Attila Pasztor

Charmonium spectral functions from 2+1 flavour lattice QCD

A bstractFinite temperature charmonium spectral functions in the pseudoscalar and vector channels are studied in lattice QCD with 2+1 flavours of dynamical Wilson quarks, on fine isotropic lattices (with a lattice spacing of 0.057fm), with a non-physical pion mass of mπ ≈ 545 MeV. The highest temperature studied is approximately 1.4Tc. Up to this temperature no significant variation of the spectral function is seen in the pseudoscalar channel. The vector channel shows some temperature dependence, which seems to be consistent with a temperature dependent low frequency peak related to heavy quark transport, plus a temperature independent term at ω > 0. These results are in accord with previous calculations using the quenched approximation.

Constraining the hadronic spectrum through QCD thermodynamics on the lattice

Fluctuations of conserved charges allow to study the chemical composition of hadronic matter. A comparison between lattice simulations and the Hadron Resonance Gas (HRG) model suggested the existence of missing strange resonances. To clarify this issue we calculate the partial pressures of mesons and baryons with different strangeness quantum numbers using lattice simulations in the confined phase of QCD. In order to make this calculation feasible, we perform simulations at imaginary strangeness chemical potentials. We systematically study the effect of different hadronic spectra on thermodynamic observables in the HRG model and compare to lattice QCD results. We show that, for each hadronic sector, the well established states are not enough in order to have agreement with the lattice results. Additional states, either listed in the Particle Data Group booklet (PDG) but not well established, or predicted by the Quark Model (QM), are necessary in order to reproduce the lattice data. For mesons, it appears that the PDG and the quark model do not list enough strange mesons, or that, in this sector, interactions beyond those included in the HRG model are needed to reproduce the lattice QCD results.

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

A bstractWe 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 Nt = 6, 8, 10, 12, 16 lattices using a 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.

A multi-robot exploration algorithm based on a static Bluetooth communication chain

In the first part of the present paper, a novel communication scheme of an autonomous robot team via Bluetooth radio is investigated. In the presented solution, an autonomous unit is equipped with two independent Bluetooth radios and so a relatively fast communication is possible in the team in a static (i.e. no ad hoc) networking topology. The performance of such a network was tested by implementing a linear graph topology by NXT robots. It was found that the reliability and the speed of such a communication scheme are satisfactory and give rise to applications in a robot team control task. In the second part of the paper, an area exploration method is presented based on the static linear communication network above. The method was tested by computer simulations for various obstacle configurations and densities. It was found that the proposed method performs better than the chosen reference methods in the case of zero or low obstacle density and when high (75% or 100%) exploration ratio is required. With a simple proof, we have shown that the proposed (fixed chain-like team) exploration method is optimal in the obstacle-free case under the constraint of the connectivity with the base station.

Compass and odometry based navigation of a mobile robot swarm equipped by bluetooth communication

This study is dealing with how we built up a new chain (line) of communication with the help of some simple “Dual NXT” robots at the Kecskemét College faculty of GAMF at the Department of Information Technology. The first part of this article describes how the communication takes place between members of the robot swarm during the movement and how robots - organized in swarm - can be used in the search simulation. In the second part we present that the odometry method, which we used for the robot movement control, caused too many imprecision because we used magnetic compass sensors for controlling of robots. This simulation can serve as a starting point of a new practical application, where for example the robots do not look for and collect food in a simulated area, but look for mobile phones in a cells divided area and transport text files between two phones via bluetooth.

Repulsive baryonic interactions and lattice QCD observables at imaginary chemical potential

Abstract The first principle lattice QCD methods allow to calculate the thermodynamic observables at finite temperature and imaginary chemical potential. These can be compared to the predictions of various phenomenological models. We argue that Fourier coefficients with respect to imaginary baryochemical potential are sensitive to modeling of baryonic interactions. As a first application of this sensitivity, we consider the hadron resonance gas (HRG) model with repulsive baryonic interactions, which are modeled by means of the excluded volume correction. The Fourier coefficients of the imaginary part of the net-baryon density at imaginary baryochemical potential – corresponding to the fugacity or virial expansion at real chemical potential – are calculated within this model, and compared with the N t = 12 lattice data. The lattice QCD behavior of the first four Fourier coefficients up to T ≃ 185 MeV is described fairly well by an interacting HRG with a single baryon–baryon eigenvolume interaction parameter b ≃ 1 fm 3 , while the available lattice data on the difference χ 2 B − χ 4 B of baryon number susceptibilities is reproduced up to T ≃ 175 MeV .

Connectivity in a wireless network of mobile robots doing a searching and collecting task

This work presents a computer simulation study on the reliability of the wireless connection between a base station and a robot swarm busy in a searching and collecting task. The size of the searched area supposed to be much bigger that the wireless coverage of a robot, so the communication is modeled by a multi-hop ad hoc network. We use the idea of dividing the area into working cells, in which the robots perform an exhaustive search simultaneously. In this work the working cells are arranged in a lattice geometry bounded inside a slice of a disk. It was found that the expected value of the longest time while the communication network is disconnected increases exponentially with the radius of the searched area but does not depend considerably on the spatial density of the searched items. Besides, the robots traveling on the way to or back from the base station contribute remarkably to the connectivity of the multi-hop network.

Spectral functions of charmonium with 2+1 flavours of dynamical quarks

Finite temperature charmonium spectral functions in the pseudoscalar(PS) and vector(V) channels are studied in lattice QCD with 2+1 flavours of dynamical Wilson quarks, on fine isotropic lattices (with a lattice spacing of 0.057fm), with a non-physical pion mass of 545MeV. The highest temperature studied is approximately 1.4Tc. Up to this temperature no significant variation of the spectral function is seen in the PS channel. The V channel shows some temperature dependence, which seems to be consistent with a temperature dependent low frequency peak related to heavy quark transport, plus a temperature independent term at omega > 0. These results are in accord with previous calculations using the quenched approximation.

Recent lattice QCD results at non-zero baryon densities

At zero baryon density lattice QCD is an established tool, that provides precise theoretical results. Calculations at non zero densities however require new techniques to deal with the sign problem. In this talk I will review the recent progress that is made in lattice QCD to investigate QCD at non-vanishing baryon chemical potential.

Phenomenology of Strange Resonances

Fluctuations of conserved charges allow to study the chemical composition of hadronic matter. A comparison between lattice simulations and the Hadron Resonance Gas (HRG) model suggested the existence of missing strange resonances. To clarify this issue we calculate the partial pressures of mesons and baryons with different strangeness quantum numbers using lattice simulations in the confined phase of QCD. In order to make this calculation feasible, we perform simulations at imaginary strangeness chemical potentials. We systematically study the effect of different hadronic spectra on thermodynamic observables in the HRG model and compare to lattice QCD results. We show that, for each hadronic sector, the well established states are not enough in order to have agreement with the lattice results. Additional states, either listed in the Particle Data Group booklet (PDG) but not well established, or predicted by the Quark Model (QM), are necessary in order to reproduce the lattice data. For mesons, it appears that the PDG and the quark model do not list enough strange mesons, or that, in this sector, interactions beyond those included in the HRG model are needed to reproduce the lattice QCD results.