  A detailed study of the normal state photoemission lineshapes and
quasiparticle dispersion for the single layer
Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+\delta}$ (Bi2201) superconductor is presented. We
report the first experimental evidence of a double peak structure and a dip of
spectral intensity in the energy distribution curves (EDCs) along the nodal
direction. The double peak structure is well identified in the normal state, up
to ten times the critical temperature. As a result of the same self-energy
effect, a strong mass renormalization of the quasiparticle dispersion, i.e.
kink, and an increase of the quasiparticle lifetime in the normal state are
also observed. Our results provide unambiguous evidence on the existence of
bosonic excitation in the normal state, and support a picture where nodal
quasiparticles are strongly coupled to the lattice.
