arXiv subjects

O. Philipsen

Publications and source records attributed to O. Philipsen.

At least 37 records · Page 2Linked to original sources

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.

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arXiv2003

QCD phase diagram at small densities from simulations with imaginary mu

We review our results for the QCD phase diagram at baryonic chemical potential mu_B \leq pi T. Our simulations are performed with an imaginary chemical potential mu_I for which the fermion determinant is positive. For 2 flavors of staggered quarks, we map out the phase diagram and identify the pseudo-critical temperature T_c(mu_I). For mu_I/T \leq pi/3, this is an analytic function, whose Taylor expansion is found to converge rapidly, with truncation errors far smaller than statistical ones. The truncated series may then be continued to real mu, yielding the corresponding phase diagram for mu_B \lsim 500 MeV. This approach provides control over systematics and avoids reweighting. We outline our strategy to find the (2+1)-flavor critical point.

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arXiv2003

What mediates the longest correlation length in the QCD plasma?

When thermal QCD crosses the critical temperature from below, its pressure density rises drastically, consistent with the picture of deconfinement and the release of partons as light degrees of freedom. On the other hand, the concept of partons is a perturbative one, whereas interactions with the infrared modes in the plasma always introduce non-perturbative contributions. Here I show how partonic correlators can be defined in a gauge invariant and non-perturbative manner which applies to all energy scales. In particular, I compute the magnetic mass for hot SU(2) gauge theory and find $m_A=0.36(2)g^2T$, whose inverse for large $T$ is the largest correlation length in the system.

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arXiv2002

QCD phase diagram for small densities from simulations at imaginary mu

We present results on the QCD phase diagram for small densities without reweighting. Our simulations are performed with an imaginary chemical potential mu_I for which the fermion determinant is positive. On an 8^3x4 lattice with 2 flavors of staggered quarks, we map out the pseudo-critical temperature T_c(mu_I). For mu_I/T < pi/3, this is an analytic function whose Taylor expansion converges rapidly, with truncation errors smaller than statistical ones. The result is analytically continued to give the location of the pseudo-critical line for real mu_B<500 MeV.

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arXiv2002

The QCD phase diagram for small densities from imaginary chemical potential

We present results on the QCD phase diagram for mu_B <= pi T. Our simulations are performed with an imaginary chemical potential mu_I for which the fermion determinant is positive. On an 8^3 x 4 lattice with 2 flavors of staggered quarks, we map out the phase diagram and identify the pseudo-critical temperature T_c(mu_I). For mu_I/T <= pi/3, this is an analytic function, whose Taylor expansion is found to converge rapidly, with truncation errors far smaller than statistical ones. The truncated series may then be continued to real mu, yielding the corresponding phase diagram for mu_B <~ 500 MeV. This approach provides control over systematics and avoids reweighting. We compare it with other recent work.

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arXiv2002

Non-perturbative formulation of the static color octet potential

By dressing Polyakov lines with appropriate functionals of the gauge fields, we construct observables describing a fundamental representation static quark-antiquark pair in the singlet, adjoint and average channels of SU(N) pure gauge theory. Each of the potentials represents a gauge invariant eigenvalue of the Hamiltonian. Numerical simulations are performed for SU(2) in 2+1 dimensions. The adjoint channel is found to be repulsive at small and confining at large separations, suggesting the existence of a metastable (N^2-1)-plet bound state. For small distances and temperatures above the deconfinement transition, the leading order perturbative prediction for the ratio of singlet and adjoint potentials is reproduced by the lattice data.

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arXiv2002

On the non-perturbative gluon mass and heavy quark physics

We study a recently proposed gauge invariant, non-local pure gauge observable, which is equivalent to the gluon propagator in a certain gauge. The correlator describes a gluon coupled to static sources and decays with eigenvalues of the Hamiltonian, permitting a non-perturbative definition of a gluonic parton mass. Detailed numerical tests of the observable are performed in SU(2) gauge theories in 2+1 dimensions. In a Higgs model it reproduces the physical W-boson mass, while in a confining theory its non-local nature results in almost exclusive projection onto torelonic states. However, the gluon mass can also be related to the mass difference between the lowest gluelumps, i.e. gluon configurations bound to adjoint sources. Its value is computed in SU(2) pure gauge theory in 2+1 dimensions to be m_A=0.360(19)g^2, which plays an important role as "magnetic mass" of the four-dimensional theory at high temperatures. The same quantity is found to determine the mass splitting between vector and scalar mesons in the three-dimensional theory. In SU(3) pure gauge theory in 3+1 dimensions the corresponding gluelump splitting yields m_A\approx 370 MeV. Possible relations of this quantity with the QCD heavy meson spectrum are discussed.

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arXiv2001

Non-perturbative parton mass for the gluon

A gauge invariant, non-local observable is constructed in pure gauge theory, which is identical to the gluon propagator in a particular gauge, permitting to define a non-perturbative parton mass for the gluon. This mass can be shown to be related to the 1P-1S mass splitting of heavy quarkonia. Preliminary numerical results for 3d SU(2) yield m_A=0.37(6)g^2, while from the \bar{b}b spectrum one infers m_A\approx 420 MeV for QCD.

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arXiv2001

QCD thermodynamics at finite density

Recent progress in extending finite temperature lattice QCD simulations from the $T$ axis into the finite density plane is reviewed. The covered topics are a calculation of the transition line by multidimensional reweighting, screening mass calculations in dimensionally reduced QCD and simulations at imaginary $μ$.

hep-ph↗

arXiv2001

Testing imaginary vs. real chemical potential in finite-temperature QCD

One suggestion for determining the properties of QCD at finite temperatures and densities is to carry out lattice simulations with an imaginary chemical potential whereby no sign problem arises, and to convert the results to real physical observables only afterwards. We test the practical feasibility of such an approach for a particular class of physical observables, spatial correlation lengths in the quark-gluon plasma phase. Simulations with imaginary chemical potential followed by analytic continuation are compared with simulations with real chemical potential, which are possible by using a dimensionally reduced effective action for hot QCD. We find that for imaginary chemical potential the system undergoes a phase transition at |mu/T| \approx pi/3, and thus observables are analytic only in a limited range. However, utilising this range, relevant information can be obtained for the real chemical potential case.

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arXiv2000

Static correlation lengths in QCD at high temperature and finite density

A brief review is given of the sign problem in finite density lattice QCD and various attempts to overcome it. To date there is still no solution to this problem which would work for realistic QCD. The main focus then is on the deconfined phase, where QCD can be described by a dimensionally reduced effective action. After summarizing derivation and validity of the effective theory, it is demonstrated that it can be simulated efficiently in the presence of a chemical potential for quarks $μ/T\lsim 4$. Direct comparison of simulations with imaginary and real $μ$ suggests that equilibrium plasma properties could be analytically continued from 4d QCD simulations at imaginary $μ$.

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arXiv2000

Debye Screening in the QCD plasma

Various definitions for the QCD Debye mass and its evaluation are reviewed in a non-perturabtive framework for the study of screening of general static sources. While it is possible to perturbatively integrate over scales $\sim T$ and thus construct a 3d effective theory, the softer scales $\sim gT$ and $\sim g^2T$ are strongly coupled for temperatures $\lsim 10^7$ GeV and require lattice simulations. Within the effective theory, a lattice treatment of screening at finite quark densities $μ\lsim 4/T$ is also possible.

hep-ph↗

arXiv2000

Static correlation lengths in QCD at high temperatures and finite densities

We use a perturbatively derived effective field theory and three-dimensional lattice simulations to determine the longest static correlation lengths in the deconfined QCD plasma phase at high temperatures (T\gsim 2 Tc) and finite densities (μ\lsim 4 T). For vanishing chemical potential, we refine a previous determination of the Debye screening length, and determine the dependence of different correlation lengths on the number of massless flavours as well as on the number of colours. For non-vanishing but small chemical potential, the existence of Debye screening allows us to carry out simulations corresponding to the full QCD with two (or three) massless dynamical flavours, in spite of a complex action. We investigate how the correlation lengths in the different quantum number channels change as the chemical potential is switched on.

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arXiv2000

Adjoint string breaking in 4d SU(2) Yang-Mills theory

We compute the static potential of adjoint sources in SU(2) Yang-Mills theory in four dimensions by numerical Monte Carlo simulations. Following a recent calculation in 2+1 dimensions, we employ a variational approach involving string and gluelump operators and obtain clear evidence for string breaking and the saturation of the potential at large distances. For the string breaking scale we find $r_b \approx 1.25{\rm fm}, 2.3 r_0$, or in units of the lightest glueball, $r_b m_{0++} \approx 9.7$. We furthermore resolve the first excitation of the flux-tube and observe its breaking as well. The result for $r_b$ is in remarkable quantitative agreement with the three-dimensional one.

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arXiv1999

The spectrum of the three-dimensional adjoint Higgs model and hot SU(2) gauge theory

We compute the mass spectrum of the SU(2) adjoint Higgs model in 2+1 dimensions at several points located in the (metastable) confinement region of its phase diagram. We find a dense spectrum consisting of an almost unaltered repetition of the glueball spectrum of the pure gauge theory, and additional bound states of adjoint scalars. For the parameters chosen, the model represents the effective finite temperature theory for pure SU(2) gauge theory in four dimensions, obtained after perturbative dimensional reduction. Comparing with the spectrum of screening masses obtained in recent simulations of four-dimensional pure gauge theory at finite temperature, for the low lying states we find quantitative agreement between the full and the effective theory for temperatures as low as T = 2 Tc. This establishes the model under study as the correct effective theory, and dimensional reduction as a viable tool for the description of thermodynamic properties. We furthermore compare the perturbative contribution O(g.T) with the non-perturbative contributions O(g^2.T) and O(g^3.T) to the Debye mass. The latter turns out to be dominated by the scale g^2.T, whereas higher order contributions are small corrections.

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arXiv1999

Cooling, Physical Scales and the Vacuum Structure of Y-M Theories

We present a cooling method controlled by a physical cooling radius that defines a scale below which fluctuations are smoothed out while leaving physics unchanged at all larger scales. This method can be generally used as a gauge invariant low pass filter to extract the physics from noisy MC configurations. Here we apply this method to study topological properties of lattice gauge theories where it allows to retain instanton--anti-instanton pairs.

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arXiv1999

String Breaking in Non-Abelian Gauge Theories with Fundamental Matter Fields

We present clear numerical evidence for string breaking in three-dimensional SU(2) gauge theory with fundamental bosonic matter through a mixing analysis between Wilson loops and meson operators representing bound states of a static source and a dynamical scalar. The breaking scale is calculated in the continuum limit. In units of the lightest glueball we find $r_{\rm b} m_G\approx13.6$. The implications of our results for QCD are discussed.

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arXiv1999

The non-perturbative QCD Debye mass from a Wilson line operator

According to a proposal by Arnold and Yaffe, the non-perturbative g^2T-contribution to the Debye mass in the deconfined QCD plasma phase can be determined from a single Wilson line operator in the three-dimensional pure SU(3) gauge theory. We extend a previous SU(2) measurement of this quantity to the physical SU(3) case. We find a numerical coefficient which is more accurate and smaller than that obtained previously with another method, but still very large compared with the naive expectation: the correction is larger than the leading term up to T ~ 10^7 T_c, corresponding to g^2 ~ 0.4. At moderate temperatures T ~ 2 T_c, a consistent picture emerges where the Debye mass is m_D ~ 6T, the lightest gauge invariant screening mass in the system is ~ 3T, and the purely magnetic operators couple dominantly to a scale ~ 6T. Electric (~ gT) and magnetic (~ g^2T) scales are therefore strongly overlapping close to the phase transition, and the colour-electric fields play an essential role in the dynamics.

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