  While condensed matter systems host both Fermionic and Bosonic
quasi-particles, reliably predicting and empirically verifying topological
states is only mature for Fermionic electronic structures, leaving topological
Bosonic excitations sporadically explored. This is unfortunate, as Bosonic
systems such a phonons offer the opportunity to assess spinless band structures
where nodal lines can be realized without invoking special additional symetries
to protect against spin-orbit coupling. Here we combine first-principles
calculations and meV-resolution inelastic x-ray scattering to demonstrate the
first realization of parity-time reversal ($\mathcal{PT}$) symmetry protected
helical nodal lines in the phonon spectrum of MoB$_{2}$. This structure is
unique to phononic systems as the spin-orbit coupling present in electronic
systems tends to lift the degeneracy away from high-symmetry locations. Our
study establishes a protocol to accurately identify topological Bosonic
excitations, opening a new route to explore exotic topological states in
crystalline materials.
