.) in agreement with our previous results. For the cross-section times branching ratio for charm production and semileptonic decay we obtain a value of $$\sigma \times BR\left[ {D(\bar D) \to v_e (\bar v_e ) X} \right] = 1.9 \pm 0.2 \pm 0.2\mu b$$ per nucleon. We find no evidence for v or v interactions. The $$(v_\tau + \bar v_\tau )$$ flux is less than 21% of the total prompt neutrino flux. We derive an improved limit on the branching ratio $$\pi ^0 \to v\bar v$$ of 6.5×10, and as a verification of the universality of the neutral weak coupling we find $$g_{v_e \bar v_e } /g_{v_\mu \bar v_\mu } = 1.05_{ - 0.18}^{ + 0.15} $$ . [ABSTRACT FROM AUTHOR]

We present here the final results of experiments searching for neutrino oscillations, carried out by the CHARM Collaboration. The data - taking took place in 1983. The first experiment was performed by exposing two detectors simultaneously to the CERN PS low energy v beam. In the second experiment the full CHARM detector was exposed to the wide-band horn-focused v beam of the CERN SPS. Complete details of the experiments and data reduction are presented. No statistically significant signals for neutrino oscillations were observed. Our 90% CL limits in the appearance experiment ( v→ v) exclude Δ m≧0.19 eV for complete mixing (sin2θ=1), and sin2θ≧0.008 for the region Δ m≧30 eV. These results, and the limits observed for ( v→ v) (disappearance of ( v), are in agreement with those of most other experiments but exclude part of the region previously reported as a possible indication of v→ v oscillations. [ABSTRACT FROM AUTHOR]

New measurements of the total crosssections of charged-current interactions of muonneutrinos and antineutrinos on isoscalar nuclei have been performed. Data were recorded in an exposure of the CHARM detector in an 160 GeV narrow-band beam. The antineutrino flux was determined from the measurements of the pion and kaon flux, and independently from the muon flux measured in the shield; the two methods are found to agree. The neutrino flux was determined from the muon flux ratio for v and $$\bar v_\mu $$ runs which was normalized to the antineutrino flux. The cross-section slopes thus determined are The momentum sum of the quarks in the nucleon and the ratio of sea quark to total quark momentum are derived from the measurements. [ABSTRACT FROM AUTHOR]

The cross-section ratio of neutral-current and charged-current semileptonic interactions of muon-neutrinos on isoscalar nuclei has been measured with the result: R=0.3093±0.0031 for hadronic energy larger than 4 GeV. From this ratio we determined the electroweak mixing angle sin θ, where m is the charm-quark mass in GeV/c. Comparison with direct measurements of m and m determines the radiative shift of the intermediate boson mass Δ r=0.077±0.025(exp.)±0.038(syst.), in agreement with the prediction. Assuming the validity of the electroweak standard theory we determined ϱ=0.990−0.013( m−1.5)±0.009(exp.)±0.003(theor.). [ABSTRACT FROM AUTHOR]

The question whether scalar-type interactions contribute to weak interactions at large momentum transfer has been investigated by a measurement of the longitudinal polarization of positive muons produced in charged-current interactions of high-energy antineutrinos with iron. At an average momentum transfer < Q>=4 GeV the muon spin is found to be oriented forward with respect to the muon momentum vector, with an average polarization of 1.10±0.24, consistent with positive helicity. A limit on scalar contributions of σ/σ <7% at the 95% confidence level can be deduced. A search for violation of time reversal invariance which could manifest itself by a polarization component perpendicular to the muon production plane gave a limit of σ/σ <16% (95% c.l.). It is concluded that the weak leptonic charged current retains its dominant vector and axial vector structure at large momentum transfers. [ABSTRACT FROM AUTHOR]

Rare decays of beauty particles were studied in several charmless modes using the data collected with the DELPHI detector at LEP from 1991 to 1994. These decays are mediated by both tree level b→ u and one-loop penguin b→ s, d transitions. Evidence for charmless B decays was obtained in two body hadronic modes. The branching ratios of B to π π or K π and B to ρ π or K* π were found to be (2.8±0.2)×10 and (1.7±0.2)×10 respectively. The fraction of these decays with a charged kaon in the final state that is not from the spectator s quark, was measured to be 0.58±0.18. Upper limits were set at 90% confidence level on the branching ratios for three and four body charmless hadronic decays in the range of (1-3)×10, for inclusive radiative b→ sγ decays at 5.4×10, for the exclusive radiative decays B → K*(892) γ and B → φ(1020) γ at 2.1×10 and 7.0×10 respectively, and for dineutrino decays $$b \to s\nu \bar \nu $$ , in the exclusive channels $$B_d^0 \to K^* (892)^0 \nu \bar \nu $$ and $$B_s^0 \to \phi (1020)\nu \bar \nu $$ at 1.0×10 and 5.4×10 respectively. The limits on dineutrino decays constrain theories with a new U(1) gauge boson coupling predominantly to the third family of fermions. [ABSTRACT FROM AUTHOR]

A search for high mass photon pairs from the processes $$e^ + e^ - \to \ell ^ + \ell ^ - \gamma \gamma $$ , $$e^ + e^ - \to q\bar q\gamma \gamma $$ and $$e^ + e^ - \to \nu \bar \nu \gamma \gamma $$ with the DELPHI detector at LEP I is reported. From a data sample containing 3.5 million hadronic Z decays, collected by DELPHI during the years 1991 to 1994, 79 events with two charged leptons and two isolated photons were selected with photon pair masses above 10 GeV/c, where 76±6 events were predicted from standard sources. In the same data sample, no $$\nu \bar \nu \gamma \gamma $$ candidates were found and no accumulation of events was visible for γγ masses above 10 GeV/c in the $$q\bar q\gamma \gamma $$ channel. Upper limits at 95% confidence level on the Z branching ratios for the three different channels were extracted from the data. In the mass region m >30 GeV/c the limits obtained are between 3×10 and 4×10. [ABSTRACT FROM AUTHOR]

The interference between initial and final state radiation in the process e e → μ μ at √ s≈M 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 $${\mathcal{O}}(\alpha )$$ 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 has been extracted. The interpretation of this result is discussed. There is an additional uncertainty in the estimate of Γ from as yet uncalculated higher order interference terms. By assuming a value of Γ consistent with the world average, the data were used to estimate the size of these uncalculated corrections. [ABSTRACT FROM AUTHOR]

A measurement of the mass difference, Δm , between the two physical B states has been obtained from the analysis of the impact parameter distribution of a lepton emitted at large transverse momentum ( p ) relative to the jet axis and from the analysis of the flight distance distribution of secondary vertices tagged by either a high p 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 lepton, a charged kaon or the mean jet charge. With 1.7 million hadronic Z decays recorded by DELPHI between 1991 and 1993, Δm is found to be:. [ABSTRACT FROM AUTHOR]