Narrow Results

We propose the enhancement of Λ_c yield in heavy ion collisions at RHIC and LHC as a novel signal for the existence of diquarks in the strongly coupled quark-gluon plasma produced in these collisions as well as in the Λ_c. Assuming that stable bound diquarks can exist in the quark-gluon plasma, we argue that the yield of Λ_c would be increased by two-body collisions between [ud] diquarks and c quarks, in addition to normal three-body collisions among u, d and c quarks. A quantitative study of this effect based on the coalescence model shows that including the contribution of diquarks to Λ_c production indeed leads to a substantial enhancement of the Λ_c/D ratio in heavy ion collisions.

We study the color confinement of QCD with the Coulomb gauge in lattice simulations. The color-Coulomb instantaneous interaction in color-singlet channel causes a linearly rising potential at large distances. We also estimate the other color-channel forces for two-quark states, i.e. color-octet, color-sextet and color-antitriplet channels. Moreover we investigate the volume dependence of color-nonsinglet potentials on finite lattices.

We consider the interplay of the pentaquark states and strange tetraquark states in the decay ${\Lambda }_b^0\to K^{-}J/\psi p$. Possible existence of ($cs\bar{c}ū$)-states is taken up and their manifestation in the $K^{-}J/\psi$-channel is discussed. It is emphasized that these exotic mesons can imitate broad bumps in the $pJ/\psi$-channel.

The diquark–diquark–antiquark model describes pentaquark states both in terms of quarks and hadrons. The latest LHCb data for pentaquarks with open charm emphasize the importance of hadron components in the structure of pentaquarks. We discuss pentaquark states with hidden charm $P(\bar{c}cuud)$ and those with open charm $P(ūussc)$ which were discovered recently in LHCb data ($J/\mathrm{\Psi }p$ and ${\mathrm{\Xi }}_c^{+}{K}^{-}$ spectra correspondingly). Considering the observed states as members of the lowest ($s$-wave) multiplet, we discuss the mass splitting of states and the dumping of their widths.

We discuss the hadroproductions of Λ_c, , D and in the framework of the constituent quark–diquark cascade model taking into account the valence quark annihilation. The spectra of Λ_c and in pA, Σ^-A and π^-A collisions are well explained by the model using the values of parameters used in hadroproductions of D and . It is shown that the role of valence diquark in the incident baryon is important for productions as well as for Λ_c production.

Since LEPS collaboration reported the first evidence of Θ⁺ pentaquark in early 2003, eleven other experimental groups have confirmed this exotic state while many other groups did not see any signal. If this state is further established by future high statistical experiments, its discovery shall be one of the most important events in hadron physics for the past three decades. This exotic baryon with such a low mass and so narrow a width imposes a big challenge to hadron theorists. Up to now, there have appeared more than two hundred theoretical papers trying to interpret this charming state. I will review some important theoretical developments on pentaquarks based on my biased personal views.

I first give a brief overview of the status of exotic hadron search so far. Then I review the recent theoretical development on pentaquarks. Finally I emphasize that none of theoretical formalisms predicts the existence of Θ pentaquark very reliably. None of them explains its narrow width in a natural way.

The diquark structure in baryons is commonly accepted as a reasonable approximation which can much simplify the picture of baryons and reduce the length of calculations. However, a diquark by no means is a point-like particle, even though it is treated as a whole object. Therefore, to apply the diquark picture to phenomenological calculations, at the effective vertices for the diquark–gauge-boson interactions, suitable form factors must be introduced to compensate the effects caused by the inner structure of the diquark. It is crucial to derive the appropriate form factors for various interactions. In this work, we use the Bethe–Salpeter equation to derive such form factors and numerically evaluate their magnitudes. Our results not only qualitatively, but also quantitatively confirm the form factors phenomenologically introduced in literature.

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, , with the spin S_D = 1 and isospin I_D = 1 and (ii) scalar one, , with the spin S_D = 0 and isospin I_D = 0. We consider several schemes for the composed baryons: (1) with different diquark masses, , (2) with and overlapping and 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.

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) , with the spin S_D = 1 and isospin I_D = 1 and (ii) , with S_D = 0 and I_D = 0. Here we try to resolve the problem of the low-lying 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 states in the mass region ≲2000 MeV. We see that the inclusion of the SU(6) constraints on states with angular momentum L = 1 results in a shift of the lightest isobar to ~2300 MeV. The scheme with the SU(6) constraints for low-lying and states (with L = 0, 1) is presented in detail.

We review the experimental searches for new particles in the dijet mass spectrum conducted at the CERN , the Fermilab Tevatron Collider, and the CERN Large Hadron Collider. The theory of the QCD background and new particle signals is reviewed, with emphasis on the choices made by the experiments to model the background and signal. The experimental techniques, data, and results of dijet resonance searches at hadron colliders over the last quarter century are described and compared. Model independent and model specific limits on new particles decaying to dijets are reviewed, and a detailed comparison is made of the recently published limits from the ATLAS and CMS experiments.

The Bialas–Bzdak model of elastic proton–proton scattering is generalized to the case when the real part of the parton–parton level forward scattering amplitude is nonvanishing. Such a generalization enables the model to describe well the dip region of the differential cross-section of elastic scattering at the intersecting storage rings (ISR) energies, and improves significantly the ability of the model to describe also the recent TOTEM data at LHC energy. Within this framework, both the increase of the total cross-section, as well as the decrease of the location of the dip with increasing colliding energies, is related to the increase of the quark–diquark distance and to the increase of the "fragility" of the protons with increasing energies. In addition, we present and test the validity of two new phenomenological relations: one of them relates the total p+p cross-section to an effective, model-independent proton radius, while the other relates the position of the dip in the differential elastic cross-section to the measured value of the total cross-section.

The Bialas–Bzdak model of elastic proton–proton scattering assumes a purely imaginary forward scattering amplitude, which consequently vanishes at the diffractive minima. We extended the model to arbitrarily large real parts in a way that constraints from unitarity are satisfied. The resulting model is able to describe elastic pp scattering not only at the lower ISR energies but also at in a statistically acceptable manner, both in the diffractive cone and in the region of the first diffractive minimum. The total cross-section as well as the differential cross-section of elastic proton–proton scattering is predicted for the future LHC energies of , 14, 15 TeV and also to 28 TeV. A nontrivial, significantly nonexponential feature of the differential cross-section of elastic proton–proton scattering is analyzed and the excitation function of the nonexponential behavior is predicted. The excitation function of the shadow profiles is discussed and related to saturation at small impact parameters.

Exotic mesons with hidden strange $(s\bar{s})$ and heavy quark pairs $(Q\bar{Q})$, where $Q=c$, $b$, are considered as diquark–antidiquark systems, $(Qs)\cdot (\bar{Q}\bar{s})$. Taking into account that these states can recombinate into two-meson ones, we study the interplay of these states in terms of the dispersion relation $D$-function technique. The classification of exotic mesons is discussed, coefficients for decay modes are given, predictions for new states are presented. The nonet structure for $(Qq)\cdot (\bar{Q}\bar{q})$, $(Qs)\cdot (\bar{Q}\bar{s})$, $(Qq)\cdot (\bar{Q}\bar{s})$-states $(q=u,d)$ is suggested.

Nonstrange and strange pentaquarks with hidden charm are considered as diquark–diquark–antiquark composite systems. Spin and isospin content of such exotic states is discussed and masses are evaluated.

We study the spectra of the doubly charmed tetraquark states in a diquark–antidiquark model. The doubly charmed tetraquark states form an antitriplet and sextet configurations according to flavor SU(3) symmetry. For the tetraquark state $[q{q}^{\prime }][\bar{c}\bar{c}]$, we show the mass for both bound and excited states. The two-body decays of tetraquark states $T^{cc}[0^{+}]$ and $T^{cc}[1^{--}]$ to charmed mesons have also been studied. In the end, the doubly charmed tetraquarks decays to a charmed baryon and a light baryon have been studied in the SU(3) flavor symmetry.

The mass spectra of light-heavy tetraquarks $cq\bar{c}\bar{q}$ ($q=u,d$) are computed in a nonrelativistic diquark model with one-gluon exchange plus confining potential. In the diquark model, a $cq\bar{c}\bar{q}$ state is regarded to be made of a light-heavy diquark ($qc$) and an antidiquark $\bar{q}\bar{c}$ in antitriplet and triplet color configuration, respectively. The masses of charm mesons were calculated in order to fit the model parameters used to create the masses of tetraquarks and therefore enhance the model’s reliability. The masses of $(cq\bar{c}\bar{q})$ tetra-quark states are determined to be in the range of 3.8–4.7GeV, which is consistent with the experimentally reported charmonium-like states. In particular, the $Z_c(3900)$, $Z_c(4430)$ and $\psi (4660)$ tetraquarks, which have been seen experimentally, may all be described by our model.

We investigate the influence of retardation effects on covariant 3-dimensional wave functions for bound hadrons. Within a quark-(scalar) diquark representation of a baryon, the four-dimensional Bethe–Salpeter equation is solved for a 1-rank separable kernel which simulates Coulombic attraction and confinement. We project the manifestly covariant bound state wave function into three dimensions upon integrating out the non-static energy dependence and compare it with solutions of three-dimensional quasi-potential equations obtained from different kinematical projections on the relative energy variable. We find that for long-range interactions, as characteristic in QCD, retardation effects in bound states are of crucial importance.

We discuss the relation between the polarization of inclusively produced (anti)hyperons and the incident baryon states in the framework of the constituent quark–diquark cascade model. We assume that there is an intrinsic diquark–antidiquark state in the incident baryon, in which the intrinsic diquark immediately fragments into a non-leading baryon and the antidiquark behaves as a valence constituent. It is also assumed that the valence (anti)diquark in the incident nucleon tends to combine selectively with a spin-down sea quark and, on the other hand, the spin-up valence quark in the projectile is chosen by a sea (anti)diquark in preference to the spin-down valence quark. It is found that the incident spin-1/2 baryon is mainly composed of a spin-0 valence diquark and a valence quark, and contains an intrinsic diquark–antidiquark state with a probability of about 7%.

One of the main problems in baryon spectroscopy is the presence of many so-called "missing resonances". A possible solution to this problem is to describe two correlated quarks inside the baryons by means of the diquark effective degree of freedom. Three different quark-diquark model are developed and compared. Some theoretical predictions are given and compared with the available experimental data.