Directional detection of dark matter in universal bound states

It has been suggested that several small-scale structure anomalies in Lambda CDM cosmology can be solved by strong self-interaction between dark matter particles. It was shown by Braaten and Hammer [Phys. Rev. D 88, 063511 (2013)] that the presence of a near threshold S-wave resonance can make the scattering cross section at nonrelativistic speeds come close to saturating the unitarity bound. This can result in the formation of a stable bound state of two asymmetric dark matter particles (which we call darkonium). The work of Laha and Braaten [Phys. Rev. D, 89, 103510 (2014)] studied the nuclear recoil energy spectrum in dark matter direct detection experiments due to this incident bound state. Here we study the angular recoil spectrum and show that it is uniquely determined up to normalization by the S-wave scattering length. Observing this angular recoil spectrum in a dark matter directional detection experiment will uniquely determine many of the low-energy properties of dark matter independent of the underlying dark matter microphysics.