A novel spectral broadening from vector--axial-vector mixing in dense matter

The presence of baryonic matter leads to the mixing between transverse $\rho$ and $a_1$ mesons through a set of $\omega \rho a_1$-type interactions, which results in the modification to the dispersion relation. We show that a clear enhancement of the vector spectral function appears below $\sqrt{s}=m_{\rho }$ for small three-momenta of the $\rho$ meson, and thus the vector spectrum exhibits broadening. We also discuss its relevance to dilepton measurements.