Image: The conference photo for the 60th edition of the “Rencontres de Moriond: Electroweak Interactions and Unified Theories”. I’m somewhere on the left side of the zero. Credit: Moriond.
This March, I had the pleasure of attending the “Rencontres de Moriond: Electroweak Interactions and Unified Theories” conference (“Moriond” for short). It is one of the most important dates in the Particle Physics calendar. Every year, researchers try their best to ensure their most exciting results are ready in time for Moriond, and theorists and experimentalists alike wait with bated breath to see what new results will come out.
You may be imagining a glitzy conference center with thousands of attendees. The reality is different: Moriond is a meeting of only about 140 people, which is held in the Planibel Hotel of the La Thuile ski resort. How did this come to be the most important conference of the year for collider physicists, direct detection dark matter scientists, neutrino physicists, hadron spectroscopy nerds, and all sorts of weird and wonderful theorists? Here is my take.
The first answer is the long-running nature of the meeting. This year was the 60th edition of Moriond. Bear in mind that the field of particle physics is less than 130 years old (I count the discovery of the electron in 1897 by J. J. Thompson as the start). Moriond has been a constant metronome for our (sometimes vertiginously fast, sometimes frustratingly slow) progress in understanding what the universe is made of. Indeed, Moriond started off as a handful of physicists renting a chalet together in 1966, in the Moriond village of the French Alps. That was before the Standard Model was developed. Gluons and the W/Z bosons had not even been imagined, let alone the Higgs boson. Quarks were a weird mathematical trick to categorise hadrons and mesons: no one believed they were real. Moriond has been going every year since then, and the original location was soon outgrown. The conference has moved around a few times before settling in La Thuile. The organisers keep meticulous records of attendance, and there is a log book. One of this year’s attendees has been to 41 editions, a joint “high score”. He probably recalls the Z boson being discovered.
The link to the past is one reason for the popularity of Moriond; but that link is no good if it is not anchored in the present and looking to the future. So, attendance of the conference is carefully curated: around half of the attendees each year should be first-timers. Moreover, physicists in positions of responsibility, such as the physics coordinators and group conveners of the ATLAS and CMS experiments, are encouraged to attend (that’s how I got my invite this year!) Finally there is a special emphasis on early-career researchers: there is a famous set of sessions reserved for PhD students and Post-Docs, who are invited to present the results they have worked on. This creates a unique environment where the promising young scientists of tomorrow mix with the giants of yesterday, and the leaders of today. Together they take stock of the field.
Another reason why the conference is so enduring is that it is a winning format. The location in the Alps has always been a key factor in the identity of the meeting. La Thuile is high in altitude and hence there is a reliable chance of bluebird skiing with white snow and blue skies even in late March. Instead of 9am-5pm, the day is split into two blocks: 8:30am-12:30pm and 5pm-8pm. In the afternoon, physicists go out together to hit the slopes*, before coming back to more conference sessions late into the evening. This may seem extravagant, but it actually is a very efficient way to get typically reserved people to socialise and exchange ideas. Chairlifts act as intellectual speed-dating arenas for scientists. You can imagine the 10-minute conversations while dangling over the powdered ravines:
“What did you take away from today’s session?”,
” What are you working on these days?”,
“You are finishing your PhD soon, you must be looking for your next job?”
And even
“I’ve had this crazy idea, maybe we could explore it further together…”
Moriond is an incredibly effective as a tool for reflection, networking, team-building and driving new ideas. It was an exceptional pleasure to be there, representing ATLAS as the Exotics group convener. From ATLAS Exotics, we had four new results shown: a search for new particles decaying to top-quark pairs, a search for long-lived particles decaying produced with a muon, a search for new particles by inferring the missing mass from proton collision remnants, and a search for quantum black holes decaying to leptons and jets. None of them unfortunately showed signs of new physics, but each succeeded in helping us narrow down what options for beyond-the-Standard-Model theories are still viable . Many of the search results were only approved for public release days or even hours before they were shown, an effect we affectionately call “the Moriond Rush”. CMS also showed a lot of new search results, in some cases already blowing our results out of the water: for instance they also released a similar search for particles decaying to top quark pairs, which was much more sensitive than ours, and from which we now need to learn.
I said that Moriond is a time to take stock of the field… here are my key take-aways.
- Some persistent anomalies in precision measurements of b-quark to s-quark decays remain. There has been much excitement about this in the last half-decade, but many of the anomalies have dissolved with more data. But not all. There is a lot of work going on to try to pin down if these remaining tensions could be a hint of new physics at a higher scale than we can reach directly with the LHC, or if it is a lack of control of the underlying theory predictions.
- Neutrino masses remain mysterious. We don’t know if the neutrino mass hierarchy is “normal” (electron neutrino < muon neutrino < tau neutrino) or “inverted”, and it’s a confusing picture. Several experiments individually prefer the inverted ordering, but if you combine their results, the normal ordering becomes more likely. We should be able to get a definitive answer on this question in the next decade thanks to new experiments coming online.
- Direct searches for dark matter are becoming so sensitive that neutrinos will become a major source of background. The experimental sensitivity is now touching this neutrino “floor”. While it was often said this was the experimental limit for current technologies, thoughts are now moving to how the experiments can measure and control those backgrounds.
- Direct searches for new particles have so far revealed no persistent excesses, but continue to gain in sensitivity and explore new models.
- Precision measurements of the Standard Model processes, the top-quark and the Higgs boson are testing the Standard Model like never before. Some decays of the Higgs boson which seems experimentally out of reach (like its decay to muons or c-quarks) are now coming into focus. The observation of the Higgs self-coupling, which is one of the major deliverables of the LHC experimental programme, may even happen sooner than expected. A lot of this progress has been driven by the deployment of Graph Neural Network machine-learning technologies.
There were no breakthroughs or major discoveries announced this year **. But that’s ok, discoveries can’t happen every year. The field moves forward, and perhaps next year, or the year after, or in ten years, we will have the next field-changing result. Whenever that happens, there is a good chance that Moriond is the place where the breakthrough is announced.
*It goes without saying that the skiing (ski hire, ski lift passes) is not paid for by the conference but by each individual. For those who don’t know how to ski, there are group lessons organised. And plenty of folks don’t choose not to ski at all and just enjoy a break and the mountain air.
**The first observation of the Ξ+cc , which made headlines, is cool, but it is not a new fundamental particle, it’s a rare process which is predicted by the Standard Model.
PS: Many thanks to Emma for proofreading my draft 🙂
Louie D. Corpe 