arXiv subjects

Christian Schmidt

Publications and source records attributed to Christian Schmidt.

At least 109 records · Page 6Linked to original sources

arXiv2010

Net-baryon number fluctuations in (2+1)-flavor QCD

We present a lattice study of net-baryon number fluctuations in (2+1)-flavor QCD. The results are based on a Taylor expansion of the pressure with respect to the baryon chemical potential. We calculate higher moments of the net-baryon number fluctuations and compare with the corresponding resonance gas results. We find that for temperature below 0.9T_c the fluctuations seem to agree with the hadron resonance gas predictions. Close to T_c, higher moments are increasingly more sensitive to the critical behavior of the QCD phase transition. Furthermore, we estimate the radius of convergence of the Taylor series as well as the curvature of the transition line in the temperature chemical potential plane.

hep-lat↗

arXiv2009

Development of a GEM-TPC prototype

The use of GEM foils for the amplification stage of a TPC instead of a con- ventional MWPC allows one to bypass the necessity of gating, as the backdrift is suppressed thanks to the asymmetric field configuration. This way, a novel continuously running TPC, which represents one option for the PANDA central tracker, can be realized. A medium sized prototype with a diameter of 300 mm and a length of 600 mm will be tested inside the FOPI spectrometer at GSI using a carbon or lithium beam at intermediate energies (E = 1-3AGeV). This detector test under realistic experimental conditions should allow us to verify the spatial resolution for single tracks and the reconstruction capability for displaced vertexes. A series of physics measurement implying pion beams is scheduled with the FOPI spectrometer together with the GEM-TPC as well.

physics.ins-det↗

arXiv2009

Hadronic fluctuations and correlations at nonzero chemical potential

We present a lattice study of fluctuations and correlations among the conserved charges baryon number and strangeness in (2+1)-flavor QCD. The lattice calculations are based on a Taylor expansion of the pressure. Results are presented at zero and nonzero density on lattices with four and six time slices, corresponding to a lattice spacing of $a\approx 0.25$ fm and $a\approx 0.16$ fm, respectively. The quark masses are almost physical, i.e. the light quark mass has been chosen to one tenth of the physical strange quark mass while the strange quark mass has been set to its physical value. We find that all analyzed fluctuations and correlations develop a peak at nonzero density, when they are treated in an expansion up to sixth order. Especially below the critical temperature ($T_c$) fluctuations and correlations increase drastically, whereas above $T_c$ they are rather unaffected.

hep-lat↗

arXiv2009

The QCD equation of state from improved staggered fermions

We calculate the equation of state in 2+1 flavor QCD at finite temperature with physical strange quark mass and almost physical light quark masses using lattices with temporal extent $N_τ=8$. Calculations have been performed with two different improved staggered fermion actions, the asqtad and p4 actions. Overall, we find good agreement between results obtained with these two $O(a^2)$ improved staggered fermion discretization schemes. A comparison with earlier calculations on coarser lattices is performed to quantify systematic errors in current studies of the equation of state. We also present results for observables that are sensitive to deconfining and chiral aspects of the QCD transition on $N_τ=6$ and 8 lattices. We find that deconfinement and chiral symmetry restoration happen in the same narrow temperature interval.

hep-lat↗

arXiv2008

QCD thermodynamics at zero and non-zero density

We present results on the QCD equation of state, obtained with two different improved dynamical staggered fermion actions and almost physical quark masses. Lattice cut-off effects are discussed in detail as results for three different lattice spacings are available now, i.e. results have been obtained on lattices with temporal extent of $N_τ=4,6$ and 8. Furthermore we discuss the Taylor expansion approach to non-zero baryon chemical potential by means of an expansion of the pressure. We use the expansion coefficients to calculate various fluctuations and correlations among hadronic charges. We find that the correlations reproduce the qualitative behavior of the resonance gas model below $T_c$ and start to agree with the free gas predictions for $T\gsim 1.5T_c$.

hep-lat↗

arXiv2008

Non-zero density QCD by the Taylor expansion method: The isentropic equation of state, hadronic fluctuations and more

We discuss the Taylor expansion approach to non-zero baryon chemical potential ($μ_B$) and present results on expanion coefficients of the pressure and energy density up to the 6-th order in $μ_B$. Calculations have been performed with (2+1)-flavor of improved staggerd fermions (p4fat3) and almost physical masses on lattices with temporal extent of four and six time slices. We use the expansion coefficients to construct the isentropic equation of state on lines of constant entropy per baryon number and various different hadronic fluctuations. Furthermore, we estimate the radius of convegence of our expansion, which can be seen as a method to analyze the structure of the QCD phase diagram.

hep-lat↗

arXiv2008

Calculating Quark Number Susceptibilities with Domain-Wall Fermions

We present results from calculations of different quark number and hadronic susceptibilities on 2+1-flavor dynamical domain wall ensembles. We find that the iso-spin and electric charge susceptibilities are especially well suited to determine the transition temperature, as these quantities show only small statistical errors. Moreover, the transition values of the coupling obtained from iso-spin and electrical charge susceptibilities are in good agreement with the one obtained from the chiral condensate.

hep-lat↗

arXiv2007

Bulk thermodynamics and charge fluctuations at non-vanishing baryon density

We present results on bulk thermodynamic quantities as well as net baryon number, strangeness and electric charge fluctuations in QCD at non-zero density and temperature obtained from lattice calculations with almost physical quark masses for two values of the lattice cut-off $aT=1/4$ and 1/6 . We show that with our improved p4fa3-action the cut-off effects are under control when using lattices with a temporal extent of 6 or larger and that the contribution to the equation of state, which is due to a finite chemical potential is small for $μ_q/T<1$. Moreover, at vanishing chemical potential, i.e. under conditions almost realized at RHIC and the LHC, quartic fluctuations of net baryon number and strangeness are large in a narrow temperature interval characterizing the transition region from the low to high temperature phase. At non-zero baryon number density, strangeness fluctuations are enhanced and correlated to fluctuations of the net baryon number. If strangeness is furthermore forced to vanish, as it may be the case in systems created in heavy ion collisions, strangeness fluctuations are significantly smaller than baryon number fluctuations.

hep-lat↗

arXiv2007

The density of states method at non-zero chemical potential

We study the QCD phase diagram by first principle lattice calculations at so far unreached high densities. For this purpose we employ the density of states method. Unimproved staggered fermions, which describe four quark flavors in the continuum are used in this analysis. Though the method is quite expensive, small lattices show an indication for a triple-point connecting three different phases on the phase diagram.

hep-lat↗

arXiv2007

QCD thermodynamics at zero and non-zero density

We present recent results on thermodynamics of QCD with almost physical light quark masses and a physical strange quark mass value. These calculations have been performed with an improved staggerd action especially designed for finite temperature lattice QCD. In detail we present a calculation of the transition temperature, using a combined chiral and continuum extrapolation. Furthermore we present preliminary results on the interaction measure and energy density at almost realistic quark masses. Finally we disscuss the response of the pressure to a finite quark chemical potential. Within the Taylor expansion formalism we calculate quark number susceptibilities and leading order corrections to finite chemical potential. This is particularly usefull for mapping out the critical region in the QCD phase diagram.

hep-lat↗

arXiv2006

Lattice QCD at Finite Density

I discuss different approaches to finite density lattice QCD. In particular, I focus on the structure of the phase diagram and discuss attempts to determine the location of the critical end-point. Recent results on the transiton line as function of the chemical potential ($T_c(μ_q)$) are reviewed. Along the transition line, hadronic fluctuations have been calculated, which can be used to characterize properties of the Quark Gluon plasma and eventually can also help to identify the location of the critical end-point in the QCD phase diagram on the lattice and in heavy ion experiments. Furthermore, I comment on the structure of the phase diagram at large $μ_q$.

hep-lat↗

arXiv2006

QCD Thermodynamics with Almost Realistic Quark Masses

Ongoing calculations on the QCDOC supercomputer at Brookhaven National Laboratory and the APEnext installation at the University of Bielefeld aim to determine the critical temperature of the QCD phase transition as well as the equation of state with almost realistic quark masses. We will discuss preliminary results of the quark mass and cut-off dependence of order parameters, susceptibilities, static quark potentials and the critical temperature in (2+1)-flavor QCD. All these quantities are of immediate interest for heavy ion phenomenology.

hep-lat↗

arXiv2005

The QCD phase diagram at finite density

We study the density of states method to explore the phase diagram of the chiral transition on the tempeature and quark chemical potential plane. Four quark flavours are used in the analysis. Though the method is quite expensive small lattices show an indication for a triple-point connecting three different phases on the phase diagram.

hep-lat↗

arXiv2004

Evolution of Electronic and Vibrational Polarity of NaF Nanocrystals from Diatomic to Bulk: A Density Functional Study

Density functional theory (DFT) is used to study vibrations, electrical dipole moments, and polarizabilities of NaF clusters. Because of prior experimental and theoretical studies, this is a good model system for tracking the evolution of the properties from diatomic molecule to bulk crystal. The ratio of vibrational to electronic contributions to the polarizability increases dramatically with size N in the closed shell clusters (NaF)_N. The open shell system Na_14F_13 has a greatly enhanced electronic polarizability. Contrary to previous studies on this system which treated only the outer electron by quantum mechanics, we find the O_h cubic structure to be stable relative to the polar distorted structures such as C_3v.

cond-mat.other↗

arXiv2004

Density-Functional-Based Determination of Vibrational Polarizabilities in Molecules

We develop a direct derivation for the primary contribution to the vibrational polarizability for molecules, clusters and other finite systems. The vibrational polarizability is then calculated within the generalized gradient approximation to the density functional theory for a variety of molecules and clusters. The agreement between theory and experiment is quite good. The results show that for small ionic molecules and clusters, inclusion of the vibrational polarizability is necessary to achieve quantitative accuracy.

cond-mat.other↗

arXiv2004

Chiral perturbation theory for twisted mass QCD

Quantum Chromodynamics on a lattice with Wilson fermions and a chirally twisted mass term for two degenerate quark flavours is considered in the framework of chiral perturbation theory. The pion masses and decay constants are calculated in next-to-leading order including terms linear in the lattice spacing a. We treat both unquenched and partially quenched QCD. We also discuss the phase structure of twisted mass lattice QCD.

hep-lat↗

arXiv2004

Chiral perturbation theory for partially quenched twisted mass lattice QCD

Partially quenched Quantum Chromodynamics with Wilson fermions on a lattice is considered in the framework of chiral perturbation theory. Two degenerate quark flavours are associated with a chirally twisted mass term. The pion masses and decay constants are calculated in next-to-leading order including terms linear in the lattice spacing $a$.

hep-lat↗

arXiv2004

Chiral perturbation theory for lattice QCD with a twisted mass term

Quantum Chromodynamics on a lattice with Wilson fermions and a chirally twisted mass term for two degenerate quark flavours is considered in the framework of chiral perturbation theory. The pion mass and decay constant are calculated in next-to-leading order including terms linear in the lattice spacing $a$.

hep-lat↗