  We present evidence for observation of virtual electromagnetic fields in the
radio domain from experiment T926 at the Fermilab Meson Test Beam Facility.
Relativistic protons with 120 GeV energy traversed a sealed electromagnetic
cavity and were observed in the radio regime of 200MHz-GHz. Closely related to
ordinary Cherenkov radiation, which we also measured, the virtual fields
require no acceleration for their existence. The experiment is also the first
observation of fields from hadronic showers, an independent and new
confirmation of coherent radio emission from ultra-relativistic particles.
Conditions of very low signal to noise were overcome by a novel and unbiased
filtering strategy that exploits exhaustive studies of correlations in the
noise backgrounds. Linear scaling of the signal region with the number of beam
particles provides evidence of coherence. Extrapolation to measurement of the
field of a single relativistic proton charge is consistent within errors. Our
study also illustrates new data processing methods that may be applied broadly
in conditions of extremely low signal to noise.
