The production of W
+W
− pairs is analysed in a data sample collected by ALEPH at a mean centre-of-mass energy of 182.7 GeV, corresponding to an integrated luminosity of 57 pb
−1. Cross sections are ...given for different topologies of W decays into leptons or hadrons. Under Standard Model assumptions for the W-pair production and decay, the W-pair cross section is measured to be
15.57±0.62
(stat.)±0.29
(syst.)
pb.
Using also the W-pair data samples collected by ALEPH at lower centre-of-mass energies, the decay branching ratio of the W boson into hadrons is measured to be
B(W→hadrons)=68.93±1.21
(stat.)±0.51
(syst.)%
, allowing a determination of the CKM matrix element
|V
cs
|=1.043±0.058
(stat.)±0.026
(syst.)
. The agreement of the cross sections with the Standard Model prediction allows a limit to be set on the W decay rate to undetectable final states.
The data collected by ALEPH at LEP at centre-of-mass energies ranging from 181 to 184 GeV, corresponding to an integrated luminosity of 57 pb
−1, are analysed to search for pair-produced neutral ...Higgs bosons h and A, in the
b
b
̄
b
b
̄
and
τ
+
τ
−
b
b
̄
final states. Two events are found in the data with 2.5 expected from standard model processes. When combined with the lower energy data collected by ALEPH and with earlier reported searches for associated hZ production, these analyses are interpreted in the context of the minimal supersymmetric extension of the standard model (MSSM). For standard choices of MSSM parameter sets, this combination results in 95% C.L. exclusion lower limits of 72.2 and 76.1 GeV/
c
2 for
m
h and
m
A, irrespective of tan
β. A scan of the MSSM parameter space is performed in which the model parameters are varied over wide ranges. For low values of tan
β, i.e., for
1<
tanβ≲
2
, the limit on
m
h of ∼88 GeV/
c
2 is shown to be robust, being satisfied in essentially all of the physically allowed domain.
A search for the pseudoscalar meson ηb 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 Γγγ(ηb)×BR(ηb→4charged particles)<48 eV, Γγγ(ηb)×BR(ηb→6charged particles)<132 eV are obtained at 95% confidence level, which correspond to upper limits of 9.0% and 25% on these branching ratios.
The final results of the ALEPH search for the Standart Model Higgs boson at LEP, with data collected in the year 2000 at center-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 GeV/c2 Standard Model Higgs boson.
The final results of the searches for the neutral Higgs bosons od the MSSM are also reported, in terms of limits on mh, mA and tanβ.
Limits are also set on mh in the case of invisible decays.
A study of events with photons and missing energy has been performed with the data sample obtained with the ALEPH detector at centre-of-mass energies from 161 to 184 GeV, corresponding to a total ...integrated luminosity of about 80 pb
−1. The measured distributions are in agreement with Standard Model predictions, leading to constraints on WW
γ gauge coupling parameters Δ
κ
γ
and
λ
γ
. The results from the fit to the cross sections and to the energy and angular distributions of the photons are: Δ
κ
γ
=0.05
+1.15
−1.10(stat)±0.25(syst),
λ
γ
=−0.05
+1.55
−1.45(stat)±0.30(syst).
Searches for pair production of squarks, sleptons, charginos and neutralinos are performed with the data collected by the ALEPH detector at LEP at centre-of-mass energies from 188.6 to 201.6 GeV. No ...evidence for any such signals is observed in a total integrated luminosity of about 410 pb
−1. The negative results of the searches are translated into exclusion domains in the space of the relevant MSSM parameters, which improve significantly on the constraints set previously. Under the assumptions of gaugino and sfermion mass unification, these results allow a 95% C.L. lower limit of 37 GeV/
c
2 to be set on the mass of the lightest neutralino for any tan
β and sfermion mass. Additional constraints in the MSSM parameter space are derived from the negative results of ALEPH searches for Higgs bosons. The results are also interpreted in the framework of minimal supergravity.
The data collected in 1998 by ALEPH at LEP at a centre-of-mass energy of 188.6 GeV, corresponding to an integrated luminosity of 176.2 pb
−1, are analysed to search for invisible decays of a Higgs ...boson produced in the reaction
e
+
e
−→
h
Z
. The number of events found in the data and their properties are in agreement with the Standard Model expectation. This search results in an improved 95% C.L. lower limit on the Higgs boson mass of 95.4 GeV/
c
2, assuming it decays totally invisibly and for a production cross section equal to that of the Standard Model.
Single top production via flavour changing neutral currents in the reactions
e
+e
−
→
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. Upper limits at 95% CL on single top production cross sections at
s
=189
–202 GeV are derived. A model-dependent limit on the sum of branching ratios BR(t→Zc)+BR(t→Zu)<17% is obtained.
Data collected at centre-of-mass energies from 189 GeV to 202 GeV by the ALEPH detector at LEP corresponding to an integrated luminosity of 411 pb
−1, are analysed in a search for the scalar top in ...the decay channels
t
̃
→
c/uχ
for small mass differences between the stop and the lightest neutralino. No evidence for deviations from the Standard Model expectation is found and a lower limit of 59 GeV/
c
2 is set for the stop mass, independent of the stop to neutralino mass difference and of the stop lifetime.
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