Measurement of the photon structure function at ALEPH

The photon structure function F2gamma has been measured with data taken by the ALEPH collaboration at LEP centre-of-mass energies sqrt s = 91 GeV with < Q^2 > of 9.9, 20.7 and 284 GeV^2 and sqrt s = 183 GeV with of 13.7 and 56.5 GeV^2. For the data at sqrt s = 183 GeV a two-dimensional unfolding method employing the principle of maximum entropy is used, which reduces the errors compared to one-dimensional methods.Comment: 5 pages, 5 figures, To be published in proceedings "PHOTON99

ALEPH Tau Spectral Functions and QCD

Hadronic $\tau$ decays provide a clean laboratory for the precise study of quantum chromodynamics (QCD). Observables based on the spectral functions of hadronic $\tau$ decays can be related to QCD quark-level calculations to determine fundamental quantities like the strong coupling constant, quark and gluon condensates. Using the ALEPH spectral functions and branching ratios, complemented by some other available measurements, and a revisited analysis of the theoretical framework, the value \asm = 0.345 \pm 0.004_{\rm exp} \pm 0.009_{\rm th} is obtained. Taken together with the determination of \asZ from the global electroweak fit, this result leads to the most accurate test of asymptotic freedom: the value of the logarithmic slope of $\alpha_s^{-1}(s)$ is found to agree with QCD at a precision of 4%. The value of \asZ obtained from $\tau$ decays is \asZ = 0.1215 \pm 0.0004_{\rm exp} \pm 0.0010_{\rm th} \pm 0.0005_{\rm evol} = 0.1215 \pm 0.0012.Comment: 14 pages, 5 figures, invited talk at the Tau06 International Workshop, Pisa, September 19-22 200

Chiral-invariant CP-violating Effective Interactions in Z Decays to three Jets

Tests of CP violation by appropriate momentum correlations in $Z\to 3$ jets and in particular in $Z\to b\bar bX$ probe CP-violating effective couplings -- that manifest themselves as form factors -- which conserve the quark chirality and quark flavour. By giving two examples we show that such couplings can be induced at one-loop order in extensions of the Standard Model with CP violation beyond the Kobayashi-Maskawa phase. In one of the models we compute the chirality-conserving part of the CP-violating $Zb{\bar b}$-gluon amplitude for massless $b$ quarks, determine the resulting effective dimension $d=6$ couplings in the local limit, and discuss the possible size of the effects. Finally we show that in models with excited quarks the chiral-invariant CP-violating effective interactions could be quite large if appropriate couplings are of a size characteristic of a strong interactionComment: 14 pages, LaTeX with 4 postscript figures, epsf macro include

Probabilistic Particle Flow Algorithm for High Occupancy Environment

Algorithms based on the particle flow approach are becoming increasingly utilized in collider experiments due to their superior jet energy and missing energy resolution compared to the traditional calorimeter-based measurements. Such methods have been shown to work well in environments with low occupancy of particles per unit of calorimeter granularity. However, at higher instantaneous luminosity or in detectors with coarse calorimeter segmentation, the overlaps of calorimeter energy deposits from charged and neutral particles significantly complicate particle energy reconstruction, reducing the overall energy resolution of the method. We present a technique designed to resolve overlapping energy depositions of spatially close particles using a statistically consistent probabilistic procedure. The technique is nearly free of ad-hoc corrections, improves energy resolution, and provides new important handles that can improve the sensitivity of physics analyses: the uncertainty of the jet energy on an event-by-event basis and the estimate of the probability of a given particle hypothesis for a given detector response. When applied to the reconstruction of hadronic jets produced in the decays of tau leptons using the CDF-II detector at Fermilab, the method has demonstrated reliable and robust performance.Comment: Accepted by Nuclear Instruments and Methods

CP Violation Beyond the Standard Model and Tau Pair Production in e+e−e^+ e^- Collisions

We show that the CP-violating dipole form factors of the tau lepton can be of the order of $\alpha/\pi$ in units of the length scale set by the inverse $Z$ boson mass. We propose a few observables which are sensitive to these form factors at LEP2 and higher e^+e^- collision energies.Comment: 11 pages LaTeX + 2 figure

The spin structure of the Lambda hyperon in quenched lattice QCD

It has been suggested to use the production of Lambda hyperons for investigating the nucleon spin structure. The viability of this idea depends crucially on the spin structure of the Lambda. Using nonperturbatively O(a) improved Wilson fermions in the quenched approximation we have studied matrix elements of two-quark operators in the Lambda. We present results for the axial vector current, which give us the contributions of the u, d, and s quarks to the Lambda spin.Comment: Lattice2001(matrixelement), 3 pages, 2 figure

Strong Coupling Constant from Scaling Violations in Fragmentation Functions

We present a new determination of the strong coupling constant alpha_s through the scaling violations in the fragmentation functions for charged pions, charged kaons, and protons. In our fit we include the latest e+e- annihilation data from CERN LEP1 and SLAC SLC on the Z-boson resonance and older, yet very precise data from SLAC PEP at center-of-mass energy sqrt(s)=29 GeV. A new world average of alpha_s is given.Comment: 10 pages, 3 eps figue

Supersymmetric Dark Matter in the Light of LEP 1.5

We discuss the lower limit on the mass of the neutralino $\chi$ that can be obtained by combining data from $e^+e^-$ annihilation at LEP and elsewhere with astrophysical and theoretical considerations. Loopholes in the purely experimental analysis of ALEPH data from the Z peak and LEP 1.5, which appear when $\mu<0$ for certain values of the sneutrino mass $m_{\tilde\nu}$ and the ratio $\tan\beta$ of supersymmetric Higgs vacuum expectation values, may be largely or totally excluded by data from lower-energy $e^+e^-$ data, the hypothesis that most of the cosmological dark matter consists of $\chi$ particles, and the assumption that electroweak symmetry breaking is triggered by radiative corrections due to a heavy top quark. The combination of these inputs imposes m_{\chi} \ge 21.4~\gev, if soft supersymmetry-breaking masses are assumed to be universal at the grand-unification scale.Comment: 22 pages. one Latex file + nine eps figure

Lepton flavor conserving Z -> l^+ l^-$ decays in the general two Higgs doublet model

We calculate the new physics effects to the branching ratios of the lepton flavor conserving decays Z -> l^+ l^- in the framework of the general two Higgs Doublet model. We predict the upper limits for the couplings |\bar{\xi}^{D}_{N,\mu\tau}| and |\bar{\xi}^{D}_{N,\tau\tau}| as 3\times 10^2 GeV and 1\times 10^2 GeV, respectively.Comment: 9 pages, 3 figure

A Note on the QCD Corrections to Forward--Backward Asymmetries of Heavy--Quark Jets in Z Decays

The measurement of the forward-backward asymmetries of heavy quarks provides one of the most precise determinations of $\sin^2\theta_W$ in $Z$ decays. We discuss in detail the one--loop QCD radiative corrections to these asymmetries. Results are given for single heavy--quark jet asymmetries and asymmetries of the thrust axis, as well as for heavy--quark two--jet final states.}Comment: 11pp, latex, 1 uuencoded eps figur