General

What does CLIC stand for? Why is this accelerator “compact”?

CLIC stands for the Compact Linear Collider. Despite a main accelerating part of 44 km in length, the accelerator is “compact” due to its high accelerating gradient of 100 MV/m. To achieve the same centre-of-mass energy with LHC acceleration (5 MV/m) would require a distance of 840 km! Or alternatively, 700 km of LEP2 acceleration (6 MV/m).

Why linear acceleration?

When particles change direction (as they must in a circular collider) they emit photons and lose energy. This effect, called synchrotron radiation, can be avoided by accelerating particles in a straight line. The challenge of linear acceleration is to achieve a very high acceleration gradient, because, unlike in circular machines, the particles pass through a linear accelerator only once.

Is CLIC necessary after the LHC?

The LHC is a fantastic accelerator to make discoveries with – as proven by the Higgs observation. The LHC will hopefully give even more answers or hints to unanswered questions in particle physics, such as: What is the nature of dark matter? Why is the universe made of matter, while antimatter must have initially existed in equal amounts? Do all forces of nature unify? However, it is unlikely that the LHC will give us a complete understanding of all these topics. After the LHC, CLIC will be able to measure the Higgs properties with superior accuracy. This is needed to fully understand the connection between the Higgs field and matter, or to obtain indirect indications of new physics effects at even higher energies. CLIC also has the possibility to see new particles that may escape detection at the LHC. As CLIC collides electrons with positrons (anti-electrons), it provides complementary information to the LHC that mostly collides protons with protons. Electron-positron collisions will yield more accurate information. In addition, proton-proton and electron-positron collisions are actually governed by different elementary forces.

Why should CLIC be built in stages?

Building CLIC in stages, from the middle out, allows physicists to begin performing research at lower centre-of-mass energies (with a shorter accelerator), whilst engineers continue to construct the rest of the accelerator. Tuning the accelerator to a few key energy settings, at the lower, middle and higher ends, allows to study a wealth of interesting physics signals. If the accelerator would be tuned immediately to the highest energy, one would miss out on some interesting physics signals that are guaranteed to exist along the way (for example some Higgs-related processes).

What are the centre-of-mass energies going to be for the different stages?

The energies of the three stages are not yet fixed. The first stage will be around 350 – 380 GeV, in order to profit from known interesting physics processes in this region. The second stage is planned to be around 1.5 TeV. This is the highest energy possible with one drive beam system, so it is attractive from a cost perspective. However, future findings at the LHC may give strong arguments for a slightly higher / lower energy. The third stage will be the top energy: 3 TeV.

How much power will CLIC use?

Designed to be a high luminosity, high energy linear collider, CLIC will inevitably need high power. Compared to an accelerator using superconducting technology, CLIC nevertheless has very low power consumption in stand-by or “waiting-for-beam” mode. A preliminary analysis of the overall CLIC energy consumption per year for the various stages shows that the first stage of CLIC would be similar to LHC, and the second stage similar to the total CERN energy consumption. However, work is on-going in several domains (overall re-baselining, permanent magnets, air-handling etc.) to further reduce the anticipated power consumption of CLIC.

How much will CLIC cost?

To build the first stage of the accelerator is estimated to cost about 30% more than the cost for the LHC. Most of this cost is in excavating the tunnels, installing general services, in the main- and drive-beam production and in the two-beam modules. The LHC construction cost was comparatively cheap because it used, to a large extent, the pre-existing LEP tunnels and infrastructure. To build a detector for CLIC is estimated to cost approximately the same as one of the LHC experiments ATLAS or CMS. Most of the detector cost is in the calorimeters, the superconducting coil and the yoke.

When will CLIC be built?

The LHC is currently foreseen to take data until ~2035. This includes a major investment for an intensity upgrade, which is due to be completed by 2026. After 2026 CERN could possibly start investing in the construction of CLIC.