The FCC, half a century on

The community has spoken: the electron–positron Future Circular Collider (FCC-ee) is the preferred next flagship project for CERN. As an initiator of the concept of a circular Higgs factory in 2011, I was elated by this outcome. But it also made me wonder: why did it take so long? To answer this, we need to travel back 50 years.

1976 was an eventful year for particle physics. The open charm was discovered at SPEAR, while the J/ψ earned Burton Richter and Samuel Ting their Nobel Prize. The same year, Richter authored both the first yellow report on a large e⁺e^– (LEP) colliding ring and the first paper proposing a linear e⁺e^– collider. Gargamelle’s measurement of the ratio of Z-induced over W-induced neutrino interactions had allowed the Standard Model (SM) – by then a familiar name – to predict the masses of the W and Z bosons. And Carlo Rubbia, synergy wizard, proposed to undercut the e⁺e^– aficionados by converting the SPS into a proton–antiproton collider. The W and Z bosons were duly discovered in 1983.

In 1987, following the La Thuile Workshop on Physics at Future Accelerators and before LEP had even been completed, Rubbia called a general meeting in CERN’s main auditorium to discuss the future beyond LEP. Two contenders stood out: a 5 TeV e⁺e^– linear collider in a new 30 km tunnel (CLIC), or a 20 TeV pp collider (LHC) with the advantage of fitting in the already financed and nearly finished LEP tunnel. The relative physics merits of the two machines were compared on supersymmetry and Higgs compositeness. The LHC was chosen, and CLIC became a priority R&D programme.

We had a whale of a time at LEP, establishing that light neutrinos are exactly three, measuring the Z mass to six digits and predicting the top-quark mass through radiative corrections a few months before its Tevatron discovery.

By 1996, the LHC was approved and a small group at CERN was considering what could follow it. The listed options were a high-energy future LHC (FLHC), CLIC, and a 4 TeV muon collider. A first 0.5–1 TeV linear collider (LC) was assumed to be done elsewhere. In that context, a circular e⁺e^– machine was mentioned only as a top factory add-on to the FLHC programme, with a design extrapolated from LEP and a performance well below the LC. The prevailing assumption was that the LHC would detect the Higgs boson and supersymmetry, if either existed.

A breakthrough came in 1999, when the asymmetric B factories PEP-II and KEKB, with separated e⁺ and e^– rings and continuous top-up injection, demonstrated luminosities orders of magnitude higher than LEP. Meanwhile, LEP Higgs-hunted fiercely until the end of 2000, setting a lower mass limit at 114.5 GeV, while precision measurements set an upper mass limit of about 180 GeV.

The hunt is on

Come the summer of 2011, the LHC experiments were taking data at 7 TeV. The hunt for the Higgs and the supersymmetric particles was on, and the first limits already constrained the Higgs mass below that of a W pair. That clarified the required centre-of-mass energy of a Higgs factory: a relatively low-energy e⁺e^– collider would do. Locating it in the LEP/LHC tunnel was an obvious possibility, and had already been discussed in the corridors of the EPS-HEP conference in Grenoble that July. Five months later, applying the B-factory design principles, a Higgs factory fitting in the LHC tunnel was evaluated. “LEP3” offered luminosity significantly higher than the linear collider, and the advantage of running several experiments simultaneously. On the downside, its maximum energy fell short of the top-pair-production threshold… and the LHC tunnel was already occupied.

None of this was a straight line. It took 15 years for the physics to make the case on its own terms

Presented with this evaluation, some members of the CERN Scientific Policy Committee suggested that an e⁺e^– Higgs factory more than triple the size of LEP would make a great initial step towards a higher-energy version of the LHC, which was already under consideration. Inserting the Higgs factory in a 100 km tunnel did magic. With its large bending radius, the machine reached the top-pair threshold while covering a wide range of energies and luminosities. It delivered large statistics at the Z pole, with 6 × 10¹² visible Z decays, and transverse polarisation for exquisite energy calibration up to the W-pair threshold. Those were key ingredients in achieving a vertiginous potential for statistical and systematic improvement, by up to a factor of 500 or more over the old LEP precision measurements. The Z run, it was later realised, would turn FCC-ee into a flavour factory for b and tau precision studies, and also enable unique searches for feebly coupled particles in the 5–80 GeV mass range.

What lies beyond

When the Higgs came, it did so at 125 GeV – too high for most incarnations of supersymmetry, too low for theories of a composite Higgs, and consistent with an unchanged SM up to very high energies. It raised the question of what lies behind the SM and at which energies, making it essential for the future of particle physics to include an extensive programme of precision measurements, in the hope of detecting deviations from the SM that could guide the next steps. The Higgs itself was also of obvious interest, and a lepton collider the natural way to study it.

These arguments were summarised in two contributions to the 2013 update of the European Strategy and led to the recommendation of a costed design study of FCC-hh and FCC-ee. The 2020 update continued it as a feasibility study, which led in turn to the 2025 recommendation.

None of this was a straight line. It took 15 years for the physics to make the case on its own terms, and the conversation had been on, in one form or another, for 35 years before that. The Higgs mass, the absence of supersymmetry and the precision reach all pointed to a machine in the best CERN tradition, where the most is made of the resources through a strong synergy between infrastructure planning and physics opportunities.