Centre d'études et d'expertise sur les risques, l'environnement, la mobilité et l'aménagement

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We describe the physics potential of e + e − linear colliders in this report. These machines are planned to operate in the first phase at a center-of-mass energy of 500 GeV, before being scaled up to about 1 TeV. In the second phase of the operation, a final energy of about 2 TeV is expected. The machines will allow us to perform precision tests of the heavy particles in the Standard Model, the top quark and the electroweak bosons. They are ideal facilities for exploring the properties of Higgs particles, in particular in the intermediate mass range. New vector bosons and novel matter particles in extended gauge theories can be searched for and studied thoroughly. The machines provide unique opportunities for the discovery of particles in supersymmetric extensions of the Standard Model, the spectrum of Higgs particles, the supersymmetric partners of the electroweak gauge and Higgs bosons, and of the matter particles. High precision analyses of their properties and interactions will allow for extrapolations to energy scales close to the Planck scale where gravity becomes significant. In alternative scenarios, like compositeness models, novel matter particles and interactions can be discovered and investigated in the energy range above the existing colliders up to the TeV scale. Whatever scenario is realized in Nature, the discovery potential of e + e − linear colliders and the high-precision with which the properties of particles and their interactions can be analysed, define an exciting physics programme complementary to hadron machines.

The phenomenology of an additional U(1) neutral gauge boson ZЈ coupled to the third family of fermions is discussed. One might expect such a particle to contribute to processes where taus, b and t quarks are produced. Precision data from CERN LEP1 put severe constraints on the mixing and heavy-boson mass. We find that the effects of such a particle could not be observed at hadronic colliders, be it at the Fermilab Tevatron or the CERN LHC, because of the QCD background. At LEP2 and future e ϩ e Ϫ linear colliders, one could instead hope to observe such effects, in particular, for bb final states.

New neutral vector bosons Z decaying to charged gauge boson pairs W + W − are predicted in many scenarios of new physics, including models with an extended gauge sector such as E6, leftright symmetric Z LRS and the sequential standard model Z SSM . For these benchmark models we calculate and present theoretical expectations for different values of the Z mass M2 and mixing parameter ξ. Our results are based on the narrow width approximation which allows to make a convenient comparison of experiment to theoretical benchmark models. The diboson production allows to place stringent constraints on the Z-Z mixing angle and the Z mass, which we determine by using data from pp collisions at √ s = 13 TeV recorded by the ATLAS detector at the CERN LHC, with integrated luminosity of ∼ 36 fb −1 . By comparing the experimental limits to the theoretical predictions for the total cross section of Z resonant production and its subsequent decay into W + W − pairs, we show that the derived constraints on the mixing angle for the benchmark models are of the order of a few ×10 −4 , i.e., greatly improved with respect to those derived from the global analysis of electroweak data. We combine the limits derived from diboson production data with those obtained from the Drell-Yan process in order to significantly extend the exclusion region in the M2-ξ parameter plane. Also, we demonstrate that further improvement on the constraining of this mixing can be achieved through analysis of the full set of Run II data. * boboilya@yandex.by † Per.Osland@uib.no ‡ pankov@ictp.it arXiv:1809.08933v2 [hep-ph]