Particle theorists already started bidding for the most amazing discoveries that await us there. We'll surely find supersymmetry, dark matter, extra dimensions, hairy black holes, a fistful of Higgs bosons, a few bosons more, and the beard of John Ellis. This game seems to ignore the fact that circumstances have changed after the first run of the LHC. Whatever the energy of the new machine, 30, 100, 1000 TeV, we currently do not have a no-lose theorem for a discovery of new particles.
This is an unprecedented situation in collider physics.
For each new collider at the energy frontier in the last 30 years such a firm no-lose theorem existed.
- The LHC was certain to discover the Higgs boson,
- The Tevatron was certain to discover the top quark,
- LEP was certain to discover supersymmetry,*
- The SppS was certain to find the W and Z bosons.**
* There is this apocryphal story of a prominent Italian physicist who vowed to cut off his testicle if superpartners are not found at LEP. It isn't clear whether he had balls to do it.
** Older readers may remember that the SppS was certain to also discover the top quark and supersymmetry. Just in case, Rubbia initially discovered both of these along with the W boson.
Now it's all different: we have a consistent and experimentally established theory of fundamental interactions that in principle may be valid all the way to the Planck scale. There is no strong case that new particles have to show up at accessible energies, and several precision measurements hint to the contrary. In this situation, banging the drum, like we did before the LHC, is dangerous since we may not have a big discovery to cover for that later. It seems wiser to focus on more certain returns from a 100 TeV collider. That is to say, even if there's no new particles within reach, what shall we learn? Of course, for all of us in the particle community, exploring a new unchartered territory is interesting per se. But we need more to convince others. The argument that we need a 100 TeV collider to prove that electroweak fine-tuning is larger than 0.01% is good. As a joke to relax the atmosphere. Certainly, the case for the new collider is stronger than that. Some ideas that are being bandied around are precision Higgs physics, double Higgs production, rare Higgs and top decays, non-perturbative electroweak physics, or WW scattering. These topics can be made more concrete and several more items can be added to the list.
Or we can study the impact of the 100 TeV collider on the CMSSM parameter and hope that no one will call the bluff...