Top-quark physics at the first CLIC stage

The Compact Linear Collider (CLIC) is a mature option for a future electron-positron collider operating at centre-of-mass energies of up to 3 TeV. CLIC will be built and operated in a staged approach with three centre-of-mass energies currently assumed to be 380 GeV, 1.5 TeV and 3 TeV. This contribution discusses the prospects for precision measurements of top-quark properties at the first stage of CLIC, based on detailed simulation studies, taking into account luminosity spectra and beam induced backgrounds, full detector simulation based on Geant4, final state reconstruction based on particle flow approach with PandoraPFA, jet clustering with the VLC algorithm as implemented in the FastJet package, and flavour tagging with LcfiPlus. Based on a dedicated centre-of-mass energy scan around the top-quark pair production threshold, the top-quark mass can be determined with a precision of about 50 MeV in a theoretically well-defined manner. This scan is also sensitive to the top-quark width and Yukawa coupling. Other approaches to extract the top-quark mass at the first stage of CLIC make use of ISR photons or the direct reconstruction of the top quarks. Precise measurements of the differential top-quark pair production cross sections at 380 GeV, for different electron beam polarisations, allow the study of top-quark couplings to electroweak gauge bosons sensitive to new physics mass scales beyond 10 TeV. The large number of top-quark pairs produced also allows competitive searches for Flavour Changing Neutral Current (FCNC) top-quark decays with charm quarks in the final state. Exclusion limits expected for 500 fb$^{-1}$ collected at the first stage of CLIC are presented for $t\rightarrow ch$, $t\rightarrow c\gamma$ and $t\rightarrow c E_{miss}$ channels, reaching down to $4.7 \cdot 10^{-5}$ for BR($t\rightarrow c\gamma$).Comment: 4 pages, 3 figures, to be submitted to proceedings of ICHEP'2018, presented on behalf of the CLICdp Collaboratio

Measurement of the leptoquark Yukawa couplings in e+e- collisions at TESLA

Measurement of the Yukawa couplings of the first-generation leptoquarks has been studied for e+e- collisions at TESLA, at sqrt(s)=800 GeV. By combining measurements from different production and decay channels, determination of Yukawa couplings with precision on the few per-cent level is possible. TESLA will be sensitive to very small leptoquark Yukawa couplings not accessible at LHC, down to lambda ~ 0.05 [e]. Distinction between left-handed and right-handed Yukawa couplings is feasible even for leptoquark masses very close to the pair-production kinematic limit.Comment: 20 pages, 15 figure

Benchmarking the Inert Doublet Model for e+ e- colliders

We present benchmarks for the Inert Doublet Model, a Two Higgs Doublet Model with a dark matter candidate. They are consistent with current constraints on direct detection, including the most recent bounds from the XENON1T experiment and relic density of dark matter, as well as with known collider and low-energy limits. We focus on parameter choices that promise detectable signals at lepton colliders via pair-production of H+H- and HA. For these we choose a large variety of benchmark points with different kinematic features, leading to distinctly different final states in order to cover the large variety of collider signatures that can result from the model.Comment: 21 pages, 1 figure, 5 tables; v2: corresponds to published version. Comments on future prospects adde

IDM benchmarks for the LHC and future colliders

We present cross-section expectations for various processes and collider options, for benchmark scenarios of the Inert Doublet Model, a Two Higgs Doublet Model with a dark matter candidate. The proposed scenarios are consistent with current dark matter constraints, including the most recent bounds from the XENON1T experiment and relic density, as well as with known collider and low-energy limits. These benchmarks, chosen in earlier work for studies at $e^+e^-$ colliders, exhibit a variety of kinematic features that should be explored at current and future runs of the LHC. We provide cross sections for all relevant production processes at 13 TeV, 27 TeV and 100 TeV proton collider, as well as for a possible 10 TeV and 30 TeV muon collider.Comment: 44 pages, 11 figures; v2: corresponds to published journal version (slight style differences

Study of the Higgs-boson decays into WW and ZZ at the Photon Collider

Production of the Standard Model Higgs-boson at the Photon Collider at TESLA is studied for the Higgs-boson masses above 150 GeV. Simulation of signal and background processes takes into account realistic luminosity spectra and detector effects. In the considered mass range, large interference effects are expected in the W+W- decay channel. By reconstructing W+W- and ZZ final states, not only the h->gamma gamma partial width can be measured, but also the relative phase of the scattering amplitude. This opens a new window for the precise determination of the Higgs-boson couplings. Models with heavy, fourth-generation fermions and with enlarged Higgs sector (2HDM (II)) are considered.Comment: 19 pages, 18 figures; updated version with improved precision of estimate