0000000000125379
AUTHOR
Johnson Dl
Production of Charmed Mesons In Z Decays
The production of charmed mesons $$\mathop {D^0 }\limits^{( - )} $$ ,D ± , andD *± is studied in a sample of 478,000 hadronicZ decays. The production rates are measured to be $$\begin{gathered} \frac{{\Gamma ({\rm Z} \to D^{* \pm } X)}}{{\Gamma _{had} }} = 0.187 \pm 0.015(\exp .) \pm 0.013(BR), \hfill \\ \frac{{\Gamma ({\rm Z} \to D^ \pm X)}}{{\Gamma _{had} }} = 0.251 \pm 0.026(\exp .) \pm 0.025(BR), \hfill \\ \frac{{\Gamma ({\rm Z} \to \mathop {D^0 }\limits^{( - )} X)}}{{\Gamma _{had} }} = 0.518 \pm 0.052(\exp .) \pm 0.035(BR), \hfill \\ \end{gathered} $$ where the errors from this analysis are separated from those coming from theD branching ratios (BR). TheD *± momentum distribution is ex…
Search for CP violation in the decay Z → τ+τ−
Data collected by ALEPH in the years 1990, 1991 and 1992 have been used to update a previous search for CP violation in the decay of the Z into tau(+)tau(-). The measurement of the weak dipole form factor of the tau lepton has been performed by studying correlations between the tau leptons. No signal of CP violation was found. The weak dipole form factor is found to be ($) over tilde d(tau) = (+0.15 +/- 0.58(stat) +/- 0.38(sys)) 10(-17)e . cm, obtained with 19628 identified tau(+)tau(-) events. This gives an upper limit on the weak dipole form factor of j\($) over tilde d(tau)\ < 1.5 . 10(-17)e . cm at the 95% confidence level.
Production ofK 0 and Λ in hadronic Z decays
Measurements of the inclusive cross-sections forK 0 and Λ production in hadronic decays of the Z are presented together with measurements of two-particle correlations within pairs of Λ andK 0. The results are compared with predictions from the hadronization models Jetset, based on string fragmentation, and Herwig, based on cluster decays. TheK 0 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 correlatio…
The forward-backward asymmetry for charm quarks at the Z pole
From 1.4 million hadronic Z decays collected by the ALEPH detector at LEP, an enriched sample of Z --> c $$($) over bar c events is extracted by requiring the presence of a high momentum D*(+/-). The charm quark forward-backward charge asymmetry at the Z pole is measured to be A(FB)(0,c) = (8.0 +/- 2.4)% corresponding to an effective electroweak mixing angle of sin(2) theta(W)(eff) = 0.2302 +/- 0.0054.
Observation of monojet events and tentative interpretation
Abstract A data sample corresponding to almost two million hadronic Z decays collected by the ALEPH detector at LEP has been searched for monojet events. Three events were found, in agreement with the expectation from the process e + e − → γ * v v ¯ , with γ * → f f ¯ . Two events are hadronic, the third one being an e + e − pair. All monojet masses are in excess of 3 GeV/ c 2 , and two of the event have large transverse momenta: 18.5 and 20.3 GeV/ c . These kinematic characteristics are quite unlikely in the process e + e − → γ * v v ¯ . The probability of their occurence increases substantially when processes involving further Z or W exchanges are taken into account, but still remains at …
Heavy Flavor Production and Decay With Prompt Leptons In the Aleph Detector
In 431 000 hadronicZ decays recorded with the ALEPH detector at LEP, the yields of electrons and muons in events with one or more prompt leptons have been analysed to give information on the production and decay of heavy quarks. The fractions of $$b\bar b$$ and $$c\bar c$$ events are measured to be 0.219±0.006±0.005 and 0.165±0.005±0.020, and the corresponding forward-backward asymmetries at theZ mass are measured to be 0.090±0.013±0.003 and 0.111±0.021±0.018, after QED and QCD corrections. Measurements for the semileptonic branching ratios BR $$(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…
Measurement of the Effective b quark Fragmentation Function At the Z resonance
Using a sample of about 1.46 million hadronic Z decays collected between 1991 and 1993 with the ALEPH detector at LEP, the energy distribution of the B-0 and B-+/- mesons produced at the Z resonance is measured by reconstructing semileptonic decays B --> l nu(l)D(X) or B --> l nu(l)D*(+)(X). The charmed mesons are reconstructed through the decay modes D-0 --> K(-)pi(+), D-0, --> K(-)pi(+)pi(-)pi(+),D+ --> K(-)pi(+)pi(+), and D*(+) --> D-0 pi(+). The neutrino energy is estimated from the missing energy in the lepton hemisphere. Accounting for B* and B** production, the shape of the scaled energy distribution x(E)((b)) for mesons containing a b quark is compared to the predictions of differen…
Study of the subjet structure of quark and gluon jets
Measurements of the subjet structure of quark and gluon jets in hadronic Z decays are presented. The analysis is based on one million hadronic events recorded by the ALEPH detector. Roughly symmetric three-jet events are selected with a coarse jet-resolution cut-off, y(1). Gluon jets are identified with a purity of 94.6% in those events where evidence of long-lived heavy-flavour hadrons in the other two jets is found. The jets are then analyzed using a smaller cut-off y(0) (< y(1)) so that subjets are resolved. The properties of the jets (subjet multiplicities [N-q], [N-g] and rates R(n)(g(q)) for n = 1,2,3,4) are determined and are found to be in good agreement with the expectations of per…
Michel Parameters and tau neutrino Helicity From Decay Correlations In Z->tau+tau-
The Michel parameters and the average tau-neutrino helicity are measured using correlations between the decays of the tau(+) and tau(-) produced on the Z resonance and observed in the ALEPH detector at LEP. The Michel parameters, rho(e), eta(e), xi(e), (delta xi)(e), are determined from tau --> e ($) over bar upsilon(e) upsilon(tau) with e = (e, mu), and the average tau neutrino helicity, (h(upsilon(tau))), from tau --> h tau with h = (pi, rho, a(1)). The results obtained with e-mu universality are: rho(e) = 0.751 +/- 0.039 +/- 0.022, eta(e) = -0.04 +/- 0.15 +/- 0.11, xi(e) = 1.18 +/- 0.15 +/- 0.06, (delta xi)(e) = 0.88 +/- 0.11 +/- 0.07, and the average tau neutrino helicity (h(upsilon(tau…
Measurements of the b baryon lifetime
Using about 1.5 million hadronic Z decays recorded with the ALEPH detector, the lifetime of the b baryons has been measured using two independent data samples. From a maximum likelihood fit to the impact parameter distribution of leptons in 519 Lambda l(-) combinations containing a b baryon sample of 290 decays, the measured b baryon lifetime is tau(b-baryon) = 1.05(-0.11)(+0.12)(stat) +/-0.09(syst) ps. The lifetime of the Lambda(b)(0) baryon from a maximum likelihood fit to the proper time distribution of 58 Lambda(c)(+)l(-) candidates containing a Lambda(b)(0) sample of 44 decays, is tau(Lambda b0) = 1.02(-0.18)(+0.23)(stat) +/- 0.06(syst) ps.