Search results for "ALEPH"
showing 10 items of 160 documents
MEASUREMENT OF ALPHA(S) FROM SCALING VIOLATIONS IN FRAGMENTATION FUNCTIONS IN E(+)E(-) ANNIHILATION
1995
A study of scaling violations in fragmentation functions performed by the ALEPH collaboration at LEP is presented. Data samples enriched in uds, c, b and gluon jets, respectively, together with measurements of the longitudinal and transverse inclusive cross sections are used to extract the fragmentation function for the gluon and for each flavour. The measurements are compared to data from experiments at energies between 22 GeV and 91 GeV and scaling violations consistent with QCD predictions are observed. From this, a measurement of the strong coupling constant alpha(s) (M(z)) = 0.126 +/- 0.009 is obtained.
Exclusive production of pion and kaon meson pairs in two photon collisions at LEP
2003
Exclusive production of pi and K meson pairs in two photon collisions is measured with ALEPH data collected between 1992 and 2000. Cross-sections are presented as a function of cos theta* and invariant mass, for \ cos theta* \ < 0.6 and invariant masses between 2.0 and 6.0 GeV/c(2) (2.25 and 4.0 GeV/c(2)) for pions (kaons). The shape of the distributions are found to be well described by QCD predictions but the data have a significantly higher normalization. (C) 2003 Published by Elsevier B.V.
Properties of Hadronic Z-decays and Test of QCD Generators
1992
Distributions are presented of event shape variables, jet roduction rates and charged particle momenta obtained from 53 000 hadronic Z decays. They are compared to the predictions of the QCD + hadronization models JETSET, ARIADNE and HERWIG, and are used to optimize several model parameters. The JETSET and ARIADNE coherent parton shower (PS) models with running alpha(s) and string fragmentation yield the best description of the data. The HERWIG parton shower model with cluster fragmentation fits the data less well. The data are in better agreement with JETSET PS than with JETSET O(alpha(s)2) matrix elements (ME) even when the renormalization scale is optimized.
Inclusive π±, K± and(p,p¯) differential cross-sections at the Z resonance
1995
Inclusive π±, K± and $$(p,\bar p)$$ differential cross-sections in hadronic decays of the Z have been measured as a function ofz=P hadron/P beam, 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.
Study of the subjet structure of quark and gluon jets
1995
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…
Searches for scalar top and scalar bottom quarks at LEP2
1997
Searches for scalar top and bottom quarks have been performed with data collected by the ALEPH detector at LEP. The data sample consists of 21.7 pb^-1 taken at sqrt{s} = 161, 170, and 172~GeV and 5.7 pb^-1 taken at sqrt{s} = 130 and 136~GeV. No evidence for scalar top quarks or scalar bottom quarks was found in the channels stop --> c chi, stop --> b l snu, and sbottom --> b chi. For the channel stop --> c chi a limit of 67 GeV/c^2 has been set on the scalar top quark mass, independent of the mixing angle between the supersymmetric partners of the left and right-handed states of the top quark. This limit assumes a mass difference between the stop and the chi of at least 10 GeV/c…
Study of the fragmentation of b quarks into B mesons at the Z peak
2001
The fragmentation of b quarks into B mesons is studied with four million hadronic Z decays collected by the ALEPH experiment during the years 1991-1995. A semi-exclusive reconstruction of B->l nu D(*) decays is performed, by combining lepton candidates with fully reconstructed D(*) mesons while the neutrino energy is estimated from the missing energy of the event. The mean value of xewd, the energy of the weakly-decaying B meson normalised to the beam energy, is found to be mxewd = 0.716 +- 0.006 (stat) +- 0.006 (syst) using a model-independent method; the corresponding value for the energy of the leading B meson is mxel = 0.736 +- 0.006 (stat) +- 0.006 (syst). The reconstructed spectra …
Heavy Flavor Production and Decay With Prompt Leptons In the Aleph Detector
1994
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…
Search for pentaquarks states in Z decays
2004
Exotic hadrons made of five quarks (pentaquarks) are searched for in hadronic Z decays collected by the ALEPH detector at LEP. No significant signal is observed. At 95% C.L. upper limits are set on the production rates N of such particles and their charge-conjugate state per Z decay: N Θ(1535)+·BR(Θ(1535)+→pK S0)<6.2×10-4, N Ε(1862)-·BR(Ε(1862)-→Ε -π-)<4.5×10-4, N Ε(1862)0·BR(Ε(1862)0→Ε -π+)<8.9×10-4, N Θc(3100)0·BR(Θc(3100) 0→D*-p)<6.3×10-4, N Θc(3100)0·BR(Θc(3100) 0→D-p)<31×10-4. © 2004 Elsevier B.V. All rights reserved.
Inclusive D* production in gamma-gamma collisions: including the single-resolved contribution with massive quarks
2003
We have calculated the next-to-leading order cross section for the inclusive production of charm quarks as a function of the transverse momentum p_T and the rapidity in two approaches using massive or massless charm quarks. For the single-resolved cross section we have derived the massless limit from the massive theory. We find that this limit differs from the genuine massless version with MS-bar factorization by finite corrections. By adjusting subtraction terms we establish a massive theory with MS-bar subtraction which approaches the massless theory very fast with increasing transverse momentum. With these results and including the equivalent results for the direct cross section obtained…