R&D for Future Circular Lepton Colliders



The INFN experiment RD-FA was born in 2017 to gather the various research activities on the subject of possible experiments performed at future accelerators or new techniques applied to the acceleration of particles. The main focus was the study of a detector concept for electron-positron circular accelerators, called IDEA, and the proposal for a muon collider.

In 2020, the project RD-FCC, dedicated to the future circular electron-positron colliders alone, stemmed from RD-FA.

The project consists in building a new 100 km tunnel that would host at first an electron-positron collider, followed later by a proton-proton one. One proposal is at CERN (FCC) and another in the People’s Republic of China, P.R.C. (CEPC+SPPC).


The Higgs boson was the last missing brick of the Standard Model and it was discovered in 2012 by the ATLAS and CMS collaborations at the Large Hadron Collider (LHC), CERN. It has a mass of 125 GeV, spin zero, no electric charge and it is responsible for the mass of elementary particles. The future 100 km circumference collider will open to the possibility to deeply investigate the properties of this peculiar particle. Such a circular lepton collider will be able to provide an impressive instantaneous luminosity with the primary goal of producing high statistics samples for an unprecedented precision study of the Higgs boson properties and of the Standard Model, as well as for the search of evidence of new physics (the so-called beyond the Standard Model sector).

The Future Circular Collider–e+e (FCC-ee) is the CERN project for the realization of the new very large electron-positron collider of 100 km circumference. It is the first of a two stage program: after the FCC-ee, the same tunnel will host an hadron collider (FCC-hh).

The studies for the possible scenario post-LHC started in 2010-2013, with both hadron and lepton collider hypotheses, that in 2014 merged in the FCC project.

The foreseen instantaneous luminosity is maximum at the the Z peak, where the design value is 2.3 × 1036 cm-2 s-1, 2.8 × 1035 cm-2 s-1 at WW threshold, 8.5 × 1034 cm-2 s-1 at ZH and 1.8 × 10^34 cm-2 s-1 at the ttbar threshold. The luminosity at the Z pole is about 105 times the LEP luminosity at the same energy and this will make measurements with a statistical uncertainty to the level of 10-5, around 300 times smaller than at LEP, possible.

The FCC-ee Conceptual Design Report was published in 2019. In June 2021 the FCC feasibility study begun. In the end, a Feasibility Study Report (FSR) will be published, before the next European Particle Physics Strategy update in 2026. If the FSR will be approved, the tunnel excavation could start during the operations of the High Luminosity LHC (HL-LHC) run, with a possible start of data taking for FCC-ee after the HL-LHC shut down.


The Circular Electron-Positron Collider (CEPC) project has been proposed in 2012 by the Chinese particle physicists, to study the feasibility of a large-scale electron-positron collider in the P.R.C.

The CEPC will require the construction of brand new facilities: various sites in the country are currently being evaluated to select the best host of the new laboratory.

The double-ring collider, with a circumference of 100 km, will have two interaction points. It will operate as Higgs factory with ZH pair production (Ecms = 240 GeV), as Z factory at the Z pole (Ecms = 91.2 GeV) and as W factory with WW pair production (Ecms = 160 GeV). The instantaneous luminosity will reach the maximum values of 3 × 1034 cm-2 s-1, 3.2 × 1035 cm-2 s-1 and 1. × 1035 cm-2 s-1 in the three modes respectively. From the design machine parameters, the collection of a sample of over one million Higgs bosons is expected, one trillion Z bosons and 100 million W bosons. Additionally, a large number of events containing charm and bottom quarks, as well as tau leptons, will provide the physicists with a huge amount of data in the flavor and tau sectors.

It is foreseen to achieve a precision of the order of 0.1-1 % on the measurement of most of the Higgs couplings, about one order of magnitude better than what expected at HL-LHC. Also the electroweak observable measurements will be receive an improvement of the same order of magnitude with respect to their present precision.

The CEPC Conceptual Design Report was published by the end of 2018. The next milestone is set in 2022 with the publication of a Technical Design Report (TDR). If approved, the schedule sets the beginning of the construction in 2022 and its completion in 2030. The data taking will start in 2030 and last about ten years. Next, the upgrade to the SPPC is expected in 2040.


The Innovative Detector for Electron-positron Accelerators (IDEA) is the detector concept for an innovative, general purpose spectrometer, designed to investigate electron-positron annihilations in a wide range of center of mass energies, as the one deliverable at FCC-ee/CEPC.

IDEA is an hermetic detector, to be placed at the collider interaction point. Three ideal regions can be identified: the central cylindrical-shaped barrel and the two symmetric end-caps, one at each extremity. The detector is composed by various systems: the central tracker, the magnet, the pre-shower, the calorimeter and the muon counter (from inner to outer distance from the central axis).

The central tracker consists of a micro-vertex detectors, a large drift chamber and the so-called silicon wrapper:

  • the micro-vertex is based on the pixel and micro-strip silicon detector technology;
  • the drift chamber has an outer radius of 2 m. Thanks to its large dimension, more than 100 measurement points are available along the charged particle track. It also provides an excellent dE/dx measurement which is used in the particle identification;
  • the wrapper is a silicon micro-strip detector surrounding the drift chamber.

The whole central tracker is immersed in a 2 T magnetic field, generated by a very thin, solenoidal, superconducting magnet.

The pre-shower, placed between the magnet and the calorimeter, allows for the identification and measurement of electromagnetic showers which start in the solenoid material. It is composed by a matrix of a new type of Micro Pattern Gaseous Detector (MPGD), the micro-RWELL.

The Dual Readout (DR) calorimeter, immediately around the pre-shower, can measure simultaneously the electromagnetic and hadronic components of the showers originating in the calorimeter medium. It has an excellent energy resolution, around 30%/√E, for hadronic jets. The energy resolution of the electromagnetic component is at the level of 10%/√E. Moreover, the calorimeter high granularity allows for a good separation capability also in the case of two close showers, as the ones generated by the decay of neutral pions.

The muon counter is the second system in the spectrometer based on micro-RWELL technology. Just like the pre-shower, these chambers have a readout anode segmented in strips, but with a larger strip pitch, i.e. strip spacing. It is composed by three cylindrical planes with increasing radii from the beam pipe, inside the iron return yoke of the magnet. Each plane provides a spatial measurement of the coordinates x and y (in the plane transverse to the beam pipe) with a resolution of about 400 micron. The combined information from the three planes allows for an independent tracking of the charged particles in the region 5-6 m far from the interaction point. This excellent resolution enables also the identification of the secondary vertices, from the decay of long living neutral particles.

IDEA detector was originally proposed by the researchers of various INFN groups (Bari, Bologna, Ferrara, Lecce, LNF, Milano, Pavia, Pisa, Roma III, Torino). Later collaborators from other countries, e.g. United Kingdom, France, Switzerland, Croatia, Russia, South Korea and P.R.C., joined the effort.