Chiral density wave versus pion condensation at finite density

The quark-meson model is often used as an effective low-energy model for QCD to study the chiral transition at finite temperature $T$, baryon chemical potential $\mu_B$, and isospin chemical potential $\mu_I$. The parameters of the model are determined by matching the meson and quark masses, as well as the pion decay constant to their physical values using the on-shell and modified minimal subtraction schemes. In this paper, we study the possibility of different phases at zero temperature. In particular, we investigate the competition between an inhomogeneous chiral condensate and a pion condensate. For the inhomogeneity, we use a chiral-density wave ansatz. For a sigma mass of $600$ MeV, we find that an inhomogeneous chiral condensate exist only for pion masses below approximately 37 MeV. We also show that due to our parameter fixing, the onset of pion condensation takes place exactly at $\mu_I={1\over2}m_{\pi}$ in accordance with exact results.Comment: 14 pages and 11 figs. v2: Minor changes, matches published version in PR

On-shell parameter fixing in the quark-meson model

The quark-meson model is often used as an effective low-energy model for QCD to study the chiral transition at finite temperature T and baryon chemical potential μB. The parameters in the quark-meson model can be found by expressing them in terms of the sigma mass mσ, the pion mass mπ, the constituent quark mass mq and the pion decay constant fπ. In practice, this matching is done at tree level, which is inconsistent once loop effects of the effective potential are taken into account. We show how to properly perform the matching in the quark-meson model by using the on-shell and the minimal subtraction renormalization schemes relating the physical masses and the pion decay constant to the running mass parameter and couplings. We map out the phase diagram in the μB–T plane and compare our results with other approximations.acceptedVersio

Inhomogeneous chiral condensate in the quark-meson model

acceptedVersionpublishedVersio

Pion condensation and phase diagram in the Polyakov-loop quark-meson model

We use the Polyakov-loop extended two-flavor quark-meson model as a low-energy effective model for QCD to study the phase diagram in the μI–T planewhere μI is the isospin chemical potential. In particular, we focus on the Bose condensation of charged pions. At T ¼ 0, the onset of pion condensation is at μI ¼ 1 2mπ in accordance with exact results. The phase transition to a Bose-condensed phase is of second order for all values of μI and in the Oð2Þuniversality class. The chiral critical line joins the critical line for pion condensation at a point whose position depends on the Polyakov-loop potential and the sigma mass. For larger values of μI these curves are on top of each other. The deconfinement line enters smoothly the phase with the broken Oð2Þ symmetry. We compare our results with recent lattice simulations and find overall good agreement.publishedVersio

Das NetzDG in der praktischen Anwendung

What are the practical effects of the Network Enforcement Act, which came into force in 2017 and was controversial from the start? Which compliance structures have the big three social networks Facebook, YouTube and Twitter implemented with regard to the removal of illegal user content? In addition to a presentation and evaluation of previous studies, monitoring reports and NetzDG reports from the social networks, possible criteria for “over-blocking” are identified and subsumed in this third volume of the series. On the occasion of the NetzDG evaluation commissioned by the federal government, the study was carried out additionally and independently of this. It is not based on any commissioning by public or private bodies.Welche Auswirkungen hat das 2017 in Kraft getretene, von Beginn an umstrittene Netzwerkdurchsetzungsgesetz in der praktischen Anwendung? Welche Compliance-Strukturen haben die großen drei Sozialen Netzwerke Facebook, YouTube und Twitter in Bezug auf die Entfernung rechtswidriger Nutzerinhalte umgesetzt? Neben einer Darstellung und Bewertung bisheriger Studien, Monitoring-Berichte und NetzDG-Reports der Sozialen Netzwerke werden im vorliegenden dritten Band der Schriftenreihe auch mögliche Kriterien für ein “Overblocking” eruiert und subsumiert. Die Studie ist aus Anlass der von der Bundesregierung beauftragten NetzDG-Evaluation zusätzlich und unabhängig hiervon durchgeführt worden. Ihr liegt keine Beauftragung durch öffentliche oder private Stellen zugrunde