  Quantum spin liquids (QSLs) are exotic states of matter characterized by
emergent gauge structures and fractionalized elementary excitations. The
recently discovered triangular lattice antiferromagnet YbMgGaO$_4$ is a
promising QSL candidate, and the nature of its ground state is still under
debate. Here, we use neutron scattering to study the spin excitations in
YbMgGaO$_4$ under various magnetic fields. Our data reveal a dispersive spin
excitation continuum with clear upper and lower excitation edges under a weak
magnetic field ($H=2.5$ T). Moreover, a spectral crossing emerges at the
$\Gamma$ point at the Zeeman-split energy. The corresponding redistribution of
the spectral weight and its field-dependent evolution are consistent with the
theoretical prediction based on the inter-band and intra-band spinon
particle-hole excitations associated with the Zeeman-split spinon bands,
implying the presence of fractionalized excitations and spinon Fermi surfaces
in the partially magnetized YbMgGaO$_4$.
