Femtobarn

Femtobarn is a unit of area used in particle and nuclear physics to express cross sections—effective areas that quantify the probability of a specific interaction between particles during a collision. It is a submultiple of the barn, the standard unit of cross section equal to 10^-28 square meters. A femtobarn is 10^-15 of a barn, i.e., 10^-43 square meters. In square centimeters, this is 10^-39 cm^2.

Cross sections are used to predict how often a process occurs when particles collide at high energy. The event rate N relates to the instantaneous luminosity L via N = L × σ, where σ is the cross section and L is the luminosity. In modern collider experiments, integrated luminosity is commonly expressed in inverse barns, especially inverse femtobarns (fb^-1). For example, a process with a cross section of 1 fb would yield about one event for each inverse femtobarn of data; with 100 fb^-1 of data, roughly 100 events are expected if the process is not suppressed.

Femtobarn-scale cross sections are common for rare Standard Model processes and many new-physics signals. More common processes lie in the nanobarn or picobarn range, while even rarer ones can be in the femtobarn or smaller. The use of fb and fb^-1 allows experimentalists to plan data collection and interpret observed counts.

Notes: The numerical values depend on energy and collider. Precision measurements require careful treatment of luminosity, detector efficiency, and backgrounds.