Effects Of Dynamical Quarks Research Articles

Glueball spectrum from Nf = 2 lattice QCD study on anisotropic lattices**The numerical calculations were carried out on Tianhe-1A at the National Supercomputer Center (NSCC) in Tianjin and the GPU cluster at Hunan Normal University. This work is supported in part by the National Science Foundation of China (NSFC) (11575196, 11575197, 11335001, 11405053, 11405178, 11275169). Y. C., Z. L. and

The lowest-lying glueballs are investigated in lattice QCD using Nf = 2 clover Wilson fermions on anisotropic lattices. We simulate at two different and relatively heavy quark masses, corresponding to physical pion masses of mπ ∼ 938 MeV and 650 MeV. The quark mass dependence of the glueball masses has not been investigated in the present study. Only the gluonic operators built from Wilson loops are utilized in calculating the corresponding correlation functions. In the tensor channel, we obtain the ground state mass to be 2.363(39) GeV and 2.384(67) GeV at mπ ∼ 938 MeV and 650 MeV, respectively. In the pseudoscalar channel, when using the gluonic operator whose continuum limit has the form of εijk TrBiDjBk, we obtain the ground state mass to be 2.573(55) GeV and 2.585(65) GeV at the two pion masses. These results are compatible with the corresponding results in the quenched approximation. In contrast, if we use the topological charge density as field operators for the pseudoscalar, the masses of the lowest state are much lighter (around 1 GeV) and compatible with the expected masses of the flavor singlet meson. This indicates that the operator εijk TrBiDjBk and the topological charge density couple rather differently to the glueball states and mesons. The observation of the light flavor singlet pseudoscalar meson can be viewed as the manifestation of effects of dynamical quarks. In the scalar channel, the ground state masses extracted from the correlation functions of gluonic operators are determined to be around 1.4-1.5 GeV, which is close to the ground state masses from the correlation functions of the quark bilinear operators. In all cases, the mixing between glueballs and conventional mesons remains to be further clarified in the future.

Lattice calculation of coordinate-space vector and axial-vector current correlators in QCD

We study the vector and axial-vector current correlators in perturbative and non-perturbative regimes of QCD. The correlators in Euclidean coordinate space are calculated on the lattice using the M\"obius domain-wall fermion formulation at three lattice spacings covering 0.044--0.080~fm. The dynamical quark effects of $2+1$ light flavors are included. The sum $V+A$ and the difference $V-A$ of the vector ($V$) and axial-vector ($A$) current correlators calculated on the lattice after extrapolating to the physical point agree with those converted from the ALEPH experimental data of hadronic $\tau$ decays. The level of the agreement in the $V+A$ channel is about $1.3\sigma$ or smaller in the region of $|x|\ge0.4$~fm, while that in the $V-A$ channel is about $1.8\sigma$ at $|x|=0.74$~fm and smaller at other distances. We also extract the hiral condensate from the short-distance correlators on the lattice using the PCAC relation. Its result extrapolated to the chiral and continuum limit is compatible with other estimates at low energies.

Instabilities of Coulomb phases and the Gribov picture of confinement

A new picture of quark confinement based on the instability of Coulomb phase at low energy was introduced by Volodya Gribov in the early nineties. In QCD the effective [Formula: see text] coupling constant can reach very large values in the infrared regime what generates Coulomb phase instabilities. In the Gribov picture the instability leads to a vacuum decay into light quarks for coupling constants [Formula: see text] larger than a critical value [Formula: see text], for SU(N) gauge theories. The instability of Coulomb phase can be derived from first principles in any non-Abelian gauge theory for [Formula: see text], a value which is larger than the Gribov critical value. In this paper we review the analytic derivation of the Gribov mechanism from first principles and analyze the effects of dynamical quarks in the instability of the Coulomb phase. The instabilities associated to light quarks turn out to appear at larger values of [Formula: see text] than the ones induced from pure gluon dynamics, unlike it is expected in the standard Gribov scenario. The analytic results confirm the consistency of the picture where quark confinement is mainly driven by gluonic fluctuations.

Determination of|Vus|from a Lattice QCD Calculation of theK→πℓνSemileptonic Form Factor with Physical Quark Masses

We calculate the kaon semileptonic form factor f+(0) from lattice QCD, working, for the first time, at the physical light-quark masses. We use gauge configurations generated by the MILC Collaboration with Nf = 2 + 1 + 1 flavors of sea quarks, which incorporate the effects of dynamical charm quarks as well as those of up, down, and strange. We employ data at three lattice spacings to extrapolate to the continuum limit. Our result, f+(0) = 0.9704(32), where the error is the total statistical plus systematic uncertainty added in quadrature, is the most precise determination to date. Combining our result with the latest experimental measurements of K semileptonic decays, one obtains the Cabibbo-Kobayashi-Maskawa matrix element |V(us)| = 0.22290(74)(52), where the first error is from f+(0) and the second one is from experiment. In the first-row test of Cabibbo-Kobayashi-Maskawa unitarity, the error stemming from |V(us)| is now comparable to that from |V(ud)|.

Domain growth and fluctuations during quenched transition to quark-gluon plasma in relativistic heavy-ion collisions

We model the initial confinement-deconfinement transition in relativistic heavy-ion collisions as a rapid quench in view of expected rapid thermalization to a quark-gluon plasma state. The transition is studied using the Polyakov loop model, with the initial field configuration (in the confining phase) covering a small neighborhood of the confining vacuum $l\ensuremath{\simeq}0$, as appropriate for $T<{T}_{c}$. Quench is implemented by evolving this initial configuration with the effective potential at a temperature $T>{T}_{c}$. We study the formation of $Z(3)$ domain structure and its evolution during the transition as $l$ rolls down in different directions from the top of the central hill in the effective potential of $l$. When explicit $Z(3)$ symmetry-breaking effects (arising from dynamical quark effects) are small, then we find well defined $Z(3)$ domains, which coarsen in time. Remarkably, the magnitude plot of $l$ shows vacuum bubblelike configurations arising during the quench. This first-order transitionlike behavior occurs even though there is no metastable vacuum separated by a barrier from the true vacuum for the parameter values used. When the initial field configuration everywhere rolls down roughly along the same direction (as will happen with large explicit symmetry breaking) then we do not find such bubblelike configurations. However, in this case we find huge oscillations of $l$ with large length scales. We show that such large oscillations can lead to large fluctuations in the evolution of flow anisotropies compared to the equilibrium transition case.

1+1+1flavorQCD+QEDsimulation at the physical point

We present the results of $1+1+1$ flavor $\mathrm{QCD}+\mathrm{QED}$ simulation at the physical point, in which the dynamical quark effects in QED and the up-down quark mass difference are incorporated by the reweighting technique. The physical quark masses together with the lattice spacing are determined with ${m}_{{\ensuremath{\pi}}^{+}}$, ${m}_{{K}^{+}}$, ${m}_{{K}^{0}}$ and ${m}_{{\ensuremath{\Omega}}^{\ensuremath{-}}}$ as physical inputs. Calculations are carried out using a set of $2+1$ flavor QCD configurations near the physical point generated by the nonperturbatively $O(a)$-improved Wilson quark action and the Iwasaki gauge action at $\ensuremath{\beta}=1.9$ on a ${32}^{3}\ifmmode\times\else\texttimes\fi{}64$ lattice. We evaluate the values of the up, down and strange quark masses individually with nonperturbative QCD renormalization.

Hadron-quark phase transition in dense matter

The two-Equation of State (EoS) model is used to describe the hadron-quark phase transition in asymmetric matter formed at high density in heavy-ion collisions. For the quark phase, the three-flavor Nambu–Jona-Lasinio (NJL) effective theory is used to investigate the influence of dynamical quark mass effects on the phase transition. At variance to the MIT-Bag results, with fixed current quark masses, the main important effect of the chiral dynamics is the appearance of an end point for the coexistence zone. We show that a first order hadron-quark phase transition may take place in the region T = (50 – 80) MeV and ρB = (2 – 4)ρ0, which is possible to be probed in the new planned facilities, such as FAIR at GSI-Darmstadt and NICA at JINR-Dubna. From isospin properties of the mixed phase some possible signals are suggested. The importance of chiral symmetry and dynamical quark mass on the hadron-quark phase transition is stressed.

On the gravity dual of Chern-Simons-matter theories with unquenched flavor

We find solutions of type IIA supergravity which are dual to three-dimensional Chern-Simons-matter theories with unquenched fields in the fundamental representation of the gauge group (flavors). In the holographic dual the addition of flavor is performed by means of D6-branesthat are extended along the Minkowski gauge theory directions and are delocalized in the internal space in such a way that the system is $ \mathcal{N} = 1 $ supersymmetric and the flavor group is abelian. For massless flavors the corresponding geometry has the form of a product space $ Ad{S_4} \times {\mathcal{M}_6} $ , where $ {\mathcal{M}_6} $ is a six-dimensional compact manifold whose metric is obtained by squashing the Fubini-Study metric of $ \mathbb{C}{\mathbb{P}^3} $ with suitable constant factors which depend on the number of flavors. We compute the effect of dynamical quarks in several observables and, in some cases, we compare our results with the ones corresponding to the $ \mathcal{N} = 3 $ supergravity solutions generated by localized flavor branes. We also show how to generalize our results to include massive flavors.

Hadron-quark phase transition in asymmetric matter with dynamical quark masses

The two-Equation of State (EoS) model is used to describe the hadron-quark phase transition in asymmetric matter formed at high density in heavy-ion collisions. For the quark phase, the three-flavor Nambu--Jona-Lasinio (NJL) effective theory is used to investigate the influence of dynamical quark mass effects on the phase transition. At variance to the MIT-Bag results, with fixed current quark masses, the main important effect of the chiral dynamics is the appearance of an End-Point for the coexistence zone. We show that a first order hadron-quark phase transition may take place in the region T=(50-80)MeV and \rho_B=(2-4)\rho_0, which is possible to be probed in the new planned facilities, such as FAIR at GSI-Darmstadt and NICA at JINR-Dubna. From isospin properties of the mixed phase somepossible signals are suggested. The importance of chiral symmetry and dynamical quark mass on the hadron-quark phase transition is stressed. The difficulty of an exact location of Critical-End-Point comes from its appearance in a region of competition between chiral symmetry breaking and confinement, where our knowledge of effective QCD theories is still rather uncertain.

Filtered topological structure of the QCD vacuum: Effects of dynamical quarks

We systematically compare filtering methods used to extract topological structures on SU(3) lattice configurations. We show that there is a strong correlation of the topological charge densities obtained by APE and Stout smearing. To get rid of artifacts of these methods, we analyse structures that are also seen by Laplace filtering and indeed identify artifacts for strong smearing. The topological charge density in this combined analysis is more fragmented in the presence of dynamical quarks. A power law exponent that characterises the distribution of filtered topological clusters turns out to be not far off the values of an instanton gas model.

Heavy-Heavy-Light Quark Potential in Two Approaches

There are a large number of studies about the interquark potential in hadrons. The interquark potential is mainly generated by gluon dynamics reflecting the SU(3) gauge symmetry. In particular, as many lattice QCD studies, the static and quenched approximation produces the only gluonic potential. However, realistic hadrons include not only gluons but also quarks, and quarks affect the interquark potential. We can consider two types of the quark effects, dynamical quark effects and valence quark effects. An example of the dynamical quark effects is the string breaking, which is obtained by unquenched calculation. In this study, we focus on the motional effects of finite-mass valence quarks, that is, valence quarks are not static but have a finite mass. To investigate such quark effects, we define the heavy-heavy-light (QQq) quark potential VQQq(R). It is defined as the energy of QQq systems in terms of the interheavy-quark distance R. The QQq system is constructed from two heavy quarks which are infinitely heavy and spatially fixed, and one light quark which is finite mass and moving around. Since this QQq potential effectively includes the lightquark motional effect, we can investigate the finite-mass valence quark effects to the interquark force. This QQq system is also related to the doubly charmed baryon, which has been recently discovered at the SELEX experiment.1),2) To calculate the QQq potential, we adopt the two approaches, SU(3) lattice QCD3) and a non-relativistic potential model.4) Lattice QCD is the first principle calculation based on QCD. The potential model, or the quark model, is one of the most successful analytical approaches for the low-energy hadron physics.

Scaling behavior and positivity violation of the gluon propagator in full QCD

The Landau-gauge gluon propagator is studied using the coarse and fine dynamical MILC configurations. The effects of dynamical quarks are clearly visible and lead to a reduction of the nonperturbative infrared enhancement relative to the quenched case. Lattice spacing effects are studied and found to be small. The gluon spectral function is shown to clearly violate positivity in both quenched and full QCD.

Absence of evidence for pentaquarks on the lattice

We study the question of whether or not QCD predicts a pentaquark state ${\ensuremath{\Theta}}^{+}$. We use the improved, fixed point lattice QCD action which has very little sensitivity to the lattice spacing and also allows us to reach light quark masses. The analysis was performed on a single volume of size $(1.8\text{ }\text{ }\mathrm{fm}{)}^{3}\ifmmode\times\else\texttimes\fi{}3.6\text{ }\text{ }\mathrm{fm}$ with lattice spacing of $a=0.102\text{ }\text{ }\mathrm{fm}$. We use the correlation matrix method to identify the ground and excited states in the isospin 0, negative parity channel. In the quenched approximation where dynamical quark effects are omitted, we do not find any evidence for a pentaquark resonance in QCD.

On the renormalized scalar density in quenched QCD

We present a non-perturbative determination of the renormalization factor Z_S of the scalar density in quenched QCD with overlap fermions. Results are obtained at four values of the lattice spacing. By combining Z_S with results for the low-energy constant Sigma we are able to compute the renormalization group invariant scalar condensate in the continuum limit with a total accuracy of 7%, excluding dynamical quark effects. Our result translates to Sigma_msbar(2 GeV)=(285+/-9 MeV)^3 if the scale is set by the kaon decay constant. We have also performed scaling studies of the pseudoscalar decay constant and the vector mass. Our results indicate that quantities computed using overlap quarks exhibit excellent scaling behaviour, with small residual lattice artifacts.

Nucleon Decay Matrix Elements with [formula omitted] and 2 Domain-Wall Quarks

The nucleon decay matrix elements of three-quark operators are calculated with domain-wall fermions. Operators are renormalized non-perturbatively to match the MS ¯ (NDR) scheme at NLO. Quenched simulation studies involve both direct measurement of the matrix elements and the chiral Lagrangian parameters, α and β. We also report on the dynamical quark effects on these parameters.

Sea quark effects in BK from [formula omitted] clover-improved Wilson fermions

We report calculations of B_K using two flavours of dynamical clover-improved Wilson lattice fermions and look for dependence on the dynamical quark mass at fixed lattice spacing. We see some evidence for dynamical quark effects. In particular B_K decreases as the sea quark masses are reduced towards the up/down quark mass. Our meson masses are quite heavy and a firm prediction of B_K is a task for future simulations.

Sea quark effects in BKfrom Nf=2 clover-improved Wilson fermions

We report calculations of the parameter B_K appearing in the Delta S=2 neutral kaon mixing matrix element, whose uncertainty limits the power of unitarity triangle constraints for testing the standard model or looking for new physics. We use two flavours of dynamical clover-improved Wilson lattice fermions and look for dependence on the dynamical quark mass at fixed lattice spacing. We see some evidence for dynamical quark effects and in particular B_K decreases as the sea quark masses are reduced towards the up/down quark mass.

Three flavor dynamical QCD project by CP-PACS/JLQCD

The CP-PACS and JLQCD Collaborations have been made systematic studies of lattice QCD carrying out both chiral and continuum extrapolations. Importance of dynamical quark effects has been clarified by a comparison of quenched QCD and two flavor full QCD simulations. In two flavor simulations, the dynamical effects of u and d quarks are taken into account, but the third quark s is still treated in a quenched approximation. As the final step towards a fully realistic lattice simulation of QCD, two collaborations started a joint project of three flavor QCD, concentrating all big computers available. Based on a series of preparative studies of exact simulation algorithm and non-perturbative improvement coefficient, a large scale simulation of three flavor QCD has been started. I present the results of light hadron spectrum and light quark masses from the first production runs.

Lattice calculation of the lowest-order hadronic contribution to the muon anomalous magnetic moment.

We present a quenched lattice calculation of the lowest order [O(alpha(2))] hadronic contribution to the anomalous magnetic moment of the muon which arises from the hadronic vacuum polarization. A general method is presented for computing entirely in Euclidean space, obviating the need for the usual dispersive treatment which relies on experimental data for e(+)e(-) annihilation to hadrons. While the result is not yet of comparable precision to those state-of-the-art calculations, systematic improvement of the quenched lattice computation to this level is straightforward and well within the reach of present computers. Including the effects of dynamical quarks is conceptually trivial; the computer resources required are not.

Static hybrid quarkonium potential with improved staggered quarks

We are studying the effects of light dynamical quarks on the excitation energies of a flux tube between a static quark and antiquark. We report preliminary results of an analysis of the ground state potential and the Σ g ′+ and Π u potentials. We have measured these potentials on closely matched ensembles of gauge configurations, generated in the quenched approximation and with 2+1 flavors of Asqtad improved staggered quarks.