  We present an extensive study of angle-dependent transverse magnetoresistance
in bismuth, with a magnetic field perpendicular to the applied electric current
and rotating in three distinct crystallographic planes. The observed angular
oscillations are confronted with the expectations of semi-classic transport
theory for a multi-valley system with anisotropic mobility and the agreement
allows us to quantify the components of the mobility tensor for both electrons
and holes. A quadratic temperature dependence is resolved. As Hartman argued
long ago, this indicates that inelastic resistivity in bismuth is dominated by
carrier-carrier scattering. At low temperature and high magnetic field, the
threefold symmetry of the lattice is suddenly lost. Specifically, a $2\pi/3$
rotation of magnetic field around the trigonal axis modifies the amplitude of
the magneto-resistance below a field-dependent temperature. By following the
evolution of this anomaly as a function of temperature and magnetic field, we
mapped the boundary in the (field, temperature) plane separating two electronic
states. In the less-symmetric state, confined to low temperature and high
magnetic field, the three Dirac valleys cease to be rotationally invariant. We
discuss the possible origins of this spontaneous valley polarization, including
a valley-nematic scenario.
