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F. Karsch

Publications and source records attributed to F. Karsch.

At least 163 records · Page 9Linked to original sources
arXiv1998

SU(3) Gauge Theory with Adjoint Fermions

We analyze the finite temperature phase diagram of QCD with fermions in the adjoint representation. The simulations performed with four dynamical Majorana fermions, which is equivalent to two Dirac fermions, show that the deconfinement and chiral phase transitions occur at two distinct temperatures, $T_{\rm chiral} \simeq 6.65 T_{\rm deconf}$. While the deconfinement transition is first order we find evidence for a continuous chiral transition. We also present potentials for $T<T_{\rm deconf}$ and $T_{\rm deconf}<T< T_{\rm chiral}$ both for fundamental and adjoint fermion-antifermion pairs.

hep-lat↗
arXiv1998

Finite Temperature Phase Diagram of QCD with improved Wilson Fermions

We present results of an ongoing study of two flavour QCD with Wilson fermions at finite temperature. We have used tree level Symanzik improvement in both the gauge and fermion part of the action. The phase diagram was previously determined on an $8^3 \times 4$ lattice and the existence of Aoki's phase demonstrated. On our current lattice of $12^2 \times 24 \times 4$ we have extended the set of observables and studied chiral and thermal properties at light quark masses.

hep-lat↗
arXiv1998

Constituent Quarks, Diquarks and the N-Delta Mass Splitting

We analyze hadron as well as quark and diquark correlation functions in Landau gauge in order to extract information on the spin dependence of the quark-quark interaction. We find evidence that the N-Delta mass splitting can be attributed to the spin dependence of the interaction between quarks in a colour anti-triplet state with spin 0 and 1, respectively. The lightest excitations are observed in the S=0 channel. However, no evidence for a deeply bound diquark state is found.

hep-lat↗
arXiv1998

Diquark Masses from Lattice QCD

We present first results for diquark correlation functions calculated in Landau gauge on the lattice. Masses have been extracted from the long distance behaviour of these correlation functions. We find that the ordering of diquark masses with spin 0 and 1 states in colour anti-triplet and sextet channels is in accordance with instanton motivated interaction models. Although we find evidence for an attractive interaction in colour anti-triplet states with a splitting between spin 0 and spin 1 diquarks that can account for the mass splitting between the nucleon and the delta, there is no evidence for a deeply bound diquark state.

hep-lat↗
arXiv1998

String Tension and Thermodynamics with Tree Level and Tadpole Improved Actions

We calculate the string tension, deconfinement transition temperature and bulk thermodynamic quantities of the SU(3) gauge theory using tree level and tadpole improved actions. Finite temperature calculations have been performed on lattices with temporal extent N_tau = 3 and 4. Compared to calculations with the standard Wilson action on this size lattices we observe a drastic reduction of the cut-off dependence of bulk thermodynamic observables at high temperatures. In order to test the influence of improvement on long-distance observables at T_c we determine the ratio T_c/sqrt(sigma). For all actions, including the standard Wilson action, we find results which differ only little from each other. We do, however, observe an improved asymptotic scaling behaviour for the tadpole improved action compared to the Wilson and tree level improved actions.

hep-lat↗
arXiv1997

The Gluon Propagator at High Temperature: Screening, Improvement and Non-Zero Momenta

We study the gluon propagator and the singlet potential in Landau gauge in the deconfined phase of SU(2) lattice gauge theory, using both the standard Wilson action and a tree-level Symanzik improved action. From the long-distance behavior of correlation functions of temporal and spatial components of the gauge fields we extract electric (m_e) and magnetic (m_m) screening masses. For the magnetic mass we find m_m(T) = 0.456(6) g^2(T) T. The electric mass can be described by a next-to leading order ansatz, obtained from one loop resummed perturbation theory. However, the best description is given by m_e(T) = \sqrt{1.70(2)} g(T) T. The electric screening mass thus is different from its lowest order perturbative prediction even for temperatures as high as T \sim 10^4 T_c.

hep-lat↗
arXiv1997

Staggered Fermion Actions with Improved Rotational Invariance

We introduce a class of improved actions for staggered fermions which to O(p^4) and O(p^6), respectively, lead to rotationally invariant propagators. We discuss the resulting reduction of flavour symmetry breaking in the meson spectrum and comment on the improvement in the calculation of thermodynamic observables.

hep-lat↗
arXiv1997

Finite Temperature Phase Diagramm of QCD with improved Wilson fermions

We present first results of a study of two flavour QCD with Wilson fermions at finite temperature. We have used tree level Symanzik improvement in both the gauge and fermion part of the action. In a first step we explore the phase diagramm on an $8^3 \times 4$ lattice, with particular emphasis on checking Aoki's conjecture with an improved action.

hep-lat↗
arXiv1997

Screening Lengths in SU(2) Gauge Theory at Finite Temperature

We discuss the calculation of electric and magnetic screening masses in SU(2) gauge theory. The temperature dependence of these masses obtained from the long-distance behaviour of spatial correlation functions has been analyzed for temperatures up to 10000 T_c.

hep-lat↗
arXiv1997

Screened Perturbation Theory

A new perturbative scheme is proposed for the evaluation of the free energy density of field theories at finite temperature. The screened loop expansion takes into account exactly the phenomenon of screening in thermal propagators.The approach is tested in the N-component scalar field theory at 2-loop level and also at 3-loop in the large $N$ limit. The perturbative series generated by the screened loop expansion shows much better numerical convergence than previous expansions generated in powers of the quartic coupling.

hep-ph↗
arXiv1996

Thermodynamics of Four-Flavour QCD with Improved Staggered Fermions

We have calculated the pressure and energy density in four-flavour QCD using improved fermion and gauge actions. We observe a strong reduction of finite cut-off effects in the high temperature regime, similar to what has been noted before for the SU(3) gauge theory. Calculations have been performed on $16^3\times 4$ and 16^4 lattices for two values of the quark mass, $ma = 0.05$ and 0.1. A calculation of the string tension at zero temperature yields a critical temperature $T_c/\sqrtσ = 0.407 \pm 0.010$ for the smaller quark mass value.

hep-lat↗
arXiv1996

SU(3) Latent Heat and Surface Tension from Tree Level and Tadpole Improved Actions

We analyze the latent heat and surface tension at the SU(3) deconfinement phase transition with tree level and tadpole improved Symanzik actions on lattices with temporal extent $N_τ= 3$ and 4 and spatial extent $N_σ/ N_τ= 4$, 6 and 8. In comparison to the standard Wilson action we do find a drastic reduction of cut-off effects already with tree level improved actions. On lattices with temporal extent $N_τ=4$ results for the surface tension and latent heat obtained with a tree level improved action agree well with those obtained with a tadpole improved action. A comparison with $N_τ=3$ calculations, however, shows that results obtained with tadpole action remain unaffected by cut-off effects even on this coarse lattice, while the tree level action becomes sensitive to the cut-off. For the surface tension and latent heat we find $σ_I/ T_c^3 = 0.0155~(16)$ and $Δε/T_c^4 = 1.40~(9)$, respectively.

hep-lat↗
arXiv1996

The Non-Perturbative ${\cal O}(g^6)$ Contribution to the Free Energy of Hot SU(N) Gauge Theory

The non-perturbative input necessary for the determination of the ${\cal O}(g^6)$ part of the weak coupling expansion of the free energy density for SU(2) and SU(3) gauge theories is estimated. Although the perturbative information completing the contribution to this order is missing, we give arguments that the magnetic fluctuations are dominated by screened elementary magnetic gluons.

hep-lat↗
arXiv1996

A Study of Finite Temperature Gauge Theory in (2+1) Dimensions

We determine the critical couplings and the critical exponents of the finite temperature transition in SU(2) and SU(3) pure gauge theory in (2+1) dimensions. We also measure Wilson loops at $T=0$ on a wide range of $β$ values using APE smearing to improve the signal. We extract the string tension $σ$ from a fit to large distances, including a string fluctuation term. With these two entities we calculate $T_c/\sqrtσ$.

hep-lat↗
arXiv1996

Critical Higgs Mass and Temperature Dependence of Gauge Boson Masses in the SU(2) Gauge-Higgs Model

We study the effective 3-D SU(2) Gauge-Higgs model at finite temperature for Higgs-masses in the range from $60$ GeV up to $100$ GeV. The first order electroweak phase transition weakens with increasing Higgs-mass and terminates at a critical end-point. For Higgs-mass values larger than about $m_{H,c}=75.4(6)$ GeV the thermodynamic signature of the transition is described by a crossover. Close to this Higgs-mass value we investigate the vector boson propagator in Landau gauge. The calculated W-boson screening masses are compared with predictions based on gap equations.

hep-lat↗
arXiv1996

QCD Thermodynamics with Improved Actions

The thermodynamics of the SU(3) gauge theory has been analyzed with tree level and tadpole improved Symanzik actions. A comparison with the continuum extrapolated results for the standard Wilson action shows that improved actions lead to a drastic reduction of finite cut-off effects already on lattices with temporal extent $N_τ=4$. Results for the pressure, the critical temperature, surface tension and latent heat are presented. First results for the thermodynamics of four-flavour QCD with an improved staggered action are also presented. They indicate similarly large improvement factors for bulk thermodynamics.

hep-lat↗