<sod>  
 :<math>\sigma=\int d\Omega {d \sigma \over d \Omega}</math>
 
A cross section is therefore a measure of the effective surface area seen by the impinging particles, and as such is expressed in units of area. Usual units are the cm<sup>2<sup>, the barn (1 b = 10<sup>-24<sup> cm<sup>2</sup>) and the corresponding submultiples: the millibarn (1 mb = 10<sup>-3</sup> b), the microbarn (1 <math>\mu</math>b = 10<sup>-6</sup> b), the nanobarn ( 1 nb = 10<sup>-9</sup> b), and the picobarn (1 pb = 10<sup>-12</sup> b).   The cross section of two [[elementary particle|particle]]s (i.e. observed when the two particles are [[collision|colliding]] with each other) is a measure of the interaction event between the two particles.
 
 ===Relation to the [[S matrix]]===
 
 <eod> <sod> 
 :<math>\sigma=\int d\Omega {d \sigma \over d \Omega}</math>
 
A cross section is therefore a measure of the effective surface area seen by the impinging particles, and as such is expressed in units of area. Usual units are the cm<sup>2<sup>, the barn (1 b = 10<sup>-28<sup> cm<sup>2</sup>) and the corresponding submultiples: the millibarn (1 mb = 10<sup>-3</sup> b), the microbarn (1 <math>\mu</math>b = 10<sup>-6</sup> b), the nanobarn ( 1 nb = 10<sup>-9</sup> b), and the picobarn (1 pb = 10<sup>-12</sup> b).   The cross section of two [[elementary particle|particle]]s (i.e. observed when the two particles are [[collision|colliding]] with each other) is a measure of the interaction event between the two particles.
 
 ===Relation to the [[S matrix]]===
 
 <eod>