  We study the anisotropic Heisenberg (XYZ) spin-1/2 chain placed in a magnetic
field pointing along the x-axis. We use bosonization and a renormalization
group analysis to show that the model has a non-trivial fixed point at a
certain value of the XY anisotropy a and the magnetic field h. Hence, there is
a line of critical points in the (a,h) plane on which the system is gapless,
even though the Hamiltonian has no continuous symmetry. The quantum critical
line corresponds to a spin-flop transition; it separates two gapped phases in
one of which the Z_2 symmetry of the Hamiltonian is broken. Our study has a
bearing on one of the transitions of the axial next-nearest neighbor Ising
(ANNNI) chain in a transverse magnetic field. We also discuss the properties of
the model when the magnetic field is increased further, in particular, the
disorder line on which the ground state is a direct product of single spin
states.
