  A mirror world can modify in a striking way the LHC signals of the Higgs
sector. An exact or approximate Z_2 symmetry between the mirror world and our
world allows large mixing between the Higgs bosons of these worlds, leading to
production rates and branching ratios for these states that are markedly
different from the standard model and are characteristic of a mirror world. The
constraints on these Higgs boson masses from precision electroweak data differ
from the standard model bound, so that the new physics that cancels the
quadratic divergence induced by the top quark may appear at a larger scale,
possibly beyond the reach of the LHC. However, the scale of new physics needed
to cancel the quadratic divergence induced by the Higgs boson is not
significantly changed. With small breakings of the Z_2 parity, the lightest
mirror quarks (and possibly charged mirror leptons) could be the dark matter in
the universe, forming galactic halos that are stable to cooling. A possible
signal from the relic radiation density of the mirror world is also discussed.
