Bose-Einstein correlations are studied in semileptonic (
WW→q
q
̄
ℓν
) and fully hadronic (
WW→q
q
̄
q
q
̄
) W-pair decays with the ALEPH detector at LEP at centre-of-mass energies of 172, 183 and ...189 GeV. They are compared with those made at the Z peak after correction for the different flavour compositions. A Monte Carlo model of Bose-Einstein correlations based on the JETSET hadronization scheme was tuned to the Z data and reproduces the correlations in the
WW→q
q
̄
ℓν
events. The same Monte Carlo reproduces the correlations in the
WW→q
q
̄
q
q
̄
channel assuming independent fragmentation of the two W's. A variant of this model with Bose-Einstein correlations between decay products of different W's is disfavoured.
Charginos and neutralinos are searched for in the data collected by the ALEPH experiment at LEP for centre-of-mass energies up to 209 GeV. The negative result of these searches is combined with those ...from searches for sleptons and Higgs bosons to derive an absolute lower limit of 43.1 GeV/c2 on the mass of the lightest supersymmetric particle (LSP), assumed to be the lightest neutralino. This limit is obtained in the framework of the MSSM with R-parity conservation and with gaugino and sfermion mass unification at the GUT scale and assuming no mixing in the stau sector. The LSP limit degrades only slightly to 42.4 GeV/c2 if stau mixing is considered. Within the more constrained framework of minimal supergravity, the limit is 50 GeV/c2.
The e
+e
−→ZZ cross section at
s
=182.7
and 188.6
GeV
has been measured using the ALEPH detector. A cut-based analysis yields cross section measurements of
σ
ZZ
(182.7
GeV
)=0.11±
0.16
0.11
(
...stat.)±0.04
(
syst.)
pb
and
σ
ZZ
(188.6
GeV
)=0.69±
0.13
0.12
(
stat.)±0.03
(
syst.)
pb
.
A neural network-based analysis yields
σ
ZZ
(188.6
GeV
)=0.64±
0.12
0.11
(
stat.)±0.04
(
syst.)
pb
.
These measurements are consistent with the Standard Model expectations.
The data collected at a centre-of-mass energy of 188.6 GeV by ALEPH at LEP, corresponding to an integrated luminosity of 173.6 pb
−1, are analysed in a search for the scalar partners of quarks and ...leptons predicted in supersymmetric models. No evidence for any such particles was found in the decay channels
ℓ
̃
→ℓχ
,
t
̃
→
cχ
,
t
̃
→
bℓ
ν
̃
,
b
̃
→
bχ
, and
q
̃
→
qχ
. Improved mass lower limits have been obtained in the framework of the Minimal Supersymmetric Standard Model.
A search for
γγ decays of a Higgs boson is performed in the data sample collected at LEP with the ALEPH detector between 1991 and 1999. This corresponds to an integrated luminosity of 672
pb
−1 at ...centre-of-mass energies ranging from 88 to 202
GeV. The search is based on topologies arising from a Higgs boson produced in association with a fermion pair via the Higgs-strahlung process
e
+
e
−→Hf
f
̄
, with
f
f
̄
=ν
ν
̄
,
e
+
e
−,μ
+μ
−,τ
+τ
−
or
q
q
̄
. Twenty-two events are selected in the data, while 28 events are expected from standard model processes. An upper limit is derived, as a function of the Higgs boson mass, on the product of the
e
+
e
−→Hf
f
̄
cross section and the H→γγ branching fraction. In particular, a fermiophobic Higgs boson produced with the standard model cross section is excluded at 95% confidence level for all masses below
100.7
GeV/c
2
.
The production of W
+W
− pairs is analysed in a data sample collected by ALEPH at a mean centre-of-mass energy of 188.6 GeV, corresponding to an integrated luminosity of 174.2 pb
−1. Cross sections ...are given for different topologies of W decays into leptons or hadrons. Combining all final states and assuming Standard Model branching fractions, the total W-pair cross section is measured to be
15.71±0.34
(stat.)±0.18
(syst.)
pb
. Using also the W-pair data samples collected by ALEPH at lower centre-of-mass energies, the decay branching fraction of the W boson into hadrons is measured to be B(W→hadrons)=66.97±0.65(stat.)±0.32(syst.)%, allowing a determination of the CKM matrix element |
V
cs|=0.951±0.030(stat.)±0.015(syst.).
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 pb
−1. The three final states
τ
+ν
ττ
−
ν
̄
τ
,
c
s
̄
τ
−
ν
̄
τ
and
c
s
̄
s
c
̄
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
+→τ
+ν
τ
)
. In the framework of a two-Higgs-doublet model, and assuming
B(
H
+→τ
+ν
τ
)+
B(
H
+→c
s
̄
)=1
, charged Higgs bosons with masses below 79.3 GeV/
c
2 are excluded at 95% confidence level independently of the branching ratios.
A search for the Higgsstrahlung process e+e−→HZ is carried out, covering decays of the Higgs boson into any quark pair, a gluon pair or a tau pair. The analysis is based on the 630 pb−1 of data ...collected by the ALEPH detector at LEP at centre-of-mass energies from 189 to 209 GeV. A 95% C.L. lower mass limit of 109.1 GeV/c2 is obtained for a Higgs boson cross section equal to that expected from the Standard Model if the Higgs boson decays exclusively into hadrons and/or taus, irrespective of the relative branching fractions.
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