NB: All opinions expressed below are my own, not necessarily those of the ATLAS collaboration!
The LHC started physics-quality collisions in 2009. It is currently due to operate until 2041. The halfway point of its 32-year lifespan is right now: 2025. In other words, we are (already!) halfway through the LHC’s lifespan. So we have 16 years left to achieve our objectives. That might sound like a lot, but considering that doing a physics analysis takes 3-4 years from kick-off to publication, actually we only have a handful of iterations left on any given topic. So what topics should we prioritise ? In fact, what are our objectives for the rest of the LHC programme ? It’s a question which gets surprisingly little attention in the day-to-day lives of particle physicists.
I’m no chess player (I stopped playing the day I was soundly beaten by a 6-year old), but I do admire the concept of the “Endgame” that chess players use to structure their strategy. The Endgame refers to the final phase of a game, where there finite resources (relatively few pieces) left on the board, and a limited time left to play. And yet, someone has to win (or at least force a draw). Having a sound strategy for the Endgame is extremely important if a player is to achieve their objective in a game of chess. In other words, they should account for the finite time and resources available to them, and work out the most effective way to reach their objective. Hence, they also need to know what their objective is (in the case of chess, do they want to win or force a draw?) That choice will affect their strategy.
Back to the LHC. As part of my convenership in the ATLAS Exotics group, I’ve been arguing recently that we need to start forming our own Endgame strategy for the LHC. So, we need to know our objective(s), be realistic about the resources available to us, and the time left. This means making choices (we can’t do everything) and organising ourselves according to what we want to achieve. In the past, many have simply said our objective is to “leave no stone unturned”. My argument is that this is not a sound Endgame strategy and that more thought is needed.
So… first, what is our objective? Well the primary goal of the LHC was, of course, the discovery of the Higgs boson. That happened in 2012, just 3 years after the LHC turned on, so that’s in the bag. The next logical step is understanding the Higgs field structure: that involves measuring (or at least observing) Higgs self-interaction. That’s what a large fraction of the LHC analysis community is gravitating towards, because it ought to be just about achievable in the next decade, with some effort. But that can’t be the only legacy of the LHC. Indeed, the LHC is our best chance in a century to discover new particles or interactions beyond those predicted by the Standard Model. That’s what the Exotics search group is all about. I would hence formulate the objective as :
“Either discover evidence of new particles, or rule out with high confidence the existence of new particles or interactions at the energy scale of the LHC in the obvious places”.
The objective is a bit fuzzy in the formulation above (what does “in the obvious places” mean?), but with some technical language one can pin it down further. The issue is that you can always dial down the interaction strengths to “hide” new particles by making their production probability very low. However, if you dial that interaction strength too low, the model becomes “un-natural”. In other words, you are creating a new problem because you need to explain why that parameter is un-naturally small. That’s what I mean by “in the obvious places”.
Stating the objective in this way already has implications for how we organise ourselves in the next 16 years: it means that instead of trying to dig deeper and deeper into a particular set of models, we should also spend some of our resources ensuring we “cover all our bases” even if the places we look are not theoretically well motivated. On the other hand, we absolutely need to make sure that all the well-motivated places are covered up to the point where the parameters become un-natural. So, in summary, we need deep and focused sensitivity to the most plausible extensions to the Standard Model, while also ensuring we have shallow but broad sensitivity to other models which we aren’t expecting. And using new technologies like Anomaly Detection can help us achieve that. This will helps make sure that if there are any discoveries available at the LHC, that we don’t miss them. It would be a disaster scenario if a discovery was possible at the LHC, but missed because we did not look in the right place.
I won’t go into the details of exactly how we organize our strategy in ATLAS (this is, after all, collaboration-internal information), but suffice it to say that it’s time to think carefully about how we organize our programme. Searching for new particles in a scattershot and haphazard way is unlikely to yield success: we need a well-planned Endgame strategy!
Louie D. Corpe 