Breit frame
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 {{tmath| \textstyle A + B \rightarrow A + \sum C_i }}, that is experiments in which particle {{tmath| A }} scatters off particle {{tmath| B }}, possibly producing particles in the process.{{cite book |last=Griffiths |first=David |authorlink=David J. Griffiths |year=1987 |title=Introduction to elementary particles |publisher=Wiley |location=New York |isbn=978-0-471-60386-3 |page=102}} 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 {{tmath| A+\gamma \rightarrow A' }}. The Breit frame is defined as the frame in which {{tmath| 1= \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, {{tmath| A }} is the scattered hadron; while for deep inelastic scattering process, the elastically scattered parton should be considered as {{tmath| 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.{{cite book | last=Hughes | first=Vernon |last2=Iachello |first2=Francesco|last3=Kusnezov|first3=Dimitri| title=The Gregory Breit Centennial Symposium: Yale University, USA | publisher=World Scientific | publication-place=Singapore River Edge, N.J | year=2001 | isbn=978-981-02-4553-5 | page=9}}