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E. Laermann

Publications and source records attributed to E. Laermann.

At least 55 records · Page 3Linked to original sources
arXiv2005

Thermodynamics of Two Flavor QCD to Sixth Order in Quark Chemical Potential

We present results of a simulation of 2-flavor QCD on a 4x16^3 lattice using p4-improved staggered fermions with bare quark mass m/T=0.4. Derivatives of the thermodynamic grand canonical partition function Z(V,T,mu_u,mu_d) with respect to chemical potentials mu_(u,d) for different quark flavors are calculated up to sixth order, enabling estimates of the pressure and the quark number density as well as the chiral condensate and various susceptibilities as functions of mu_q = (mu_u + mu_d)/2 via Taylor series expansion. Furthermore, we analyze baryon as well as isospin fluctuations and discuss the relation between the radius of convergence of the Taylor series and the chiral critical point in the QCD phase diagram. We argue that bulk thermodynamic observables do not, at present, provide direct evidence for the existence of a chiral critical point in the QCD phase diagram. Results are compared to high temperature perturbation theory as well as a hadron resonance gas model.

hep-lat↗
arXiv2004

The pion form factor on the lattice at zero and finite temperature

We calculate the electromagnetic form factor of the pion in quenched lattice QCD. The non-perturbatively improved Sheikoleslami-Wohlert lattice action is used together with the consistently O(a) improved current. We calculate the pion form factor for masses down to m_pi = 360 MeV, extract the charge radius, and extrapolate toward the physical pion mass. In the second part, we discuss results for the pion form factor and charge radius at 0.93 T_c and compare with zero temperature results.

hep-lat↗
arXiv2004

Fluctuations in the vicinity of the phase transition line for two flavor QCD

We study the susceptibilities of quark number, isospin number and electric charge in numerical simulations of lattice QCD at high temperature and density. We discuss the equation of state for 2 flavor QCD at non-zero temperature and density. Derivatives of $\ln Z$ with respect to quark chemical potential $(μ_q)$ are calculated up to sixth order. From this Taylor series, the susceptibilities are estimated as functions of temperature and $μ_q$. Moreover, we comment on the hadron resonance gas model, which explains well our simulation results below $T_c$.

hep-lat↗
arXiv2004

Numerical study of the equation of state for two flavor QCD at non-zero baryon density

We discuss the equation of state (EoS) for two flavor QCD at non-zero temperature and density. Derivatives of $\ln Z$ with respect to quark chemical potential $μ_q$ are calculated up to sixth order. From this Taylor series, the pressure, quark number density and associated susceptibilities are estimated as functions of temperature and $μ_q$. It is found that the fluctuations in the quark number density increase in the vicinity of the phase transition temperature and the susceptibilities start to develop a pronounced peak as $μ_q$ is increased. This suggests the presence of a critical endpoint in the $(μ_q, T)$ plane. Moreover, we comment on the hadron resonance gas model, which explains well our simulation results below $T_c$.

hep-lat↗
arXiv2004

Pion structure from improved lattice QCD: form factor and charge radius at low masses

The charge form factor of the pion is calculated in lattice QCD. The non-perturbatively improved Sheikholeslami-Wohlert action is used together with the $\mathcal{O}(a)$ improved vector current. Other choices for the current are examined. The form factor is extracted for pion masses from 970 MeV down to 360 MeV and for momentum transfers $Q^2 \leq 2 \mathrm{GeV}^2$. The mean square charge radius is extracted, compared to previous determinations and its extrapolation to lower masses discussed.

hep-lat↗
arXiv2003

Numerical study of the equation of state for two flavor QCD at finite density

We discuss the equation of state for 2 flavor QCD at non-zero temperature and density. Derivatives of $\ln Z$ with respect to quark chemical potential $μ_q$ up to fourth order are calculated, enabling estimates of the pressure, quark number density and associated susceptibilities as functions of $μ_q$ via a Taylor series expansion. It is found that the fluctuations in the quark number density increase in the vicinity of the phase transition temperature and the susceptibilities start to develop a pronounced peak as $μ_q$ is increased. This suggests the presence of a critical endpoint in the $(T, μ_q)$ plane.

hep-lat↗
arXiv2003

Free Meson Spectral Functions on the Lattice

We present results from an analytic calculation of thermal meson spectral functions in the infinite temperature (free field) limit. We compare spectral functions for various lattice fermion formulations used at present in studies of in-medium properties of hadrons based on the maximum entropy method (MEM). In particular, we will present a new calculation of spectral functions performed with extended quark sources.

hep-lat↗
arXiv2003

Where is the chiral critical point in 3-flavor QCD?

We determine the location of the second order endpoint of the line of first order chiral phase transition in 3-flavor QCD at vanishing chemical potential. Using Ferrenberg-Swendsen reweighting for two values of the quark mass we determine the dependence of the transition line on the chemical potential and locate the chiral critical point. For both quantities we find a significant quark mass dependence.

hep-lat↗
arXiv2003

The equation of state for two flavor QCD at finite density

We discuss the equation of state for QCD at non-zero temperature and density. We present results of a simulation for QCD with 2 flavors of p4-improved staggered fermions. Derivatives of $\ln {\cal Z}$ with respect to quark chemical potential $μ_q$ up to fourth order are calculated, enabling estimates of the pressure, quark number density and associated susceptibilities as functions of $μ_q$ via a Taylor series expansion. We also discuss the radius of convergence of the expansion as a function of temperature. It is found that the fluctuations in the quark number density increase in the vicinity of the phase transition temperature and the susceptibilities start to develop a pronounced peak as $μ_q$ is increased. This suggests the presence of a critical endpoint in the $(T, μ_q)$ plane.

hep-lat↗
arXiv2003

Thermodynamics and in-medium hadron properties from lattice QCD

Non-perturbative studies of the thermodynamics of strongly interacting elementary particles within the context of lattice regularized QCD are being reviewed. After a short introduction into thermal QCD on the lattice we report on the present status of investigations of bulk properties. In particular, we discuss the present knowledge of the phase diagram including recent developments of QCD at non-zero baryon number density. We continue with the results obtained so far for the transition temperature as well as the temperature dependence of energy and pressure and comment on screening and the heavy quark free energies. A major section is devoted to the discussion of thermal modifications of hadron properties, taking special account of recent progress through the use of the maximum entropy method.

hep-lat↗
arXiv2003

The Equation of State for Two Flavor QCD at Non-zero Chemical Potential

We present results of a simulation of QCD on a 4x16^3 lattice with 2 continuum flavors of p4-improved staggered fermion with mass m/T=0.4. Derivatives of the thermodynamic grand potential with respect to quark chemical potential mu_q up to fourth order are calculated, enabling estimates of the pressure, quark number density and associated susceptibilities as functions of mu_q via Taylor series expansion. Discretisation effects associated with various staggered fermion formulations are discussed in some detail. In addition it is possible to estimate the radius of convergence of the expansion as a function of temperature. We also discuss the calculation of energy and entropy densities which are defined via mixed derivatives of the thermodynamic grand potential with respect to the bare couplings and quark masses.

hep-lat↗
arXiv2003

Infinite temperature limit of meson spectral functions calculated on the lattice

We analyze the cut-off dependence of mesonic spectral functions calculated at finite temperature on Euclidean lattices with finite temporal extent. In the infinite temperature limit we present analytic results for lattice spectral functions calculated with standard Wilson fermions as well as a truncated perfect action. We explicitly determine the influence of `Wilson doublers' on the high momentum structure of the mesonic spectral functions and show that this cut-off effect is strongly suppressed when using an improved fermion action.

hep-lat↗
arXiv2003

The pion form factor in improved lattice QCD

We calculate the electromagnetic form factor of the pion in lattice gauge theory. The non-perturbatively improved Sheikoleslami-Wohlert lattice action is used together with the O(a) improved current. The form factor is compared to results for other choices for the current and features of the structure of the pion deduced from the 'Bethe-Salpeter wave function' are discussed.

hep-lat↗
arXiv2003

The Status of Lattice QCD at Finite Temperature

The status of lattice QCD investigations at high temperature is reviewed. After a short introduction into thermal QCD on the lattice we report on the present understanding of the phase diagram and the equation of state, in particular in presence of dynamical quarks. We continue with a discussion of various screening lengths in the plasma phase including results from dimensionally reduced QCD. This is followed by summarizing lattice data on quark number susceptibilities and spectral densities, both of which are of immediate relevance to the interpretation of heavy ion experiments. A major section is devoted to presenting simulations of QCD at small yet phenomenologically important values for the baryon density.

hep-ph↗
arXiv2002

The quark mass and $μ$ dependence of the QCD chiral critical point

In order to study the QCD chiral critical point we investigate Binder Cumulants of the chiral condensate. The results were obtained from simulations of 3 and 2+1 flavors of standard staggered fermions and 3 flavors of p4 improved staggered fermions. The quark masses used are close to the physical quark mass. To extract the dependence on quark mass and chemical potential we apply a new reweighting technique based on a Taylor expansion of the action. The reweighting accuracy is ${\cal O}(m)$ for the standard and ${\cal O}(m^2)$, ${\cal O}(μ^2)$ for the p4 action.

hep-lat↗
arXiv2002

The QCD phase transition at high temperature and low density

We study the thermal properties of QCD in the presence of a small quark chemical potential $μ$. Derivatives of the phase transition point with respect to $μ$ are computed at $μ=0$ for 2 and 3 flavors of p-4 improved staggered fermions on a $16^3\times4$ lattice. Moreover we contrast the case of isoscalar and isovector chemical potentials, quantify the effect of $μ\not=0$ on the equation of state, and comment on the screening effect by dynamical quarks and the complex phase of the fermion determinant in QCD with $μ\not=0$.

hep-lat↗
arXiv2002

The QCD thermal phase transition in the presence of a small chemical potential

We propose a new method to investigate the thermal properties of QCD with a small quark chemical potential $μ$. Derivatives of the phase transition point with respect to $μ$ are computed at $μ=0$ for 2 flavors of p-4 improved staggered fermions with $ma=0.1,0.2$ on a $16^3\times4$ lattice. The resulting Taylor expansion is well behaved for the small values of $μ_{\rm q}/T_c\sim0.1$ relevant for RHIC phenomenology, and predicts a critical curve $T_c(μ)$ in reasonable agreement with estimates obtained using exact reweighting. In addition, we contrast the case of isoscalar and isovector chemical potentials, quantify the effect of $μ\not=0$ on the equation of state, and comment on the complex phase of the fermion determinant in QCD with $μ\not=0$.

hep-lat↗