  SU(2) lattice gauge theory with four flavors of quarks is studied at nonzero
chemical potential $\mu$ and temperature $T$ by computer simulation and
Effective Lagrangian techniques. Simulations are done on $8^4$, $8^3 \times 4$
and $12^3 \times 6$ lattices and the diquark condensate, chiral order
parameter, Wilson line, fermion energy and number densities are measured.
Simulations at a fixed, nonzero quark mass provide evidence for a tricritical
point in the $\mu$-$T$ plane associated with diquark condensation. For low $T$,
increasing $\mu$ takes the system through a line of second order phase
transitions to a diquark condensed phase. Increasing $T$ at high $\mu$, the
system passes through a line of first order transitions from the diquark phase
to the quark-gluon plasma phase. Using Effective Lagrangians we estimate the
position of the tricritical point and ascribe its existence to trilinear
couplings that increase with $\mu$ and $T$.
