CLIC's three-step plan

In early October, the Compact Linear Collider (CLIC) collaboration published its final Conceptual Design Report. Accompanying it was a strategic summary document that describes a whole new approach to the project: developing the linear e+e collider in three energy stages. Though CLIC’s future still depends on signs from the LHC, its new staged approach to high-energy electron-positron physics for the post-LHC era is nothing short of convincing.


Instead of asking for a 48-kilometre-long commitment right off the bat, the CLIC collaboration is now presenting an accelerator that can be constructed in stages. For example, it could begin as an 11-kilometre 500 GeV accelerator that could later be extended to a 27-kilometre 1.5 TeV machine. Finally, after a decade or so of data taking, it could be taken up to the full 48-kilometre 3 TeV facility (see image 2). “Not only is the approach technically and financially practical, it also offers a very convincing physics programme,” explains Lucie Linssen, who is leading CERN’s Linear Collider Detector project. “Each stage of the machine could be optimized to probe different physics issues: at the initial 500 GeV stage, CLIC would be optimised for Higgs physics and top physics; when it is brought up to higher energies it could then look for signs of rarer Higgs decays, dark matter, supersymmetry and other new physics (see image 3).”

The report also confirms CLIC technology to be sufficiently flexible and robust to withstand the engineering challenges of this staged approach. Thus, data-taking during the first energy stage and second-stage construction and tunnelling could occur simultaneously, to a large degree. Current schedules – laid out in the report by the same team that scheduled the LHC – would see the 500 GeV stage completed in time for when the LHC programme comes to an end around 2030.

Image 2: Possible CLIC location, showing the three energy stages.

But of course, a possible decision to give the CLIC project the green light is still a few years away. The future of physics is dependent on signs from the LHC, which in addition to recent successes retains a great potential for new physics at higher energies. “The physics argument for a high energy linear collider is not strong enough yet,” says Lucie. “While certain interesting Higgs processes – such as the Higgs self-coupling – would require these high energies, there have not been sufficient hints of physics beyond the Standard Model from the LHC to justify the final energy needed.”

However, should the LHC see signs of new physics, the CLIC accelerator could be the optimal project on the drawing board capable of reaching the energy levels needed. “We still hope and believe that the LHC will see more indications of physics beyond the Standard Model,” concludes Lucie. “Once the LHC has run at full energy for a few years, we should be in a position to decide how CLIC might fit into the global physics programme.”

Image 3: Interaction cross-sections for an exemplar SUSY model, SM Higgs boson (with mass 125 GeV) and SM top physics as a function of e+e centre-of-mass energy.

Electron-positron physics: a powerful investigative tool

The advantage of an electron-positron collider is simple: precision. By colliding well-understood base particles, physicists will gain access to decays that are difficult to see at the LHC.

One example is ZH production, also known as the “Higgs-strahlung”. At CLIC, the properties of the colliding electrons and positrons are known with high precision, and the resulting Z particle can be measured with matching precision. Thus, physicists would be able to deduce the Higgs mass and its coupling to other particles in a model independent way. And if an unknown particle were to enter into the equation – say, a candidate for dark matter – it would be easily and accurately spotted.

Another example is the one of the most produced Higgs decays: a Higgs into b-bbar. In practice, this gives just two jets in the detector; so even though this decay is produced frequently at the LHC, background events make it almost impossible for experiments to record it. This decay would be a lot easier to see at CLIC, as there is significantly less background and no trigger selection is needed for reading the detectors out.

In e+e collisions, the Higgs is primarily produced essentially through these two processes. The Higgs-strahlung (left) is the dominant process up to ~500 GeV, though its cross section decreases with centre-of-mass energy increases. The WW-fusion process (right) dominates at higher energies, as its cross section increases with centre-of-mass energy. Very rare Higgs decays also profit at higher centre-of-mass energies, as there is an additional increase in luminosity.


by Katarina Anthony