A Week On Topic at Fermilab

The blog’s been quiet recently, thanks to a series of unfortunate events, not the least of which were my first (known) Covid-19 infection and an ongoing struggle with a bureaucracy within the government of Massachusetts. But meanwhile there is some good news: it seems I will someday have a book published. More on that another time.

Meanwhile I have also been doing some science. Recent efforts included presenting at a workshop on the potential capabilities of the Future Circular Collider [FCC], a possible successor to the Large Hadron Collider [LHC]. Honestly, my own feeling is that the FCC is an unfortunate distraction from important LHC activities. For my part I remain focused on the latter, and on trying to remind everyone just how much remains to do with the LHC data sets from previous years.

Visiting the LPC at Fermilab

Toward that end, I’ll be at the Fermilab National Accelerator this week, near Chicago. I’ll be visiting their LHC Physics Center [LPC], which is the major US hub for the CMS experiment at the LHC. (CMS is one of the LHC’s two general purpose experiments, the other being ATLAS; these are the experiments that discovered the Higgs particle.)

I’ll be at the LPC as part of their “Topic of the Week” series. (I think this might be a sort of follow-up to an old program under which there was a “Theorist of the Week” visiting Fermilab, which I participated in back in 2008.) On the LPC website, the relevant “topic” for my visit is listed as “Dark Showers” (which I would rather call “Hidden Valley Showers”, based on my own historical involvement with the subject, but 😉 that’s up to the community to decide.) But in fact I plan to take a much broader view of Hidden Valleys/Dark Sectors [HV/DS] this week.

The question that HV/DS poses is this: if in fact there are new particles that are inert under the electromagnetic force and the strong and weak nuclear forces, but have forces of their own, then what experimental signatures might they produce that the LHC experiments have not yet searched for? It turns out that there are a wide variety of unconventional signals that can appear in LHC data, and some can be easily missed if dedicated searches are not attempted. Showers of HV/DS particles (which I briefly wrote about here) are just one of these phenomena. Of course I’m all in favor of looking for them. But there are plenty of other HV/DS-related targets, just as urgent, and often technically easier. I want to make sure to point those out to my CMS colleagues.

Why Might We Expect Hidden Valleys/Dark Sectors?

Some of you might ask whether we really expect to see such HV/DS particles at the LHC. Are they really motivated theoretically? Are they really worth looking for? I’ll give you a classic theorist’s answer later in the week, because I know some of you insist on it: “Dark matter blah blah blah, neutral naturalness yada yada, relaxion baryogenesis string theory mumble mumble…” But fundamentally I think these are the wrong questions, and I give answers to them only under duress.

Instead, it seems to me that during a period when physics theorists are fundamentally confused as to what nature is trying to tell us — a situation that’s hardly uncommon in scientific history! — we should not be focusing on what one or another of us thinks is “likely”, or what some people among us might “expect”. For 80 years, from Fermi’s theory of the weak nuclear force to the discovery of the Higgs boson, we knew, in a very rough sense, what we were looking for, and we had a sense of the range of what to expect (though theorists often missed the mark anyway.) But now we don’t. I have yet to speak to anyone in our field with a compelling argument in favor of any one scenario (including the scenario that nothing new is to be found in the LHC’s data.)

At such a time, I believe, we ought to rely less on theoretical expectations and biases, and think more about taking advantage of the information we already have around us. That includes finding the most effective methods for searching through LHC data, a resource that we ought to mine thoroughly. If there exist search strategies that can simultaneously cover a large range of reasonable but still unexplored possibilities, then by all means we should implement those strategies. Subtle phenomena will not simply jump out of a gigantic pile of data; we have to actively look for them. That’s the challenge in mining huge data sets: we can only learn the answers to those questions that we choose to ask.