Searches for the pair production of supersymmetric particles under the assumption that R-parity is violated via a single dominant LLEbar, LQDbar or UbarDbarDbar coupling are performed using the data ...collected by the ALEPH detector at LEP at centre-of-mass energies from 189 to 209Gev. The numbers of observed candidate events in the data are in agreement with the Standard Model expectation, and limits on the production cross sections and on the masses of charginos, sleptons, squarks and sneutrinos are derived.
Events containing only energetic photons are analysed in a sample of 628 pb -1 of data recorded from e+e- collisions at centre-of-mass energies between 189 and 209 GeV by the ALEPH detector at LEP. ...The $\reenunug$ and $\reeggg$ cross sections are measured and found to be in agreement with the Standard Model predictions. The number of light neutrino generations is determined to be N_\nu = 2.86 \pm 0.09. Upper limits are derived on the cross sections for photon production in the context of several supersymmetric models. Limits are also set on the parameters of models with extra spatial dimensions with contact interactions and with excited electrons.
An improved search for Bs 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 Bs 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 Bs purity. Semileptonic decays with a reconstructed Ds- meson provide a second sample with larger statistics, high average Bs 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 Bs oscillations, as the much higher statistics compensate for the low average Bs purity and poorer time resolution. A lower limit is set at 10.9 ps-1 at 95% CL, significantly lower than the expected limit of 15.7 ps-1.
This paper is an updated version of the lectures given at the XXIX International Meeting on Fundamental Physics in Sitges, Barcelona (February 2000). The measurements perforned at LEP and SLC have ...substantially improved the precision of the test of the Minimal Standard Model. The precision is such that there is sensitivity to pure weak radiative corrections. This allows to indirectly determine the top mass (m_t = 180 +- 10GeV), the W-boson mass (M_W = 80.375 +- 0.022GeV), and to set an upper limit on the Higgs boson mass of 196GeV at 95% confidence level.
A search for events with a photon pair arising from the decay of a Higgs boson produced in association with a fermion pair, is performed in 893pb^-1 of data recorded by the ALEPH detector at LEP at ...centre-of-mass energies up to 209GeV. No excess of such events is found over the expected background. An upper limit is derived on the product of the e^+e^- -> HZ cross section and the H -> gamma gamma branching fraction as a function of the Higgs boson mass. A fermiophobic Higgs boson produced with the Standard Model cross section is excluded at 95\% confidence level for all masses below 105.4GeV/c2
A search for the pseudoscalar meson eta_b is performed in two-photon interactions at LEP~2 with an integrated luminosity of 699 pb^-1 collected at e+e- centre-of-mass energies from 181 GeV to 209 ...GeV. One candidate event is found in the six-charged-particle final state and none in the four-charged-particle final state, in agreement with the total expected background of about one event. Upper limits of Gamma_gammagamma(eta_b) * BR(eta_b -> 4 charged particles) 48 eV Gamma_gammagamma(eta_b) * BR(eta_b -> 6 charged particles) 132 eV are obtained at 95\% confidence level, which correspond to 95\% confidence level upper limits of $9.0\%$ and $25\%$ on these branching ratios.
Search for Scalar Quarks in e+e- Collisions at sqrt(s) up to 209 GeV Searches for scalar top, scalar bottom and mass-degenerate scalar quarks are performed in the data collected by the ALEPH detector ...at LEP, at centre-of-mass energies up to 209 GeV, corresponding to an integrated luminosity of 675 pb-1. No evidence for the production of such particles is found in the decay channels stop->c/u chi, stop->b l snu, sbottom-> b chi, squark-> q chi or in the stop four-body decay channel stop-> b chi f f' studied for the first time at LEP. The results of these searches yield improved mass lower limits. In particular, an absolute lower limit of 63GeV/c2 is obtained for the stop mass, at 95% confidence level, irrespective of stop lifetime and decay branching ratios.
A search for charged Higgs bosons produced in pairs is performed with data collected at centre-of-mass energies ranging from 189 to 209 GeV by ALEPH at LEP, corresponding to a total luminosity of 629 ...invpb. The three final states taunutaunu, taunucs and cscs are considered. No evidence for a signal is found and lower limits are set on the mass M_H+ as a function of the branching fraction B(H to taunu). In the framework of a two-Higgs-doublet model, and assuming B(H+ to taunu + B(H+ to cs) = 1 charged Higgs bosons with masses below 79.3 Gev/c2 are excluded at 95% confidence level independently of the branching ratios.
The 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 $anb$.% Limits are also set on $mh$ in the case of invisible decays.
