\section{Conclusion}
\label{sec:conclusion}

The discovery at the LHC of a particle that resembles strongly the long-sought Higgs boson of the Standard Model has placed studies for the next large machine for high-energy physics in a new perspective. The prospects for the next decade already look quite promising: the HL-LHC is an impressive Higgs factory, with great potential for measuring many Higgs couplings with accuracies of a few per-cent. The LHC run at 13-14 TeV may well discover something else, and it would be premature to mortgage the future of high-energy physics before knowing what it reveals. In the meantime new ideas are emerging for possible future Higgs factories.

In view of the financial, technical and personnel resources needed for the next large high-energy physics instrument, it is essential to choose a strategy that provides complementarity to the LHC, with optimal capabilities beyond what can be achieved with HL-LHC, in both precision measurements and/or discovery potential. 

In our view, TLEP,  a large $\epem$ circular collider in a tunnel with 80 to 100~km circumference, would best complement the LHC, as it would provide {\it (i)} per-mil precision in measurements of Higgs couplings, {\it (ii)} unique precision in measurements of Electroweak Symmetry-Breaking parameters and the strong coupling constant, {\it (iii)} a measurement of the Z invisible width equivalent to better than 0.001 of a conventional neutrino species, and {\it (iv)} a unique search programme for rare Z, W, Higgs, and top decays. We emphasize that circular $\epem$ colliders use a mature technology that has been developed during the construction and operation of successive $\epem$ machines over 50 years, and in particular in a very similar regime at LEP2. Many of the key technical advances that make TLEP possible will be demonstrated by SuperKEKB, which has many parameters similar to TLEP. Experience with SuperKEKB will make more reliable the cost estimates, power evaluations, and luminosity predictions for TLEP. Moreover, TLEP would be a stepping-stone towards a 100 TeV pp collider in the same tunnel, and therefore provides a unique long-term vision for high-energy physics. The combination of TLEP and the VHE-LHC offers, for a great cost effectiveness, the best precision and the best search reach of all options presently on the market.

The design study of TLEP has now started, in close collaboration with the VHE-LHC design study, with worldwide collaboration from Asia, USA and Europe, and with full support from the CERN Council. The study is now included in the approved CERN Medium-Term Plan for the years 2014-2018. The first proposed step is a design study report in 2015, to be followed by a conceptual design report and a detailed cost estimate in 2018-2019. In this paper, we have taken a first look at a potentially very rich TLEP physics programme, which can serve as a baseline for a comprehensive exploration of its possibilities during this period. An informed decision on the project could then be taken in full knowledge of the LHC results at 13-14 TeV and operational experience with SuperKEKB. Technically, and if given the necessary financial and political support, TLEP could be ready for physics in 2030.