Browsing by Author "David, A."
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- Electroweak Measurements in Electron-Positron Collisions at W-Boson-Pair Energies at LEPPublication . ALEPH, DELPHI, L3, OPAL, LEP Electroweak collaboration (1644 authors); David, A.; Abreu, P.; Andringa, S.; Anjos, N.; Castro, N.; De Angelis, A.; Espirito Santo, M.C.; Goncalves, P.; Onofre, A.; Pimenta, M.; Tome, B.; Veloso, F.Electroweak measurements performed with data taken at the electron-positron collider LEP at CERN from 1995 to 2000 are reported. The combined data set considered in this report corresponds to a total luminosity of about 3 fb$^{-1}$ collected by the four LEP experiments ALEPH, DELPHI, L3 and OPAL, at centre-of-mass energies ranging from 130 GeV to 209 GeV. Combining the published results of the four LEP experiments, the measurements include total and differential cross-sections in photon-pair, fermion-pair and four-fermion production, the latter resulting from both double-resonant WW and ZZ production as well as singly resonant production. Total and differential cross-sections are measured precisely, providing a stringent test of the Standard Model at centre-of-mass energies never explored before in electron-positron collisions. Final-state interaction effects in four-fermion production, such as those arising from colour reconnection and Bose-Einstein correlations between the two W decay systems arising in WW production, are searched for and upper limits on the strength of possible effects are obtained. The data are used to determine fundamental properties of the W boson and the electroweak theory. Among others, the mass and width of the W boson, $M_W$ and $\Gamma_W$, the branching fraction of W decays to hadrons, $B(W\to\mathrm{had})$, and the trilinear gauge-boson self-couplings $g^Z_1$, $\kappa_\gamma$ and $\lambda_\gamma$ are determined to be: M_W = 80.376 \pm 0.033 ~GeV \Gamma_W = 2.195 \pm 0.083 ~GeV B(W\to\mathrm{had}) = 67.41 \pm 0.27 % g^Z_1 = 0.984 ^{+0.018}_{-0.020} \kappa_\gamma = 0.982 \pm 0.042 \lambda_\gamma = -0.022 \pm 0.019
- First Look at the Physics Case of TLEPPublication . TLEP Design Study Working Group collaboration (128 authors); David, A.; Faccioli, P.The discovery by the ATLAS and CMS experiments of a new boson with mass around 125 GeV and with measured properties compatible with those of a Standard-Model Higgs boson, coupled with the absence of discoveries of phenomena beyond the Standard Model at the TeV scale, has triggered interest in ideas for future Higgs factories. A new circular e+e- collider hosted in a 80 to 100 km tunnel, TLEP, is among the most attractive solutions proposed so far. It has a clean experimental environment, produces high luminosity for top-quark, Higgs boson, W and Z studies, accommodates multiple detectors, and can reach energies up to the t-tbar threshold and beyond. It will enable measurements of the Higgs boson properties and of Electroweak Symmetry-Breaking (EWSB) parameters with unequalled precision, offering exploration of physics beyond the Standard Model in the multi-TeV range. Moreover, being the natural precursor of the VHE-LHC, a 100 TeV hadron machine in the same tunnel, it builds up a long-term vision for particle physics. Altogether, the combination of TLEP and the VHE-LHC offers, for a great cost effectiveness, the best precision and the best search reach of all options presently on the market. This paper presents a first appraisal of the salient features of the TLEP physics potential, to serve as a baseline for a more extensive design study.
- Search for Charged Higgs bosons: Combined Results Using LEP DataPublication . ALEPH, DELPHI, L3, OPAL, LEP collaboration (1216 authors); Abreu, P.; Andringa, S.; Anjos, N.; Castro, Nuno Filipe; De Angelis, A.; Espirito Santo, M.C.; Goncalves, P.; Onofre, A.; Peralta, L.; Pimenta, M.; Tome, B.; Veloso, F.; David, A.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.
- Search for vectorlike light-flavor quark partners in proton-proton collisions at $\sqrt s$ =8 TeVPublication . CMS collaboration (2223 authors); Nayak, A.; Bargassa, P.; Beirão Da Cruz E Silva, C.; Calpas, B.; Di Francesco, A.; Faccioli, P.; Gallinaro, M.; Hollar, J.; Leonardo, N.; Lloret Iglesias, L.; Seixas, J.; Toldaiev, O.; Vadruccio, D.; Varela, J.; Vischia, P.; David, A.; Veckalns, V.; Musella, P.; Pela, J.; Delgado, A.A search is presented for heavy vectorlike quarks (VLQs) that couple only to light quarks in proton-proton collisions at s=8 TeV at the LHC. The data were collected by the CMS experiment during 2012 and correspond to an integrated luminosity of 19.7 fb−1. Both single and pair production of VLQs are considered. The single-production search is performed for down-type VLQs (electric charge of magnitude 1/3), while the pair-production search is sensitive to up-type (charge of magnitude 2/3) and down-type VLQs. Final states with at least one muon or one electron are considered. No significant excess over standard model expectations is observed, and lower limits on the mass of VLQs are derived. The lower limits range from 400 to 1800 GeV, depending on the single-production cross section and the VLQ branching fractions B to W, Z, and Higgs bosons. When considering pair production alone, VLQs with masses below 845 GeV are excluded for B(W)=1.0, and below 685 GeV for B(W)=0.5, B(Z)=B(H)=0.25. The results are more stringent than those previously obtained for single and pair production of VLQs coupled to light quarks.