Image: Me chairing a session on searches for particles beyond the Standard Model at ICHEP 2026. Photo by Giuliano Gustavino.
About a month ago, I attended the “International Conference on High Energy Physics” (or ICHEP for short), which is the largest conference in the field and happens every two years. In contrast to Moriond, which is a high-impact but very small conference, ICHEP is a behemoth of over 900 attendees, from across particle physics and related fields, and it usually held in large convention centres (instead of a ski resort). This year, it took place in the Natal convention center in Brazil. I attended as a session organiser, which meant that I shared the responsibility to organise the “Beyond the Standard Model” sessions, which were spread over three days and involved nearly 90 talks from theorists and experimentalists (from the large ATLAS and CMS collaborations but also a host of smaller collaborations and groups). In practice, this meant sifting through the submitted abstracts to select a varied programme which was representative of the state of the art and covering the whole field, and organising the talks to have a good logical flow. Moreover, it meant chairing several of the half-day sessions, ensuring speakers turned up, didn’t go over time (too much) and moderating (and in some cases, initiating) discussion after there talks.
In addition to being a session organiser, of course, I had the chance to attend many of the other talks, and see a large number of ATLAS results which my group had prepared, get shown on the big screen. In total, ATLAS exotics had four results, spanning searches for vector-like quarks, heavy neutral leptons, quantum black holes and heavy vector bosons. Some of our results are finally approved just hours before the talks where they were first shown… ICHEP ends with three days of “Plenary” summary talks which allow one to take stock of the field as a whole.
I thought I’d use this post to jot down two important messages I took home from Brazil.
No groundbreaking discoveries were announced, but some there are some anomalies to follow up
If you have a discovery to announce, ICHEP is the place to do it in this field. However, there were no results which were really earth shaking at this year’s conference (unless some group is sitting on game-changing result and maybe just couldn’t get it publication-quality in time*).
That being said, there were a few intriguing results which could have the potential to get more exciting. One of the most memorable to me, is the “ATOMKI anomaly”. Basically, about 10 years ago a group at the ATOMKI lab in Hungary, while making a nuclear physics experiment, observed an unusual excess of events when studying de-excitations of Beryllium atoms. The excess only appeared when looking at a certain opening angle of the electron-positron pairs from the de-excitation photons. That would be consistent with a the atom emitting a 17 MeV boson which travels a little distance before decaying.
Image: illustration of the original ATOMKI anomaly, taken from slide 9 of J. Feng’s excellent ICHEP talk.
That excess in itself is interesting… but nuclear physics is hard and, well, it seemed likely to many of us that it was more likely a systematic or instrumental effect of some kind than a new particle.
In the intervening decade, two things happened:
- The same group repeated the measurement with another atom’s (excited Helium’s) de-excitation, and also saw an excess… at a different opening angle, but still consistent with the 17 MeV boson.
- Different groups are searched directly for a 17 MeV boson with the same properties, and found nothing within their sensitivity ranges.
This year, at ICHEP, a group called PADME also searched for a light MeV scale boson… and reported a small (~2 sigma) excess at 17 MeV, in a region that the previous groups would not have ben sensitive to.
2 sigma is not super significant (that means there is a 1 in 20 chance that the result was a statistical fluctation: we need 1 in a billion or so for a discovery), but it is intriguing. So that is one to keep an eye on as the same group release more data in the coming years.
If you want to know more, I suggest you check out this talk and the references therein.
ATLAS and CMS need to be more careful about how we present our results: the wider field is watching
I really like it when multiple searches for similar families of particles, show their results on a same graphic. Usually different searches will be sensitive to different assumptions about a new model (such as mass of particles, their relative properties, production probabilities). These summary plots can given an at-a-glace view of the state of the field, often showing results from multiple collaborations. This comparison allows us to learn lessons from each other, identify regions where extra effort is needed, and conversely, areas where we can be pretty sure there are no new particles to be found because they have been excluded by multiple searches.
However, in collider physics, we naturally tend to make these plots focusing on the range of model parameters which we actually have a chance of reaching. It’s only natural, why make a plot showing model configurations you are not sensitive to?
Well, the honest answer is because the regions we are sensitive to are not always the ones the field cares about. Indeed, sometimes the areas of model space which are most theoretically interesting, are hardly even shown on our plots. So when we fill out a whole region on our dark matter summary plots and say, “look what a great job we’ve done”, our friends in dark matter detection rightly say “well, that’s misleading, there are plenty of places where DM could show up and be well motivated, which are not excluded or even shown on your plot”. In fact, that was a conversation I had with a colleague from the direct dark matter detection field (who used to work in collider physics with me).**
We don’t need to completely change the way we present our results, but collider physicists do need to be mindful that we are not alone in the field. Collider physicists, neutrino physicists, astrophysics, direct dark matter physicists, hadron spectroscopists, heavy ion collider physics, flavour physicists… we are all trying to answer the same questions, and we need to try to present results coherently to make sense of the questions ahead of us.
To finish off, I can’t help but post this photo of the beach in Natal with a rainbow over it. Thanks for reading! Maybe something exciting is on the horizon.
* turns out that pretty much happened. Just my luck, on the same day I finally sit down to write this article, the LUX-ZEPLIN collaboration have released a new observation of an anomalous event in their direct dark matter detection search, which has set the community alight. I’ll write my next article about that!
** I now realise he would have know about the result in *, which sort of explains why he made the comment!