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 ...have been 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
(Type II scenario) or 72.5
(Type I scenario, for pseudo-scalar masses above 12
) are excluded at the 95 % confidence level.
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.
A search has been performed for the Standard Model Higgs boson in the data sample collected with the ALEPH detector at LEP, at centre-of-mass energies up to 209
GeV. An excess of 3
σ beyond the ...background expectation is found, consistent with the production of the Higgs boson with a mass near 114
GeV/
c
2. Much of this excess is seen in the four-jet analyses, where three high purity events are selected.
A search for charginos nearly mass degenerate with the lightest neutralino is performed with the data collected by the ALEPH detector at LEP, at centre-of-mass energies between 189 and 209 GeV, ...corresponding to an integrated luminosity of 628 pb−1. The analysis is based on the detection of isolated and energetic initial state radiation photons, produced in association with chargino pairs whose decay products have little visible energy. The number of candidate events observed is in agreement with that expected from Standard Model background sources. These results are combined with those of other direct searches for charginos, and a lower limit of 88 GeV/c2 at 95% confidence level is derived for the chargino mass in the case of heavy sfermions, irrespective of the chargino-neutralino mass difference.
Study of charm production in Z decays Barate et al, R.
The European physical journal. C, Particles and fields,
09/2000, Letnik:
16, Številka:
4
Journal Article
Recenzirano
Odprti dostop
The production rates of \({\mathrm{D}}^{*\pm}\), \({\mathrm{D}}_{\rm{s}}^{* \pm}\), \({\mathrm{D}}^{\pm}\), \({\mathrm{D}}^0 / \bar{{\mathrm{D}}}^0\), \({\mathrm{D}}_{\rm{s}}^{\pm}\), and ...\(\Lambda_{\rm c}^{+}/{\bar{\Lambda}}_{\rm c}^{-}\) in \({\rm Z} \rightarrow {\mathrm c} \bar{{\mathrm c}}\) decays are measured using the LEP I data sample recorded by the ALEPH detector. The fractional energy spectrum of the \({\mathrm{D}}^{*\pm}\) is well described as the sum of three contributions: charm hadronisation, b hadron decays and gluon splitting into a pair of heavy quarks. The probability for a c quark to hadronise into a \({\mathrm{D}}^{*+}\) is found to be \(f({\mathrm c} \to{\mathrm{D}}^{*+}) = 0.233 \pm 0.010 \mathrm{(stat.)} \pm 0.011\mathrm{(syst.)}\). The average fraction of the beam energy carried by \({\mathrm{D}}^{*\pm}\) mesons in \({\rm Z} \to{\rm c \bar c}\) events is measured to be \({\langle X_E ({\mathrm{D}}^{*\pm}) \rangle}_{{\mathrm c} \bar{{\mathrm c}}} =0.4878 \pm 0.0046 \mathrm{(stat.)} \pm 0.0061 \mathrm{(syst.)}.\) The \({\mathrm{D}}^{*\pm}\) energy and the hemisphere mass imbalance distributions are simultaneously used to measure the fraction of hadronic Z decays in which a gluon splits to a \({\mathrm c} \bar{{\mathrm c}}\) pair: \(\bar{n}_{\mathrm{g} \to{\mathrm c}\bar{{\mathrm c}}} = (3.23 \pm 0.48 \mathrm{(stat.)} \pm 0.53 \mathrm{(syst.)})\%.\) The ratio of the Vector/(Vector+Pseudoscalar) production rates in charmed mesons is found to be \(P_V = 0.595\pm0.045\). The fractional decay width of the Z into \({\mathrm c}\bar{{\mathrm c}}\) pairs is determined from the sum of the production rates for various weakly decaying charmed states to be \({\rm R_c} =0.1738 \pm 0.0047{\rm (stat.)} \pm 0.0116 {\rm (syst.)}.\)
Inclusive branching ratios involving b to tau transitions are measured in approximately four million hadronic Z decays collected by the ALEPH detector at LEP. The fully-inclusive branching ratio b -> ...tau nu X and the semi-inclusive branching ratio b -> tau nu D*+/- X are measured to be (2.43 +/- 0.20 +/- 0.25)% and (0.88 +/- 0.31 +/- 0.28)%, in agreement with the standard model predictions. Upper limits on the branching fractions b -> tau nu and b -> s nu nubar are set to 8.3 10**-4 and 6.4 10**-4 at the 90\%~C.L. These results allow a 90\% C.L. lower limit of 0.40 (GeV/c**2)**-1 to be set on the tan(beta)/mH+/- ratio, in the framework of type-II two-Higgs-doublet models.
A search for selectron, smuon and stau pair production is performed with the data collected by the ALEPH detector at LEP at centre-of-mass energies up to 209 GeV. The numbers of candidate events are ...consistent with the background predicted by the Standard Model. Final mass limits from ALEPH are reported.
The cross sections and forward-backward asymmetries of hadronic and leptonic events produced in e+e- collisions at centre-of-mass energies of 130-183 GeV are presented. Results for ee, mumu, tautau, ...qq, bb and cc production show no significant deviation from the Standard Model predictions. This enable constraints to be set upon physics beyond the Standard Model such as four-fermion contact interactions, leptoquarks, Z' bosons and R-parity violating squarks and sneutrinos. Limits on the energy scale Lambda of eeff contact interactions are typically in the range from 2-10 TeV. Limits on R-parity violating sneutrinos reach masses of a few hundred GeV for large values of their Yukawa couplings.
A total of 628$\invpb$ of data collected with the ALEPH detector at centre-of-mass energies from 189 to 209\,GeV is analysed in the search for gauge mediated SUSY breaking (GMSB) topologies. These ...topologies include two acoplanar photons, non-pointing single photons, acoplanar leptons, large impact parameter leptons, detached slepton decay vertices, heavy stable charged sleptons and multi-leptons plus missing energy final states. No evidence is found for new phenomena, and lower limits on masses of supersymmetric particles are derived. A scan of a minimal GMSB parameter space is performed and lower limits are set for the next-to-lightest supersymmetric particle (NLSP) mass at 54$\gevcc$ and for the mass scale parameter $\Lambda$ at 10$\tevcc$, independently of the NLSP lifetime. Including the results from the neutral Higgs boson searches, a NLSP mass limit of 77$\gevcc$ is obtained and values of $\Lambda$ up to 16$\tevcc$ are excluded.
The anomalous weak dipole moments of the $\tau$ lepton are measured in a data sample collected by ALEPH from 1990 to 1995 corresponding to an integrated luminosity of 155~pb$^{-1}$. Tau leptons ...produced in the reaction $e^+ e^- \rightarrow \tau^+ \tau^-$ at energies close to the ${\rm Z}$ mass are studied using their semileptonic decays to $\pi$, $\rho$, $a_1 \rightarrow \pi 2\pi^0$ or $a_1 \rightarrow 3 \pi$. The real and imaginary components of both the anomalous weak magnetic dipole moment and the CP-violating anomalous weak electric dipole moment, $ {\rm Re}\,\mu_{\tau}$, ${\rm Im}\,\mu_{\tau}$, ${\rm Re}\,d_{\tau}$ and ${\rm Im}\,d_{\tau}$, are measured simultaneously by means of a likelihood fit built from the full differential cross section. No evidence of new physics is found. The following bounds are obtained (95\% CL): $|{\rm Re}\, \mu_{\tau} | 1.14 \times 10^{-3}$, $|{\rm Im}\, \mu_{\tau} | 2.65 \times 10^{-3}$, $|{\rm Re}\, d_{\tau} | 0.91 \times 10^{-3}$, and $|{\rm Im}\, d_{\tau} | 2.01 \times 10^{-3}$.
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