CLICK

These days CERN hosts the CLIC workshop. CLIC is the famous italian porno comic book, and also the name of a future linear collider that is developed here at CERN. Somewhat disappointedly, the workshop is more focused on the latter. Most of the talks report on very hard-core R&D, but there is something for a wider audience too. A nice wrap-up of physics prospects was delivered yesterday by John Ellis.

If the technology turns out feasible (which should be concluded by the end of the decade), the machine is planned for the year two thousand twenty something. It will collide electrons with positrons at 3-5 TeV center-of-mass energies. This is not a big energy gain as compared to the LHC, but the much cleaner environment of a lepton collider will open up many new opportunities.

A light Standard Model-like higgs boson will be pinned down at the LHC, but a precise study of its properties must wait for a new linear collider. CLIC seems perfectly suited for this. The dominant production mode at a lepton collider is the W fusion whose cross section strongly increases with energy (see the plot). Various rare Higgs decays may be observed and the higgs coupling can be determined quite accurately. For example, the coupling to muons will be determined at the 2% level, while that to bottom quarks at 4% level. This will be a good test of the Standard Model predictions. Also, the higgs self-coupling can be measured, for example, the triple coupling can be determined with a 10% precision.

If the higgs is not found at the LHC, CLIC remains useful. It will be able to measure WW scattering precisely (something that is very tough at the LHC) and determine once and for all if the electroweak breaking is weakly or strongly coupled. Unfortunately, John did not talk about it and only a slight mention is made in the yellow report.

Obviously, if there is some new physics at the TeV scale, CLIC will be able to explore it. Whether we encounter extra dimensions, the little higgs or John's favourite supersymmetry, CLIC will measure the masses and couplings of the new particles. Have a look at the slides for a comparison of the CLIC and ILC performances in several supersymmetric models. CLIC is indispensable if the new particles have TeV or higher masses.

Just like LEP, CLIC will be able to indirectly probe physics up to scales much higher than its center of mass energy. This can be done by searching for effects of four-fermion interactions in the process of e+e- annihilating into muons. Such four-fermion interactions would appear as an effect of heavy virtual particles and they are suppressed by the mass scale of these heavy particles. CLIC will be able to probe these operators up to the scale of a few hundred TeV. In the nightmare scenario - only the Standard Model + the higgs boson found at the LHC - CLIC may tell us if there is some new physics within the reach of the next, more powerful machine. In that case, however , the CLIC performance is not terribly better than that if the ILC, as shown on the plot. CLIC people should better pray for new physics at the LHC.

Slides are available via the workshop page. There you also find video recordings of several other talks at the workshop. For those well-motivated, here is the yellow report.