Return of the prodigal Goldstone boson

Topological Aspects of Dense Matter: Lattice Studies

Topological fluctuations change their nature in the different phases of strong interactions, and the interrelation of topology, chiral symmetry and confinement at high temperature has been investigated in many lattice studies. This review is devoted to the much less explored subject of topology in dense matter. After a short overview of the status at zero density, which will serve as a baseline for the discussion, we will present lattice results for baryon rich matter, which, due to technical difficulties, has been mostly studied in two-color QCD, and for matter with isospin and chiral imbalances. In some cases, a coherent pattern emerges, and in particular the topological susceptibility seems suppressed at high temperature for baryon and isospin rich matter. However, at low temperatures the topological aspects of dense matter remain not completely clear and call for further studies.

The role of strangeness in chiral and U ( 1 ) A restoration

We use recently derived Ward identities and lattice data for the light- and strange-quark condensates to reconstruct the scalar and pseudoscalar susceptibilities ( χ S κ , χ P K ) in the isospin 1/2 channel. We show that χ S κ develops a maximum above the QCD chiral transition, after which it degenerates with χ P K . We also obtain χ S κ within Unitarized Chiral Perturbation Theory (UChPT) at finite temperature, when it is saturated with theK 0 ∗ ( 700 ) (or κ ) meson, the dominant lowest-energy state in the isospin 1/2 scalar channel of π K scattering. Such UChPT result reproduces the expected peak structure, revealing the importance of thermal interactions, and makes it possible to examine the χ S κ dependence on the light- and strange-quark masses. A consistent picture emerges controlled by them l / m s ratio that allows one studying K - κ degeneration in the chiral, two-flavor and SU(3) limits. These results provide an alternative sign for O ( 4 ) × U ( 1 ) A restoration that can be explored in lattice simulations and highlight the role of strangeness, which regulated by the strange-quark condensate helps to reconcile the current tension among lattice results regarding U ( 1 ) A restoration.

Aspects on Effective Theories and the QCD Transition

We review recent advances in the understanding of the Quantum Chromodynamics (QCD) transition and its nature, paying special attention to the analysis of chiral symmetry restoration within different approaches based on effective theories. After presenting some of the main aspects of the current knowledge of the phase diagram from the theoretical, experimental and lattice sides, we discuss some recent problems where approaches relying on effective theories have been particularly useful. In particular, the combination of ideas such as Chiral Perturbation Theory, unitarity and Ward Identities allows us to describe successfully several observables of interest. This is particularly relevant for quantities expected to be dominated by the light meson components of the hadron gas such as the scalar and topological susceptibilities. In addition, ward identities and effective Lagrangians provide systematic results regarding chiral and U ( 1 ) A partner degeneration properties which are of great importance for the interplay between those two transitions and the nature of chiral symmetry restoration. Special attention is paid to the connection of this theoretical framework with lattice simulations.

Indirect observation of an in-medium η' mass reduction in √s NN =200 GeV Au+Au collisions

PHENIX Collaboration and STAR Collaboration data on the intercept parameter of the two-pion Bose-Einstein correlation functions in √ s{NN}=200  GeV Au+Au collisions are analyzed in terms of different models of hadronic multiplicities. To describe this combined PHENIX and STAR data set, an in-medium η' mass reduction of at least 200 MeV is needed, at the 99.9% confidence level in the considered model class. Such a significant η' mass modification may indicate the restoration of the U{A}(1) symmetry in a hot and dense hadronic matter and the return of the 9th "prodigal" Goldstone boson.

Effective restoration of the U(A)(1) symmetry in the SU(3) linear sigma model

The effective restoration of the chiral U(A)(1) symmetry in strong interactions is studied using the linear chiral SU(3)xSU(3) model at finite temperatures. We find that the disappearance of the chiral anomaly causes a considerable change in the meson mass spectrum. We propose several signals for detecting this chiral phase in ultrarelativistic heavy-ion collisions: The eta/pi(0) ratio is enhanced by an order of magnitude, the a(0) is suppressed in the K&Kmacr; mass spectrum, and the scalar kappa meson appears as a peak just below the K(*)(892) in the invariant piK mass spectrum.