Searching for Exotic Interactions between Antimatter

We show that atomic antimatter spectroscopy can be used to search for new bosons that carry spin-dependent exotic forces between antifermions. A comparison of a recent precise measurement of the hyperfine splitting of the $1$S and $2$S electronic levels of antihydrogen and bound-state quantum electrodynamics theory yields the first tests of positron-antiproton exotic interactions, constraining the dimensionless coupling strengths $g_pg_p$, $g_Vg_V$ and $g_Ag_A$, corresponding to the exchange of a pseudoscalar (axionlike), vector, or axial-vector boson, respectively. We also discuss new tests of CPT invariance with exotic spin-dependent and spin-independent interactions involving antimatter.