  The discovery by the ATLAS and CMS experiments of a new boson with mass
around 125 GeV and with measured properties compatible with those of a
Standard-Model Higgs boson, coupled with the absence of discoveries of
phenomena beyond the Standard Model at the TeV scale, has triggered interest in
ideas for future Higgs factories. A new circular e+e- collider hosted in a 80
to 100 km tunnel, TLEP, is among the most attractive solutions proposed so far.
It has a clean experimental environment, produces high luminosity for
top-quark, Higgs boson, W and Z studies, accommodates multiple detectors, and
can reach energies up to the t-tbar threshold and beyond. It will enable
measurements of the Higgs boson properties and of Electroweak Symmetry-Breaking
(EWSB) parameters with unequalled precision, offering exploration of physics
beyond the Standard Model in the multi-TeV range. Moreover, being the natural
precursor of the VHE-LHC, a 100 TeV hadron machine in the same tunnel, it
builds up a long-term vision for particle physics. Altogether, 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.
This paper presents a first appraisal of the salient features of the TLEP
physics potential, to serve as a baseline for a more extensive design study.
