Exploring Complex-Langevin Methods for Finite-Density QCD

Abstract

QCD at non-zero chemical potential ($\mu$) for quark number has a complex fermion determinant and thus standard simulation methods for lattice QCD cannot be applied. We therefore simulate this theory using the Complex-Langevin algorithm with Gauge Cooling in addition to adaptive methods, to prevent runaway behaviour. Simulations are performed at zero temperature on a $12^4$ lattice with 2 quarks which are light enough that $m_N/3$ is significantly larger than $m_\pi/2$. Preliminary results are qualitatively as expected. The quark-number density is close to zero for $\mu < m_N/3$, beyond which it increases, eventually reaching its saturation value of $3$ for $\mu$ sufficiently large. The chiral condensate decreases as $\mu$ is increased approaching zero at saturation, while the plaquette increases towards its quenched value. We have yet to observe the transition to nuclear matter at $\mu \approx m_N/3$, presumably because the runs for $\mu$ between $m_N/3$ and saturation have yet to equilibrate.