“A barn (symbol b) is a unit of area. Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is also used in all fields of high-energy physics to express the cross sections of any scattering process, and is best understood as a measure of the probability of interaction between small particles. A barn is defined as 10−28 m2 and is approximately the cross-sectional area of a uranium nucleus.” – Wikipedia
The cross-section of the uranium nucleus (1 barn) is and femto means , a thousandth of a millionth of a millionth. So a femtobarn is .
The inverse femtobarn gives you an estimate number of particle collision events per femtobarn. 1 inverse femtobarn corresponds to approximately 100 trillion () proton-proton collisions.
In 2015, the LHC collider at CERN delivered over 4 inverse femtobarns worth of proton-proton collisions to the experiments.
LHC is expected to deliver 25 inverse femtobarns in the next 6 months.
In the future, LHC will be capable of delivering 300 inverse femtobarns per year.
I will conclude by linking to Adam Falkowski’s comments on the inverse femtobarn.