arXiv subjects

Owe Philipsen

Publications and source records attributed to Owe Philipsen.

At least 73 records · Page 4Linked to original sources

arXiv2019

Testing the strength of the $\text{U}_A(1)$ anomaly at the chiral phase transition in two-flavour QCD

We study the thermal transition of QCD with two degenerate light flavours by lattice simulations using $\mathcal{O}(a)$-improved Wilson quarks. Particular emphasis lies on the pattern of chiral symmetry restoration, which we probe via the static screening correlators. On $32^3$ volumes we observe that the screening masses in transverse iso-vector vector and axial-vector channels become degenerate at the transition temperature. The splitting between the screening masses in iso-vector scalar and pseudoscalar channels is strongly reduced compared to the splitting at zero temperature and is actually consistent with zero within uncertainties. In this proceedings article we extend our studies to matrix elements and iso-singlet correlation functions. Furthermore, we present results on larger volumes, including first results at the physical pion mass.

hep-lat↗

arXiv2019

Precision computation of hybrid static potentials in SU(3) lattice gauge theory

We perform a precision computation of hybrid static potentials with quantum numbers $Λ_η^ε= Σ_g^-,Σ_u^+,Σ_u^-,Π_g,Π_u,Δ_g,Δ_u$ using SU(3) lattice gauge theory. The resulting potentials are used to estimate masses of heavy $\bar{c} c$ and $\bar{b} b$ hybrid mesons in the Born-Oppenheimer approximation. Part of the lattice gauge theory computation, which we discuss in detail, is an extensive optimization of hybrid static potential creation operators. The resulting optimized operators are expected to be essential for future projects concerning the computation of 3-point functions as e.g. needed to study spin corrections, decays or the gluon distribution of heavy hybrid mesons.

hep-lat↗

arXiv2018

Baryonic or quarkyonic matter?

During the last years it has become possible to address the cold and dense regime of QCD directly for sufficiently heavy quarks, where combined strong coupling and hopping expansions are convergent and a 3d effective theory can be derived, which allows to control the sign problem either in simulations or by fully analytic calculations. In this contribution we review the effective theory and study the $N_c$-dependence of the nuclear liquid gas transition, as well as the equation of state of baryonic matter in the strong coupling limit. We find the transition to become more strongly first order with growing $N_c$, suggesting that in the large $N_c$ limit its critical endpoint moves to high temperatures to connect with the deconfinement transition. Furthermore, to leading and next-to-leading order in the strong coupling and hopping expansions, respectively, the pressure is found to scale as $p\sim N_c$. This suggests that baryonic and quarkyonic matter might be the same at nuclear densities. Further work is needed to see whether this result is stable under gauge corrections.

hep-lat↗

arXiv2018

The QCD chiral phase transition from non-integer numbers of flavors

Attempts to extract the order of the chiral transition of QCD at zero chemical potential, with two dynamical flavors of massless quarks, from simulations with progressively decreasing pion mass have remained inconclusive because of their increasing numerical cost. In an alternative approach to this problem, we consider the path integral as a function of continuous number $N_\text{f}$ of degenerate quarks. If the transition in the chiral limit is first-order for $N_\text{f} \ge 3$, a second-order transition for $N_\text{f}=2$ then requires a tricritical point in between. This in turn implies tricritical scaling of the critical boundary line between the first-order and crossover regions as the chiral limit is approached. Non-integer numbers of fermion flavors are easily implemented within the staggered fermion discretization. Exploratory simulations at $μ=0$ and $N_\text{f} = 2.8, 2.6, 2.4, 2.2, 2.1$, on coarse $N_τ= 4$ lattices, indeed show a smooth variation of the critical mass mapping out a critical line in the $(m,N_\text{f})$-plane. For the smallest masses the line appears consistent with tricritical scaling, allowing for an extrapolation to the chiral limit.

hep-lat↗

arXiv2018

Progress on the nature of the QCD thermal transition as a function of quark flavors and masses

We investigate to which extent we can exploit the dependence of the order of the chiral transition on the number of light degenerate flavors $N_\text{f}$, re-interpreted as continuous parameter in the path integral formulation, as a means to perform a controlled chiral extrapolation and deduce the order of the transition for the case $N_\text{f}=2$, which is still under debate.

hep-lat↗

arXiv2018

Structure of hybrid static potential flux tubes in lattice Yang-Mills theory

We report about an ongoing lattice field theory project concerned with the investigation of heavy hybrid mesons. In particular we discuss our computation of the structure of hybrid static potential flux tubes in SU(2) lattice Yang-Mills theory, which is based on the squares of the chromoelectric and chromomagnetic field strength components. Our flux tube results for hybrid static potential quantum numbers $Σ_g^-$, $Σ_u^+$, $Σ_u^-$, $Π_g$, $Π_u$, $Δ_g$, $Δ_u$ are significantly different compared to the flux tube of the ordinary static potential.

hep-lat↗

arXiv2018

Computation of hybrid static potentials from optimized trial states in SU(3) lattice gauge theory

We compute hybrid static potentials in SU(3) lattice gauge theory using a method to automatically generate a large set of suitable creation operators from elementary building blocks. This method allows to find sets of creation operators, which generate trial states with large ground state overlaps for all investigated angular momentum and parity sectors. We present numerical results for hybrid static potentials with quantum numbers $Σ^-_g, Σ^+_u, Σ^-_u, Π_g, Π_u, Δ_g, Δ_u$.

hep-lat↗

arXiv2018

From the colour glass condensate to filamentation: Systematics of classical Yang-Mills theory

The non-equilibrium early time evolution of an ultra-relativistic heavy ion collision is often described by classical lattice Yang-Mills theory, starting from the colour glass condensate (CGC) effective theory with an anisotropic energy momentum tensor as initial condition. In this work we investigate the systematics associated with such studies and their dependence on various model parameters (IR, UV cutoffs and the amplitude of quantum fluctuations) which are not yet fixed by experiment. We perform calculations for SU(2) and SU(3), both in a static box and in an expanding geometry. Generally, the dependence on model parameters is found to be much larger than that on technical parameters like the number of colours, boundary conditions or the lattice spacing. In a static box, all setups lead to isotropisation through chromo-Weibel instabilities, which is illustrated by the accompanying filamentation of the energy density. However, the associated time scale depends strongly on the model parameters and in all cases is longer than the phenomenologically expected one. In the expanding system, no isotropisation is observed for any parameter choice. We show how investigations at fixed initial energy density can be used to better constrain some of the model parameters.

hep-lat↗

arXiv2018

Cluster Expansion Model for QCD Baryon Number Fluctuations: No Phase Transition at $μ_B / T < π$

A Cluster Expansion Model (CEM), representing a relativistic extension of Mayer's cluster expansion, is constructed to study baryon number fluctuations in QCD. The temperature dependent first two coefficients, corresponding to the partial pressures in the baryon number $B = 1$ and $B = 2$ sectors, are the only model input, which we fix by recent lattice data at imaginary baryochemical potential. All other coefficients are constructed in terms of the first two and required to match the Stefan-Boltzmann limit at $T \to \infty$. The CEM allows calculations of the baryon number susceptibilities $χ_k^B$ to arbitrary order. We obtain excellent agreement with available lattice data for the baryon fluctuation measures $χ_2^B$, $χ_4^B$, $χ_6^B$ and predict higher order susceptibilities, that are not yet available from Lattice QCD. The calculated susceptibilities are then used to extract the radius of convergence of the Taylor expansion of the pressure. The commonly used ratio test fails due to the non-trivial asymptotic behavior of the Taylor coefficients. At the same time, a more elaborate estimator provides finite convergence radii at all temperatures and in agreement with the singularities of Padé approximants. The associated singularities lie in the complex $μ_B/T$-plane and appear smoothly connected to the Roberge-Weiss transition at high temperatures and imaginary chemical potential. No evidence for a phase transition at $μ_B / T \lesssim π$ is found.

hep-ph↗

arXiv2017

Updates on the Columbia plot and its extended/alternative versions

We report on the status of ongoing investigations aiming at locating the deconfinement critical point with standard Wilson fermions and $N_f=2$ flavors towards the continuum limit (standard Columbia plot); locating the tricritical masses at imaginary chemical potential with unimproved staggered fermions at $N_f=2$ (extended Columbia plot); identifying the order of the chiral phase transition at $μ=0$ for $N_f=2$ via extrapolation from non integer $N_f$ (alternative Columbia plot).

hep-lat↗

arXiv2017

Computation of hybrid static potentials in SU(3) lattice gauge theory

We compute hybrid static potentials in SU(3) lattice gauge theory. We present a method to generate a large set of suitable creation operators with defined quantum numbers from elementary building blocks. We show preliminary results for several channels and discuss, which structures of the gluonic flux tube seem to be realized by the ground states in these channels.

hep-lat↗

arXiv2017

On the strength of the $U_A(1)$ anomaly at the chiral phase transition in $N_f=2$ QCD

We study the thermal transition of QCD with two degenerate light flavours by lattice simulations using $O(a)$-improved Wilson quarks. Temperature scans are performed at a fixed value of $N_t = (aT)^{-1}=16$, where $a$ is the lattice spacing and $T$ the temperature, at three fixed zero-temperature pion masses between 200 MeV and 540 MeV. In this range we find that the transition is consistent with a broad crossover. As a probe of the restoration of chiral symmetry, we study the static screening spectrum. We observe a degeneracy between the transverse isovector vector and axial-vector channels starting from the transition temperature. Particularly striking is the strong reduction of the splitting between isovector scalar and pseudoscalar screening masses around the chiral phase transition by at least a factor of three compared to its value at zero temperature. In fact, the splitting is consistent with zero within our uncertainties. This disfavours a chiral phase transition in the $O(4)$ universality class.

hep-lat↗

arXiv2016

Complex Langevin simulations of a finite density matrix model for QCD

We study a random matrix model for QCD at finite density via complex Langevin dynamics. This model has a phase transition to a phase with nonzero baryon density. We study the convergence of the algorithm as a function of the quark mass and the chemical potential and focus on two main observables: the baryon density and the chiral condensate. For simulations close to the chiral limit, the algorithm has wrong convergence properties when the quark mass is in the spectral domain of the Dirac operator. A possible solution of this problem is discussed.

hep-lat↗

arXiv2016

Towards a theoretical description of dense QCD

The properties of matter at finite baryon densities play an important role for the astrophysics of compact stars as well as for heavy ion collisions or the description of nuclear matter. Because of the sign problem of the quark determinant, lattice QCD cannot be simulated by standard Monte Carlo at finite baryon densities. I review alternative attempts to treat dense QCD with an effective lattice theory derived by analytic strong coupling and hopping expansions, which close to the continuum is valid for heavy quarks only, but shows all qualitative features of nuclear physics emerging from QCD. In particular, the nuclear liquid gas transition and an equation of state for baryons can be calculated directly from QCD. A second effective theory based on strong coupling methods permits studies of the phase diagram in the chiral limit on coarse lattices.

hep-lat↗

arXiv2016

The $N_f=2$ QCD chiral phase transition with Wilson fermions at zero and imaginary chemical potential

The order of the thermal phase transition in the chiral limit of Quantum Chromodynamics (QCD) with two dynamical flavors of quarks is a long-standing issue and still not known in the continuum limit. Whether the transition is first or second order has important implications for the QCD phase diagram and the existence of a critical endpoint at finite densities. We follow a recently proposed approach to explicitly determine the region of first order chiral transitions at imaginary chemical potential, where it is large enough to be simulated, and extrapolate it to zero chemical potential with known critical exponents. Using unimproved Wilson fermions on coarse $N_t=4$ lattices, the first order region turns out to be so large that no extrapolation is necessary. The critical pion mass $m_π^c\approx 560$ MeV is by nearly a factor 10 larger than the corresponding one using staggered fermions. Our results are in line with investigations of three-flavour QCD using improved Wilson fermions and indicate that the systematic error on the two-flavour chiral transition is still of order 100\%.

hep-lat↗

arXiv2016

Roberge-Weiss transition in $N_f=2$ QCD with staggered fermions and $N_τ=6$

The QCD phase diagram at imaginary chemical potential exhibits a rich structure and studying it can constrain the phase diagram at real values of the chemical potential. Moreover, at imaginary chemical potential standard numerical techniques based on importance sampling can be applied, since no sign problem is present. In the last decade, a first understanding of the QCD phase diagram at purely imaginary chemical potential has been developed, but most of it is so far based on investigations on coarse lattices ($N_τ=4$, $a=0.3\:$fm). Considering the $N_f=2$ case, at the Roberge-Weiss critical value of the imaginary chemical potential, the chiral/deconfinement transition is first order for light/heavy quark masses and second order for intermediate values of the mass: there are then two tricritical masses, whose position strongly depends on the lattice spacing and on the discretization. On $N_τ=4$, we have the chiral $m_π^{\text{tric.}}=400\:$MeV with unimproved staggered fermions and $m_π^{\text{tric.}}\gtrsim900\:$MeV with unimproved pure Wilson fermions. Employing finite size scaling we investigate the change of this tricritical point between $N_τ=4$ and $N_τ=6$ as well as between Wilson and staggered discretizations.

hep-lat↗

arXiv2016

Computing the static potential using non-string-like trial states

We present a method for computing the static quark-antiquark potential, which is not based on Wilson loops, but where the trial states are formed by eigenvector components of the covariant lattice Laplace operator. We have tested this method in SU(2) Yang-Mills theory and have obtained results with statistical errors of similar magnitude compared to a standard Wilson loop computation. The runtime of the method is, however, significantly smaller, when computing the static potential not only for on-axis, but also for many off-axis quark-antiquark separations, i.e. when a fine spatial resolution is required.

hep-lat↗

arXiv2016

The QCD deconfinement critical point for $N_τ=8$ with $N_f=2$ flavours of unimproved Wilson fermions

QCD at zero baryon density in the limit of infinite quark mass undergoes a first order deconfinement phase transition at a critical temperature $T_c$ corresponding to the breaking of the global centre symmetry. In the presence of dynamical quarks this symmetry is explicitly broken. Lowering the quark mass the first order phase transition weakens and terminates in a second order Z(2) point. Beyond this line confined and deconfined regions are analytically connected by a crossover transition. As the continuum limit is approached (i.e. the lattice spacing is decreased) the region of first order transitions expands towards lower masses. We study the deconfinement critical point with standard Wilson fermions and $N_f=2$ flavours. To this end we simulate several kappa values on $N_τ=8$ and various aspect ratios in order to extrapolate to the thermodynamic limit, applying finite size scaling. We estimate if and when a continuuum extrapolation is possible.

hep-lat↗