Kvantumszíndinamika téridő rácson dinamikus fermionokkal = Lattice quantum chromodynamics with dynamical fermions

2002 és 2005 között kidolgoztuk a véges kémiai potenciál mellett alkalmazható átfedést javító multiparaméteres átsúlyozás módszerét, melyet dinamikus kvarkszimulációkra alkalmaztunk. Meghatároztuk nemeltűnő barionsűrűség mellett a fázisdiagrammot és az állapotegyenletet. Vizsgáltuk zérus kémiai potenciálon az állapotegyenlet kontinuum limesz extrapolációját. A számoláshoz javított hatást használtunk dinamikus kvarkokkal. Meghatároztuk az öt kvarkot tartalmazó állapotok spektrumát. Az általunk vizsgált kvarktömegek és rácsállandó mellett kizártuk a theta pentakvark létezését. Vizsgáltuk a Z-robbanás mechanizmusát. Jóslatot adtunk erősen kölcsönható neutrínó forgatókönyvre. Meghatároztuk a kozmogenikus neutrínó fluxus alsó és a neutrínó-nukleon hatáskeresztmetszet felső korlátját. | Between 2002 and 2005 we introduced the overlap improving multi-parameter reweighting technique. This method is suitable to study non-vanishing chemical potentials. We applied the technique for dynamical quark simulations. For non-vanishing baryonic densities we determined the phase diagram and the equation of state. We studied the continuum extrapolation of the equation of state for vanishing chemical potentials. We used improved action with dynamical quarks. We determined the spectrum of five-quark states. For our quark masses and lattice spacing we excluded the existence of the theta pentaquark. We studied the Z-burst mechanism. We predicted the consequences of a strongly interacting neutrino scenario. We determined the lower bound for the cosmogenic neutrino flux and the upper bound for the neutrino-nucleon cross section

Static QˉQ\bar{Q}Q pair free energy and screening masses from correlators of Polyakov loops: continuum extrapolated lattice results at the QCD physical point

We study the correlators of Polyakov loops, and the corresponding gauge invariant free energy of a static quark-antiquark pair in 2+1 flavor QCD at finite temperature. Our simulations were carried out on $N_t$ = 6, 8, 10, 12, 16 lattices using Symanzik improved gauge action and a stout improved staggered action with physical quark masses. The free energies calculated from the Polyakov loop correlators are extrapolated to the continuum limit. For the free energies we use a two step renormalization procedure that only uses data at finite temperature. We also measure correlators with definite Euclidean time reversal and charge conjugation symmetry to extract two different screening masses, one in the magnetic, and one in the electric sector, to distinguish two different correlation lengths in the full Polyakov loop correlator

QCD finite T transition -- Comparison between Wilson and staggered results

A quantitative comparison between the finite temperature behaviour of the staggered and Wilson fermion formulations are performed. The comparison is based on a physical quantity that is expected to be quite sensitive to the fermionic features of the action. For that purpose we use the height of the peak for $d\chi_s/dT$, where $\chi_s$ is the quark number susceptibility.Comment: 6 pages. Talk presented at Lattice 200

New approach to lattice QCD at finite density; results for the critical end point on coarse lattices

All approaches currently used to study finite baryon density lattice QCD suffer from uncontrolled systematic uncertainties in addition to the well-known sign problem. We formulate and test an algorithm, sign reweighting, that works directly at finite μ = μ_{B}B/3 and is yet free from any such uncontrolled systematics. With this algorithm the only problem is the sign problem itself. This approach involves the generation of configurations with the positive fermionic weight |Re det D(μ)| where D(μ) is the Dirac matrix and the signs sign(Re det D(μ)) = ±1 are handled by a discrete reweighting. Hence there are only two sectors, +1 and −1 and as long as the average 〈±1〉 ≠ 0 (with respect to the positive weight) this discrete reweighting by the signs carries no overlap problem and the results are reliable. The approach is tested on N_{t}​ = 4 lattices with 2 + 1 flavors and physical quark masses using the unimproved staggered discretization. By measuring the Fisher (sometimes also called Lee-Yang) zeros in the bare coupling on spatial lattices L/a = 8, 10, 12 we conclude that the cross-over present at μ = 0 becomes stronger at μ > 0 and is consistent with a true phase transition at around μ_{B}​ /T ∼ 2.4

Curvature of the phase transition line in the mu-T plane

We determined the curvature of the phase transition line in the mu-T plane using a Taylor expansion in mu. The Polyakov loop and the strange quark number susceptibility were measured to locate the pseudocritical line. The analysis was carried out on Nt=4,6,8,10 lattices generated with a Symanzik improved gauge and stout-link improved (2+1) flavour staggered fermion action using physical quark masses.Comment: 7 pages, 5 figures; format adjuste

Effect of stout smearing on the phase diagram from multiparameter reweighting in lattice QCD

The phase diagram and the location of the critical endpoint (CEP) of lattice QCD with unimproved staggered fermions on a Nt=4 lattice was determined fifteen years ago with the multiparameter reweighting method by studying Fisher zeros. We first reproduce the old result with an exact algorithm (not known at the time) and with statistics larger by an order of magnitude. As an extension of the old analysis we introduce stout smearing in the fermion action in order to reduce the finite lattice spacing effects. First we show that increasing the smearing parameter ρ the crossover at μ=0 gets weaker, i.e., the leading Fisher zero gets farther away from the real axis. Furthermore as the chemical potential is increased the overlap problem gets severe sooner than in the unimproved case, therefore shrinking the range of applicability of the method. Nevertheless certain qualitative features remain, even after introducing the smearing. Namely, at small chemical potentials the Fisher zeros first get farther away from the real axis and later at around aμ_q=0.1–0.15 they start to get closer, i.e., the crossover first gets weaker and later stronger as a function of μ. However, because of the more severe overlap problem the CEP is out of reach with the smeared action

A rácselmélet részecskefizikai alkalmazásai = Particle physics applications at lattice field theory

Rácstérelméleti módszerekkel vizsgáltuk a kvantumszíndinamikát véges és zérus hőmérsékleten és kémiai potenciálon. Terjedelmi okokból a legfontosabb eredményeket tekintjük csak át az összefoglalóban. Nulla kémiai potenciál mellett meghatároztuk a hadronikus anyag és a kvark-gluon plazma közti átmenet rendjét, valamint az átmeneti homérsékletet a kontinuum limeszben. Ezen eredmények a rácstérelmélet végleges eredményeinek tekinthetők. Összehasonlítottuk a Wilson és staggered fermionokat 3 íz használatával. Nem-nulla kémiai potenciálnál meghatároztuk a kvark-antikvark potenciált valamint a fázisdiagramot nagy kémiai potenciálokra az állapotsűrűség módszer segítségével. A pion szektort megvizsgáltuk kanonikus sokaság segítségével. Meghatároztuk a mu-T fázisdiagramot kis kémiai potenciálokra 4 különböző rácsállandónál. Rácsszámolásokkal megvizsgáltuk a pentakvarkok létezését és nem találtunk erre utaló jelet. Kiszámoltuk a hadron spektrumot Wilson fermionokkal a kontinuum limeszben. Meghatároztuk a pion és kaon leptonikus bomlási állandóinak arányát. Implementáltuk a QCD kódjainkat grafikus kártyákra. Overlap fermionokat használó korábbi dinamikus algoritmusunkat továbbfejlesztettük. | We have studied quantum chromodynamics at finite and zero temperature and chemical potential using lattice field theory methods. Due to space limitations we present here only the most important results. We have determined at zero chemical potential the order of the transition from hadronic matter to the quark-gluon plasma as well as the transition temperature, both in the continuum limit. These results should be considered as final results of lattice field theory. We have compared Wilson and staggered fermions for 3 flavours. We have determined the quark-antiquark potential as well as the phase diagram for large non-zero chemical potentials using the density of states method. We have considered the pion sector using canonical ensembles. We have determined the mu-T phase diagram for small chemical potentials at 4 different lattice spacings. Using lattice simulations we have considered the problem of existence of pentaquarks and found no evidence for them. We have calculated the hadronic spectrum in the continuum limit using Wilson fermions. We have determined the ratio of the pion and kaon leptonic decay constants. We have implemented our QCD computer codes for the graphical processor cards (GPU). Our dynamical algorithms using overlap fermions have also been extended