  We have used the diffusion quantum Monte Carlo method to calculate the energy
band of the two-dimensional homogeneous electron gas (HEG), and hence we have
obtained the quasiparticle effective mass and the occupied bandwidth. We find
that the effective mass in the paramagnetic HEG increases significantly when
the density is lowered, whereas it decreases in the fully ferromagnetic HEG.
Our calculations therefore support the conclusions of recent experimental
studies [Y.-W. Tan et al., Phys. Rev. Lett. 94, 016405 (2005); M. Padmanabhan
et al., Phys. Rev. Lett. 101, 026402 (2008); T. Gokmen et al., Phys. Rev. B 79,
195311 (2009)]. We compare our calculated effective masses with other
theoretical results and experimental measurements in the literature.
