A test of virtual vacuum polarizations in a strong electromagnetic field

A strong electromagnetic field may modify the refractive index of the normal vacuum, which can be reduced to the light-light scattering amplitude via virtual vacuum polarizations in a microscopic sense. In order to observe the increase of the refractive index, Cherenkov light emissions associated with relativistic electrons into the strong field can be utilized as an experimental probe. In this letter, we will present the results of an experimental search for Cherenkov emission with a laser intensity of 10(17) W/cm(2) with a wavelength of 800 nm and an electron beam of 35.4 MeV/c, in which the expected Compton energy and indications of unexpected lower energies in the visible range were observed.