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We study the high temperature transition in pure SU(3) gauge theory and in full QCD with 3D-convolutional neural networks trained as parts of either unsupervised or semi-supervised learning problems. Pure gauge configurations are obtained with the MILC public code and full QCD are from simulations of Nf=2+1+1 Wilson fermions at maximal twist. We discuss the capability of different approaches to identify different phases using as input the configurations of Polyakov loops. To better expose fluctuations, a standardized version of Polyakov loops is also considered.
We explore the phase structure of the 1+1 dimensional Gross-Neveu model at finite number of fermion flavors using lattice field theory. Besides a chirally symmetric phase and a homogeneously broken phase we find evidence for the existence of an inhomogeneous phase, where the condensate is a spatially oscillating function. Our numerical results include a crude μ-T phase diagram.
In this work we study the 3+1-dimensional Nambu-Jona-Lasinio (NJL) model in the mean field-approximation. We carry out calculations using five different regularization schemes (two continuum and three lattice regularization schemes) with particular focus on inhomogeneous phases and condensates. The regularization schemes lead to drastically different inhomogeneous regions. We provide evidence that inhomogeneous condensates appear for all regularization schemes almost exclusively at values of the chemical potential and with wave numbers, which are of the order of or even larger than the corresponding regulators. This can be interpreted as indication that inhomogeneous phases in the 3+1-dimensional NJL model are rather artifacts of the regularization and not a consequence of the NJL Lagrangian and its symmetries.
In this work we study the 3+1-dimensional Nambu-Jona-Lasinio (NJL) model in the mean field-approximation. We carry out calculations using five different regularization schemes (two continuum and three lattice regularization schemes) with particular focus on inhomogeneous phases and condensates. The regularization schemes lead to drastically different inhomogeneous regions. We provide evidence that inhomogeneous condensates appear for all regularization schemes almost exclusively at values of the chemical potential and with wave numbers, which are of the order of or even larger than the corresponding regulators. This can be interpreted as indication that inhomogeneous phases in the 3+1-dimensional NJL model are rather artifacts of the regularization and not a consequence of the NJL Lagrangian and its symmetries.
We compute potentials of two static antiquarks in the presence of two quarks qq of finite mass using lattice QCD. In a second step we solve the Schrödinger equation, to determine, whether the resulting potentials are sufficiently attractive to host a bound state, which would indicate the existence of a stable qqb¯b¯ tetraquark. We find a bound state for qq=(ud−du)/2–√ with corresponding quantum numbers I(JP)=0(1+) and evidence against the existence of bound states with isospin I=1 or qq∈{cc,ss}.
We investigate BB̅ systems by computing potentials of two static quarks in the presence of two quarks of finite mass using lattice QCD. By solving the Schrodinger equation we check whether these potentials are sufficiently attractive to host bound states. Particular focus is put on the experimentally most promising bottomonium-like tetraquark candidate Zb± with quantum numbers I(JP) = 1(1+).
Intense ion beams with small phase space occupation (high brilliance) are mandatory to keep beam losses low in high current injector accelerators like those planned for FAIR. The low energy beam transport from the ion source towards the linac has to keep the emittance growth low and has to support the optimization of the ion source tune. The Frankfurt Neutron Source Facility FRANZ is currently under construction. An intense beam of protons (2 MeV, 200 mA) will be used for neutron production using the Li7(p,n)Be7 reaction for studies of the astrophysical s-process. A collimation channel, which can be adjusted to allow the transport of beams with a certain beam emittance, is an ideal tool to optimize the ion source tune in terms of beam brightness. Therefore a collimation channel in the Low Energy Beam Transport section will be used. Through defined apertures and transversal phase space rotation using focusing solenoids the beam halo as well as unwanted H2+ and H3+ fractions will be cut. Theoretical studies which were carried out so far and a first design of the setup will be presented.
b̄b̄ud tetraquark resonances in the Born-Oppenheimer approximation using lattice QCD potentials
(2019)
We study tetraquark resonances for a pair of static antiquarks b¯b¯ in presence of two light quarks ud based on lattice QCD potentials. The system is treated in the Born-Oppenheimer approximation and we use the emergent wave method. We focus on the isospin I = 0 channel but take different angular momenta l of the heavy antiquarks b¯b¯ into account. Further calculations have already predicted a bound state for the l = 0 case with quantum numbers I(JP) = 0(1+). Performing computations for several angular momenta, we extract the phase shifts and search for T and S matrix poles in the second Riemann sheet. For angular momentum l = 1, we predict a tetraquark resonance with quantum numbers I(JP) = 0(1−), resonance mass m = 10576+4−4 MeV and decay width Γ = 112+90−103 MeV, which decays into two B mesons.
The so-called sign problem of lattice QCD prohibits Monte Carlo simulations at finite baryon
density by means of importance sampling. Over the last few years, methods have been developed
which are able to circumvent this problem as long as the quark chemical potential is m=T <~1.
After a brief review of these methods, their application to a first principles determination of the
QCD phase diagram for small baryon densities is summarised. The location and curvature of the
pseudo-critical line of the quark hardon transition is under control and extrapolations to physical
quark masses and the continuum are feasible in the near future. No definite conclusions can as
yet be drawn regarding the existence of a critical end point, which turns out to be extremely quark
mass and cut-off sensitive. Investigations with different methods on coarse lattices show the lightmass
chiral phase transition to weaken when a chemical potential is switched on. If persisting on
finer lattices, this would imply that there is no chiral critical point or phase transition for physical
QCD. Any critical structure would then be related to physics other than chiral symmetry breaking.
The chiral critical surface is a surface of second order phase transitions bounding the region of
first order chiral phase transitions for small quark masses in the fmu;d;ms;mg parameter space.
The potential critical endpoint of the QCD (T;m)-phase diagram is widely expected to be part of
this surface. Since for m = 0 with physical quark masses QCD is known to exhibit an analytic
crossover, this expectation requires the region of chiral transitions to expand with m for a chiral
critical endpoint to exist. Instead, on coarse Nt = 4 lattices, we find the area of chiral transitions
to shrink with m, which excludes a chiral critical point for QCD at moderate chemical potentials
mB < 500 MeV. First results on finer Nt = 6 lattices indicate a curvature of the critical surface
consistent with zero and unchanged conclusions. We also comment on the interplay of phase
diagrams between the Nf = 2 and Nf = 2+1 theories and its consequences for physical QCD.