A unit of integrated luminosity, representing the number of particle collisions per femtobarn of cross-section.The inverse femtobarn (fb⁻¹) is the standard unit of integrated luminosity in particle physics, particularly at colliders like the Large Hadron Collider (LHC). It quantifies the total number of particle collisions that have occurred over a given period, normalized by the cross-section of the processes being studied. One inverse femtobarn corresponds to one collision event per femtobarn of cross-section, where a femtobarn is 10⁻⁴³ square meters — an area roughly the size of a single atomic nucleus. This unit allows physicists to compare the statistical power of different datasets and to calculate expected event rates for rare processes.
The term 'barn' was coined during World War II by physicists at Purdue University, who joked that a cross-section of 10⁻²⁸ m² was 'as big as a barn' compared to typical nuclear sizes. The femtobarn (10⁻¹⁵ barns) and its inverse became standard with the advent of high-luminosity colliders in the 1980s–1990s, though the exact origin of the inverse femtobarn as a unit is not precisely documented.
Luminosity measures how many particles pass through each other per unit area per second, and integrating it over time gives the total collision count. When multiplied by the cross-section of a specific process (e.g., Higgs boson production), the result is the expected number of such events. For example, if a process has a cross-section of 10 picobarns and the dataset is 100 fb⁻¹, one expects 1,000,000 events. The unit is practical because cross-sections in particle physics span many orders of magnitude, and using inverse femtobarns keeps the numbers manageable.
Used to report the size of datasets at the LHC and other colliders; for instance, the discovery of the Higgs boson in 2012 was based on about 5 fb⁻¹ of data at 7–8 TeV, while the full Run 2 dataset reached 140 fb⁻¹.
The total integrated luminosity of the LHC's Run 2 (2015–2018) was about 140 fb⁻¹, meaning that if you scaled the collision area to a barn, you would have filled it 140 times over — yet the Higgs boson was produced only about 3 million times in that entire dataset.
