Quark Diquark Research Articles

MOMENTUM-SPACE FADDEEV CALCULATIONS FOR GROUND-STATE TRIPLY AND DOUBLY HEAVY BARYONS IN THE CONSTITUENT QUARK MODEL

In the framework of constituent quark model, mass spectra of the ground-state baryons consisting of three or two heavy (b or c) and one light (u, d or s) quarks are calculated by solving three-body Faddeev equations. The results imply that, it is possible to obtain a unified model to describe heavy baryons spectra, as well as meson and SU(3) octet and decuplet baryon spectra. We find that, when taking into account the relativistic correction quark–diquark approximation and three-body Faddeev approach tend to give similar predictions for heavy–light systems. We also study the spin splitting of JP = (1/2+) and JP = (3/2+).

QUARK–DIQUARK SYSTEMATICS OF BARYONS AND THE SU(6) SYMMETRY FOR LIGHT STATES

We continue our attempts to systematize baryons, which are composed of light quarks (q = u, d), as quark–diquark systems. The notion of two diquarks is used: (i) [Formula: see text], with the spin SD = 1 and isospin ID = 1 and (ii) [Formula: see text], with SD = 0 and ID = 0. Here we try to resolve the problem of the low-lying [Formula: see text] states: in the last experiments the lightest state is observed at ≳2200 MeV , not at 1900–2000 MeV as it has been stated 20 years ago. We are looking for different systematization variants with the forbidden low-lying [Formula: see text] states in the mass region ≲2000 MeV . We see that the inclusion of the SU(6) constraints on [Formula: see text] states with angular momentum L = 1 results in a shift of the lightest [Formula: see text] isobar to ~2300 MeV . The scheme with the SU(6) constraints for low-lying [Formula: see text] and [Formula: see text] states (with L = 0, 1) is presented in detail.

SEARCHING FOR THE QUARK–DIQUARK SYSTEMATICS OF BARYONS COMPOSED BY LIGHT QUARKS q = u, d

Supposing quark–diquark structure of baryons, we look for systematics of baryons composed of light quarks (q = u, d). We systematize baryons using the notion of two diquarks: (i) axial–vector state, [Formula: see text], with the spin SD = 1 and isospin ID = 1 and (ii) scalar one, [Formula: see text], with the spin SD = 0 and isospin ID = 0. We consider several schemes for the composed baryons: (1) with different diquark masses, [Formula: see text], (2) with [Formula: see text] and overlapping [Formula: see text] and [Formula: see text] states (resonances), (3) with/without SU(6) constraints for low-lying states (with quark–diquark orbital momenta L = 0). In the high-mass region, the model predicts several baryon resonances at M ~ 2.0–2.9 GeV . Moreover, the model gives us the double pole structure (i.e. two poles with the same Re M but different Im M) in many amplitudes at masses M ≳ 2.0 GeV . We see also that for description of low-lying baryons (with L = 0), the SU(6) constraint is needed.

Color triplet diquarks at the LHC

We consider a class of supersymmetric models containing baryon number violating processes such as observable n–n¯ oscillations that are mediated by color triplet diquark fields. For plausible values of the diquark–quark couplings, the scalar diquark with mass between a few hundred GeV and one TeV or so can be produced in the s-channel at the LHC and detected through its decay into a top quark and a hadronic jet.

Electromagnetic form factors of the Δ in an S-wave approach

Without any further adjusting of parameters, a relativistic constituent quark model, successful in the description of the data for the nucleon elastic form factors and of the dominant contribution for the nucleon to Δ electromagnetic transition, is used here to predict the dominant electromagnetic form factors of the Δ baryon. The model is based on a simple Δ wavefunction corresponding to a quark–diquark system in an S-state. The results for E0 and M1 are consistent both with experimental results and lattice calculations. The remaining form factors E2 and M3 vanish, given the symmetric structure taken for Δ.

Formula omitted] and [formula omitted] baryon production in scalar bc diquark fragmentation

We investigate the production of the triply heavy baryons Ω b b c and Ω b c c in a scalar bc diquark fragmentation. For this propose we calculate a fragmentation function in a quark–diquark model. The fragmentation probabilities turn out to be almost one order of magnitude larger than the case when a fully perturbative picture is adopted. We also analyze a two stage fragmentation of a heavy quark into a scalar diquark followed by the fragmentation of such a scalar diquark into a triply heavy baryon.

A nuclear matter description based on quark structure of the nucleon and pion exchange

We investigate the possibility to describe nuclear matter in an approach constrained by the prominent features of quantum chromodynamics. We mapped the in-medium nucleon self-energies of a point coupling relativistic mean-field model on self-energies obtained in effective theories of QCD. More precisely, the contributions to the nucleon self-energy have been separated into the short range part, driven principally by the quark structure of the nucleon described in a quark–diquark picture, and the long range part, dictated by pion dynamics and determined using in-medium chiral perturbation theory. A saturation point, although unrealistic, is obtained without any free parameter. A realistic description of nuclear matter saturation properties has been obtained with the inclusion of a small phenomenological correction term to the short range part of the self-energy.

Wounded nucleons, wounded quarks: an update

History and recent developments of the concept of wounded hadronic constituents are summarized, with the special attention to the quark–diquark model of particle production in nucleon–nucleon and nucleus–nucleus collisions.

Further evidence for [formula omitted] from photoproduction of hyperons

We report further evidence for N(1900)P13 from an analysis of a large variety of photo- and pion-induced reactions, in particular from the new CLAS measurements of double polarization observables for photoproduction of hyperons. The data are consistent with two classes of solutions both requiring contributions from N(1900)P13 but giving different N(1900)P13 pole positions. (M−iΓ/2)=(1915±50)−i(90±25) MeV covers both solutions. The small elasticity of 10% or less explains why it was difficult to observe the state in πN elastic scattering.N(1900)P13 is a 2-star resonance which is predicted by symmetric three-quark models. In diquark–quark models, the existence of the state is not expected.

Masses of excited heavy baryons in the relativistic quark–diquark picture

The mass spectra of the excited heavy baryons consisting of two light (u,d,s) and one heavy (c,b) quarks are calculated in the heavy-quark–light-diquark approximation within the constituent quark model. The light quarks, forming the diquark, and the light diquark in the baryon are treated completely relativistically. The expansion in v/c up to the second order is used only for the heavy (b and c) quarks. The internal structure of the diquark is taken into account by inserting the diquark–gluon interaction form factor. An overall good agreement of the obtained predictions with available experimental data is found.

Transversity quark distributions in a covariant quark–diquark model

Transversity quark light-cone momentum distributions are calculated for the nucleon. We utilize a modified Nambu–Jona-Lasinio model in which confinement is simulated by eliminating unphysical thresholds for nucleon decay into quarks. The nucleon bound state is obtained by solving the relativistic Faddeev equation in the quark–diquark approximation, where both scalar and axial-vector diquark channels are included. Particular attention is paid to comparing our results with the recent experimental extraction of the transversity distributions by Anselmino et al. We also compare our transversity results with earlier spin-independent and helicity quark distributions calculated in the same approach.

Effects of a multi-quark interaction on color superconducting phase transition in an extended NJL model

We study the interplay of the chiral and the color superconducting phase transition in an extended Nambu–Jona-Lasinio model with a multi-quark interaction that produces the nonlinear chiral–diquark coupling. We observe that this nonlinear coupling adds up coherently with the ω2 interaction to either produce the chiral–color superconductivity coexistence phase or cancel each other depending on its sign. We discuss that a large coexistence region in the phase diagram is consistent with the quark–diquark picture for the nucleon whereas its smallness is the prerequisite for the applicability of the Ginzburg–Landau approach.

Production of [formula omitted] and [formula omitted] baryons in quark–diquark model

Using perturbative quantum chromodynamics and quark–diquark model of baryons, we have obtained an analytical fragmentation function which describes Ω b b c and Ω b c c baryons in b and c quarks fragmentation. The universal fragmentation probabilities, the average fragmentation parameters and the production cross sections resulting from this function are in a very good agreement with those obtained using a full perturbative regime. We conclude that the quark–diquark model of baryons works well when a scalar heavy diquark is considered in a heavy quark fragmentation into a triply heavy baryon.

EMC and polarized EMC effects in nuclei

We determine nuclear structure functions and quark distributions for 7 Li, 11 B, 15 N and 27 Al. For the nucleon bound state we solve the covariant quark–diquark equations in a confining Nambu–Jona-Lasinio model, which yields excellent results for the free nucleon structure functions. The nucleus is described using a relativistic shell model, including mean scalar and vector fields that couple to the quarks in the nucleon. The nuclear structure functions are then obtained as a convolution of the structure function of the bound nucleon with the light-cone nucleon distributions. We find that we are readily able to reproduce the EMC effect in finite nuclei and confirm earlier nuclear matter studies that found a large polarized EMC effect.

Two nucleon states in a chiral quark–diquark model

We study the ground and first excited states of nucleons in a chiral quark–diquark model. We include two quark–diquark channels of the scalar–isoscalar and axial-vector–isovector types for the nucleon states. The diquark correlation violating the spin–flavour SU(4)SF symmetry allows us to treat the two quark–diquark channels independently. Hence the two states appear as the superpositions of the two quark–diquark channels; one is the nucleon and the other is a state which does not appear in the SU(4)SF quark models. With a reasonable choice of model parameters, the mass of the excited state appears at around 1.5 GeV, which we identify with the Roper resonance N(1440).

The phases of isospin-asymmetric matter in the two-flavor NJL model

We investigate the phase diagram of isospin-asymmetric matter at T=0 in the two-flavor Nambu–Jona-Lasinio model. Our approach describes the single nucleon as a confined quark–diquark state, the saturation properties of nuclear matter at normal densities, and the phase transition to normal or color superconducting quark matter at higher densities. The resulting equation of state of charge-neutral matter and the structure of compact stars are discussed.

Nucleon quark distributions in a covariant quark–diquark model

Spin-dependent and spin-independent quark light-cone momentum distributions and structure functions are calculated for the nucleon. We utilize a modified Nambu–Jona-Lasinio model in which confinement is simulated by eliminating unphysical thresholds for nucleon decay into quarks. The nucleon bound state is obtained by solving the Faddeev equation in the quark–diquark approximation, where both scalar and axial-vector diquark channels are included. We find excellent agreement between our model results and empirical data.

Estimations of at RHIC from a QGP model with diquarks

Assuming that axial-vector and scalar diquarks exist in the quark-gluon plasma (QGP) near the critical temperature Tc, baryons can be produced from quark–diquark interactions. In RHIC conditions ( and 200 GeV), the ratio may be larger than 1, based on the concept that QGP with diquarks would exist. This unusual result might be a helpful evidence for QGP existing in RHIC.

Forward and midrapidity like-particle ratios from [formula omitted] collisions at [formula omitted

We present a measurement of π−/π+, K−/K+ and p¯/p from p+p collisions at s=200 GeV over the rapidity range 0<y<3.4. For pT<2.0 GeV/c we see no significant transverse momentum dependence of the ratios. All three ratios are independent of rapidity for y≲1.5 and then steadily decline from y∼1.5 to y∼3. The π−/π+ ratio is below unity for y>2.0. The p¯/p ratio is very similar for p+p and 20% central Au+Au collisions at all rapidities. In the fragmentation region the three ratios seem to be independent of beam energy when viewed from the rest frame of one of the protons. Theoretical models based on quark–diquark breaking mechanisms overestimate the p¯/p ratio up to y≲3. Including additional mechanisms for baryon number transport such as baryon junctions leads to a better description of the data.

A diquark model for pentaquark baryons and tetraquark mesons

In the spirit of quark–diquark model, used earlier for the nucleon [Ann. Phys. (N.Y.) 108 (1977) 198] we investigate here some properties of the newly discovered exotic pentaquark systems, viz., θ + ( 1540 ) and (say) ζ 0 ( 3099 ) . For this purpose, in the first stage, we consider these multiquark states as three-body systems—two diquarks of ( u d ) -type and an antiquark with a Coulomb-type color force between them. In the second stage, we replace this three-body system with an effective two-body system in which the two bound diquarks constitute the core and the extra (fifth) antiquark (valence) revolves around this core in a Coulomb field. After accounting for the extended character of the diquark from the earlier work, we compute the form factors for these pentaquark systems and make some remarks about their magnetic moments. Predictions are also made for the form factors of the four-quark (tetraquark) mesons, i.e., for the diquark–antidiquark bound systems. In both θ + ( 1540 ) and ζ 0 ( 3099 ) the valance antiquarks are found to be embedded in the core unlike the case of nucleon.