Search

The electroweak mixing angle is determined with high precision from measurements of the mean difference between forward and backward hemisphere charges in hadronic decays of the Z. A data sample of 2.5 million hadronic Z decays recorded over the period 1990 to 1994 in the ALEPH detector at LEP is used. The mean charge separation between event hemispheres containing the original quark and antiquark is measured for $$b\bar b$$ and $$c\bar c$$ events in subsamples selected by their long lifetimes or using fast D*'s. The corresponding average charge separation for light quarks is measured in an inclusive sample from the anticorrelation between charges of opposite hemispheres and agrees with predictions of hadronisation models with a precision of 2%. It is shown that differences between light quark charge separations and the measured average can be determined using hadronisation models, with systematic uncertainties constrained by measurements of inclusive production of kaons, protons and Λ's. The separations are used to measure the electroweak mixing angle precisely as sin ϑ =0.2322±0.0008(exp.stat.) ±0.0007(exp.syst.)±0.0008(sep.). The first two errors are due to purely experimental sources whereas the third stems from uncertainties in the quark charge separations. [ABSTRACT FROM AUTHOR]

The four-jet final state is analyzed to search for hadronic decays of pair-produced heavy particles. The analysis uses the ALEPH data collected at LEP in November 1995 at centre-of-mass energies of 130 and 136 GeV, corresponding to a total integrated luminosity of 5.7 pb. An excess of four-jet events is observed with respect to the standard model predictions. In addition, these events exhibit an enhancement in the sum of the two di-jet masses around 105 GeV/ c . The properties of these events are studied and compared to the expectations from standard processes and to pair production hypotheses. [ABSTRACT FROM AUTHOR]

The lifetimes of the B and B mesons have been measured with the Aleph detector at LEP, using approximately 3 million hadronic Z decays collected in the period 1991-1994. In the first of three methods, semileptonic decays of B and B mesons were partially reconstructed by identifying events containing a lepton with an associated D* or $$\bar D^0 $$ meson. The second method used fully reconstructed B and B mesons. The third method, used to measure the B lifetime, employed a partial reconstruction technique to identify B → D* π X decays. The combined results are. [ABSTRACT FROM AUTHOR]

Using 22 pb of data collected at LEP in 1992 on the peak of the Z resonance, the ALEPH collaboration has measured the polarisation of the tau leptons decaying into $$ev\bar v, \mu v\bar v$$ , πν, ρν and a ν from their individual decay product distributions. The measurement of the tau polarisation as a function of the production polar angle yields the two parameters N and N , where, in terms of the axial and vector couplings g and g , N =2 g g/( g +g). This analysis follows to a large extent the methods devised for the 1990 and 1991 data but with improvements which bring a better understanding of the systematic uncertainties. Combining the 1992 measurements with our previously published results yields N =0.136±0.012±0.009 and N =0.129±0.016±0.005. Assuming e−τ universality, the measurements imply an effective weak mixing angle of sin θ =0.2332±0.0014. [ABSTRACT FROM AUTHOR]

Charged particle multiplicity distributions have been measured with the ALEPH detector in restricted rapidity intervals | Y| ≤0.5, 1.0, 1.5, 2.0 along the thrust axis and also without restriction on rapidity. The distribution for the full range can be parametrized by a log-normal distribution. For smaller windows one finds a more complicated structure, which is understood to arise from perturbative effects. The negative-binomial distribution fails to describe the data both with and without the restriction on rapidity. The JETSET model is found to describe all aspects of the data while the width predicted by HERWIG is in significant disagreement. [ABSTRACT FROM AUTHOR]

Earlier measurements at LEP of isolated hard photons in hadronic Z decays, attributed to radiation from primary quark pairs, have been extended in the ALEPH experiment to include hard photon production inside hadron jets. Events are selected where all particles combine democratically to form hadron jets, one of which contains a photon with a fractional energy z≥0.7. After statistical subtraction of non-prompt photons, the quark-to-photon fragmentation function, D( z), is extracted directly from the measured 2-jet rate. By taking into account the perturbative contributions to D( z) obtained from an O( ααs) QCD calculation, the unknown non-perturbative component of D( z) is then determined at high z. Provided due account is taken of hadronization effects near z=1, a good description of the other event topologies is then found. [ABSTRACT FROM AUTHOR]

Data on the inclusive production of the neutral vector mesons ρ (770), ω(782), K*(892), and φ(1020) in hadronic Z decays recorded with the ALEPH detector at LEP are presented and compared to Monte Carlo model predictions. Bose-Einstein effects are found to be important in extracting a reliable value for the ρ production rate. An average ρ multiplicity of 1.45±0.21 per event is obtained. The ω is detected via its three pion decay mode ω→ π π π and has a total rate of 1.07±0.14 per event. The multiplicity of the K* is 0.83±0.09, whilst that of the φ is 0.122±0.009, both measured using their charged decay modes. The measurements provide information on the relative production rates of vector and pseudoscalar mesons, as well as on the relative probabilities for the production of hadrons containing u, d, and s quarks. [ABSTRACT FROM AUTHOR]

A data sample of about 3.0 million hadronic Z decays collected by the ALEPH experiment at LEP in the years 1991 through 1994 is used to make an inclusive selection of B hadron events. In this event sample 4227±140±252 B* mesons in the decay B*→ Bγ and 1944±108±161 B** mesons decaying into a B meson and a charged pion are reconstructed. Here and in the following B** denotes the eight L=1( bū) and ( bd) states and their charge conjugate. For the well established B* meson the following quantities are obtained: ΔM=M *−M=(45.30±0.35±0.87) MeV/c and N */( N + N *)=(77.1±2.6±7.0)%. The angular distribution of the photons in the B* rest frame is used to measure the relative contribution of longitudinal B* polarization states to be σ /( σ + σ )=(33±6±5)%. In the M( Bπ)− M( B) mass difference resonance structure is observed at (424±4±10) MeV/c, for which the shape and position is consistent with the expectation for B** states decaying into B(*) π . The relative production rate is determined to be $$\begin{gathered} \frac{{BR(Z \to b \to B_{u,d}^{ * * } )}}{{BR(Z \to b \to B_{u,d} )}} \hfill \\ = [27.9 \pm 1.6(stat) \pm 5.9(syst) {}_{ - 5.6}^{ + 3.9} (model)]\% \hfill \\ \end{gathered} $$ where the third error reflects the uncertainty due to different production and decay models for the broad B** states. [ABSTRACT FROM AUTHOR]

D mesons produced in Z → bb events were separated from the Z → cc component using a lifetime tag. Using a sample of 1.5 million hadronic Z decays collected with the Aleph detector the B and D yields have been measured: $$\begin{gathered} \hfill B(b \to \bar B_s^0 )B(\bar B_s^0 \to D_s^ + ) = 0.088 \pm 0.020(stat.) \\ \hfill \pm 0.020(stat.) \\ \hfill B(c \to D_s^ + ) = 0.088 \pm 0.020(stat.)_{ - 0.016}^{ + 0.019} (syst.) \\ \end{gathered} $$ The B lifetime was measured in a B enriched sample reconstructing the decay length from the vertex of the D with a hadron from the B decay. The result obtained is: $$\tau _{B_s } = 1.61_{ - 0.29}^{ + 0.30} (stat.)_{ - 0.016}^{ + 0.018} (syst.)$$ ps. [ABSTRACT FROM AUTHOR]

Inclusive π, K and $$(p,\bar p)$$ differential cross-sections in hadronic decays of the Z have been measured as a function of z= P/ P, the scaled momentum. The results are based on approximately 520 000 events measured by the ALEPH detector at LEP during 1992. Charged particles are identified by their rate of ionization energy loss in the ALEPH Time Projection Chamber. The position, ξ, of the peak in the ln(1/ z) distribution is determined, and the evolution of the peak position with centre-of-mass energy is compared with the prediction of QCD. [ABSTRACT FROM AUTHOR]

The process ee→, where l is a charged or a neutral lepton and f any charged fermion, is analyzed. The study uses the ALEPH data collected at LEP from 1989 to 1993 at centre-of-mass energies between 88 and 95 GeV, corresponding to almost two million hadronic Z decays and to a total integrated luminosity of 79 pb. For all channels, the data agree well with the standard model expectation both in shape and normalization. The indication of an excess in the $$e^ + e^ - \to \tau ^ + \tau ^ - f\bar f$$ channel, reported by ALEPH in 1991, is not confirmed. [ABSTRACT FROM AUTHOR]

Measurements of the inclusive cross-sections for K and Λ production in hadronic decays of the Z are presented together with measurements of two-particle correlations within pairs of Λ and K. The results are compared with predictions from the hadronization models Jetset, based on string fragmentation, and Herwig, based on cluster decays. The K spectrum is found to be harder than predicted by both models, while the Λ spectrum is softer than predicted. The correlation measurements are all reproduced well by Jetset, while Herwig misses some of the qualitative features and overestimates the size of the $$\Lambda \bar \Lambda $$ correlation. Finally, the possibility of Bose-Einstein correlation in the K K system is discussed. [ABSTRACT FROM AUTHOR]

Precise values of the Z resonance cross sections and lepton pair forward-backward asymmetries are measured from a sample of 1.3 million Z decays into hadrons and charged leptons taken with the ALEPH detector at LEP. A silicon-tungsten luminosity calorimeter was installed in ALEPH in 1992, permitting the determination of the absolute luminosity to a systematic precision of 0.15% (experimental) and 0.25% (theoretical). This is combined with a better understanding of the hadronic event selection and an improved tau pair selection. Including the ALEPH results of the past three years one obtains σ =(41.60±0.16) nb, and R=20.69±0.09. The corresponding number of light neutrino species is N =2.983±0.034. The lepton pair forward-backward asymmetry is used to derive the ratio of vector to axial-vector couplings of leptons: g(M)/g(M)=0.0058 ±0.0011. An application of these results to constrain one model with extra Z bosons is presented. [ABSTRACT FROM AUTHOR]

In 431 000 hadronic Z decays recorded with the ALEPH detector at LEP, the yields of electrons and muons in events with one or more prompt leptons have been analysed to give information on the production and decay of heavy quarks. The fractions of $$b\bar b$$ and $$c\bar c$$ events are measured to be 0.219±0.006±0.005 and 0.165±0.005±0.020, and the corresponding forward-backward asymmetries at the Z mass are measured to be 0.090±0.013±0.003 and 0.111±0.021±0.018, after QED and QCD corrections. Measurements for the semileptonic branching ratios BR $$(b \to \ell ^ - \bar vX)$$ and BR ( b→ cℓ vX) yield 0.114±0.003±0.004 and 0.082±0.003±0.012, respectively. The dilepton events enable measurement of the b mixing parameter, Χ=0.114±0.014±0.008. Results are also presented for the energy variation of the $$b\bar b$$ asymmetry and the parameters required to describe heavy quark fragmentation. From the asymmetry measurements, the effective electroweak mixing angle is sinθ=0.2333±0.0022. [ABSTRACT FROM AUTHOR]

The production of charmed mesons $$\mathop {D^0 }\limits^{( - )} $$ , D, and D is studied in a sample of 478,000 hadronic Z decays. The production rates are measured to be where the errors from this analysis are separated from those coming from the D branching ratios (BR). The D momentum distribution is extracted separately for $$Z \to c\bar c$$ and $$Z \to b\bar b$$ events with the help of event shape variables. It is consistent with the prediction of the JETSET Monte Carlo program after adjustment of the charm fragmentation function. Constraining the shape of the $$Z \to b\bar b$$ contribution, the average fraction of the beam energy taken by a D meson produced in the fragmentation of a charm quark is extracted by a parametric fit to be < X>=0.495±0.011±0.007. Evidence for D ( D(2420) and/or D(2460)) production is found in the $$D^{* \pm } \pi ^ \mp $$ channel, accounting for a fraction (18±5±2)% of all D production. The relative production of vector and pseudoscalar mesons is dicussed, together with the possible effects of D production. The c-quark forward-backward Z-pole asymmetry is detrmined from that of high momentum D to be A=(7.7±4.4)%. [ABSTRACT FROM AUTHOR]

The 132 pb of data collected by ALEPH from 1991 to 1994 have been used to analyze η and ω production in τ decays. The following branching fractions have been measured: $$B\left( {{\tau ^ - } \to {\nu _\tau }\omega {h^ - }} \right) = \left( {1.91 \pm 0.07 \pm 0.06} \right) \times {10^{ - 2}},$$ $$B\left( {{\tau ^ - } \to {\nu _\tau }\omega {h^ - }{\pi ^0}} \right) = \left( {4.3 \pm 0.6 \pm 0.5} \right) \times {10^{ - 3}},$$ $$B\left( {{\tau ^ - } \to {\nu _\tau }\eta {K^ - }} \right) = \left( {2.9_{ - 1.2}^{ + 1.3} \pm 0.7} \right) \times {10^{ - 4}},$$ $$B\left( {{\tau ^ - } \to {\nu _\tau }\eta {h^ - }{\pi ^0}} \right) = \left( {1.8 \pm 0.4 \pm 0.2} \right) \times {10^{ - 3}}$$ and the 95% C.L. limit B(τ → νηπ) < 6.2 × 10 has been obtained. The ωπ and ηππ rates and dynamics are found in agreement with the predictions made from ee∼ - annihilation data with the help of isospin invariance (CVC). [ABSTRACT FROM AUTHOR]

Indirect limits on the mass of the lightest neutralino are derived from the results of searches for charginos, neutralinos, and sleptons performed with data taken by the ALEPH Collaboration at centre-of-mass energies near the Z peak and at 130 and 136 GeV. Within the context of the Minimal Supersymmetric Standard Model and when $$M_{\tilde \nu } \geqslant 200 GeV/c^2 $$ , the bound M > 12.8 GeV/ c at the 95% confidence level applies for any tan β. The impact of lighter sneutrinos is presented in the framework of SUSY grand unified theories; a massless neutralino is allowed only for a narrow range of tan β, μ, and the scalar mass parameter m . Finally, by including Higgs mass constraints and requiring that radiative electroweak symmetry breaking occur, more stringent bounds on M as a function of tan β are derived. [ABSTRACT FROM AUTHOR]