  We have undertaken deep optical imaging observations of three 6.2<z<6.5
quasar fields in the i' and z' filters. These data are used to search for
foreground galaxies which are gravitationally lensing the quasars and distant
galaxies physically associated with the quasars. Foreground galaxies are found
closer than 5 arcsec from the lines-of-sight of two of the three quasars.
However, the faintness of these galaxies suggests they have fairly low masses
and provide only weak magnifications (mu<1.1). No convincing galaxies
physically associated with the quasars are found and the number of i'-band
dropouts is consistent with that found in random fields. We consider the
expected dark matter halo masses which host these quasars under the assumption
that a correlation between black hole mass and dark matter halo mass exists. We
show that the steepness of the high-mass tail of the halo mass function at this
redshift, combined with realistic amounts of scatter in this correlation, lead
to expected halo masses substantially lower than previously believed. This
analysis can explain the lack of companion galaxies found here and the low
dynamical mass recently published for one of the quasars.
