Effective Mass of the Positron in Sodium

The band effective mass of the positron in sodium is computed by the standard method of Wigner, Seitz, and Bardeen. The result is $m^{*}=1.06m$, where $m$ is the free-particle mass. Combining it with the results of the many-body calculations of the effective mass of a positron interacting with a Fermi sea of electrons, one can conclude that the theoretical value is much smaller than the experimentally observed effective mass.