  Heretofore undiscovered spin-0 or spin-1 bosons can mediate exotic
spin-dependent interactions between standard-model particles. Here we carry out
the first search for semileptonic spin-dependent interactions between matter
and antimatter. We compare theoretical calculations and spectroscopic
measurements of the hyperfine structure of antiprotonic helium to constrain
exotic spin- and velocity-dependent interactions between electrons and
antiprotons.
