CERN Accelerating science

A highlight from the CLIC annual workshop: an update to the baselining strategy

by Daniel Hynds

The Compact Linear Collider (CLIC) is a proposed e+e- collider to be built at CERN in order to serve the needs of the particle physics community beyond the high-luminosity LHC. The accelerator is foreseen to be built in several stages, allowing lower-energy phases to take data during construction of the full-length accelerating structure. A re-optimisation of the accelerator planning is currently underway, following more detailed physics studies which have shown the influence of the centre-of-mass energy during the first phase operation on the physics program. The new staging baseline, presented at the recent CLIC workshop in January of this year [1], foresees operation at centre-of-mass energies of 380 GeV, 1.5 TeV and 3 TeV [2]. 

As expected, the CLIC physics program drives the energy staging of the machine construction. In the first phase, a focus on precision Higgs physics is planned alongside top physics, with several analyses exhibiting competing demands on the exact centre of mass energy. Subsequent stages at higher energies are concerned primarily with searches for new physics, as well as measurements of rare Higgs processes in order to extract quantities such as the Higgs self-coupling. 

One of the driving systematics on Higgs measurements at CLIC is the accuracy of the HZ production cross-section, measured at low energy in a model-independent way by observing the Z recoil mass in Higgsstrahlung events (a measurement unique to lepton colliders).  A subtle trade-off between cross-section, detector resolution and background rejection at different energies leads to an optimum first-stage energy of around 350 GeV. For top physics, a threshold scan with modest luminosity around the top pair production energy is planned, in addition to precision measurements of the top couplings which may provide evidence of new physics processes. The dependence of such measurements on the collision energy favours higher energy running, leading to a compromise first stage operation of 380 GeV. 

A new optimisation of the accelerator construction has now been prepared using the above baseline, to be presented as input to the next European Strategy Update. With the imminent release of the CLIC Higgs physics expectations, the focus in CLIC will now turn to top physics and physics beyond the Standard Model, taking into account the latest results from LHC operations, where we all await eagerly for the first glimmers of new physics to be realised!