Bridging the 8.6km gap between ATLAS and CMS with long-lived particles

This week I had a new result out, a bit of an unusual one. This is not strictly speaking a paper, just a set of plots (you can find the complete set here https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PUBNOTES/ATL-PHYS-PUB-2025-002/)
summarising the state of “Hidden Sector” neutral long-lived particle searches. What’s really cool about them is that, for the first time for this type of search, we show both ATLAS and CMS results on the same plot.

Normally each of the LHC experiments is responsible for summarising its own results, but that can lead to differences in presentation style or reference model, which make it difficult to take stock of the experimental situation. Indeed, small differences in how the results are presented mean that one can’t always make an apples to apples comparison. So the reason I am proud of this result is that I was able to work with a counterpart on CMS (Alberto Escalante del Valle) to work out the differences such that we could really compare apples to apples. That was the easy part. The hard part was then navigating both the ATLAS and CMS approval processes in parallel to ensure the plots could be made public. The ATLAS approval process alone is notoriously painful to get through, so you can imagine the coordination needed to get this result out. But we got out of the labyrinth and our lovely joint summary plots can now see the light of day! And what’s more, now that we have a template, similar comparisons for other exotic particle models are in the works.

In terms of the physics, there are a lot of interesting things to say, but I’ll keep my summary to a few lines. Between ATLAS and CMS, we ca exclude the notion that Higgs bosons are decaying to exotic long-lived particles more than 1% of the time so long as the new particles decay on average before 100m from where they were produced (when almost all of them decay outside of the detector) and after 1mm (if they decay too soon we are swamped by experimental backgrounds). In the example shown, CMS does better at low lifetimes while ATLAS does better at higher lifetimes. My own analysis (labelled ATLAS Calorimeter) lives somewhere in the middle of the range. But at both ends of the lifetime spectrum, we have lots of work to do. For the lowest lifetimes, 1 in 3 higgs bosons could be decaying to new particles without us realising it! And for the highness lifetimes, that figure could be 1/10. My objective over the lifetime of the LHC is that we could exclude the entirely lifetime range to the 1% level. An ambitious target, which we have 15 years to hit!