First Measurements of the Unique Influence of Spin on the Energy Loss of Ultrarelativistic Electrons in Strong Electromagnetic Fields

Abstract

Although some authors have claimed that the effect is not detectable, we show experimentally for the first time that as the quantum parameter x grows beyond 1, an increasingly large part of the hard radiation emitted arises from the spin of the electron. Results for the energy loss of electrons in the energy range 35-243 GeV incident on a W single crystal are presented. Close to the axial direction the strong electromagnetic fields induce a radiative energy loss which is significantly enhanced compared to incidence on an amorphous target. In such continuously strong fields, the radiation process is highly nonperturbative for ultrarelativistic particles and a full quantum description is needed. The remarkable effect of spin flips and the energy loss is connected to the presence of a field comparable in magnitude to the Schwinger critical field, E 0 m 2 c 3 ͞eh, in the rest frame of the emitting electron. DOI: 10.1103/PhysRevLett.87.054801 PACS numbers: 41.60. -m, 12.20. -m, 41.75.Ht, 78.70. -g Under small angles of incidence to a crystal, the strong electric fields of the nuclear constituents add coherently such as to obtain a macroscopic, continuous field of the order E Ӎ 10 11 V͞cm. This is evidenced by, e.g., the channeling phenomenon [1] or the so-called "doughnut scattering" [2]. Therefore, in the rest frame of an ultrarelativistic electron with a Lorentz factor of g Ӎ 10 5 , the field encountered becomes comparable to the critical (or Schwinger-) field, E 0 m 2 c 3 ͞eh 1.32 3 10 16 V͞cm, corresponding to a magnetic field B 0 4.41 3 10 9 T. Here, m is the rest mass of the electron, c is the speed of light, e is the elementary charge, andh is Planck's constant divided by 2p. The incident particle moves in these immensely strong fields over distances up to that of the crystal thickness, i.e., up to several mm. Thereby the behavior of charged particles in strong fields as E 0 can be investigated.Strong field effects can be investigated by other means. One example is in heavy ion collisions where the field becomes comparable to the Schwinger field, but the collision is of extremely short duration. Another -technically demanding -example is in multi-GeV electron collisions with terawatt laser pulses where nonlinear Compton scattering and so-called "Breit-Wheeler" pair production are observed [3]. In nature, near-critical fields are believed to be present in the vicinity of pulsars.However, as we point out below, in order to investigate the effect of the spin on the radiation spectrum, the electron must interact with the strong field over large distances. So crystals present unique tools for the investigation of the influence of spin on the radiation spectrum.Already in the late 1960s, Baier and Katkov [4] calculated the photon spectrum emitted by "particles of arbitrary spin moving in an arbitrary electromagnetic field." However, the realization that the spin influences the spectrum significantly in this context lay dormant for many years and was not discussed as an observable phenomenon although radiation in...

Experimental investigations of synchrotron radiation at the onset of the quantum regime

“…However, apart from a previous study performed with a tungsten target without the ability to measure the spectral composition of the emitted photons [4], such calculations have hitherto not been experimentally tested in the relevant region of values of .…”

“…Due to the Lorentz transformation to the rest frame of the penetrating electron, the strong electric field in the crystal appears as a strong magnetic field B ¼ E lab , in which the magnetic moment of the electron achieves an energy of E ¼ À ÁB, giving rise to emission connected to a spin-flip transition. In an ''elementary treatment'', which is not completely correct but ''has direct intuitive appeal and works surprisingly well for electrons'' [3], the spin-flip transitions of electrons with ¼ eℏ=2mc have an energy ÁE ¼ eℏB=mc in the rest system [4]. Transformation back to the laboratory yields a factor such that the result is…”

“…In a previous experiment by Kirsebom et al [4] values of the associated strong-field parameters above unity were probed in a tungsten crystal. These results are plotted together with our lead glass The radiation enhancement as a function of the associated strong-field parameter s for a Ge h110i crystal detected by a lead glass calorimeter and W h111i lead glass measurements [4]. The data have been fitted by the approximate quantum suppression formula, Eq.…”

“…In crystals, electric fields of the order 10 11 V=cm, corresponding to magnetic fields of a few 10 4 T and originating from the screened nuclear fields, are achievable along crystallographic axes (see below). Thus, using particles with $ 10 5 as, e.g., the 100 GeV electrons obtainable at the CERN SPS, experimental investigations of synchrotron radiation in the quantum regime become possible.…”

Classical relativistic model for spin dependence in a magnetized electron gas

“…There is extensive recent literature on spin dependence in plasmas, motivated partly by application to neutrino emission from the interiors of dense stars [1][2][3][4][5][6], partly by potential applications of nonlinear waves in quantum plasmas [7][8][9], and partly by more speculative suggested applications [10][11][12][13][14]. Spin dependence in a plasma introduces a new source of dispersion that is intrinsically quantum mechanical.…”