  Revealing the quantum properties of matter requires a high degree of
experimental control accompanied by a profound theoretical understanding. At
ultracold temperatures, quantities that appear continuous in everyday life,
such as the motional angular momentum of two colliding particles, become
quantized, leaving a measurable imprint on experimental results. Embedding a
single particle within a larger quantum bath at lowest temperatures can result
in resonant partial-wave dependent interaction, whose strength near zero energy
is dictated by universal threshold scaling laws. Hybrid atom-ion systems have
emerged as a novel platform in which a single charged atom in an ultracold bath
serves as a well-controlled impurity of variable energy. However, entering the
low-energy s-wave regime and exploring the role of higher-partial-wave
scattering within has remained an open challenge. Here, we immerse a Barium ion
in a cloud of ultracold spin-polarized Lithium atoms, realize tunable collision
energies below the s-wave limit and explore resonant higher-partial-wave
scattering by studying the energy dependence of Feshbach resonances. Utilizing
precise electric field control, we tune the collision energy over four orders
of magnitude, reaching from the many-parital-wave to the s-wave regime. At the
lowest energies, we probe the energy dependence of an isolated s-wave Feshbach
resonance and introduce a theoretical model that allows to distinguish it from
higher-partial-wave resonances. Additionally, at energies around the p-wave
barrier, we find and identify an open-channel f-wave resonance, consistent with
threshold laws. Our findings highlight and benchmark the importance of
higher-partial-wave scattering well within the s-wave regime and offer control
over chemical reactions and complex many-body dynamics in atom-ion ensembles -
on the level of individual angular momentum quanta.
