  We examine the magnetic field dependent excitations of the dimerized spin-1/2
chain, copper nitrate, with antiferromagnetic intra-dimer exchange $J_1=0.44$
meV and exchange alternation $\alpha=J_2/J_1=0.24$. Magnetic excitations in
three distinct regimes of magnetization are probed through inelastic neutron
scattering at low temperatures. At low and high fields there are three and two
long-lived magnon-like modes, respectively. The number of modes and the
anti-phase relationship between the wave-vector dependent energy and intensity
of magnon scattering reflect the distinct ground states: A singlet ground state
at low fields $\mu_0H < \mu_0H_{c1} = 2.8$~T and an $S_z=1/2$ product state at
high fields $\mu_0H > \mu_0H_{c2} = 4.2$~T. In the intermediate field regime, a
continuum of scattering for $\hbar\omega\approx J_1$ is indicative of a
strongly correlated gapless quantum state without coherent magnons.
