arXiv subjects

Nils Strodthoff

Publications and source records attributed to Nils Strodthoff.

85 records · Page 5Linked to original sources

arXiv2014

Chiral symmetry breaking in continuum QCD

We present a quantitative analysis of chiral symmetry breaking in two-flavour continuum QCD in the quenched limit. The theory is set-up at perturbative momenta, where asymptotic freedom leads to precise results. The evolution of QCD towards the hadronic phase is achieved by means of dynamical hadronisation in the non-perturbative functional renormalisation group approach. We use a vertex expansion scheme based on gauge-invariant operators and discuss its convergence properties and the remaining systematic errors. In particular we present results for the quark propagator, the full tensor structure and momentum dependence of the quark-gluon vertex, and the four-fermi scatterings.

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arXiv2014

Transport Coefficients in Yang--Mills Theory and QCD

We calculate the shear viscosity over entropy density ratio $η/s$ in Yang--Mills theory from the Kubo formula using an exact diagrammatic representation in terms of full propagators and vertices using gluon spectral functions as external input. We provide an analytic fit formula for the temperature dependence of $η/s$ over the whole temperature range from a glueball resonance gas at low temperatures, to a high-temperature regime consistent with perturbative results. Subsequently we provide a first estimate for $η/s$ in QCD.

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arXiv2014

Towards quantitative precision in the chiral crossover: masses and fluctuation scales

We investigate the relation between the physical pion pole and screening masses and the mesonic fluctuation scale in low-energy QCD, which relates to the curvature of the mesonic effective potential. This relation is important for the correct relative weight of quantum, thermal and density fluctuations. Hence, it governs the location of phase boundaries as well as the phase structure of QCD. The identification of the correct physics scales is also primarily important for the correct adjustment of the parameters of effective models for low-energy QCD. It is shown that subject to an appropriate definition of the latter, all these scales agree at vanishing temperature, while they deviate from each other at finite temperature.

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arXiv2014

Spectral functions from the functional renormalization group at finite temperature and density

We present a viable method to obtain real-time quantities such as spectral functions or transport coefficients at finite temperature and density within a non-perturbative Functional Renormalization Group approach. Our method is based on a thermodynamically consistent truncation of the flow equations for 2-point functions with analytically continued frequency components in the originally Euclidean external momenta. We demonstrate its feasibility by calculating the mesonic spectral functions in the quark-meson model at different temperatures and quark chemical potentials, in particular around the critical endpoint in the phase diagram of the model.

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arXiv2014

Spectral Functions from the Functional Renormalization Group

In this article we wish to present a new method to obtain spectral functions at finite temperature and density from the Functional Renormalization Group (FRG). The FRG offers a powerful non-perturbative tool to deal with phase transitions in strong-interaction matter under extreme conditions and their fluctuation properties. Based on a thermodynamically consistent truncation we derive flow equations for pertinent two-point functions in Minkowski space-time. We demonstrate the feasibility of the method by calculating mesonic spectral functions in hot and dense hadronic matter using the quark-meson model as a simple example.

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arXiv2014

Polyakov-Quark-Meson-Diquark Model for two-color QCD

We present an update on the phase diagram of two-color QCD from a chiral effective model approach based on a quark-meson-diquark model using the Functional Renormalization Group (FRG). We discuss the impact of perturbative UV contributions, the inclusion of gauge field dynamics via a phenomenological Polyakov loop potential, and the impact of matter backcoupling on the gauge sector. The corresponding phase diagram including these effects is found to be in qualitative agreement with recent lattice investigations.

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arXiv2014

Real-Time Correlation Functions in the O(N) Model from the Functional Renormalization Group

In the framework of the functional renormalization group (FRG) we present a simple truncation scheme for the computation of real-time mesonic n-point functions, consistent with the derivative expansion of the effective action. Via analytic continuation on the level of the flow equations we perform calculations of mesonic spectral functions in the scalar O(N) model, which we use as an exploratory example. By investigating the renormalization-scale dependence of the 2-point functions we shed light on the nature of the sigma meson, whose spectral properties are predominantly of dynamical origin.

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arXiv2013

Finite-Temperature Spectral Functions from the Functional Renormalization Group

We present a method to obtain spectral functions at finite temperature from the Functional Renormalization Group. Our method is based on a thermodynamically consistent truncation of the flow equations for 2-point functions with analytically continued frequency components in the originally Euclidean external momenta. For the uniqueness of this continuation at finite temperature we furthermore implement the physical Baym-Mermin boundary conditions. Results are presented for mesonic spectral functions obtained from a two-flavor quark-meson model.

hep-lat↗

arXiv2012

Fluctuations in the quark-meson model for QCD with isospin chemical potential

We study the two-flavor quark-meson (QM) model with the functional renormalization group (FRG) to describe the effects of collective mesonic fluctuations on the phase diagram of QCD at finite baryon and isospin chemical potentials, $μ_B$ and $μ_I$. With only isospin chemical potential there is a precise equivalence between the competing dynamics of chiral versus pion condensation and that of collective mesonic and baryonic fluctuations in the quark-meson-diquark model for two-color QCD at finite baryon chemical potential. Here, finite $μ_B=3μ$ introduces an additional dimension to the phase diagram as compared to two-color QCD, however. At zero temperature, the ($μ_I,μ$)-plane of this phase diagram is strongly constrained by the "Silver Blaze problem." In particular, the onset of pion condensation must occur at $μ_I= m_π/2$, independent of $μ$ as long as $μ+ μ_I$ stays below the constituent quark mass of the QM model or the liquid-gas transition line of nuclear matter in QCD. In order to maintain this relation beyond mean field it is crucial to compute the pion mass from its timelike correlator with the FRG in a consistent way.

hep-ph↗

arXiv2012

Quark-meson-diquark model for two-color QCD

We introduce a two-flavor quark-meson-diquark model for two-color QCD and its extensions to include gauge-field dynamics as described by the Polakov loop. Grand potential and phase structure are being studied both in mean-field approximation and with the functional renormalization group. The model provides an explicit example for the importance of baryonic degrees of freedom: When they are omitted, the phase diagram closely resembles that of the corresponding (Polyakov)-quark-meson models for QCD, in particular including their critical endpoint. In order to reproduce the well established main features based on the symmetries and breaking patterns of two-color QCD, however, they must be included and there is no critical endpoint. The competing dynamics of collective mesonic and baryonic fluctuations is well described by the functional renormalization group equation in lowest order derivative expansion for the effective potential which we solve numerically on a two-dimensional grid in field space.

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arXiv2010

Universal Aspects of Deconfinement in 2+1 Dimensions

The 2+1 dimensional pure SU(N) gauge theories with N <= 4 are candidates for applying the powerful tools of scaling and universality to their deconfinement transitions at finite temperature. The corresponding 2 dimensional q-state Potts models with q <= 4 have 2nd order transitions, and we can exploit many exact results to obtain accurate critical couplings, transition temperatures, critical exponents, and the leading behavior of the continuum string tension near the phase transition on one side, together with its dual on the other. Thereby, the self-duality of the 2d spin models is reflected in a duality between spacelike vortices and confining electric fluxes. We also discuss the relevance of center symmetry and the corresponding vortices for confinement in full QCD when the electromagnetic interactions of fractionally charged quarks are included.

hep-ph↗

arXiv2010

SU(3) Deconfinement in (2+1)d from Twisted Boundary Conditions and Self-Duality

We study the pure SU(3) gauge theory in 2+1 dimensions on the lattice using 't Hooft's twisted boundary conditions to force non-vanishing center flux through the finite volume. In this way we measure the free energy of spacelike center vortices as an order parameter for the deconfinement transition. The transition is of 2nd order in the universality class of the 2d 3-state Potts model, which is self-dual. This self-duality can be observed directly in the SU(3) gauge theory, and it can be exploited to extract critical couplings with high precision in rather small volumes. We furthermore obtain estimates for critical exponents and the critical temperature in units of the dimensionful continuum coupling. Finally, we also apply our methods to the (2+1)d SU(4) gauge theory which was previously found to have a weak 1st order transition. We nevertheless observe at least approximate q = 4 Potts scaling at length scales corresponding to the lattice sizes used in our simulations.

hep-lat↗

arXiv2010

Universal Aspects of Deconfinement: Interfaces, Flux Tubes and Self-Duality in 2+1 Dimensions

We study center vortex free energies and 't Hooft's electric fluxes on the lattice in 2+1 dimensions, where SU(2) for example, is in the universality class of the 2d Ising model. This places a wealth of exact results at our fingertips. In particular, spacelike center vortices in SU(2) near criticality correspond to spin interfaces in the 2d Ising model, whose universal scaling functions are known exactly. We exploit this to locate the deconfinement transition with unprecedented precision and subsequently for a finite size scaling analysis, where the self-duality of the $2d$ spin model is reflected in a duality between the spacelike vortices and confining electric fluxes. The corresponding relation between the string tension and its dual in the high temperature phase is arguably the simplest example of a universal amplitude ratio. Around the transition, both can be efficiently extracted from the exact results with a global one-parameter fit which allows straightforward continuum extrapolation.

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