Top-quark physics at the first CLIC stage

The Compact Linear Collider (CLIC) is a mature option for a future electron-positron col- lider 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. The energy of the initial stage was chosen to optimize the physics po- tential in terms of Higgs-boson and top-quark measurements. 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 in- duced 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 measure- ments 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. Expected limits on new physics contributions described in terms of Effective Field Theory (EFT) operator coefficients are presented, showing sensitivity of the first CLIC stage to 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 $\mathrm{t}\rightarrow \mathrm{cH}$, $\mathrm{t}\rightarrow \mathrm{c}\gamma$ and $\mathrm{t}\rightarrow \mathrm{cE\!\!/}$ channels, reaching down to $4.7.10^{-5}$ for BR ($t\rightarrow c\gamma$).