  In conventional superconductors, a gap exists in the energy absorption
spectrum only below the transition temperature (Tc), corresponding to the
energy price to pay for breaking a Cooper pair of electrons. In high-Tc cuprate
superconductors above Tc, an energy gap called the pseudogap exists, and is
controversially attributed either to pre-formed superconducting pairs, which
would exhibit particle-hole symmetry, or to competing phases which would
typically break it. Scanning tunnelling microscopy (STM) studies suggest that
the pseudogap stems from lattice translational symmetry breaking and is
associated with a different characteristic spectrum for adding or removing
electrons (particle-hole asymmetry). However, no signature of either spatial or
energy symmetry breaking of the pseudogap has previously been observed by
angle-resolved photoemission spectroscopy (ARPES). Here we report ARPES data
from Bi2201 which reveals both particle-hole symmetry breaking and dramatic
spectral broadening indicative of spatial symmetry breaking without long range
order, upon crossing through T* into the pseudogap state. This symmetry
breaking is found in the dominant region of the momentum space for the
pseudogap, around the so-called anti-node near the Brillouin zone boundary. Our
finding supports the STM conclusion that the pseudogap state is a
broken-symmetry state that is distinct from homogeneous superconductivity.
