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We report a high-resolution terahertz spectroscopic study of quantum spindynamics in the antiferromagnetic Heisenberg-Ising spin-chain compoundBaCo$_2$V$_2$O$_8$ as a function of temperature and longitudinal magneticfield. Confined spinon excitations are observed in an antiferromagnetic phasebelow $T_N\simeq 5.5$ K. In a field-induced gapless phase above $B_c=3.8$ T, weidentify many-body string excitations as well as low-energy fractionalpsinon/antipsinon excitations by comparing to Bethe-Ansatz calculations. In thevicinity of $B_c$, the high-energy string excitations are found to bedynamically dominant over the fractional excitations.

Quantum Critical Dynamics of a Heisenberg-Ising Chain in a Longitudinal Field: Many-Body Strings versus Fractional Excitations