The full LEP-1 data set collected with the ALEPH detector at the
Z pole during 1991–1995 is analysed in order to measure the
τ
decay branching fractions. The analysis follows the global method used ...in the published study based on 1991–1993 data, but several improvements are introduced, especially concerning the treatment of photons and
π
0
's. Extensive systematic studies are performed, in order to match the large statistics of the data sample corresponding to over 300
000 measured and identified
τ
decays. Branching fractions are obtained for the two leptonic channels and 11 hadronic channels defined by their respective numbers of charged particles and
π
0
's. Using previously published ALEPH results on final states with charged and neutral kaons, corrections are applied to the hadronic channels to derive branching ratios for exclusive final states without kaons. Thus the analyses of the full LEP-1 ALEPH data are combined to yield a complete description of
τ
decays, encompassing 22 non-strange and 11 strange hadronic modes. Some physics implications of the results are given, in particular related to universality in the leptonic charged weak current, isospin invariance in
a
1
decays, and the separation of vector and axial-vector components of the total hadronic rate. Finally, spectral functions are determined for the dominant hadronic modes and updates are given for several analyses. These include: tests of isospin invariance between the weak charged and electromagnetic hadronic currents, fits of the
ρ
resonance lineshape, and a QCD analysis of the non-strange hadronic decays using spectral moments, yielding the value
α
s
(
m
τ
2
)
=
0.340
±
0
.
005
exp
±
0
.
014
th
. The evolution to the
Z mass scale yields
α
s
(
M
Z
2
)
=
0.1209
±
0.0018
. This value agrees well with the direct determination from the
Z width and provides the most accurate test to date of asymptotic freedom in the QCD gauge theory.
A search for the production and non-standard decay of a Higgs boson, h, into four taus through intermediate pseudoscalars, a, is conducted on 683 pb
−1
of data collected by the ALEPH experiment at ...centre-of-mass energies from 183 to 209 GeV. No excess of events above background is observed, and exclusion limits are placed on the combined production cross section times branching ratio,
. For
m
h
< 107 GeV/
c
2
and 4 <
m
a
< 10 GeV/
c
2
,
ξ
2
> 1 is excluded at the 95% confidence level.
The RD48 (ROSE) collaboration has succeeded to develop radiation hard silicon detectors, capable to withstand the harsh hadron fluences in the tracking areas of LHC experiments. In order to reach ...this objective, a defect engineering technique was employed resulting in the development of Oxygen enriched FZ silicon (DOFZ), ensuring the necessary O-enrichment of about 2×10
17 O/cm
3 in the normal detector processing. Systematic investigations have been carried out on various standard and oxygenated silicon diodes with neutron, proton and pion irradiation up to a fluence of 5×10
14
cm
−2 (1
MeV neutron equivalent). Major focus is on the changes of the effective doping concentration (depletion voltage). Other aspects (reverse current, charge collection) are covered too and the appreciable benefits obtained with DOFZ silicon in radiation tolerance for charged hadrons are outlined. The results are reliably described by the “Hamburg model”: its application to LHC experimental conditions is shown, demonstrating the superiority of the defect engineered silicon. Microscopic aspects of damage effects are also discussed, including differences due to charged and neutral hadron irradiation.
The hadronic final states observed with the ALEPH detector at LEP in e+e- annihilation are analysed using 730 pb-1 of data collected between 91 and 209 GeV in the framework of QCD. In particular ...event-shape variables and inclusive charged particle spectra are measured. The energy evolution of quantities derived from these measurements is compared to analytic QCD predictions. The mean charged particle multiplicity, the charged particle momentum spectrum and its peak position are compared to predictions of the modified-leading-logarithmic approximation. The strong coupling constant alpha_s is determined from a fit of the QCD prediction to distributions of six event-shape variables at eight centre-of-mass energies. A study of non-perturbative power law corrections is presented
Deuteron and anti-deuteron production in Z decays has been observed in the ALEPH experiment at LEP. The production rate of anti-deuterons is measured to be (5.9±1.8±0.5)×10−6 per hadronic Z decay in ...the anti-deuteron momentum range from 0.62 to 1.03 GeV/c. The coalescence parameter B2, which characterizes the likelihood of anti-deuteron production, is measured to be 0.0033±0.0013 GeV2 in Z decays. These measurements indicate that the production of anti-deuterons is suppressed in e+e− collisions compared to that in pp and photoproduction collisions.
The fragmentation of b quarks into B mesons is studied with four million hadronic Z decays collected by the ALEPH experiment during the years 1991–1995. A semi-exclusive reconstruction of
B→ℓ
νD
(★) ...decays is performed, by combining lepton candidates with fully reconstructed
D
(★) mesons while the neutrino energy is estimated from the missing energy of the event.
The mean value of
x
B
wd, the energy of the weakly-decaying B meson normalised to the beam energy, is found to be
〈x
B
wd
〉=0.716±0.006
(
stat)±0.006
(
syst),
using a model-independent method; the corresponding value for the energy of the leading B meson is
〈x
B
L
〉=0.736±0.006
(
stat)±0.006
(
syst)
. The reconstructed spectra are compared with different fragmentation models.
The flavour changing neutral current decay
b→
sγ has been detected in hadronic Z decays collected by ALEPH at LEP. The signal is isolated in lifetime-tagged
b
b
̄
events by the presence of a hard ...photon associated with a system of high momentum and high rapidity hadrons. The background processes are normalised from the data themselves. The inclusive branching ratio is measured to be
(3.11±0.80
stat
±0.72
syst
)×10
−4,
consistent with the Standard Model expectation via penguin processes.
The mass of the W boson is determined from the direct reconstruction of W decays in WW→qq̄qq̄ and WW→ℓνqq̄ events in e+e- collisions at LEP. The data sample corresponds to an integrated luminosity of ...683 pb-1 collected with the ALEPH detector at centre-of-mass energies up to 209 GeV. To minimise any effect from colour reconnection a new procedure is adopted in which low energy particles are not considered in the mass determination from the qq̄qq̄ channel. The combined result from all channels is \(m_{\text{W}}=80.440 \pm0.043{\text{(stat.)}} \pm0.024{\text{(syst.)}} \pm0.009{\text{(FSI)}} \pm0.009{\text{(LEP)}} \text{GeV/}c^2, \)where FSI represents the possible effects of final state interactions in the qq̄qq̄ channel and LEP indicates the uncertainty in the beam energy. From two-parameter fits to the W mass and width, the W width is found to be \(\Gamma_{\text{W}} = 2.14 \pm0.09{\text{(stat.)}} \pm0.04{\text{(syst.)}} \pm0.05{\text{(FSI)}} \pm0.01{\text{(LEP)}} \text{GeV}. \)
Exotic hadrons made of five quarks (pentaquarks) are searched for in hadronic Z decays collected by the ALEPH detector at LEP. No significant signal is observed. At 95% C.L., upper limits are set on ...the production rates N of such particles and their charge-conjugate state per Z decay: NΘ(1535)+⋅BR(Θ(1535)+→pKS0)<6.2×10−4,NΞ(1862)−−⋅BR(Ξ(1862)−−→Ξ−π−)<4.5×10−4,NΞ(1862)0⋅BR(Ξ(1862)0→Ξ−π+)<8.9×10−4,NΘc(3100)0⋅BR(Θc(3100)0→D*−p)<6.3×10−4,NΘc(3100)0⋅BR(Θc(3100)0→D−p)<31×10−4.
Cross sections, angular distributions and forward-backward asymmetries are presented, of two-fermion events produced in e+e- collisions at centre-of-mass energies from 189 to 209 GeV at LEP, measured ...with the ALEPH detector. Results for e+e-, μ+μ-, τ+τ-, qq̄, bb̄ and cc̄ production are in agreement with the standard model predictions. Constraints are set on scenarios of new physics such as four-fermion contact interactions, leptoquarks, Z′ bosons, TeV-scale quantum gravity and R-parity violating squarks and sneutrinos.
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