The CERN Courier today has a long interview with the omnipresent Nima Arkani-Hamed, discussing the current state of HEP physics. About the motivations for a next-generation collider project, I’m pretty much in agreement with him: the main argument is for a Higgs factory that would allow a much more detailed study of the Higgs, and if at all possible, an appropriate machine should be built (see more here). He agrees that the SUSY and extra dimensions models used to get people excited about the LHC can’t reasonably be used again for a higher-energy machine:
Is supersymmetry still a motivation for a new collider?
Nobody who is making the case for future colliders is invoking, as a driving motivation, supersymmetry, extra dimensions or any of the other ideas that have been developed over the past 40 years for physics beyond the Standard Model. Certainly many of the versions of these ideas, which were popular in the 1980s and 1990s, are either dead or on life support given the LHC data, but others proposed in the early 2000s are alive and well.
The last reference is to his favored split SUSY models, which I think few people besides him find compelling.
About WIMP dark matter he seems to be claiming that a 100 TeV machine has always been what is needed to find it:
There is a funny perception, somewhat paralleling the absence of supersymmetry at the LHC, that the simple paradigm of WIMP dark matter has been ruled out by direct-detection experiments. Nope! In fact, the very simplest models of WIMP dark matter are perfectly alive and well. Once the electroweak quantum numbers of the dark-matter particles are specified, you can unambiguously compute what mass an electroweak charged dark-matter particle should have so that its thermal relic abundance is correct. You get a number between 1–3 TeV, far too heavy to be produced in any sizeable numbers at the LHC. Furthermore, they happen to have miniscule interaction cross sections for direct detection. So these very simplest theories of WIMP dark matter are inaccessible to the LHC and direct-detection experiments. But a 100 TeV collider has just enough juice to either see these particles, or rule out this simplest WIMP picture.
I don’t remember ever hearing, pre-LHC, from him or anyone else, this argument that the most likely WIMP dark matter models are inaccessible to the LHC or to direct detection experiments. For many years, most of the direct detection experimental results came with plots showing a “prediction” of SUSY WIMP dark matter (see for example here, figure 5), in a mass range of 100-500 GeV, at a cross section measurable (and now ruled out by) experiments like XENON1T (see here).
Arkani-Hamed likes to make the following argument, which I think most current HEP theory graduate students may find hard to swallow:
How do you view the status of particle physics?
There has never been a better time to be a physicist. The questions on the table today are not about this-or-that detail, but profound ones about the very structure of the laws of nature. The ancients could (and did) wonder about the nature of space and time and the vastness of the cosmos, but the job of a professional scientist isn’t to gape in awe at grand, vague questions – it is to work on the next question. Having ploughed through all the “easier” questions for four centuries, these very deep questions finally confront us: what are space and time? What is the origin and fate of our enormous universe? We are extremely fortunate to live in the era when human beings first get to meaningfully attack these questions. I just wish I could adjust when I was born so that I could be starting as a grad student today!
There’s something to be said for entering a field at a time when it is finally able to “meaningfully attack” difficult and fundamental questions. The issue though is whether anyone has any good ideas that will make headway against such questions. The Standard Model was in place by the mid-70s, and by the time I was a graduate student in the early 80s, the “what are space and time? what is the origin and fate of our enormous universe?” questions were already on everyone’s mind as the next things to be thinking about. Starting in 1984, the superstring revolution promised a way to answer these questions.
35 years later, the current generation of graduate students has the same questions to think about, but a long history of failed attempts to consider. In addition, there’s the sad story of the unwillingness of leading figures of the field to admit to the failure of the 1984 revolution, and widespread multiverse pseudo-science (often promoted by Arkani-Hamed) to overcome. The only argument that I can see that this is a good time to start an HEP theory career is that it’s hard to see how things can get worse…
For some commentary about the interview by Tommaso Dorigo, concentrating on the positive case for a new collider as a tool to study the Higgs, see here.