by Panos Charitos. Published: 29 March 2014

The FCC Kick-off meeting took place in Geneva earlier in February as a response to one of the directions emphasized by the European Strategy for Particle Physics. But is it the right time to start exploring this direction?

Michelangelo Mangano says: “The LHC will saturate its exploration potential in about 20 years from now. At that time, CERN must have chosen its next project, and have begun its construction, in order for the new facility to start operating within 5-10 years after the end of the LHC. This timescale, the complexity of the FCC project, and the desire to compare it against the other option whose design is already well advanced, i.e. CLIC, require that its study begins now.”

From a general point of view, the FCC seems as the obvious direction to go, given its ultimate access to energies far beyond anything that can be directly probed in the near future. Each of the suggested possibilities – proton-proton, electron-positron or electron-proton – has its own specific virtues. The results from the future runs of the LHC will better pin down the value of their potential contributions.

The race for the Future Circular Collider has just started. In the photo, Michelangelo Mangano while finishing his race in Ultra Trail du Mont Blanc 2012. 

Michelangelo notes: “The precision study of the properties of the Higgs particle will remain for a long time a keystone of the experimental programme, at the LHC and beyond. It is a well-defined challenge, which provides a set of clear benchmarks to assess the value of a future facility, and to compare with each other alternative proposals. In addition, a properly ambitious future facility must promise great progress in the continued direct search for new phenomena, in particular for the new particles and interactions addressing the so-called hierarchy problem, as well as for the origin of dark matter and of the matter-antimatter asymmetry.  Both CLIC and the FCC have a potential to fulfill these goals: the precision Higgs studies and the exploration of new physics. It is fantastic that CERN has decided to consider and study in depth both these future avenues.”

Michelangelo thinks that pushing for the highest possible energies is absolutely necessary to complete the study of electroweak interactions. There are several questions related to physics at the TeV scale, whose answer is critical for our knowledge, and which we cannot guarantee the LHC has sufficient energy to address. “The structure of the theory in a phase in which electroweak symmetry is unbroken can only be probed at energies well above the TeV in the partonic center-of-mass frame. For example, it is above the TeV that the collision of W boson pairs becomes sensitive to the possible existence of an underlying strong dynamics in the Higgs sector. This fact has been known since a long time, and motivated the design of the SSC at the energy of 40 TeV. With the knowledge we gained since then, from the discovery of the Higgs boson at 126 GeV, from the direct measurement of its couplings and from precision electroweak data, we know that we need to study WW collisions at energies significantly larger than what accessible at the LHC in order to gain new information. Likewise, we know that higher energies are needed in order to test precisely the structure of Higgs selfcouplings and of the Higgs potential.”

The study of electroweak interactions must also address other questions. A particularly fascinating one is whether the phase transition between unbroken and broken electroweak symmetry, which took place during the big bang, generated the baryon asymmetry we observe in today’s universe. “In the SM, the phase transition induced by a Higgs boson heavier than 60 GeV is too weak to generate the required asymmetry. For this to happen, a 126 GeV Higgs boson must have other interactions beyond the SM, and additional sources of CP violation should be present. The scale of these phenomena must be within few TeV, i.e. not too far from the electroweak breaking scale. Models have been proposed for these new phenomena, but their direct manifestation could escape detection at the LHC, if the masses are too large. It is conceivable, however, that the FCC could give a conclusive answer to the question whether the baryon asymmetry was generated at the electroweak phase transition.”

Likewise it is expected that a collider at 100 TeV could provide conclusive answers to the question of whether the Higgs sector is made of a single Higgs or whether there are other Higgs bosons at the TeV scale, and whether physics at the electroweak scale is responsible for dark matter. Michelangelo adds: “The study of the possibilities of a 100 TeV pp collider has just started. The studies done for the SSC 25 years ago were limited because of the knowledge we had at the time. The top quark, neutrino masses and the Higgs boson had not been discovered, we did not have all the information from electroweak and flavour precision data, searches for supersymmetry and other theories beyond the SM only gave minimal constraints, and little was known about dark matter. We have to restart thinking about the opportunities offered by experiments at 100 TeV, and many new ideas will certainly come up within the next few years”.

For many physicists the discussion for a future collider might be a shot in the dark. Mangano reminds me that this is, after all, the most fundamental thing in scientific research. “We have found all the elements of the Standard Model but we still do not know what lies beyond the SM, in spite of the certainty that there has to be something. The existence of dark matter, the neutrino masses and the baryon asymmetry of the Universe are just a few indicators pointing to something beyond the SM. However, we still cannot single out a preferred model of new physics among the many that could explain all of these observations, and thus there is no single accelerator or experiment that can guarantee a discovery. In this situation, my preference goes to a facility where a very ambitious discovery programme is accompanied by a rich and broad programme of solid measurements that will enrich our understanding of fundamental interactions. The FCC is precisely such a facility.”

The heavy-ion community will also benefit from moving to these higher energies as a more powerful collider will create new interesting opportunities. The quark-gluon plasma will slightly change because the temperature will increase to about a GeV, and the charm quark will also likely become thermalized. “The equation of state of the plasma will be affected and monitoring that it is properly modeled at those higher energies is an interesting target. Furthermore, there will be a great increase in the cross sections for hard probes, namely very high Q2 processes like production of jets, W and Z bosons, or heavy quarks. The use of such hard probes to monitor the behaviour of the quark-gluon plasma will become much more powerful”.

You might ask whether theoretical physicists currently have the tools that are needed to study and understand new phenomena that might be observed as we move to these energies. Michelangelo argues that this is not a constraint. “Theoretical progress is required even today, in order to fully exploit the LHC potential for precision measurements of the Higgs. The Higgs discovery has stimulated an immense effort of the theoretical community, pushing the boundaries of our ability to calculate with great accuracy extremely complex processes. Calculations that used to take several years can now be done in few minutes, thanks to the discovery of new theoretical formalisms. Having witnessed how our field advanced beyond the most optimistic expectations of just few years ago, I consider shortsighted to set limits to the progress that will take place in the next 20-30 years, in time to meet the challenges of the new accelerators.” He continues: “As for the development of new ideas for physics beyond the Standard Model: this is a process that evolves in a natural way, as a result of what the experiments will measure at the LHC, in neutrino and flavour physics, and observing the sky. Thirty years from now, once the new experiments will be ready, theory will also be ready.”