  We report a high-resolution terahertz spectroscopic study of quantum spin
dynamics in the antiferromagnetic Heisenberg-Ising spin-chain compound
BaCo$_2$V$_2$O$_8$ as a function of temperature and longitudinal magnetic
field. Confined spinon excitations are observed in an antiferromagnetic phase
below $T_N\simeq 5.5$ K. In a field-induced gapless phase above $B_c=3.8$ T, we
identify many-body string excitations as well as low-energy fractional
psinon/antipsinon excitations by comparing to Bethe-Ansatz calculations. In the
vicinity of $B_c$, the high-energy string excitations are found to be
dynamically dominant over the fractional excitations.
