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Research Abstracts Online
January 2009 - March 2010

Main TOC ....... College TOC

University of Minnesota Twin Cities
Institute of Technology
School of Physics and Astronomy
Department of Physics
Theoretical Physics Institute

PI: Philippe de Forcrand, Adjunct Fellow

Quantum Chromodynamics at Finite Density

Quantum chromodynamics (QCD) predicts that quarks are confined at low temperature and low density. But the theory predicts that confinement is lost under extreme conditions: at high temperature, a plasma of quarks and gluons forms; at high densities and low temperatures, Cooper pairs of two quarks condense and color superconductivity takes place. In addition to these three regimes, other phases are likely to exist. 

At non-zero quark density, the numerical simulations have to face the "sign problem”: the integral to sample is over an oscillatory function, and importance sampling fails. Information can only be obtained on small volumes and for small densities.

To circumvent this problem, these researchers have been pursuing an approach where the simulations are performed with an imaginary chemical potential μ, where the sign problem is absent. Results are fitted with a truncated Taylor expansion, which can be trivially continued back to real μ. In this way, they have determined the pseudo-critical temperature Tc(μ), and the order of the phase transition along this pseudo-critical line. The results for the latter are opposite to standard expectations: instead of becoming stronger with μ, the transition becomes weaker. Since, at μ = 0, QCD with real-world quark masses has only a crossover at Tc = 0) between the confined and deconfined regimes, the result implies that the transition will remain crossover when μ ≠ 0. Instead, the standard expectation is that the μ = 0 crossover turns into a phase transition at a critical point (TE, μE), and an experimental search for this critical point is underway. Therefore, confirming or disproving this finding is an important question.

To firm up their result, the researchers plan to study in detail the phase diagram for imaginary μ: it appears to feature two tricritical points, which will put serious constraints on the possible phase structure at real μ.

Group Members

Owe Philipsen, Institute for Theoretical Physics, Westfaelische Wilhelms-Universität, Münster, Germany
Urs Wenger, Institute for Theoretical Physics, ETZ Zürich, Zürich, Switzerland