1,721,061 research outputs found
Final results of the searches for neutral Higgs bosons in e+ e- collisions at s**(1/2) up to 209-GeV
Get PDFThe final results of the ALEPH search for the Standard Model Higgs boson at LEP, with data collected in the year 2000 at centre-of-mass energies up to 209 GeV, are presented. The changes with respect to the preceding publication are described and a complete study of systematic effects is reported. The findings of this final analysis confirm the preliminary results published in November 2000 shortly after the closing down of the LEP collider: a significant excess of events is observed, consistent with the production of a 115 \Gcs Standard Model Higgs boson. The final results of the searches for the neutral Higgs bosons of the MSSM are also reported, in terms of limits on \mh, \mA and \tanb. Limits are also set on \mh in the case of invisible decays
Improved search for oscillations
No full textAn improved search for oscillations is performed in the ALEPH
data sample collected during the first phase of LEP, and reprocessed in 1998.
Three analyses based on complementary event selections are presented.
First, decays of mesons into hadronic flavour eigenstates are fully reconstructed.
This selection yields a small sample of candidates with excellent decay length and momentum resolution
and high average purity.
Semileptonic decays with a reconstructed meson provide a second sample with larger
statistics, high average purity, but a poorer momentum and decay length resolution
due to the partial decay reconstruction.
Finally, semileptonic b-hadron decays are inclusively selected and yield the data sample with the highest
sensitivity to oscillations, as
the much higher statistics
compensate for the low average purity and poorer time resolution.
A lower limit is set at
at , significantly lower than the expected
limit of
Measurement of the inclusive D production in collisions at LEP
No full textThe inclusive production of D mesons in two-photon collisions
is measured with the ALEPH detector at centre-of-mass energies
from 183 GeV to 209 GeV. A total of
D meson events are observed from an integrated luminosity of
. Contributions from direct and single-resolved
processes are separated using the ratio of the transverse momentum
of the D to the visible invariant mass
of the event.
Differential cross sections of D production as functions of
and the pseudorapidity
are measured in the range
and . They are compared to next-to-leading
order (NLO) perturbative QCD calculations.
The extrapolation of the integrated visible D cross section to the
total charm cross section, based on the Pythia Monte Carlo program,
yields \sigma (\mathrm{e}^+\mathrm{e}^-\rightarrow \mathrm{e}^+\mathrm{e}^-
\mathrm{c}\bar{\mathrm{c}}) _ { \left=197 {\rm GeV} }
= 731 \pm 74_{\mathrm{stat}} \pm
47_{\mathrm{syst}} \pm 157_{\mathrm{extr}} {\rm pb}
Production of Orbitally Excited Charm Mesons in Semileptonic B Decays
No full textA sample of 3.6 million hadronic Z decays recorded between 1991 and 1995 with the ALEPH detector at LEP is used to investigate semileptonic decays of B mesons into final states involving orbitally excited charm mesons. Topological vertex criteria are used to search for decays involving narrow D states as well as wide D resonances and non-resonant D () ß final states. The sum of the branching ratios for these processes is measured to be Br(B ! Dß` \Gamma ) + Br(B ! D ß` \Gamma ) = (2:26 \Sigma 0:29(stat) \Sigma 0:33(syst))%; which accounts for a significant fraction of the deficit between inclusive measurements and the sum of exclusive semileptonic B decay modes. (Submitted to Zeitschrift fur Physik) The ALEPH Collaboration D. Buskulic, I. De Bonis, D. Decamp, P. Ghez, C. Goy, J.-P. Lees, A. Lucotte, M.-N. Minard, J.-Y. Nief, P. Odier, B. Pietrzyk Laboratoire de Physique des Particules (LAPP), IN 2 P 3 -CNRS, 74019 Annecy-le-Vieux Cedex, France M.P. Casado, M. Ch..
Searches for R-Parity Violating Decays of Supersymmetric Particles in <i>e</i><sup>+</sup><i>e</i><sup>−</sup> Collisions at Centre-of-mass Energies from 189 to 202 GeV
No full textExperimental High-Energy Physic
A convergent reformulation of perturbative QCD
Get PDFWe present and explore a new formulation of perturbative QCD based not on the renormalised coupling but on the dimensional transmutation parameter of the theory and the property of asymptotic scaling. The approach yields a continued function, the iterated function being that involved in the solution of the two-loop β-function equation. In the so-called large-b limit the continued function reduces to a continued fraction and the successive approximants are diagonal Padé approximants. We investigate numerically the convergence of successive approximants using the leading-b approximation, motivated by renormalons, to model the all-orders result. We consider the Adler D-function of vacuum polarisation, the Polarised Bjorken and Gross-Llewellyn Smith sum rules, the (unpolarised) Bjorken sum rule, and the Minkowskian quantities R(_r) and the R-ratio of e(^+)e(^-) annihilation. In contrast to diagonal Fade approximants the truncated continued function method gives remarkably stable large-order approximants in cases where infra-red renormalon effects are important. We also use the new approach to determine the QCD fundamental parameters from the R(_r) and the R-ratio measurements, where we find Ā(^(3))(_MS)=516±48 MeV (which yields a(_s)(µ=m(_r))=0.360(^+0.021)(_=0.020)), and Ā(^(5))(_MS)=299(^+6)(_-7) MeV (which yields a(_s)(µ=m(_zo)=0.1218±0.0004), respectively. The evolution of the former value to the m(_zo) energy results in a(_s)(µ= m(_zo)) = 0.123 ± 0.002. These values are in line with other determinations available in the literature. We implement the Complete Renormalisation Group Improvement (CORGI) scheme throughout all the calculations. We report on how the mathematical concept of Stieltjes series can be used to assess the convergence of Padé approximants of perturbative series. We find that the combinations of UV renormalons which occur in perturbative QCD may or may not be Stieltjes series depending on the renormalisation scheme used
Combined Results on b-hadronn Production Rates and Decay Properties
No full textExperimental High-Energy Physic
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