Barn (unit) - Inverse Femtobarn

The "inverse femtobarn" (fb−1) is a measurement of particle collision events per femtobarn of target cross-section, and is the conventional unit for time-integrated luminosity.

In a particle accelerator two streams of particles, with cross-sectional areas measured in femtobarns, are directed to collide over a period of time. The total number of collisions is directly proportional to the luminosity of the collisions measured over this time. Therefore, the collision count can be calculated by multiplying the integrated luminosity by the sum of the cross-section for those collision processes. This count is then expressed as inverse femtobarns for the time period (e.g., 100 fb−1 in nine months). Inverse femtobarns are often quoted as an indication of particle collider productivity.

Fermilab has produced 10 fb−1 in the first decade of the 21st century. Fermilab's Tevatron took about 4 years to reach 1 fb−1 in 2005, while two of CERN's LHC experiments, ATLAS and CMS, reached over 5 fb−1 of proton-proton data in 2011 alone.

Usage example

As a simplified example, if a beamline runs for 8 hours (28,800 seconds) at an instantaneous luminosity of 300 × 1030 cm−2s−1 = 300 μb−1s−1, then it will gather data totaling an integrated luminosity of 8,640,000 μb−1 = 8.64 pb−1 = 0.00864 fb−1 during this period.