Electroweak measurements in electron–positron collisions at w-boson-pair energies at lep

Contains fulltext : 121524.pdf (preprint version ) (Open Access

Search for single top production in e+ e- collisions at s**(1/2) = 189-GeV - 202-GeV

The single top production via flavour changing neutral currents in the reactions ee -> t c/u is searched for in approximately 411 pb-1 of data collected by ALEPH at centre-of-mass energies in the range between 189 and 202 GeV. In total, 58 events are selected in the data to be compared with 50.3 expected from Standard Model backgrounds. No deviation from the Standard Model expectation is observed. A 95\%~CL upper limit of 0.72 pb on the single top production cross section at 202 GeV is derived assuming a top mass of 174 GeV/c^2 and a 100% branching ratio of the top decay into bW. A model dependent limit on the flavour-changing couplings kZ and kg is obtained by combining all centre-of-mass energies

Search for the standard model Higgs boson at the LEP2 Collider near GeV

During 1997 the ALEPH experiment at LEP gathered 57 pb-1 of data at centre-of-mass energies near 183 GeV. These data are used to look for possible signals from the production of the Standard Model Higgs boson in the reaction e+e--->HZ. No evidence of a signal is found in the data; seven events are selected, in agreement with the expectation of 7.2 events from background processes. This observation results in an improved lower limit on the mass of the Higgs boson: mH>87.9 GeV/c2 at 95% confidence level

Measurement of triple gauge boson couplings at 172-GeV

The triple gauge-boson couplings, αWΦ, αW and αBΦ, have been measured using 34 semileptonically and 54 hadronically decaying W+W− candidate events. The events were selected in the data recorded during 1996 with the ALEPH detector at 172 GeV, corresponding to an integrated luminosity of 10.65 pb−1. The triple gauge-boson couplings have been measured using optimal observables constructed from kinematic information of W+W− events. The results are in agreement with the Standard Model expectation

Three-prong \u3c4 decays with charged kaons

Final states with charged kaons in three-prong \u3c4 decays are studied by exploiting the particle identification from the dE/dx measurement. The results are based on a sample of about 1.6 7 105 detected \u3c4 pairs collected with the ALEPH detector between 1991 and 1995 around the Z peak. The following branching ratios have been measured: B(\u3c4- \u2192 K 12K+\u3c0 12\u3bd\u3c4) = (1.63 \ub1 0.21 \ub1 0.17) 7 10 123, B(\u3c4 12 \u2192 K 12\u3c0+\u3c0 12\u3bd\u3c4) = (2.14 \ub1 0.37 \ub1 0.29) 7 10 123, B(\u3c4 12 \u2192 K 12K+\u3c0 12\u3c00\u3bd\u3c4) = (0.75 \ub1 0.29 \ub1 0.15) 7 10 123, and B(\u3c4 12 \u2192 K 12\u3c0+\u3c0 12\u3c00\u3bd\u3c4) = (0.61 \ub1 0.39 \ub1 0.18) 7 10 123. The first two measurements are more precise than the current world averages, while the last two channels are investigated for the first time. The 95% C.L. upper limit on the branching ratio for the decay \u3c4 12 \u2192 K 12K+K 12\u3bd\u3c4 is 0.19 7 10 123. A study of intermediate states occurring in the K 12K+\u3c0 12\u3bd\u3c4 and K 12\u3c0+\u3c0 12\u3bd\u3c4 decays is also presented

Search for Charged Higgs bosons: Combined Results Using LEP Data

The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have searched for pair-produced charged Higgs bosons in the framework of Two Higgs Doublet Models (2HDMs). The data of the four experiments are statistically combined. The results are interpreted within the 2HDM for Type I and Type II benchmark scenarios. No statistically significant excess has been observed when compared to the Standard Model background prediction, and the combined LEP data exclude large regions of the model parameter space. Charged Higgs bosons with mass below 80 GeV/c^2 (Type II scenario) or 72.5 GeV/c^2 (Type I scenario, for pseudo-scalar masses above 12 GeV/c^2) are excluded at the 95% confidence level