  Magnetoelastic interaction couples spin and lattice degrees of freedom and
plays a key role in thermal transport properties of magnetic insulators. In the
Kitaev quantum spin liquid, the low energy excitations are charge neutral
Majorana fermions, which transform the magnetoelasctic interaction into
Majorana-phonon scattering. Motivated by anomalous thermal properties of the
Kitaev quantum spin liquid candidate RuCl$_3$, in this letter, we combine meV
resolution inelastic x-ray scattering and theoretical calculation to examine
the Majorana-phonon scattering. We analytically derive the velocity-dependent
Majorana-phonon scattering and find a divergence when the acoustic phonons and
the itinerant Majorana fermions have the same velocity. Based on the
experimentally determined acoustic phonon velocity in RuCl$_3$, we estimate the
range in the Kitaev interaction for which divergent Majorana-phonon scattering
can happen. Our result opens a new avenue to uncover fractionalized
quasiparticles in the Kitaev quantum spin liquid and emphasizes the critical
role of lattice excitations in RuCl$_3$.

Strongly Correlated Electrons;Materials Science