Quark States Research Articles

On the infrared behavior of Landau Gauge Yang–Mills theory with a fundamentally charged scalar field

Recently it has been shown that infrared singularities of Landau gauge QCD can confine static quarks via a linearly rising potential. We show that the same mechanism can also provide a confining interaction between charged scalar fields in the fundamental representation. This confirms that within this scenario static confinement is a universal property of the gauge sector even though it is formally represented in the functional equations of the matter sector. The simplifications compared to the fermionic case make the scalar system an ideal laboratory for a detailed analysis of the confinement mechanism in numerical studies of the functional equations as well as in gauge-fixed lattice simulations.

Stability of the pentaquark in a naive string model

The pentaquark is studied in a simple model of confinement in which the quarks and the antiquark are linked by flux tubes of minimal cumulated length and the Coulomb-like interaction, the spin-dependent terms, and the antisymmetrization constraints are neglected. The ground state is found to be stable against spontaneous dissociation into a meson and a baryon, both in the case of five equal-mass constituents and for a static quark or antiquark surrounded by four equal masses.

Wilson loops stability in the gauge/string correspondence

We study the stability of some classical string worldsheet solutions employed for computing the potential energy between two static fundamental quarks in confining and non-confining gravity duals. We discuss the fixing of the diffeomorphism invariance of the string action, its relation with the fluctuation orientation and the interpretation of the quark mass substraction worldsheet needed for computing the potential energy in smooth (confining) gravity background. We consider various dual gravity backgrounds and show by a numerical analysis the existence of instabilities under linear fluctuations for classical string embedding solutions having positive length function derivative $L'(r_0)>0$. Finally we make a brief discussion of 't Hooft loops in non-conformal backgrounds.

G-Matrix Approach to Hyperon-Nucleus Systems

The properties of the new YN · YY interaction model ESC08 are studied on the basis of the G-matrix theory. G-matrix calculations in nuclear matter are performed in the cases of ΛN -, ΣN -, ΞN -a ndΛΛ -starting channels. In the ΛN case, the spin-spin and spinorbit terms are shown to be of reasonable strengths. The folding-model analyses are done extensively for observed spectra of Λ hypernuclei. The experimental energy spectra are nicely reproduced. The shell-model analyses for typical Λ hypernuclei are performed with the Gmatrix interactions. In the ΣN case, the repulsive values of UΣ are obtained owing to the effect of the quark Pauli-forbidden states which are taken into account phenomenologically in ESC08. In the ΞN case, the moderately attractive values of UΞ are obtained mainly due to the strongly attractive contributions in 3 S1 T =1 ΞN -ΛΣ -ΣΣ coupled states, which leads to extensive existence of Ξ hypernuclei. In the ΛΛ case, the BΛΛ values derived from ESC08 are found to be consistent with the experimental values.

Mass–spin relation for quark anti-quark bound states in non-commutative Yang–Mills theory

We investigate the relationship between mass and spin of quark anti-quark bound state in non-commutative gauge theory with N = 4 supersymmetry. In the large N and large 't Hooft coupling limit, these bound states correspond to rotating open strings ending on a D-brane embedded into the supergravity background dual of the non-commutative field theory. Although the physical configuration of the open strings are drastically deformed by the non-commutativity, we find that the relation between the energy and the angular momentum of the bound state is unaffected by the non-commutativity parameter. We also clarify the holographic interpretation of quark anti-quark potential in non-commutative gauge theories.

Meson correlation functions for a model with nonlocal four-fermion interaction

We study meson correlation functions for several models with a four-fermion quark interaction. We show that although the system average energy and quark condensate, as previously noted, are singular, the meson observables are finite, completely recognizable, and comparable to the experimental data on the energy scale. This permits using a wide set of Hamiltonians to model nonequilibrium states of quark and hadronic systems, which is a relevant problem in studying the physics of ultrarelativistic heavy-ion collisions. We obtain analytic expressions for the meson correlation functions in the Keldysh model.

NONRELATIVISTIC QUARK–ANTIQUARK POTENTIAL: SPECTROSCOPY OF HEAVY-QUARKONIA AND EXOTIC SUSY QUARKONIA

The experiments at LHC have shown that the SUSY (exotic) bound states are likely to form bound states in an entirely similar fashion as ordinary quarks form bound states, i.e. quarkonium. Also, the interaction between two squarks is due to gluon exchange which is found to be very similar to that interaction between two ordinary quarks. This motivates us to solve the Schrödinger equation with a strictly phenomenological static quark–antiquark potential: [Formula: see text] using the shifted large N-expansion method to calculate the low-lying spectrum of a heavy quark with antisbottom [Formula: see text] and sbottom with antisbottom [Formula: see text] bound states with [Formula: see text] is set free. To have a full knowledge on spectrum, we also give the result for a heavier as well as for lighter sbottom masses. As a test for the reliability of these calculations, we fix the parameters of this potential by fitting the spin-triplet (n3S1) and center-of-gravity l≠0 experimental spectrum of the ordinary heavy quarkonia [Formula: see text], [Formula: see text] and [Formula: see text] to few MeV. Our results are compared with other models to gauge the reliability of these predictions and point out differences.

Free energies of heavy quarks in full-QCD lattice simulations with Wilson-type quark action

The free energy between a static quark and an antiquark is studied by using the color-singlet Polyakov-line correlation at finite temperature in lattice QCD with 2+1 flavors of improved Wilson quarks. From the simulations on 32 3 × 12, 10, 8, 6, 4 lattices in the high temperature phase, based on the fixed scale approach, we find that, the heavy-quark free energies at short distance converge to the heavy-quark potential evaluated from the Wilson loop at zero temperature, in accordance with the expected insensitivity of short distance physics to the temperature. At long distance, the heavy-quark free energies approach to twice the single-quark free energies, implying that the interaction between heavy quarks is screened. The Debye screening mass obtained from the long range behavior of the free energy is compared with the results of thermal perturbation theory.

Bottom-hadron mass splittings from static-quark action on2+1-flavor lattices

We calculate bottom-hadron mass splittings with respect to ${B}_{d}$ and ${\ensuremath{\Lambda}}_{b}$ using full QCD with $2+1$ flavors of dynamical Kogut-Susskind sea quarks and domain-wall valence quarks along with a static heavy quark. Our lattices have spatial volume of $(2.5\text{ }\text{ }\mathrm{fm}{)}^{3}$ with lattice spacing about 0.124 fm and a range of pion masses as low as 291 MeV. Our results are in agreement with experimental observations and other lattice calculations within our statistical and systematic errors. In particular, we find the mass of the ${\ensuremath{\Omega}}_{b}$ to be consistent with the recent CDF measurement. We also predict the mass for the as yet unobserved ${\ensuremath{\Xi}}_{b}^{\ensuremath{'}}$ to be 5955(27) MeV.

Configuration Mixing of Quark States in Baryons

Mixing angles are used to describe the $SU(6)\otimes O(3)$ symmetry breaking in $\left[70,~1^-\right]$ multiplet in the sector of the lowest mass nucleon resonances, which are investigated extensively in constituent quark models for baryon spectroscopy. The transition amplitudes for the meson photoproduction off nucleon can also be expressed in terms of the mixing angles to take into account the configuration mixing. Those amplitudes are derived as a function of the mixing angles between $|N^2P_M{J}^-\rangle$ and $|N^4P_M{J}^-\rangle$ states, with $J$=1/2 and 3/2, for the processes $\gamma p \to \eta p,~K^+ \Lambda,~K^+ \Sigma^\circ,~K^\circ \Sigma^+$. The present status of our knowledge on the mixing angles between $S_{11}(1535)$ and $S_{11}(1650)$ ($\theta _{S}$), as well as between $D_{13}(1520)$ and $D_{13} (1700)$ ($\theta _{D}$) is reported. Since these resonances play very important role in the threshold region for both $\eta$ and kaon production mechanisms, they are expected to provide crucial tests of different quark models for the baryon spectroscopy.

Meson wave function from holographic models

We consider the light-front wave function for the valence quark state of mesons using the AdS/CFT correspondence, as has been suggested by Brodsky and Teramond. Two kinds of wave functions, obtained in different holographic Soft-Wall models, are discussed.

A relativistic structure of one-meson and one-gluon exchange forces and a lower excitation spectrum of the nucleon and Δ

A lower excitation spectrum of the nucleon and Δ is calculated in a relativistic chiral quark model. Corrections to the baryon mass spectrum from the second-order self-energy and exchange diagrams induced by pion and gluon fields are estimated in the field-theoretical framework. Convergent results for the self-energy terms are obtained when including the intermediate quark and antiquark states with a total momentum up to j = 25/2. Relativistic one-meson and color-magnetic one-gluon exchange forces are shown to generate spin 0, 1, 2, etc operators, which couple the lower and the upper components of the two interacting valence quarks and yield reasonable matrix elements for the lower excitation spectrum of the nucleon and Δ. The only contribution to the ground-state nucleon and Δ comes from the spin-1 operators, which correspond to the exchanged pion or gluon in the l = 1 orbit, thus indicating that both pion exchange and color-magnetic gluon exchange forces can contribute to the spin of baryons. Is is shown also that the contribution of the color-electric component of the gluon fields to the baryon spectrum is enormously large (more than 500 MeV with a value of αs = 0.65) and one needs to restrict to very small values of the strong coupling constant or to exclude completely the gluon-loop corrections to the baryon spectrum. With this restriction, the calculated spectrum reproduces the main properties of the data; however it needs further contribution from the two-pion exchange and instanton-induced exchange (for the nucleon sector) forces in consistence with the realistic NN-interaction models.

Masses, fermions and generalized D-dimensional unitarity

We extend the generalized D-dimensional unitarity method for numerical evaluation of one-loop amplitudes by incorporating massive particles. The issues related to extending the spinor algebra to higher dimensions, treatment of external self-energy diagrams and mass renormalization are discussed within the context of the D-dimensional unitarity method. To validate our approach, we calculate in QCD the one-loop scattering amplitudes of a massive quark pair with up to three additional gluons for arbitrary spin states of the external quarks and gluons.

Formula omitted]-coupling in the static heavy quark limit

By means of QCD simulations on the lattice, we compute the coupling of the heavy–light mesons to a soft pion in the static heavy quark limit. The gauge field configurations used in this calculations include the effect of Nf=2 dynamical Wilson quarks, while for the static quark propagator we use its improved form (so-called HYP). On the basis of our results we obtain that the coupling gˆ=0.44±0.03−0.00+0.07, where the second error is flat (not Gaussian).

Global aspects of the scalar meson puzzle

A generalized linear sigma model for low-energy QCD is employed to study the quark structure of eight low-lying scalar isomultiplets as well as eight low-lying pseudoscalar isomultiplets. The model, building on earlier work, assumes the possible mixing of quark antiquark states with others made of two quarks and two antiquarks. No a priori assumption is made about the quark contents of the states, which emerge as predictions. An amusing and contrasting pattern for the quark structure is found; the lighter conventional pseudoscalars are, as expected, primarily of two-quark type whereas the lighter scalars have very large four-quark admixtures. The new feature of the present paper compared to earlier ones in this series involves the somewhat subtle and complicated effects of SU(3) flavor breaking. They do not alter the general pattern of two-quark vs four-quark mixing obtained in the SU(3)symmetric case but, of course, give a more detailed picture.

Quark statistics shed light on Universe's symmetry

Quark statistics shed light on Universe's symmetry

Quark statistics shed light on Universe's symmetry

Quark statistics shed light on Universe's symmetry

Bottom hadron mass splittings in the static limit from [formula omitted] flavour lattice QCD

Dynamical 2 + 1 flavour lattice QCD is used to calculate the splittings between the masses of mesons and baryons containing a single static heavy quark and domain-wall light and strange quarks. Our calculations are based on the dynamical domain-wall gauge field configurations generated by the RBC and UKQCD Collaborations at a spatial volume of (2.7 fm) 3 and a range of quark masses with a lightest value corresponding to a (partially-quenched) pion mass of 275 MeV. When extrapolated to the physical values of the light quark masses, the results of our calculations are generally in good agreement with experimental determinations in the bottom sector. However, the static limit splittings between the Ω b − baryon and other bottom hadrons tend to slightly underestimate those obtained using the recent DØ measurement of the Ω b − .

Nonperturbative dynamics in the chromomagnetic QCD vacuum above the deconfinement temperature

We investigate the residual chromomagnetic nonperturbative interaction between colour sources which survives above the deconfinement temperature. This interaction is demonstrated to be attractive enough to support bound states of quarks (and gluons). Although, for heavy quarks, such bound states appear to be very shallow and thus to dissociate easily, for light quarks (and gluons), the interaction is found to be strong enough to lead to quark (gluon) pair creation in the vacuum and thus to a complete rebuild of the vacuum state.

Fluctuations of the baryonic flux-tube junction from effective string theory

In quenched QCD, where the dynamic creation of quark-antiquark pairs out of the vacuum is neglected, a confined baryonic system composed of three static quarks exhibits stringlike behavior at large interquark separation, with the formation of flux tubes characterized by the geometry of the so-called $Y$ ansatz. We study the fluctuations of the junction of the three flux tubes, assuming the dynamics to be governed by an effective bosonic string model. We show that the asymptotic behavior of the effective width of the junction grows logarithmically with the distance between the sources, with the coefficient depending on the number of joining strings, on the dimension of spacetime and on the string tension.