The open session at the beginning of the recent CLIC workshop, held at CERN 3-7 Feb, focused on preparing for CERN's long-term future. The High Luminosity upgrade of the LHC confirms CERN's presence at the high energy frontier for the next two decades. However, a series of options are already being explored concerning the next accelerator to be built at CERN. The Future Circular Collider (FCC) study joins the Compact Linear Collider (CLIC) study, which has already been under way for a number of years.

The FCC study comes in response to the European Strategy for Particle Physics recommending a feasibility study on future fundamental research projects at CERN. Building a 100 TeV circular hadron collider or a 3 TeV linear lepton collider certainly requires different accelerator development. However, the detector research and development for future high energy machines is, in many ways, quite similar.

Future high energy collider detectors will profit from highly granular calorimetry to enable the use of particle flow techniques in jet reconstruction. A primary goal of the CLIC calorimeter design is the ability to separate W and Z bosons at the 2.5 σ level, which requires jet energy resolutions of 3.5% across the energy range of interest (100 GeV - 1 TeV). To achieve this, ~30 layers of 5x5 mm cells in the ECAL and ~60 layers of 3x3 cm cells in the HCAL are currently foreseen. 

To enable particle flow reconstruction techniques, both electromagnetic and hadronic calorimeters must exist within the detector's magnetic field. At CLIC, a field strength of 4-5 T is anticipated in order to facilitate track separation in jets and provide good momentum resolution. The overall engineering aspects for such a large and strong magnet, for example the possibility of a superconducting coil in a cryostat 5.5m in diameter, are a common development goal for all future detectors.

 A possible detector layout for the future Compact Linear Collider.

The high centre of mass energy coupled with the desire to fit both calorimeters inside the magnetic field leads to high density calorimeter designs. For the future CLIC detector, tungsten is foreseen as the absorber. This dense metal has a radiation length five times smaller than that of steel, in addition to a significantly shorter nuclear interaction length. In order to reduce backgrounds from the pile up of minimum bias events (FCC) and beam induced backgrounds (CLIC), hit time stamping with 1-25 ns accuracy is required for future detectors. This calls for both fast sensor response times and fast front-end electronic readout times. Possible technologies for the active layers include silicon, scintillator and gaseous detectors like RPCs, all of which are under assessment in the CLIC detector group.

High precision tracking will be required for all future detectors. Minimising multiple scattering is a shared goal, and areas of development include low-mass detector support systems and low-mass cooling. The CLIC beam structure allows the detector to be power-pulsed, thereby reducing the heat load on the cooling system and allowing the possibility of air flow cooling. Power-pulsing isn't feasible for detectors at circular storage rings, so alternative options such as micro-channel cooling are being pursued.

One difference between the two environments concerns radiation. High luminosity hadron colliders produce extremely high radiation levels in the inner regions of the detector, requiring radiation-hard technology. The CLIC vertex detector, on the other hand, will be exposed to a radiation level 10⁴ times lower than the vertex detectors at the LHC. CLIC is currently pursuing a hybrid approach, with an ultimate goal of 50 μm thick silicon sensors bonded to 50 μm thick readout ASICs. Through-silicon-via technology will be used to achieve seamless tiling of sensors. Last year the first CLICpix demonstration chip was produced, using 65 nm microelectronics technology. It was designed primarily for the requirements of the vertex detector at CLIC, with the prospect of re-using some of its building blocks for pixel detector readout chips at the LHC and FCC. 

Optimising the layout of the vertex and tracking detectors has produced some interesting similarities between possible future detectors. The CMS phase 2 upgrade plans to use double-sided sensors as a hardware track trigger. At CLIC we are also considering double-sided sensors in the vertex detector, because using the same support for two layers reduces the material budget. Initial simulation studies of the geometric effect of three double-sided layers versus five single-sided layers showed a similar performance in terms of impact parameter resolution and flavour tagging. This is an interesting case of the same technology concept having different uses in different environments.

Future high energy colliders will produce events with high boost, due to their parton nature (FCC) and to beamstrahlung (CLIC). Therefore, detectors at both types of collider will require well-instrumented, complex forward regions. One concept for an FCC detector foresees extending the forward coverage to η = 5 by adding a smaller version of LHCb to either end of the detector, including a dipole magnet and additional calorimeters and muon detectors. At CLIC the forward region is complicated further by the presence of the final beam focusing magnets, which must exist close to the collision point for optimal luminosity. Two forward calorimeters, LumiCal and BeamCal, must function in a high-radiation environment, with doses up to 1 MGy per year. This is similar to conditions in which LHC beam monitors currently operate.

The exact nature of the next high energy collider at CERN will be chosen according to physics results of the LHC run 2, which should confirm the top priority for our next facility; precise measurements or the highest possible centre of mass energy. However, regardless of the next high energy collider's shape and particles, the challenges of an FCC detector mirror some of those already under consideration for CLIC. Collaboration between projects, smart ideas and new techniques are required.