The finite temperature transition for 2-flavour lattice QCD at finite isospin density

We simulate 2-flavour lattice QCD at finite isospin chemical potential mu_I, for temperatures close to the finite temperature transition from hadronic matter to a quark-gluon plasma. The mu_I dependence of the transition coupling is observed and used to estimate the decrease in the transition temperature with increasing mu_I. These simulations are performed on an 8^3 times 4 lattice at 3 different quark masses. Our estimate of the magnitude of the fluctuations of the phase of the fermion determinant at small quark-number chemical potential mu, suggest that the position of the small mu and small mu_I transitions should be the same for mu_I=2mu, and we argue that the nature of these transitions should be the same. For all mu_I < m_pi the smoothness of these transitions and the values of the Binder cumulant B_4, indicate that these transitions are mere crossovers, and show no sign of the expected critical endpoint. For mu_I > m_pi and a small isospin (I_3) breaking term lambda, we do find evidence of a critical endpoint which would indicate that, for lambda=0, there is a tricritical point on the phase boundary where the pion condensate evaporates, where this phase transition changes from second to first order.