  Quantum spin liquid involves fractionalized quasipariticles such as spinons
and visons. They are expressed as itinerant Majorana fermions and $Z_2$ fluxes
in the Kitaev model with bond-dependent exchange interactions on a honeycomb
spin lattice. The observation has recently attracted attention for a candidate
material $\alpha$-RuCl$_3$, showing spin liquid behaviour induced by a magnetic
field. Since the observable spin excitation is inherently composed of the two
quasiparticles, which further admix each other by setting in the magnetic field
as well as non-Kitaev interactions, their individual identification remains
challenging. Here we report an emergent low-lying spin excitation through
nuclear magnetic and quadrupole resonance measurements down to $\sim 0.4$ K
corresponding to $1/500$ of the exchange energy under the finely tuned magnetic
field across the quantum critical point. We determined the critical behaviour
of low-lying excitations and found evolution of two kinds of the spin gap at
high fields. The two excitations exhibit repulsive magnetic field dependence,
suggesting anti-crossing due to the hybridization between fractionalized
quasiparticles.
