Higgs Searches at LEP and at the Tevatron

After years of efforts to push the LEP performance to, and indeed beyond, the limits of what had been believed possible, hints of signal of a Higgs boson at 115GeV/c2 appeared in June 2000, were confirmed in September, and confirmed again in November. Spending an additional six-month period with LEP would have given the unambiguous opportunity of a fundamental discovery. Instead, this possibilty was handed over to the Tevatron, for which at least si more years will be needed to confirm the existence of a Higgs boson around 115GeV/c2. The upgrades performed at LEP and needed at the Tevatron, together with the physics outcomes, are briefly mentioned in turn.Comment: 20 pages, 9 figures, Physics in Collisions, June 2001, Seou

Top-quark electroweak couplings at the FCC-ee

An optimal-observable analysis of the lepton angular and energy distributions from top-quark pair production with semi-leptonic decays in e+e- collisions is used to predict the potential sensitivity of the FCC-ee to the couplings of the top quark to the photon and the Z.Comment: 18 pages, 5 figures. To be published in JHEP (accepted for publication). Between version 2 and version 3, the suggestions of the JHEP referee were implemented both in the text (in Sections "Results and discussion" and "Summary and outlook") and in Figures 3 and

Direct measurement of alpha_QED(mZ) at the FCC-ee

When the measurements from the FCC-ee become available, an improved determination of the standard-model "input" parameters will be needed to fully exploit the new precision data towards either constraining or fitting the parameters of beyond-the-standard-model theories. Among these input parameters is the electromagnetic coupling constant estimated at the Z mass scale, alpha_QED(mZ). The measurement of the muon forward-backward asymmetry at the FCC-ee, just below and just above the Z pole, can be used to make a direct determination of alpha_QED(mZ) with an accuracy deemed adequate for an optimal use of the FCC-ee precision data.Comment: Version 4: Corrigendum added (2 pages). Accepted for publication in JHE

Closing the light sbottom mass window from a compilation of e+e- -> hadron data

The e+e- -> hadron cross section data from PEP, PETRA, TRISTAN, SLC and LEP, at centre-of-mass energies between 20 to 209 GeV, are analysed to search for the production of a pair of light sbottoms decaying hadronically via R-parity-violating couplings. This analysis allows the 95%C.L. exclusion of such a particle if its mass is below 7.5 GeV/c2. The light sbottom mass window is closed.Comment: 19 pages, 8 figures, 10 tables To be published in Physics Letters. This new version has been reshuffled with respect to the original version to include the suggestions from the Referees of Physics Letters. In particular, the e+e- -> hadron data compilation and combination method are now described in detail, to avoid references to unpublished material. A more realistic treatment of systematic uncertainties also improved the limit slightl

SUSY Decays of Higgs Particles

Among the possible decay modes of Higgs particles into supersymmetric states, neutralino and chargino decays play a prominent r\^ole. The experimental opportunities of observing such decay modes at LEP2 and at future e+e- linear colliders are analyzed within the frame of the Minimal Supersymmetric extension of the Standard Model. For heavy Higgs particles, the chargino/neutralino decay modes can be very important, while only a small window is open for the lightest CP-even Higgs particle. If charginos/neutralinos are found at LEP2, such decay modes can be searched for in a small area of the parameter space, and invisible decays may reduce the exclusion limits of the lightest CP-even Higgs particle slightly; if charginos/neutralinos are not found at LEP2 in direct searches, the Higgs search will not be affected by the SUSY particle sector.Comment: 13 pages including 4 figures, uses latex and (e)psfig.st

Topological Polaritons in a Quantum Spin Hall Cavity

We study the topological structure of matter-light excitations, so called polaritons, in a quantum spin Hall insulator coupled to photonic cavity modes. We identify a topological invariant in the presence of time reversal (TR) symmetry, and demonstrate the existence of a TR-invariant topological phase. We find protected helical edge states with energies below the lower polariton branch and characteristic uncoupled excitonic states, both detectable by optical techniques. Applying a Zeeman field allows us to relate the topological index to the double coverage of the Bloch sphere by the polaritonic pseudospin.Comment: 5 pages + 4 pages supplemental material, 3 figure

Mechanical Properties Characterization of Welded Automotive Steels

Among the various welding technologies, resistance spot welding (RSW) and laser beam welding (LBW) play a significant role as joining methods for the automobile industry. The application of RSW and LBW for the automotive body alters the microstructure in the welded areas. It is necessary to identify the mechanical properties of the welded material to be able to make a reliable statement about the material behavior and the strength of welded components. This study develops a method by which to determine the mechanical properties for the weldment of RSW and LBW for two dual phase (DP) steels, DP600 and DP1000, which are commonly used for the automotive bodies. The mechanical properties of the resistance spot weldment were obtained by performing tensile tests on the notched tensile specimen to cause an elongation of the notched and welded area in order to investigate its properties. In order to determine the mechanical properties of the laser beam weldment, indentation tests were performed on the welded material to calculate its force-penetration depth-curve. Inverse numerical simulation was used to simulate the indentation tests to determine and verify the parameters of a nonlinear isotropic material model for the weldment of LBW. Furthermore, using this method, the parameters for the material model of RSW were verified. The material parameters and microstructure of the weldment of RSW and LBW are compared and discussed. The results show that the novel method introduced in this work is a valid approach to determine the mechanical properties of welded high-strength steel structures. In addition, it can be seen that LBW and RSW lead to a reduction in ductility and an increase in the amount of yield and tensile strength of both DP600 and DP1000

Symmetry properties of Penrose type tilings

The Penrose tiling is directly related to the atomic structure of certain decagonal quasicrystals and, despite its aperiodicity, is highly symmetric. It is known that the numbers 1, $-\tau$, $(-\tau)^2$, $(-\tau)^3$, ..., where $\tau =(1+\sqrt{5})/2$, are scaling factors of the Penrose tiling. We show that the set of scaling factors is much larger, and for most of them the number of the corresponding inflation centers is infinite.Comment: Paper submitted to Phil. Mag. (for Proceedings of Quasicrystals: The Silver Jubilee, Tel Aviv, 14-19 October, 2007

Search for the Standard Model Higgs Boson at LEP

One of the missions of the LEP program was the search for the Standard Model Higgs Boson. The skillful operation of the machine in the year 2000, the final year of operation, has allowed the four collaborations ALEPH, DELPHI, L3 and OPAL to collect 536/pb of data at center-of-mass energies of 206 GeV or higher. This data is used to probe the existence of the Higgs boson up to a mass of around 115 GeV. Tantalizing candidates have been observed in excess over the Standard Model predictions, but without enough statistical power to claim a discovery. A Higgs boson lighter than 114.4 GeV is hence excluded at 95% confidence level.Comment: 5 pages, 6 figures, to be published in the proceedings of the "5th International Conference on Hyperons, Charm and Beauty Hadrons