  Superconductivity originates from the formation of bound (Cooper) pairs of
electrons that can move through the lattice without resistance below the
superconducting transition temperature $T_c$. Electron Cooper pairs in most
superconductors form anti-parallel spin singlets with total spin $S=0$,
although they can also form parallel spin-triplet Cooper pairs with $S=1$ and
an odd parity wavefunction. Spin-triplet pairing is important because it can
host topological states and Majorana fermions relevant for quantum computation.
Because spin-triplet pairing is usually mediated by ferromagnetic (FM) spin
fluctuations, uranium based materials near an FM instability are considered to
be ideal candidates for realizing spin-triplet superconductivity. Indeed,
UTe$_2$, which has a $T_c\approx 1.6$ K, has been identified as a candidate for
a chiral spin-triplet topological superconductor near an FM instability,
although it also has antiferromagnetic (AF) spin fluctuations. Here we use
inelastic neutron scattering (INS) to show that superconductivity in UTe$_2$ is
coupled to a sharp magnetic excitation, termed resonance, at the Brillouin zone
boundary near AF order. Because the resonance has only been found in
spin-singlet unconventional superconductors near an AF instability, its
observation in UTe$_2$ suggests that AF spin fluctuations may also induce
spin-triplet pairing or that electron pairing in UTe$_2$ has a spin-singlet
component.
