Concept of formation length in radiation theory |
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| Institution: | 1. NASA/Goddard Space Flight Center, Greenbelt Road, Greenbelt, MD 20771, USA;2. Space Science Center, University of New Hampshire, Durham, NH 03824, USA;3. Facultad de Matemática, Astronomía y Fisica, Universidad de Córdoba, Córdoba 5008, Argentina;4. Pacific Northwest National Laboratory, Richland, WA 99352, USA;5. Dipartimento di fisica, Università Degli Studi de Trieste, Treste, Italy;6. Department of Physics, University of Maryland Baltimore County, Baltimore, MD 21250, USA;1. Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, 01328 Dresden, Germany;2. Dipartimento di Scienze Fisiche, Informatiche e Matematiche, Università degli Studi di Modena e Reggio Emilia, Via Campi 213/A, I-41125 Modena, Italy;3. INFN, Sezione di Bologna, Via Irnerio 46, I-40126 Bologna, Italy;4. Department of Physics and Astronomy, Uppsala University, 75108 Uppsala, Sweden;5. Escuela de Matemáticas, Universidad Nacional de Colombia Sede Medellín, Carrera 65 # 59A–110, Medellín, Colombia;6. Instituto de Física y Matemáticas, Universidad Michoacana de San Nicolás de Hidalgo, Edificio C-3, Apdo. Postal 2-82, C.P. 58040, Morelia, Michoacán, Mexico;1. Cornell Laboratory for Accelerator-based Science and Education, Cornell University, Ithaca, NY 14853, United States;2. Physics Department, California Polytechnic State University, San Luis Obispo, CA 93407, United States |
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| Abstract: | The features of electromagnetic processes are considered which connected with finite size of space region in which final particles (photon, electron–positron pair) are formed. The longitudinal dimension of the region is known as the formation length. If some external agent is acting on an electron while traveling this distance the emission process can be disrupted. There are different agents: multiple scattering of projectile, polarization of a medium, action of external fields, etc. The theory of radiation under influence of the multiple scattering, the Landau–Pomeranchuk–Migdal (LPM) effect, is presented. The probability of radiation is calculated with an accuracy up to “next to leading logarithm” and with the Coulomb corrections taken into account. The integral characteristics of bremsstrahlung are given, it is shown that the effective radiation length increases due to the LPM effect at high energy. The LPM effect for pair creation is also presented. The multiple scattering influences also on radiative corrections in a medium (and an external field too) including the anomalous magnetic moment of an electron and the polarization tensor as well as coherent scattering of a photon in a Coulomb field. The polarization of a medium alters the radiation probability in soft part of spectrum. Specific features of radiation from a target of finite thickness include: the boundary photon emission, interference effects for thin target, multi-photon radiation. The theory predictions are compared with experimental data obtained at SLAC and CERN SPS. For electron–positron colliding beams following items are discussed: the separation of coherent and incoherent mechanisms of radiation, the beam-size effect in bremsstrahlung, coherent radiation and mechanisms of electron–positron creation. |
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