  In this article, we report on inelastic neutron scattering measurements on a
quasi-1D antiferromagnet BaCo$_2$V$_2$O$_8$ under a transverse magnetic field
applied along the (0,1,0) direction. Combining results of inelastic neutron
scattering experiments, analytical analysis, and numerical simulations, we
precisely studied the $E_8$ excitations appearing in the whole Brillouin zone
at $B_c^{1D}\approx 4.7$ T. The energy scan at $Q=(0,0,2)$ reveals a match
between the data and the theoretical prediction of energies of multiple $E_8$
excitations. Furthermore, dispersions of the lightest three $E_8$ particles
have been clearly observed, confirming the existence of the $E_8$ particles in
BaCo$_2$V$_2$O$_8$. Our results lay down a concrete ground to systematically
study the physics of the exotic $E_8$ particles.
