  Precision measurements of the vortex phase diagram in single crystals of the
layered superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ in oblique
magnetic fields confirm the existence of a second phase transition, in addition
to the usual first order vortex lattice melting line $H_{m}(T)$. The transition
has a strong first order character, is accompanied by strong hysteresis, and
intersects the melting line in a tricritical point ($H_{m}^{\perp}$,
$H^{\parallel}_{cr}$). Its field dependence and the changing character of the
melting line at the tricritical point strongly suggest that the ground state
for magnetic fields closely aligned with the superconducting layers is a
lattice of uniformly tilted vortex lines.
