  We use a tunable laser ARPES to study the electronic properties of the
prototypical multiband BCS superconductor MgB2. Our data reveal a strong
renormalization of the dispersion (kink) at ~65 meV, which is caused by
coupling of electrons to the E2g phonon mode. In contrast to cuprates, the 65
meV kink in MgB2 does not change significantly across Tc. More interestingly,
we observe strong coupling to a second, lower energy collective mode at binding
energy of 10 meV. This excitation vanishes above Tc and is likely a signature
of the elusive Leggett mode.
