  Lead-halide perovskite (LHP) semiconductors are emergent optoelectronic
materials with outstanding transport properties which are not yet fully
understood. We find signatures of large polaron formation in the electronic
structure of the inorganic LHP CsPbBr$_3$ by means of angle-resolved
photoelectron spectroscopy. The experimental valence band dispersion shows a
hole effective mass $0.26\pm0.02\,\,m_e$, 50% heavier than the bare mass $m_0
=0.17 m_e$ predicted by density functional theory. Calculations of
electron-phonon coupling indicate that phonon dressing of the carriers mainly
occurs via distortions of the Pb-Br bond with a Fr\"ohlich coupling parameter
$\alpha=1.82$. A good agreement with our experimental data is obtained within
the Feynmann polaron model, validating a viable theorical method to predict the
carrier effective mass of LHPs ab-initio.
