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

The classical description of synchrotron radiation fails at large Lorentz factors, gamma, for relativistic electrons crossing strong transverse magnetic fields B. In the rest frame of the electron this field is comparable to the so-called critical field B-0 = 4.414 x 10(9) T. For chi = gamma B/B-0 similar or equal to 1 quantum corrections are essential for the description of synchrotron radiation to conserve energy. With electrons of energies 10-150 GeV penetrating a germanium single crystal along the axis, we have experimentally investigated the transition from the regime where classical synchrotron radiation is an adequate description, to the regime where the emission drastically changes character; not only in magnitude, but also in spectral shape. The spectrum can only be described by quantum synchrotron radiation formulas. Apart from being a test of strong-field quantum electrodynamics, the experimental results are also relevant for the design of future linear colliders where beamstrahlung-a closely related process-may limit the achievable luminosity