  The stellar initial mass function (IMF) describes the distribution of stellar
masses that form in a given star formation event. The long main-sequence
lifetimes of low-mass stars mean that the IMF in this regime (below $\sim 1
\rm{M}_\odot$) can be investigated through star counts. Ultrafaint dwarf
galaxies are low-luminosity systems with ancient, metal-poor stellar
populations. We investigate the low-mass IMF in four such systems (Reticulum
II, Ursa Major II, Triangulum II, and Segue 1), using Hubble Space Telescope
imaging data that reaches to $\lesssim 0.2 \rm{M}_\odot$ in each galaxy. The
analysis techniques that we adopt depend on the number of low-mass stars in
each sample. We use Kolmogorov-Smirnov tests for all four galaxies to determine
whether their observed apparent magnitude distributions can reject a given
combination of IMF parameters and binary fraction for the underlying
population. We forward model 1000 synthetic populations for each combination of
parameters, and reject those parameters only if each of the 1000 realizations
reject the null hypothesis. We find that all four galaxies reject a variety of
IMFs, and the IMFs that they cannot reject include those that are identical, or
similar, to that of the stellar populations of the Milky Way. We determine the
best-fit parameter values for the IMF in Reticulum II and Ursa Major II and
find that the IMF in Reticulum II is generally consistent with that of the
Milky Way, while the IMF in Ursa Major II is more bottom heavy. The
interpretation of the results for Ursa Major II is complicated by possible
contamination from two known background galaxy clusters.
