However, experimental particle physicists often use a modified definition of rapidity relative to a beam axis where is the component of momentum along the beam axis. This is the rapidity of the boost along the beam axis which takes an observer from the lab frame to a frame in which the particle moves only perpendicular to the beam. Related to this is the concept of pseudorapidity. Rapidity relative to a beam axis can also be expressed as See also Bondi k-calculus Lorentz transformation Pseudorapidity Proper velocity Theory of relativity Notes and references Varićak V (1910), (1912), (1924) See Vladimir Varićak#Publications Borel E (1913) La théorie de la relativité et la cinématique, Comptes Rendus Acad Sci Paris 156 215-218; 157 703-705 Vladimir Karapetoff (1936)"Restricted relativity in terms of hyperbolic functions of rapidities", American Mathematical Monthly 43:70. Frank Morley (1936) "When and Where", The Criterion, edited by T.S. Eliot, 15:200-2009. Wolfgang Rindler (2001) Relativity: Special, General, and Cosmological, page 53, Oxford University Press. Shaw, Ronald (1982) Linear Algebra and Group Representations, v. 1, page 229, Academic Press . (see page 17 of e-link) Special relativity