Luminosity measures how intensely a collider produces collisions. Instantaneous luminosity is the collision producing rate at a given moment, in cm-2s-1; integrated luminosity is that rate summed over time, measured in inverse femtobarns (fb-1), and it is effectively the size of the dataset. The HL-LHC exists to multiply both.
The Machine's Speedometer
Energy tells you what a collider can reach; luminosity tells you how often it gets a chance. Formally, instantaneous luminosity relates a process's probability (its cross section) to the number of events you actually see: events per second = luminosity x cross section. The LHC's original design value was 1x1034 cm-2s-1. The upgraded machine will hold five times that, with headroom to 7.5x1034.
What sets the value is intuitive even if the units are not. Pack more protons per bunch, collide more bunches per second, squeeze each bunch into a smaller spot, and overlap the bunches more completely at the crossing. Every piece of HL-LHC hardware attacks one of those four levers: brighter injected beams, stronger focusing magnets for a smaller spot, and crab cavities for a fuller overlap.
Integrated Luminosity: The Dataset
Discoveries do not care about any single moment; they care about the total. Integrated luminosity accumulates over months and years, and its unit, the inverse femtobarn, has a useful translation: one fb-1 corresponds to roughly 1014 proton proton collisions at LHC energies. Some reference points:
- Run 1 (2010 to 2013): about 30 fb-1, enough to discover the Higgs boson.
- Run 2 (2015 to 2018): about 140 fb-1 per experiment.
- Run 3 (2022 to 2026): more than Runs 1 and 2 combined, pushing the total toward 450 fb-1.
- HL-LHC era (2030 to ~2041): target of roughly 3,000 fb-1, about ten times everything before it.
That last multiplication is the whole argument for the upgrade. A process too rare to distinguish from background in 300 fb-1 can become a solid measurement in 3,000.
Levelling: Spending the Beam Wisely
Left alone, luminosity decays through a fill: collisions literally burn protons out of the beams, and the rate can halve within hours. The HL-LHC introduces levelling as its standard mode. The machine begins with the beams deliberately focused less than it could manage, then tightens the squeeze continuously to hold luminosity flat at 5x1034 for hours. Total data per fill goes up, and the detectors get a steady rate they can be tuned for instead of a spike and a long fade.
The Price: Pileup
Luminosity has a cost the public rarely hears about. At 5x1034, every 25 nanosecond bunch crossing contains up to 140 to 200 separate proton collisions on top of each other, a mess called pileup. Run 3 averaged around 60. Disentangling one interesting event from 199 simultaneous mundane ones is why ATLAS and CMS spent Long Shutdown 3 installing new trackers and picosecond timing detectors. The upgrade is really two projects: make more collisions, and stay able to read them.
How Luminosity Is Measured
Quoting a luminosity implies someone measured it, and the technique has a good pedigree. The absolute calibration comes from van der Meer scans, named for Simon van der Meer, the accelerator physicist who shared the 1984 Nobel Prize. The two beams are swept across each other in small steps while the collision rate is recorded, which maps the beams' overlap directly and turns the machine itself into its own measuring instrument. Between scans, dedicated luminometers in each experiment count simple, well understood processes in real time, which is how the control room can display luminosity live and how the levelling system knows exactly how hard to squeeze. The experiments ultimately know their collected luminosity to around one percent, which is what makes precision physics possible at all.
One Number to Watch
When the machine returns in 2030, press releases will quote peak luminosity records, but the number that matters compounds quietly: integrated fb-1, week after week, for a decade. The countdown on our homepage measures the time until that accumulation resumes; the Run 4 guide covers what the first years of it will chase.