  One of the primary goals of nuclear physics is to understand the force
between nucleons, which is a necessary step for understanding the structure of
nuclei and how nuclei interact with each other. Rutherford discovered the
atomic nucleus in 1911, and the large body of knowledge about the nuclear force
since acquired was derived from studies made on nucleons or nuclei. Although
antinuclei up to antihelium-4 have been discovered and their masses measured,
we have no direct knowledge of the nuclear force between antinucleons. Here, we
study antiproton pair correlations among data taken by the STAR experiment at
the Relativistic Heavy Ion Collider and show that the force between two
antiprotons is attractive. In addition, we report two key parameters that
characterize the corresponding strong interaction: namely, the scattering
length (f0) and effective range (d0). As direct information on the interaction
between two antiprotons, one of the simplest systems of antinucleons, our
result provides a fundamental ingredient for understanding the structure of
more complex antinuclei and their properties.
