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Le LHC, le plus grand accélérateur de particules du monde, le plus complexe instrument scientifique jamais construit, a permis la mise en évidence du mythique boson de Higgs, une particule d un type nouveau prévue par la théorie, mais qui n' avait encore jamais pu être observée. Quelles en sont les conséquences ? La théorie doit-elle être révisée ? Cet événement spectaculaire devra permettre de répondre à des questions cruciales que se posent les physiciens concernant les lois qui régissent la structure de la matière et les interactions fondamentales entre particules élémentaires (quarks et leptons).
Full LEP-I data collected by the ALEPH detector during 1991-1995 running are analyzed in order to measure the $\tau$ decay branching fractions and the hadronic spectral functions. The analysis follows the global method used in the published study based on 1991-1993 data, with several improvements, especially concerning the treatment of photons and $\pi^0$'s. Extensive systematic studies are performed, in order to match the large statistics of the data sample corresponding to 327148 measured and identified $\tau$ decays. Preliminary values for the branching fractions are obtained for the 2 leptonic channels and 11 hadronic channels defined by their respective numbers of charged particles and $\pi^0$'s. Using previously published ALEPH results on final states with charged and neutral kaons, corrections are applied so that branching ratio...
Some interesting developments in astroparticle physics are discussed, taking advantage of novel experimental approaches. Multi-messenger astronomy is likely to open new insights on the physics of the most violent events in the universe. Examples are taken using UHE cosmic rays, gamma-ray bursts and the exploration of dark matter using high energy photons. A generic case involving the detection of gravitational waves and neutrinos from the next galactic supernova is worked out in more detail, emphasizing its implications in particle and stellar collapse physics.
Hadronic decays of the $\tau$ lepton provide a clean environment to study hadron dynamics in an energy regime dominated by resonances. The interesting information is captured in the spectral functions. Recent results from ALEPH on exclusive channels are presented, with emphasis on the $\pi \pi^0$ final state which plays a crucial role for the determination of the hadronic contribution to the muon anomalous magnetic moment. A comparison between $2\pi$ spectral functions obtained in $\tau$ decays (after corrections for isospin-breaking) and $e^+e^-$ annihilation reveals some discrepancy in the line shape of the $\rho$ resonance which can be attributed to different pole mass values for the charged and neutral $\rho$'s, which are determined through a robust fitting procedure. However, after applying this correction, the normalization of th...
The past decade has seen very significant progress in the measurement of the cross section for the annihilation process e+e−→hadrons. Different experimental approaches have helped achieve a more complete and fairly consistent picture. I provide a short review of the procedures used to obtain a continuous description of cross sections combining all available results from experiments. These data are necessary input to calculations of hadronic vacuum polarization effects that use dispersion integrals in an energy regime in which perturbative QCD cannot be applied. Applications include the hadronic contribution to the muon magnetic anomaly prediction in the Standard Model and the running of the electromagnetic coupling through the scale dependence of α(s), particularly its value at the Z boson mass, which is necessary for electroweak preci...
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