  In this paper, we bring out several potential signatures of negative mass
regimes while investigating an expanding spin-orbit (SO) coupled spinor $F=1$
Bose-Einstein condensates by analyzing the dispersion relation of the
single-particle quantum system. In SO-coupled spinor condensates, a negative
mass parameter generates a wave packet that propagates in the opposite
direction of the momentum. We analyze the dynamics of spin waves analytically
and present a simple approach to investigate the expansion of spinor
condensates. In particular, we examine the dynamics when both masses are
negative, which results in the spinor condensates splitting into two
counter-propagating self-interfering packets (SIPs). Using numerical
simulations of the coupled Gross-Pitaevskii equations, we demonstrate the
density expansion and self-interference patterns with and without magnetization
for repulsive and attractive interactions with different coupling parameters.
The highlight of our investigation is that we are able to unearth several
phenomena observed in experiments, such as self-interfering packets, pileup,
modulation instability, slow down, self-trapping, and gap solitons. In
particular, the gap soliton exists at the gap created by the intersection of
two negative masses.
