title: Direct measurement of the formation length of photons.
abstract: We report the first observation of a shoulder in the radiation spectrum from GeV electrons in a structured target consisting of two thin and closely spaced foils. The position of the shoulder depends on the target spacing and is directly connected to the finite formation length of a low-energy photon emitted by an ultrarelativistic electron. With the present setup it is possible to control the separation of the foils on a μm scale and hence measure interference effects caused by the macroscopic dimensions of the formation length. Several theoretical groups have predicted this effect using different methods. Our observations have a preference for the modified theory by Blankenbecler but disagree with the results of Baier and Katkov.
authors.affiliation: Department of Physics and Astronomy, Aarhus University, Aarhus C, Denmark. kka@phys.au.dk
authors.forename: Kristoffer K
authors.initials: KK
authors.lastname: Andersen
authors.forename: Søren L
authors.initials: SL
authors.lastname: Andersen
authors.forename: Jakob
authors.initials: J
authors.lastname: Esberg
authors.forename: Helge
authors.initials: H
authors.lastname: Knudsen
authors.forename: Rune
authors.initials: R
authors.lastname: Mikkelsen
authors.forename: Ulrik I
authors.initials: UI
authors.lastname: Uggerhøj
authors.forename: Pietro
authors.initials: P
authors.lastname: Sona
authors.forename: Alessio
authors.initials: A
authors.lastname: Mangiarotti
authors.forename: Tjeerd J
authors.initials: TJ
authors.lastname: Ketel
authors.forename: Sergio
authors.initials: S
authors.lastname: Ballestrero