In particle physics, the Breit frame (also known as infinite-momentum frame or IMF) is a frame of reference used to describe scattering experiments of the form A + B → A + ∑ C i {\displaystyle A+B\rightarrow A+\sum C_{i}} , that is experiments in which particle A scatters off particle B, possibly producing particles C i {\displaystyle C_{i}} in the process. The frame is defined so that the particle A has its momentum reversed in the scattering process.

Another way of understanding the Breit frame is to look at the elastic scattering A + γ → A ′ {\displaystyle A+\gamma \rightarrow A'} . The Breit frame is defined as the frame in which p → A + p → A ′ = 0 {\displaystyle {\vec {p}}_{A}+{\vec {p}}_{A'}=0} . There are different occasions when Breit frame can be useful, e.g., in measuring the electromagnetic form factor of a hadron, A {\displaystyle A} is the scattered hadron; while for deep inelastic scattering process, the elastically scattered parton should be considered as A {\displaystyle A} . It is only in the latter case the Breit frame gets related to infinite-momentum frame.

It is named after the American physicist Gregory Breit.