We report resonant inelastic x-ray scattering (RIXS) measurements on the prototypical hexagonal boron nitride ($h$-BN) layered compound. The RIXS results at the B and N $K$ edges have been combined with electron energy loss spectroscopy (EELS) experiments and ab initio calculations within the framework of the Bethe-Salpeter equation of many-body perturbation theory. By means of this tight interplay of different spectroscopies, the lowest longitudinal exciton of $h$-BN has been identified. Moreover, its qualitatively different dispersions along the $\mathrm{\Gamma }\mathrm{K}$ and the $\mathrm{\Gamma }\mathrm{M}$ directions of the Brillouin zone have been determined. Our study advocates soft x-ray RIXS and EELS to be a promising combination to investigate electronic excitations in materials.

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• Dielectric properties
• Plasmons
• Boron nitride
• Electron energy loss spectroscopy
• GW method
• Resonant inelastic x-ray scattering