The QCD phase diagram for small densities from imaginary chemical potential

We present results on the QCD phase diagram for mu(B) less than or equal to piT. Our simulations are performed with an imaginary chemical potential mu = imu(I) for which the fermion determinant is positive. On an 8(3) x 4 lattice with 2 flavors of staggered quarks, we map out the phase diagram and identify the (pseudo-) critical temperature T-c(mu(I)). For mu(I)/T less than or equal to pi/3, this is an analytic function, whose Taylor expansion is found to converge rapidly, with truncation errors far smaller than statistical ones. The truncated series may then be continued to real mu, yielding the corresponding phase diagram for mu(B) less than or similar to 500 MeV. This approach provides control over systematics and avoids reweighting. We compare it with other recent work. (C) 2002 Elsevier Science B.V. All rights reserved.