  Various satellite-borne missions are being planned whose goal is to measure
the polarization of a large number of gamma-ray bursts (GRBs). We show that the
polarization pattern predicted by current models of GRB emission can be
drastically modified by the existence of very light axion-like particles
(ALPs), which are present in many extensions of the Standard Model of particle
physics. Basically, the propagation of photons emitted by a GRB through cosmic
magnetic fields with a domain-like structure induces photon-ALP mixing, which
is expected to produce a strong modification of the original photon
polarization. Because of the random orientation of the magnetic field in each
domain, this effect strongly depends on the orientation of the photon line of
sight. As a consequence, photon-ALP conversion considerably broadens the
original polarization distribution. Searching for such a peculiar feature
through future high-statistics polarimetric measurements is therefore a new
opportunity to discover very light ALPs.
