Refine
Year of publication
Document Type
- Conference Proceeding (35)
- Article (17)
Language
- English (52)
Has Fulltext
- yes (52)
Is part of the Bibliography
- no (52)
Keywords
- Lattice QCD (3)
- Lattice Quantum Field Theory (2)
- Effective Field Theories (1)
- FOS: Physical sciences (1)
- Finite baryon density (1)
- High Energy Physics - Lattice (hep-lat) (1)
- High Energy Physics - Phenomenology (hep-ph) (1)
- High Energy Physics - Theory (hep-th) (1)
- Hybrid Monte Carlo algorithm (1)
- Lattice field theory (1)
Institute
- Physik (52)
- Frankfurt Institute for Advanced Studies (FIAS) (4)
- ELEMENTS (2)
- Informatik (1)
Lattice QCD and functional methods are making significant progress in constraining the QCD phase diagram. As an important milestone, the chiral phase transition with massless u, d-quarks at zero density is now understood to be of second order for all strange quark masses, and a smooth crossover as soon as mu,d, ≠ 0. Together with information on fluctuations and refined reweighted simulations, this bounds a possible critical point to be at µB/T ≲3. On the other hand, an approximately chiral-spin symmetric temperature window has been discovered above the chiral crossover, Tch<T ≳3Tch, with distinct correlator multiplet patterns and a pion spectral function suggesting resonance-like degrees of freedom, which dissolve graduallly with temperature.
Phase transitions in a non-perturbative regime can be studied by ab initio Lattice Field Theory methods. The status and future research directions for LFT investigations of Quantum Chromo-Dynamics under extreme conditions are reviewed, including properties of hadrons and of the hypothesized QCD axion as inferred from QCD topology in different phases. We discuss phase transitions in strong interactions in an extended parameter space, and the possibility of model building for Dark Matter and Electro-Weak Symmetry Breaking. Methodological challenges are addressed as well, including new developments in Artificial Intelligence geared towards the identification of different phases and transitions.
Effective three-dimensional Polyakov loop theories derived from QCD by strong coupling and hopping expansions are valid for heavy quarks and can also be applied to finite chemical potential μ, due to their considerably milder sign problem. We apply the Monte-Carlo method to the Nf=1,2 effective theories up to O(κ4) in the hopping parameter at μ=0 to determine the critical quark mass, at which the first-order deconfinement phase transition terminates. The critical end point obtained from the effective theory to order O(κ2) agrees well with 4-dimensional QCD simulations with a hopping expanded determinant by the WHOT-QCD collaboration. We also compare with full QCD simulations and thus obtain a measure for the validity of both the strong coupling and the hopping expansion in this regime.
The thermodynamics of QCD with sufficiently heavy dynamical quarks can be described by a three-dimensional Polyakov loop effective theory, obtained after a truncated character and hopping expansion. We investigate the resulting phase diagram for low temperatures by mean field methods. Taking into account chemical potentials for both baryon number and isospin, we obtain clear signals for a liquid-gas type transition to baryon matter at μI=0 and a Bose-Einstein condensation transition at μB=0, as well as for their connection when both chemical potentials are non-zero.
The global center symmetry of quenched QCD at zero baryonic chemical potential is broken spontaneously at a critical temperature Tc leading to a first-order phase transition. Including heavy dynamical quarks breaks the center symmetry explicitly and weakens the first-order phase transition for decreasing quark masses until it turns into a smooth crossover at a Z(2)-critical point. We investigate the Z(2)-critical quark mass value towards the continuum limit for Nf=2 flavors using lattice QCD in the staggered formulation. As part of a continued study, we present results from Monte-Carlo simulations on Nτ=8,10 lattices. Several aspect ratios and quark mass values were simulated in order to obtain the critical mass from a fit of the Polyakov loop to a kurtosis finite size scaling formula. Moreover, the possibility to develop a Ginzburg-Landau effective theory around the Z(2)-critical point is explored.
For the exploration of the phase diagram of QCD, effective Polyakov loop theories derived from lattice QCD provide a valuable tool in the heavy quark mass regime. In practice, the evaluation of these theories is complicated by the appearance of long-range and multipoint interaction terms. On the other hand, it is well known that for theories with such kind of interactions mean field approximations can be expected to yield reliable results. Here, we apply this framework to the critical endpoint of the deconfinement transition and results are compared to the literature. This treatment can also be used to investigate the phase diagram at non-zero baryon and isospin chemical potential.
Approaching the continuum limit of the deconfinement critical point for Nf=2 staggered fermions
(2022)
Quenched QCD at zero baryonic chemical potential undergoes a first-order deconfinement phase transition at a critical temperature Tc, which is related to the spontaneous breaking of the global center symmetry. The center symmetry is broken explicitly by including dynamical quarks, which weaken the first-order phase transition for decreasing quark masses. At a certain critical quark mass, which corresponds to the Z(2)-critical point, the first-order phase transition turns into a smooth crossover. We investigate the Z(2)-critical quark mass for Nf=2 staggered fermions on Nτ=8,10 lattices, where larger Nτ correspond to finer lattices. Monte-Carlo simulations are performed for several quark mass values and aspect ratios in order to extrapolate to the thermodynamic limit. We present final results for Nτ=8 and preliminary results for Nτ=10 for the critical mass, which are obtained from fitting to a kurtosis finite size scaling formula of the absolute value of the Polyakov loop.
The so-called Columbia plot summarises the order of the QCD thermal transition as a function of the number of quark flavours and their masses. Recently, it was demonstrated that the first-order chiral transition region, as seen for Nf∈[3,6] on coarse lattices, exhibits tricritical scaling while extrapolating to zero on sufficiently fine lattices. Here we extend these studies to imaginary baryon chemical potential. A similar shrinking of the first-order region is observed with decreasing lattice spacing, which again appears compatible with a tricritical extrapolation to zero.
Recently, an approximate SU(4) chiral spin-flavour symmetry was observed in multiplet patterns of QCD meson correlation functions, in a temperature range above the chiral crossover. This symmetry is larger than the chiral symmetry of massless QCD, and can only arise effectively when colour-electric quark-gluon interactions dynamically dominate the quantum effective action. At temperatures about three times the crossover temperature, these patterns disappear again, indicating the screening of colour-electric interactions, and the expected chiral symmetry is recovered. In this contribution we collect independent evidence for such an intermediate temperature range, based on screening masses and the pion spectral function. Both kinds of observables behave non-perturbatively in this window, with resonance-like peaks for the pion and its first excitation disappearing gradually with temperature. Using symmetry arguments and the known behaviour of screening masses at small densities, we discuss how this chiral spin symmetric band continues into the QCD phase diagram.
Spectral functions encode a wealth of information about the dynamics of any given system, and the determination of their non-perturbative characteristics is a long-standing problem in quantum field theory. Whilst numerical simulations of lattice QCD provide ample data for various Euclidean correlation functions, the inversion required to extract spectral functions is an ill-posed problem. In this work, we pursue previously established constraints imposed by field locality at finite temperature T, namely that spectral functions possess a non-perturbative representation which generalises the well-known Källén-Lehmann spectral form to T > 0. Using this representation, we analyse lattice QCD data of the spatial pseudo-scalar correlator in the temperature range 220–960 MeV, and obtain an analytic expression for the corresponding spectral function, with parameters fixed by the data. From the structure of this spectral function we find evidence for the existence of a distinct pion state above the chiral pseudo-critical temperature Tpc, and contributions from its first excitation, which gradually melt as the temperature increases. As a non-trivial test, we find that the extracted spectral function reproduces the corresponding temporal lattice correlator data for T = 220 MeV.