429 research outputs found
Flow equations for spectral functions at finite external momenta
In this work we study the spatial-momentum dependence of mesonic spectral
functions obtained from the quark-meson model using a recently proposed method
to calculate real-time observables at finite temperature and density from the
Functional Renormalization Group. This non-perturbative method is
thermodynamically consistent, symmetry-preserving and based on an analytic
continuation from imaginary to real time on the level of the flow equations for
2-point functions. Results on the spatial-momentum dependence of the pion and
sigma spectral function are presented at different temperatures and densities,
in particular near the critical endpoint in the phase diagram of the
quark-meson model.Comment: 13 pages, 7 figure
Spectral Functions for the Quark-Meson Model Phase Diagram from the Functional Renormalization Group
We present a method to obtain spectral functions at finite temperature and
density 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. We demonstrate the feasibility of the method by calculating the
mesonic spectral functions in the quark-meson model along the temperature axis
of the phase diagram, and at finite quark chemical potential along the
fixed-temperature line that crosses the critical endpoint of the model.Comment: 11 pages, 5 figures, 1 tabl
The low-temperature behavior of the quark-meson model
We revisit the phase diagram of strong-interaction matter for the two-flavor
quark-meson model using the Functional Renormalization Group. In contrast to
standard mean-field calculations, an unusual phase structure is encountered at
low temperatures and large quark chemical potentials. In particular, we
identify a regime where the pressure decreases with increasing temperature and
discuss possible reasons for this unphysical behavior.Comment: 7 pages, 7 figures, v2 corresponds to published versio
Spectral Functions from the Functional Renormalization Group
We present results for in-medium spectral functions obtained within the
Functional Renormalization Group framework. The analytic continuation from
imaginary to real time is performed in a well-defined way on the level of the
flow equations. Based on this recently developed method, results for the sigma
and the pion spectral function for the quark-meson model are shown at finite
temperature, finite quark-chemical potential and finite spatial momentum. It is
shown how these spectral function become degenreate at high temperatures due to
the restoration of chiral symmetry. In addition, results for vector- and
axial-vector meson spectral functions are shown using a gauged linear sigma
model with quarks. The degeneration of the and the spectral
function as well as the behavior of their pole masses is discussed.Comment: CPOD 2017 Proceeding
In-Medium Spectral Functions of Vector- and Axial-Vector Mesons from the Functional Renormalization Group
In this work we present first results on vector and axial-vector meson
spectral functions as obtained by applying the non-perturbative functional
renormalization group approach to an effective low-energy theory motivated by
the gauged linear sigma model. By using a recently proposed analytic
continuation method, we study the in-medium behavior of the spectral functions
of the and mesons in different regimes of the phase diagram. In
particular, we demonstrate explicitly how these spectral functions degenerate
at high temperatures as well as at large chemical potentials, as a consequence
of the restoration of chiral symmetry. In addition, we also compute the
momentum dependence of the and spectral functions and discuss the
various time-like and space-like processes that can occur.Comment: 18 pages, 13 figures, 1 tabl
Online railway delay management: Hardness, simulation and computation
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Delays in a railway network are a common problem that railway companies face in their daily operations. When a train is delayed, it may either be beneficial to let a connecting train wait so that passengers in the delayed train do not miss their connection, or it may be beneficial to let the connecting train depart on time to avoid further delays. These decisions naturally depend on the global structure of the network, on the schedule, on the passenger routes and on the imposed delays. The railway delay management (RDM) problem (in a broad sense) is to decide which trains have to wait for connecting trains and which trains have to depart on time. The offline version (i.e. when all delays are known in advance) is already NP-hard for very special networks. In this paper we show that the online railway delay management (ORDM) problem is PSPACE-hard. This result justifies the need for a simulation approach to evaluate wait policies for ORDM. For this purpose we present TOPSU—RDM, a simulation platform for evaluating and comparing different heuristics for the ORDM problem with stochastic delays. Our novel approach is to separate the actual simulation and the program that implements the decision-making policy, thus enabling implementations of different heuristics to ‘‘compete’’ on the same instances and delay distributions. We also report on computational results indicating the worthiness of developing intelligent wait policies. For RDM and other logistic planning processes, it is our goal to bridge the gap between theoretical models, which are accessible to theoretical analysis, but are often too far away from practice, and the methods which are used in practice today, whose performance is almost impossible to measure.EU/FP6/021235-2/EU/Algorithms for Robust and on-line Railway optimisation: Improving the validity and reliability of large-scale systems/ARRIVA
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