arXiv subjects

Owe Philipsen

Publications and source records attributed to Owe Philipsen.

At least 127 records · Page 7Linked to original sources

arXiv2011

Centre symmetric 3d effective actions for thermal SU(N) Yang-Mills from strong coupling series

We derive three-dimensional, Z(N)-symmetric effective actions in terms of Polyakov loops by means of strong coupling expansions, starting from thermal SU(N) Yang-Mills theory in four dimensions on the lattice. An earlier action in the literature, corresponding to the (spatial) strong coupling limit, is thus extended by several higher orders, as well as by additional interaction terms. We provide analytic mappings between the couplings of the effective theory and the parameters $N_τ,β$ of the original thermal lattice theory, which can be systematically improved. We then investigate the deconfinement transition for the cases SU(2) and SU(3) by means of Monte Carlo simulations of the effective theory. Our effective models correctly reproduce second order 3d Ising and first order phase transitions, respectively. Furthermore, we calculate the critical couplings $β_c(N_τ)$ and find agreement with results from simulations of the 4d theory at the few percent level for $N_τ=4-16$.

hep-lat↗

arXiv2011

Strong-coupling effective action(s) for SU(3) Yang-Mills

We apply strong-coupling expansion techniques to finite-temperature lattice pure gauge theory, obtaining dimensionally reduced $Z_N$-symmetric effective theories. The analytic mappings between the effective couplings and the original one, viz. $β$, allow to estimate the transition point $β_c$ of the 4D theory for a large range of the imaginary-time extent $N_τ$ of the lattice. We study the models for SU(3) via Monte Carlo simulation, finding satisfactory agreement with the critical point of the original theories especially at low $N_τ$. We have fixed an error in the group measure used in arXiv:1010.0951 and provide here the correct numerical results.

hep-lat↗

arXiv2010

Towards the N_f=2 deconfinement transition temperature with O(a) improved Wilson fermions: An update

We give an update on our current project to determine the transition temperature and the order of the deconfinement transition in the chiral limit of two flavour QCD. We use nonperturbatively O(a) improved Wilson fermions of the Sheikholeslami-Wohlert type, employing the efficient deflation accelerated DDHMC algorithm. We start at lattices with N_t>=12 and pion masses below 600 MeV, aiming at chiral and continuum limits with light quarks.

hep-lat↗

arXiv2010

Constraints for the QCD phase diagram from imaginary chemical potential

We present unambiguous evidence from lattice simulations of N_f=3 QCD for two tricritical points in the (T,m) phase diagram at fixed imaginary μ/T=iπ/3 mod. 2π/3, one in the light and one in the heavy quark regime. Together with similar results in the literature for N_f=2 this implies the existence of a chiral and of a deconfinement tricritical line at those values of imaginary chemical potentials. These tricritical lines represent the boundaries of the analytically continued chiral and deconfinement critical surfaces, respectively, which delimit the parameter space with first order phase transitions. It is demonstrated that the shape of the deconfinement critical surface is dictated by tricritical scaling and implies the weakening of the deconfinement transition with real chemical potential. A qualitatively similar effect holds for the chiral critical surface.

hep-lat↗

arXiv2010

Effective Polyakov-loop theory for pure Yang-Mills from strong coupling expansion

Lattice Yang-Mills theories at finite temperature can be mapped onto effective 3d spin systems, thus facilitating their numerical investigation. Using strong-coupling expansions we derive effective actions for Polyakov loops in the $SU(2)$ and $SU(3)$ cases and investigate the effect of higher order corrections. Once a formulation is obtained which allows for Monte Carlo analysis, the nature of the phase transition in both classes of models is investigated numerically, and the results are then used to predict -- with an accuracy within a few percent -- the deconfinement point in the original 4d Yang-Mills pure gauge theories, for a series of values of $N_τ$ at once.

hep-lat↗

arXiv2010

Thermal transition temperature from twisted mass QCD

We present the current status of lattice simulations with N_f=2 maximally twisted mass Wilson fermions at finite temperature. In particular, the determination of the thermal transition temperature is discussed.

hep-lat↗

arXiv2010

Towards the N_f=2 deconfinement transition temperature with O(a) improved Wilson fermions

A lot of effort in lattice simulations over the last years has been devoted to studies of the QCD deconfinement transition. Most state-of-the-art simulations use rooted staggered fermions, while Wilson fermions are affected by large systematic uncertainties, such as coarse lattices or heavy sea quarks. Here we report on an ongoing study of the transition, using two degenerate flavours of nonperturbatively $\Ord(a)$ improved Wilson fermions. We start with $N_{t}=12$ and 16 lattices and pion masses of 600 to 450 MeV, aiming at chiral and continuum limits with light quarks.

hep-lat↗

arXiv2010

Strong coupling series for QCD at finite temperature and density

We discuss the use of strong coupling expansions for Yang-Mills theory and QCD at finite temperature and density. In particular we consider the onset of temperature effects for the free energy and screening masses, derive the hadron resonance gas model from first principles and compute the weakening of the deconfinement transition with chemical potential.

hep-lat↗

arXiv2010

Cutoff effects of Wilson fermions on the QCD equation of state to O(g^2)

We compute the O(g^2) contribution to the thermodynamic pressure for Wilson fermions in the standard, the twisted mass, and clover improved formulation in lattice perturbation theory, including finite mass effects. We compare the continuum approaches of these discretizations for the massive ideal and interacting gas. In all cases, for N_t \geq 8 cutoff effects of Wilson type fermions are comparable to those of staggered fermions, but asymptotic scaling requires $N_τ>10$.

hep-lat↗

arXiv2010

The deconfinement transition of finite density QCD with heavy quarks from strong coupling series

Starting from Wilson's action, we calculate strong coupling series for the Polyakov loop susceptibility in lattice gauge theories for various small N_τin the thermodynamic limit. Analysing the series with Padé approximants, we estimate critical couplings and exponents for the deconfinement phase transition. For SU(2) pure gauge theory our results agree with those from Monte-Carlo simulations within errors, which for the coarser N_τ=1,2 lattices are at the percent level. For QCD we include dynamical fermions via a hopping parameter expansion. On a N_τ=1 lattice with N_f=1,2,3, we locate the second order critical point where the deconfinement transition turns into a crossover. We furthermore determine the behaviour of the critical parameters with finite chemical potential and find the first order region to shrink with growing μ. Our series moreover correctly reflects the known Z(N) transition at imaginary chemical potential.

hep-lat↗

arXiv2010

Constraining the QCD phase diagram by tricritical lines at imaginary chemical potential

We present unambiguous evidence from lattice simulations of QCD with three degenerate quark species for two tricritical points in the (T,m) phase diagram at fixed imaginary μ/T=iπ/3 mod 2π/3, one in the light and one in the heavy mass regime. These represent the boundaries of the chiral and deconfinement critical lines continued to imaginary chemical potential, respectively. It is demonstrated that the shape of the deconfinement critical line for real chemical potentials is dictated by tricritical scaling and implies the weakening of the deconfinement transition with real chemical potential. The generalization to non-degenerate and light quark masses is discussed.

hep-lat↗

arXiv2010

The pressure of strong coupling lattice QCD with heavy quarks, the hadron resonance gas model and the large N limit

In this paper we calculate the pressure of pure lattice Yang-Mills theories and lattice QCD with heavy quarks by means of strong coupling expansions. Dynamical fermions are introduced with a hopping parameter expansion, which also allows for the incorporation of finite quark chemical potential. We show that in leading orders the results are in full agreement with expectations from the hadron resonance gas model, thus validating it with a first principles calculation. For pure Yang-Mills theories we obtain the corresponding ideal glueball gas, in QCD with heavy quarks our result equals that of an ideal gas of mesons and baryons. Another finding is that the Yang-Mills pressure in the large N limit is of order $\sim N^0$ to the calculated orders, when the inverse 't Hooft coupling is used as expansion parameter. This property is expected in the confined phase, where our calculations take place.

hep-lat↗

arXiv2009

Lattice QCD Thermodynamics on the Grid

We describe how we have used simultaneously ${\cal O}(10^3)$ nodes of the EGEE Grid, accumulating ca. 300 CPU-years in 2-3 months, to determine an important property of Quantum Chromodynamics. We explain how Grid resources were exploited efficiently and with ease, using user-level overlay based on Ganga and DIANE tools above standard Grid software stack. Application-specific scheduling and resource selection based on simple but powerful heuristics allowed to improve efficiency of the processing to obtain desired scientific results by a specified deadline. This is also a demonstration of combined use of supercomputers, to calculate the initial state of the QCD system, and Grids, to perform the subsequent massively distributed simulations. The QCD simulation was performed on a $16^3\times 4$ lattice. Keeping the strange quark mass at its physical value, we reduced the masses of the up and down quarks until, under an increase of temperature, the system underwent a second-order phase transition to a quark-gluon plasma. Then we measured the response of this system to an increase in the quark density. We find that the transition is smoothened rather than sharpened. If confirmed on a finer lattice, this finding makes it unlikely for ongoing experimental searches to find a QCD critical point at small chemical potential.

cs.DC↗

arXiv2009

Screened perturbation theory for 3d Yang-Mills theory and the magnetic modes of hot QCD

Perturbation theory for non-abelian gauge theories at finite temperature is plagued by infrared divergences which are caused by magnetic soft modes ~g^2T, corresponding to gluon fields of a 3d Yang-Mills theory. While the divergences can be regulated by a dynamically generated magnetic mass on that scale, the gauge coupling drops out of the effective expansion parameter requiring summation of all loop orders for the calculation of observables. Some gauge invariant possibilities to implement such infrared-safe resummations are reviewed. We use a scheme based on the non-linear sigma model to estimate some of the contributions ~g^6 of the soft magnetic modes to the QCD pressure through two loops. The NLO contribution amounts to ~10% of the LO, suggestive of a reasonable convergence of the series.

hep-ph↗

arXiv2009

Towards a determination of the chiral critical surface of QCD

The chiral critical surface is a surface of second order phase transitions bounding the region of first order chiral phase transitions for small quark masses in the {m_{u,d}, m_s,μ} parameter space. The potential critical endpoint of the QCD (T,μ)-phase diagram is widely expected to be part of this surface. Since for μ=0 with physical quark masses QCD is known to exhibit an analytic crossover, this expectation requires the region of chiral transitions to expand with μfor a chiral critical endpoint to exist. Instead, on coarse N_t=4 lattices, we find the area of chiral transitions to shrink with μ, which excludes a chiral critical point for QCD at moderate chemical potentials μ_B < 500 MeV. First results on finer N_t=6 lattices indicate a curvature of the critical surface consistent with zero and unchanged conclusions. We also comment on the interplay of phase diagrams between the N_f=2 and N_f=2+1 theories and its consequences for physical QCD.

hep-ph↗

arXiv2009

Towards the chiral critical surface of QCD

The critical endpoint of the QCD phase diagram is usually expected to belong to the chiral critical surface, i.e. the surface of second order transitions bounding the region of first order chiral phase transitions for small quark masses in the {m_{u,d}, m_s,μ} parameter space. For μ=0, QCD with physical quark masses is known to be an analytic crossover, requiring the region of chiral transitions to expand with μfor a critical endpoint to exist. Instead, on coarse N_t=4 lattices, we find the area of chiral transitions to shrink with μ, which excludes a chiral critical point for QCD at moderate chemical potentials μ_B < 500 MeV. First results on finer N_t=6 lattices indicate a curvature of the critical surface consistent with zero and unchanged conclusions.

hep-ph↗

arXiv2009

Heavy Quark Thermalization in Classical Lattice Gauge Theory: Lessons for Strongly-Coupled QCD

Thermalization of a heavy quark near rest is controlled by the correlator of two electric fields along a temporal Wilson line. We address this correlator within real-time, classical lattice Yang-Mills theory, and elaborate on the analogies that exist with the dynamics of hot QCD. In the weak-coupling limit, it can be shown analytically that the dynamics on the two sides are closely related to each other. For intermediate couplings, we carry out non-perturbative simulations within the classical theory, showing that the leading term in the weak-coupling expansion significantly underestimates the heavy quark thermalization rate. Our analytic and numerical results also yield a general understanding concerning the overall shape of the spectral function corresponding to the electric field correlator, which may be helpful in subsequent efforts to reconstruct it from Euclidean lattice Monte Carlo simulations.

hep-ph↗

arXiv2008

The curvature of the critical surface (m_ud,m_s)^{crit}(mu): a progress report

At zero chemical potential mu, the order of the temperature-driven quark-hadron transition depends on the quark masses m_{u,d} and m_s. Along a critical line bounding the region of first-order chiral transitions in the (m_{u,d},m_s) plane, this transition is second order. When the chemical potential is turned on, this critical line spans a surface, whose curvature at mu=0 can be determined without any sign or overlap problem. Our past measurements on N_t=4 lattices suggest that the region of quark masses for which the transition is first order shrinks when mu is turned on, which makes a QCD chiral critical point at small mu/T unlikely. We present results from two complementary methods, which can be combined to yield information on higher-order terms. It turns out that the O(mu^4) term reinforces the effect of the leading O(mu^2) term, and there is strong evidence that the O(mu^6) and O(mu^8) terms do as well. We also report on simulations underway, where the strange quark is given its physical mass, and where the lattice spacing is reduced.

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