  The fundamental excitations in an antiferromagnetic chain of spins-1/2 are
spinons, de-confined fractional quasiparticles that when combined in pairs,
form a triplet excitation continuum. In an Ising-like spin chain the continuum
is gapped and the ground state is N{\'e}el ordered. Here, we report high
resolution neutron scattering experiments, which reveal how a magnetic field
closes this gap and drives the spin chains in \YPP\ to a critical, disordered
Luttinger-liquid state. In \YPP\ the effective spins-1/2 describe the dynamics
of large, Ising-like Yb magnetic moments, ensuring that the measured
excitations are exclusively longitudinal, which we find to be well described by
time-dependent density matrix renormalization group calculations. The
inter-chain coupling leads to the confinement of spinons, a condensed matter
analog of quark confinement in quantum chromodynamics. Insensitive to
transverse fluctuations, our measurements show how a gapless, dispersive
longitudinal mode arises from confinement and evolves with magnetic order.
