On my recent trip to CERN, the lab that hosts the Large Hadron Collider, I had the opportunity to stop by the CERN control centre [CCC]. There the various particle accelerator operations are managed by accelerator experts, who make use of a host of consoles showing all sorts of data. I’d not been to the CCC in person — theoretical physicists congregate a few kilometers away on another part of CERN’s campus — although back in the LHC’s very early days, when things ran less smoothly, I used to watch some of the CCC’s monitoring screens to see how the accelerator was performing.
The atmosphere in the control room was relatively quiet, as the proton-proton collisions for the year 2024 had just come to an end the previous day. Unlike 2023, this has been a very good year. There’s a screen devoted to counting the number of collisions during the year, and things went so well in 2024 it had to be extended, for the first time, by a “1” printed on paper.
The indication “123/fb” means “123-collisions-per-femtobarn”, while one-collision-per-femtobarn corresponds to about 1014 = 100,000,000,000,000 proton-proton collisions. In other words, the year saw more than 12 million billion proton-proton collisions at each of the two large-scale experiments, ATLAS and CMS. That’s about double the best previous year, 2018.
Yes, that’s a line of bottles that you can see on the back wall in the first photo. Major events in the accelerator are often celebrated with champagne, and one of the bottles from each event is saved for posterity. Here’s one from a few weeks ago that marked the achievement of 100-collisions-per-femtobarn.
With another one and a half seasons to go in Run 3 of the LHC, running at 13.6 TeV of energy per collision (higher than the 13 TeV per collision in Run 2 from 2015 to 2018, and the 7 and 8 TeV per collision in Run 1 from 2010 to 2012), the LHC accelerator folks continue to push the envelope. Much more lies ahead in 2029 with Run 4, when the collision rate will increase by another big step.
2 Responses
“123 collisions per femtobarn” doesn’t make sense as an interpretation. The proton-proton cross section at those energies is much higher than that. The 123 inverse femtobarns is a measure of integrated luminosity over the entire run, not a per-collision statistic. The math on that being about 10^16 p-p collisions is correct, however.
Not sure I follow your logic.
I think we agree that
(1 pb) * (123 inverse femtobarn) = (1000 fb) * (123 inverse fb) = 123000 events
Now, if that’s so, why don’t we agree that “events” = “collisions that produce this particular process”, and thus conclude
(1 pb for process X) * (123 inverse femtobarn integrated luminosity)
= (1000 femtobarn cross section for process X) * (123 collisions per femtobarn )
= 123000 collisions that produce process X ?
The point is that the proton-proton cross section is 100 millibarns = 10^14 femtobarns. The analogous equation I just used then applies, and gives the 10^16 that you agree with. And in that equation, 123 inv fb *means* 123 collisions per femtobarn of cross section.