Brief Thoughts on Future Circular Colliders

The January/February 2019 issue of the CERN Courier introduces the Future Circular Collider Conceptual Design Reports and offers commentary on the European and Chinese initiatives. Here is an extended version of my contribution to the commentary.

1. Why is a 100-km collider (regardless of where it is built) the right choice for the field now?

Let’s begin with two observations: (1) There is great interest in learning as much as we can about the Higgs boson and the mechanism of electroweak symmetry breaking. If a high-luminosity electron–positron Higgs factory were to drop out of the sky tomorrow, the line of users would be very long. (2) In contrast to the state of knowledge before the construction of the Large Hadron Collider (and the aborted Superconducting Super Collider), when the electroweak (TeV) scale was a clear landmark, we have not yet identified the next important energy scale.

A very large tunnel, say 100 km in circumference, would be a very fruitful investment for the field for many decades. It would enable a number of ambitious instruments, evolving with our technological capabilities and our scientific imperatives. A very-high-energy hadron collider is a vessel of discovery that would, for example, help us study the role of the Higgs boson in taming the high-energy behavior of longitudinal gauge-boson (WW) scattering. [For additional examples, see C. Quigg, “Dream Machines,” Reviews of Accelerator Science and Technology 10, (2019), arXiv:1808.06036; I pose many more questions in “Perspectives and Questions,” doi: 10.5281/zenodo.1458236 (October 2018).]

2. Is the preference in the first instance for a lepton or for a hadron collider?

If we interpret 100 km circumference to mean 100 TeV c.m. energy for a proton–proton collider, then we have not yet mastered the magnet technology needed. On the other hand, a significant step from the LHC, say a quadrupling of energy, could be readily achieved within current practice. In contrast, what is needed for an electron–positron collider to reach top-pair threshold is essentially ready now. If the International Linear Collider were to go forward as a Higgs factory, we would have to consider carefully whether a circular electron machine, arguably with superior capabilities, should be our top priority.

3. Is it beneficial or detrimental to the field as a whole to have two competing proposals for a major collider at this time?

It is a very significant validation of the scientific promise opened by a 100-km ring for scientists of different regions to express the same judgment.

4. From a theorist’s point of view, how much does it matter if the post-LHC machine is built in Asia over Europe or vice versa?

CERN has earned an exemplary reputation for inclusiveness and openness, which go hand in hand with scientific excellence. Any region, nation, and institution that aims to host a world-leading instrument must strive for a similar environment.

5. And from the point of view of the long-term future of the field?

I look forward to a world in which every region helps to advance science, people of many origins and backgrounds bring their talents, and science and reason inform the lives of people everywhere.

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