  Negative effective masses can be realised by engineering the dispersion
relation in a variety of quantum systems. A recent experiment with spin-orbit
coupled Bose-Einstein condensates has shown that a negative effective mass can
halt the free expansion of the condensate and produce fringes in the density
[M. Khamehchi et al., Phys. Rev. Lett. 118, 155301 (2017)]. Here, we show that
the observed fringes are due to the negativity of only one of the two effective
mass parameters that characterise the dispersion, which leads to previously
predicted self-interference of the wave packet. We show how confgurations are
nevertheless accessible to spin-orbit coupled Bose-Einstein condensates where
both mass parameters controlling the propagation and diffusion of the
condensate are negative, resulting in the novel phenomenon of
counter-propagating self-interfering packets.
