Concentration total amount of all "other" constituents in a mixture: If formula_6 is much smaller than formula_26, the mole ratio is almost identical to the mole fraction. The SI unit is mol/mol. However, the deprecated parts-per notation is often used to describe small mole ratios. The mass fraction formula_33 is the fraction of one substance with mass formula_2 to the mass of the total mixture formula_35, defined as: The SI unit is kg/kg. However, the deprecated parts-per notation is often used to describe small mass fractions. The mass ratio formula_37 is defined as the mass of a constituent formula_2 divided by
Cross section (physics) of the colliding system are , and , before and after the collision respectively, the differential cross section is given by where the on-shell matrix is defined by in terms of the S-matrix. Here is the Dirac delta function. The computation of the S-matrix is the main goal of the scattering theory. Although the SI unit of total cross sections is m, smaller units are usually used in practice. In nuclear and particle physics, the conventional unit is the barn b, where 1 b = 10 m = 100 fm. Smaller prefixed units such as mb and μb are also