a measure of the likelihood of a given process occurring at an accelerator. The idea is that two objects with a larger cross-sectional area are more likely to hit one another. So, larger cross-sections mean that a process is more likely to occur. Cross-sections are measured in barns, 10^-28 m². A barn is an extremely large cross-section in particle physics. Many interesting cross-sections are measured in pb (picobarns), which are equal to 10-12 barns.