An already long history
At the times when the existence of the Higgs boson, let alone its mass, was unknown, it was widely accepted that LEP would be the last e+e- storage ring collider at the high energy frontier. At high energies, synchrotron radiation rapidly limits the energy reach of such machines, and the risk of missing the top quark or the Higgs boson was just too high.
This situation changed radically in 2011 when the first hints for a light Higgs boson started to appear. A first note discussed an e+e- collider, dubbed LEP3, that could be (re-)installed in the LHC tunnel. By combining the experience gained on LEP2 and the B factory ideas, and fixing the total synchrotron radiation power to be no more than 100 MW, a luminosity of over 1034cm-2s-1 at a centre-of-mass energy of 240 GeV could be contemplated in up to four interaction points. The machine, however, was still limited in energy.
When the study of a 80-100 km tunnel was undertaken at CERN, it was soon realized that the e+e- collider that would fit in there is just remarkable: 1) the luminosity scales proportionally to the accelerator radius; 2) a centre-of-mass energy in excess of the top-pair threshold can be reached allowing this machine to produce all standard model particles with unequalled statistics; 3) the energy spread is reduced, hence beam transverse polarization can be envisioned at least up to the WW threshold; 4) by using all the RF power of 100 MW, the machine performance at the Z peak is simply mind-boggling – a Tera-Z factory becomes realistically feasible.
After two years of LEP3 and TLEP studies, which shaped the FCC scientific case,the FCC-ee is now part and parcel of the FCC design study.
If you wish to learn more about the ambitious spirit of the former TLEP design team, take a look: