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The Future electron-positron Circular Collider (FCC-ee), formerly known as TLEP (Triple LEP), is under study at CERN, Geneva, to propose a design and evaluate the performance of a high-luminosity, high-precision circular electron-positron collider. The FCC-ee machine would be hosted in an 80-to-100 km tunnel in the Geneva area. It is aimed at delivering unprecedentedly high statistics at centre-of-mass energies ranging from 90 to 350 GeV to detectors placed in several interaction points, with a possible lower-energy run at 60 GeV and a possible high-energy upgrade up to 500 GeV. The "First Look at the Physics Case for TLEP" of the FCC-ee was published in December 2013.

The FCC-ee project is part and parcel of the Future Circular Collider (FCC) design study at CERN, that had its kick-off meeting on the 12-15 February 2014 at the University of Geneva, and will hold its first annual meeting at the end of March 2015 in Washington. In this context, the FCC-ee will be the first step towards the long-term goal of the European strategy for particle physics, namely a 100 TeV proton-proton collider (denoted FCC-hh), with an optional proton-electron collision period (named FCC-eh). The two machines are therefore foreseen to be hosted in the same tunnel.

This endeavor was already put forward by the ICFA beam-dynamics workshop at FNAL (November 2012), where the design study of a circular Higgs factory was proposed. As alluded to above, it is also in line with the recent update of the European Strategy, approved at the end of May 2013 by the CERN Council, which recommends to develop a proposal for an ambitious post-LHC accelerator project at the high-energy frontier, and recalls the strong scientific case for an electron-positron collider that can study the properties of the Higgs boson and other particles with unprecented precision.

Motivations
With the discovery of the Higgs boson at the LHC (CERN) in 2012, the Standard Model of particle physics has been completed. Nevertheless, it is not a complete description of the Universe: several experimental facts show that there is new physics beyond the Standard Model, such as the existence of dark matter, the origin of the baryon asymmetry of the universe and the existence and smallness of neutrino masses. The relatively small mass of the discovered Higgs has triggered new ideas for future Higgs factories.

The FCC-ee is actually much more than a Higgs factory, it is in addition a Z, W, and top quark factory. Its high luminosity would provide large samples of these particles: up to 1013 Z, several 108 W and several million Higgs bosons and top quarks. Such samples would allow the measurement of their properties with unprecedented accuracy, would enable the study of rare Z, W, Higgs boson and top decays, and would eventually provide sensitivity to new physics at a much higher energy scale, typically up to 100 TeV. Besides the precise study of the electroweak symmetry breaking, direct searches for rare processes might unveil the existence of yet unknown particles, beyond what can be observed directly at the LHC. For example, the existence of sterile neutrinos can only be tested with the large statistics delivered at the FCC-ee. If proven, these sterile neutrinos would not only provide an even more complete standard theory for particle physics, but could also give an definitive explanation to some of the most fundamental questions, like dark matter, baryon asymmetry of the universe, and neutrino masses.

Accelerator
The FCC-ee collider complex would consist of an accelerator ring and a storage ring. The accelerator ring delivers continuous top-up injection to the storage ring, so that a constant level of luminosity is provided in collisions. The storage ring has a superconducting RF system and low β* insertions and operates at fixed field. Multi-bunch operations are necessary for high luminosity below the top quark energy, so separated beam pipes should be foreseen for electron-positron beams. This basic design allows very large luminosities to be contemplated in up to four interaction points.

The set of preliminary baseline parameters for the four energies of interest (Z peak, WW threshold, HZ cross-section maximum, top-pair threshold) in a head-on collision scheme, compared to the LEP collider at CERN, can be found in the FCC website. The relative contribution of beamstrahlung to the total energy spread (and bunch length) is smaller at low energies (Z peak and WW threshold). This sets a limit on achievable luminosity, which seems to be impossible to overcome in head-on collisions. Therefore, FCC-ee is undertaking a crab-waist collision scheme study, which would achieve higher beam-beam parameters and therefore higher luminosity.

Energy and luminosity
A 12 GV RF system is designed to compensate for the energy loss by synchrotron radiation at the centre-of-mass energy √s = 350 GeV, at which a luminosity of 1.3 x 1034cm-2s-1 can be delivered at each interaction point (IP), in a configuration with four IPs. At lower centre-of-mass energies, the energy losses decrease steeply like 1/Ebeam4, and the RF power can be used to accelerate a much larger number of bunches, from about 100 bunches at 350 GeV all the way to almost 17,000 bunches at the Z pole. As a result, the luminosity increases approximately like 1/Ebeam3 when the centre-of-mass energy decreases.



The machine would primarily run at the four centre-of-mass energies of interest: the Z pole (√s ~ 91 GeV); the WW threshold (√s  ~ 161 GeV); the HZ cross-section maximum (√s  ~ 240 GeV); and the top-pair threshold (√s  ~ 350 GeV). The expected luminosity performance of FCC-ee leads to the production of a Tera Z (~1013 Z decays, i.e., almost a million times more than LEP was able to produce), several Oku W (several 108 W decays), several Mega Higgs (several 106 Higgs decays) and a Mega top (several 106 top quark pairs). Typically, the baseline physics programme of FCC-ee would consist of a few years at the Z pole, one or two years at the WW threshold, five years as a Higgs factory and five years at the t tbar threshold.

Other centre-of-mass energies are being considered to maximally exploit the potential of the machine: for example, a run at √s = 125 GeV would allow the coupling of the Higgs boson to electrons to be measured, and the invisible width of the Z to be determined with a precision at least 10 times better than at LEP; and a run at √s = 60 GeV would open the possibility to measure the electromagnetic coupling constant with a precision deemed adequate for a safe theoretical interpretation of the wealth of experimental measurements promised at 90 GeV and above.

For Electroweak precision measurements, the availability of a very precise beam energy measurement is an essential ingredient, uniquely provided by circular machines. The FCC-ee can achieve transverse beam polarization at the Z peak and WW threshold which, with the resonant depolarisation method, delivers a beam energy measurement with unparalleled accuracy, typically 100 keV per measurement. A practical scenario in which a few dedicated non-colliding bunches would be depolarized every 30 minutes will allow the beam energy to be measured continuously.

Possible timescale and physics programme


The FCC-ee and the FCC-hh design studies are being conducted in close coordination, with the purpose of providing maximum flexibility for the installation of the two machines, while keeping possible a concurrent operation. The aim of the study is to produce a conceptual design report (CDR) by 2018 in time for the next update of the European Strategy. By that time, the first results of the nominal energy run of the LHC, which will be crucial for defining the strategy for High Energy Physics for the next 20-30 years, will be available. The combination of the FCC-ee and the FCC-hh provide, for a great cost effectiveness, the best precision and the best search reach of all projects for future colliders on the market.

If the case remains as strong as today, and with appropriate political and financial support, a decision could be taken immediately and the tunnel excavation could start at the beginning of the next decade. The construction and installation of the collider and the detectors would then proceed in parallel with the High-Luminosity LHC (HL-LHC) running. It could therefore be technically envisioned to start commissioning for the first FCC-ee physics run as early as in 2035.