  The coherent photons induced by relativistic heavy ions are highly linearly
polarized, in close analogy to the linear polarization of gluons in a large
nucleus. We proposed to measure the photon polarization through azimuthal
asymmetries in dilepton production in ultra-peripheral collisions. Our
prediction for the asymmetries were soon confirmed by the STAR experiment with
high precision. We refined our analysis recently by including the final state
soft photon radiation effect beyond the double leading logarithm approximation.
The azimuthal asymmetries and acoplanarity at relatively high transverse
momentum provide unique opportunities to test the resummation formalism thanks
to the extremely high photon flux in UPCs. Our results clearly show the
feasibility to access the sub-leading resummation effects in UPCs at the RHIC
and LHC.
