  Observing how electronic states in solids react to a local symmetry breaking
provides insight into their microscopic nature. A striking example is the
formation of bound states when quasiparticles are scattered off defects. This
is known to occur, under specific circumstances, in some metals and
superconductors but not, in general, in the charge-density-wave (CDW) state.
Here, we report the unforeseen observation of bound states when a magnetic
field quenches superconductivity and induces long-range CDW order in
YBa$_2$Cu$_3$O$_y$. Bound states indeed produce an inhomogeneous pattern of the
local density of states $N(E_F)$ that leads to a skewed distribution of Knight
shifts which is detected here through an asymmetric profile of $^{17}$O NMR
lines. We argue that the effect arises most likely from scattering off defects
in the CDW state, which provides a novel case of disorder-induced bound states
in a condensed-matter system and an insightful window into charge ordering in
the cuprates.
