  At the international linear collider, measurement of the beam profile at the
interaction point is a key issue to achieve high luminosity. We report a
simulation study on a new beam profile monitor, called the pair monitor, which
uses the hit distribution of the electron-positron pairs generated at the
interaction point. We obtained measurement accuracies of 5.1%, 10.0%, and 4.0%
for the horizontal, vertical, and longitudinal beam size, respectively, for 50
bunch crossings.
