A femtobarn. Unitwise, it's an area, a measure of the probability of an interaction, the "literal" cross section of the things you try to hit. Luminosity, ofte

A femtobarn. Unitwise, it's an area, a measure of the probability of an interaction, the "literal" cross section of the things you try to hit. Luminosity, often measured in inverse (femto) barn, is the other side: The number of tries, given by the number of particles you collided. The product of these is the number of events of this type which occur on average. So if you measured for 10 inverse femtobarn integrated luminosity, a process which has a cross section of 1 femtobarn should have happened 10 times.