Quark-antiquark Pair Creation Research Articles

Detailed derivation of the $\mathbf{^3P_0}$ strong decay model applied to baryons

Abstract We provide an in-detail derivation, through the $^3P_0$ pair creation model, of the transition matrix for a baryon decaying into a meson baryon system. The meson's analysis was successfully conducted in Ref.~\cite{Segovia:2012cd} and we extend the same formalism to the baryon sector, focusing on the $\Delta(1232)\to \pi N$ strong decay width because all hadrons involved in the reaction are very well established, the two hadrons in the final state are stable avoiding further analysis, all quarks are light and so equivalent, and the decay width of the process is relatively well measured. Taking advantage of a very common Rayleigh-Ritz variational method for solving the 2- and 3-body Sch"odinger bound-state equation in which the hadron's radial wave functions are expanded in terms of a Gaussian basis, the expression of the invariant matrix element can be related with the mean-square radii of hadrons involved in the decay. We use their experimental measures in such a way that only the strength of the quark-antiquark pair creation from the vacuum is a free parameter. This is then taken from our previous study of strong decay widths in the meson sector~\cite{Segovia:2012cd}, obtaining a quite compatible result with experiment for the calculated $\Delta(1232)\to \pi N$ decay width and indicating a possible path towards a unified and global description of the baryon and meson strong decay widths within the same constituent quark model framework. Finally, this research work has been developed in setting the stage for a novel raft of applications to exotic hadrons, \emph{i.e.} the description of baryon's coupling to meson-baryon thresholds, one of the mechanism that is thought to be responsible of providing either a large renormalization to naive states or genuine dynamically-generated meson-baryon molecules.Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd

Charmonium states in a coupled-channel model

We systematically investigate the mass spectrum and two-body open-charm strong decays of charmonium states in a coupled-channel model where the 3P0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3P_0$$\\end{document} quark-antiquark pair creation mechanism is employed. The results of masses, mass shifts, proportions of the cc¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$c\\bar{c}$$\\end{document} component, and open-charm decay widths are provided. The S-D wave mixing angles and di-electric decay widths for vector mesons are also presented. Based on our results, we find that the ψ(3770)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\psi (3770)$$\\end{document}, ψ(4040)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\psi (4040)$$\\end{document}, ψ(4160)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\psi (4160)$$\\end{document}, ψ(4360)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\psi (4360)$$\\end{document}, and ψ(4415)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\psi (4415)$$\\end{document} can be assigned as the 13D1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1^3D_1$$\\end{document}-, 33S1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3^3S_1$$\\end{document}-, 23D1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2^3D_1$$\\end{document}-, 43S1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$4^3S_1$$\\end{document}-, and 33D1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3^3D_1$$\\end{document}-dominated charmonium states, respectively. The ψ3(3842)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\psi _3(3842)$$\\end{document} is a good candidate of the ψ3(1D)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\psi _3(1D)$$\\end{document} charmonium state. The calculated mass and strong decay width of χc1(2P)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\chi _{c1}(2P)$$\\end{document} with significant continuum contribution (∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim $$\\end{document}57%) favor the charmonium interpretation for the mysterious χc1(3872)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\chi _{c1}(3872)$$\\end{document}. When considering the large uncertainty in the observed decay width, the possibility to assign the χc0(3860)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\chi _{c0}(3860)$$\\end{document} as the χc0(2P)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\chi _{c0}(2P)$$\\end{document} charmonium state cannot be ruled out. One may describe well the properties of χc2(3930)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\chi _{c2}(3930)$$\\end{document} with the χc2(2P)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\chi _{c2}(2P)$$\\end{document} charmonium. The predictions on properties of other cc¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$c\\bar{c}$$\\end{document} states can be tested by future experiments.

Open charm and charmonium states in strong magnetic fields

The mass modifications of the open charm ([Formula: see text] and [Formula: see text]) mesons, and their effects on the decay widths [Formula: see text] as well as of the charmonium state, [Formula: see text] to open charm mesons ([Formula: see text]), are investigated in the presence of strong magnetic fields. These are studied accounting for the mixing of the pseudoscalar ([Formula: see text]) and vector ([Formula: see text]) mesons ([Formula: see text], [Formula: see text] mixings), with the mixing parameter, [Formula: see text] of a phenomenological three-point ([Formula: see text]) vertex interaction determined from the observed radiative decay width of [Formula: see text]. For charged [Formula: see text] mixing, this parameter is dependent on the magnetic field, because of the Landau level contributions to the vacuum masses of these mesons. The masses of the charged [Formula: see text] and [Formula: see text] mesons modified due to [Formula: see text] mixing, in addition, have contributions from the lowest Landau levels in the presence of a strong magnetic field. The effects of the magnetic field on the decay widths are studied using a field theoretical model of composite hadrons with quark (and antiquark) constituents. The matrix elements for these decays are evaluated using the light quark–antiquark pair creation term of the free Dirac Hamiltonian for the constituent quark field, with explicit constructions for the charmonium state [Formula: see text], the open charm ([Formula: see text], [Formula: see text], [Formula: see text]) mesons and the pion states in terms of the constituent quark fields. The parameter for the charged [Formula: see text] mixing is observed to increase appreciably with increase in the magnetic field. This leads to dominant modifications to their masses, and hence the decay widths of charged [Formula: see text] as well as [Formula: see text] at large values of the magnetic field. The modifications of the masses and decay widths of the open and hidden charm mesons in the presence of strong magnetic fields should have observable consequences on the production of the open charm ([Formula: see text] and [Formula: see text]) mesons as well as of the charmonium states resulting from noncentral ultra-relativistic heavy ion collision experiments.

Strange mesons in strong magnetic fields

The masses of the strange mesons ([Formula: see text], [Formula: see text] and [Formula: see text]) are investigated in the presence of strong magnetic fields. The changes in the masses of these mesons arise from the mixing of the pseudoscalar and vector mesons in the presence of a magnetic field. For the charged mesons, these mass modifications are in addition to the contributions from the lowest Landau energy levels to their masses. The decay widths, [Formula: see text] and [Formula: see text], in the presence of the magnetic field are studied using a field theoretic model of composite hadrons with constituent quarks/antiquarks. The model uses the free Dirac Hamiltonian in terms of the constituent quark fields as the light quark–antiquark pair creation term and explicit constructions for the meson states in terms of the constituent quarks and antiquarks to study the decay processes. The pseudoscalar–vector (PV) meson mixing leads to a drop (rise) in the mass of the pseudoscalar (longitudinal component of the vector) meson. It is observed that the mass modifications for the hidden strange mesons, arising from [Formula: see text] mixing, are quite prominent, whereas the neutral open strange mesons have only marginal changes in their masses due to mixing. The mass modifications of the charged open strange mesons are due to interplay between the Landau-level contributions and the PV mixing, and, the latter effect is observed to dominate at large values of the magnetic field. The vector meson decay widths ([Formula: see text] and [Formula: see text]) involving the charged mesons are observed to be quite different from the widths involving neutral mesons, due to the additional contribution for the charged mesons from the Landau levels.

Spectral functions of strange vector mesons in asymmetric hyperonic matter

We study the medium modifications of the spectral functions as well as production cross-sections of the strange vector mesons ($\phi$, $K^*$ and $\bar {K^*}$) in isospin asymmetric strange hadronic matter. These are obtained from the in-medium masses of the open strange mesons and the decay widths $\phi \rightarrow K\bar K$, $K^* \rightarrow K\pi$ and $\bar {K^*} \rightarrow {\bar K}\pi$ in the hadronic medium. The decay widths are computed using a field theoretic model of composite hadrons with quark/antiquark constituents, from the matrix element of the light quark-antiquark pair creation term of the free Dirac Hamiltonian between the initial and final states. The matrix element is multiplied with a coupling strength parameter for the light quark-antiquark pair creation, which is fitted to the observed vacuum decay width of the decay process. There are observed to be substantial modifications of the spectral functions as well as production cross-sections of these vector mesons due to isospin asymmetry as well as strangeness of the hadronic medum at high densities. These studies should have observable consequences, e.g. in the yield of the hidden and open strange mesons arising from the isospin asymmetric high energy heavy ion collisions at the Compressed baryonic matter (CBM) experiments at the future facility at GSI.

Unquenching the Quark Model in a Nonperturbative Scheme

In recent years, the discovery in quarkonium spectrum of several states not predicted by the naive quark model has awakened a lot of interest. A possible description of such states requires the enlargement of the quark model by introducing quark-antiquark pair creation or continuum coupling effects. The unquenching of the quark models is a way to take these new components into account. In the spirit of the Cornell Model, this is usually done by coupling perturbatively a quark-antiquark state with definite quantum numbers to the meson-meson channel with the closest threshold. In this work we present a method to coupled quark-antiquark states with meson-meson channels, taking into account effectively the nonperturbative coupling to all quark-antiquark states with the same quantum numbers. The method will be applied to the study of the X(3872) resonance and a comparison with the perturbative calculation will be performed.

Extended-soft-core baryon-baryon model ESC16. I. Nucleon-nucleon scattering

Background: The Nijmegen extended-soft-core (ESC) model ESC16, as well as its predecessors ESC04--ESC08, describe the nucleon-nucleon ($NN$), hyperon-nucleon ($YN$), and hyperon-hyperon/nucleon ($YY/\mathrm{\ensuremath{\Xi}}\phantom{\rule{4pt}{0ex}}N$) interactions in a unified way using broken SU(3) symmetry. SU(3) symmetry serves to connect the $NN$ with the $YN$ and the $YY$ channels. In the spirit of the Yukawa approach to the nuclear force problem, the interactions are studied from the meson-exchange picture viewpoint, using generalized soft-core Yukawa functions. The meson exchanges are supplemented with diffractive contributions due to multiple-gluon exchanges. The extended-soft-core (ESC) meson-exchange interactions consist of local and nonlocal potentials due to (i) one-boson exchanges (OBE), which are the members of nonets of pseudoscalar, vector, scalar, and axial-vector mesons, (ii) diffractive exchanges, (iii) two-pseudoscalar exchange (PS-PS), and (iv) meson-pair exchange (MPE). The OBE and MPE vertices are regulated by Gaussian form factors producing potentials with a soft behavior near the origin. The assignment of the cutoff masses for the BBM vertices is dependent on the SU(3) classification of the exchanged mesons for OBE and a similar scheme for MPE.Purpose: The evolution of the ESC approach to the ESC16 model for the baryon-baryon (BB) interactions of the SU(3) flavor octet of baryons (N, $\mathrm{\ensuremath{\Lambda}}$, $\mathrm{\ensuremath{\Sigma}}$, and $\mathrm{\ensuremath{\Xi}}$) is described and presented. In this first of a series of papers, the $NN$ model and results are reported in detail.Methods: Important nonstandard ingredients in the OBE sector in the ESC models are (i) the axial-vector meson potentials, and (ii) a zero in the scalar- and axial-vector meson form factors. Furthermore, the strange scalar $\ensuremath{\kappa}$ meson is treated within the scheme of the Gell-Mann-Okubo mass relations, and like the $\ensuremath{\rho}$ and $\ensuremath{\epsilon}$ treated as a broad meson. The multiple-gluon exchanges are elaborated further by adding contributions due to odd number of gluon exchanges. A novel contribution is the incorporation of structural effects due to the quark core of the baryons. In establishing the parameters of the model a simultaneous fit to $NN$ and $YN$ channels has been performed. The meson-baryon coupling constants are calculated via SU(3) using the coupling constants of the $NN\ensuremath{\bigoplus}YN$ analysis as input. In ESC16 the couplings are kept completely SU(3) symmetric. About 25 physical coupling parameters and 8 cutoff and diffractive masses were searched.Results: In the fit to $NN$ and $YN$ many parameters are essentially fixed by the $NN$ data. A few, but severely constrained parameters, e.g., $F/(F+D)$ ratios, are left for determination of the $YN$ interactions and the $YY$ experimental indications. The simultaneous fit of the ESC models to the $NN$- and $YN$-scattering data with a single set of parameters has achieved excellent results for the $NN$ and $YN$ data, and for the $YY$ data in accordance with the experimental indications for $\mathrm{\ensuremath{\Lambda}}\mathrm{\ensuremath{\Lambda}}$ and $\mathrm{\ensuremath{\Xi}}N$. In the case of ESC16, the version discussed here, the achievements are: (i) For the selected 4313 $pp$ and $np$ scattering data with energies $0\ensuremath{\le}{T}_{\text{lab}}\ensuremath{\le}350\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$, the model reaches a fit having ${\ensuremath{\chi}}^{2}/{N}_{\text{data}}=1.10$. (ii) The deuteron binding energy and all the $NN$ scattering lengths are fitted very nicely. (iii) The $YN$ data are described very well with ${\ensuremath{\chi}}^{2}/{N}_{\text{data}}=1.04$, giving at the same time a description of the $\mathrm{\ensuremath{\Xi}}N$ cross sections in agreement with the experimental indications.Conclusions: The ESC approach leads to an excellent description of the $NN$ and $YN$ data, and for the scarce $YY$ data. The added innovations as well as the treatment of mass broken SU(3) make it possible to keep the meson coupling parameters and the $F/(F+D)$ ratios of the model qualitatively in accordance with the predictions of the $^{3}P_{0}$-dominated quark-antiquark pair creation (QPC) model. The information about estimates of (i) the $\mathrm{\ensuremath{\Lambda}}$- and $\mathrm{\ensuremath{\Sigma}}$-nuclear well-depth, and (ii) the $\mathrm{\ensuremath{\Lambda}}\mathrm{\ensuremath{\Lambda}}$ hypernuclei played an important role in the form of using constraints. In particular, the experimental indications for the $\mathrm{\ensuremath{\Lambda}}\mathrm{\ensuremath{\Lambda}}$-attraction and the $\mathrm{\ensuremath{\Sigma}}$-nuclear well-depth were directive.

A Multiquark Approach to Excited Hadrons and Regge Trajectories

We propose a novel approach to construction of hadron spectroscopy. The case of light nonstrange mesons is considered. By assumption, all such mesons above 1 GeV appear due to creation of constituent quark-antiquark pairs inside π or ρ(ω) mesons. These spin-singlet or triplet pairs dictate the quantum numbers of formed resonance. The resulting classification of light mesons turns out to be in a better agreement with the experimental observations than the standard quark model classification. It is argued that the total energy of quark components should be proportional to the hadron mass squared rather than the linear mass. As a byproduct a certain relation expressing the constituent quark mass via the gluon and quark condensate is put forward. We show that our approach leads to an effective mass counting scheme for meson spectrum and results in the linear Regge and radial Regge trajectories by construction. An experimental observation of these trajectories might thus serve as evidence not for string but for multiquark structure of highly excited hadrons.

Dual Meissner Effect and Quark Confinement Potential in SU(3) Dual QCD Formalism

The mechanism of color confinement has been studied in the framework of SU (3) color gauge theory in terms of abelian fields and monopoles extracted by adopting magnetic symmetry. The existence of the mechanism of color confinement corresponds to the dual Meissner effect caused by monopoles. The two length scales, i.e. the penetration depth and coherence length are defined to demonstrate the scaling nature of the QCD vacuum and their ratio defines the Ginzburg-Landau parameter indicating the border of type I and type II dual superconductor. The existence of these two length scales describes the intrinsic shape of the confining flux-tube and is a characteristic of the dual superconductor model of confinement in QCD. As a result, the quark confining potential has been computed and the resulting expression of string tension has been constructed in the infrared sector of SU (3) Dual QCD formulation. Moreover, with the introduction of dynamical quarks the flux tube breaks and leads to the creation of quark anti-quark pairs. Finite temperature quark confining potential and the associated string tension has also been extracted which demonstrates a considerable reduction in the vicinity of critical temperature showing agreement with the recent lattice studies.

Decay widths of bottomonium states in matter: A field theoretic model for composite hadrons

We compute the in-medium partial decay widths of the bottomonium states to open bottom mesons ($B\bar B$) using a field theoretical model for composite hadrons with quark constituents. These decay widths are calculated by using the explicit constructions for the bottomonium states and the open bottom mesons ($B$ and $\bar B$), and, the quark antiquark pair creation term of the free Dirac Hamiltonian written in terms of the constituent quark field operators. These decay widths in the hadronic medium are calculated as arising from the mass modifications of the bottomonium states and the $B$ and $\bar B$ mesons, obtained in a chiral effective model. The decay amplitude in the present model is multiplied with a strength parameter for the light quark pair creation, which is fitted from the observed vacuum partial decay width of the bottomonium state, $\Upsilon (4S)$ to $B\bar B$. The effects of the isospin asymmetry, the strangeness fraction of the hadronic matter on the decay widths, arising due to the mass modifications due to these effects, have also been studied. There is observed to be appreciable effects from density, and the effects from isospin asymmetry on the parital decay widths of $\Upsilon \rightarrow B\bar B$ are observed to be quite pronounced at high densities. These effects should show up in the asymmetric heavy ion collisions in Compressed baryonic matter (CBM) experiments planned at the future facility at FAIR. The study of the $\Upsilon$ states will, however, require access to energies higher than the energy regime planned at the CBM experiment. The density effects on the decay widths of the bottomonium states should show up in the production of these states, as well as, in dilepton spectra at the Super Proton Synchrotron (SPS) energies.

Self-energies of octet and decuplet baryons due to the coupling to the baryon-meson continuum

We present an unquenched quark model calculation of the mass shifts of ground-state octet and decuplet baryons due to the coupling to the meson-baryon continuum. All ground-state baryons and pseudoscalar mesons are included in our calculation as intermediate states. The $q\bar{q}$ pair creation effects are taken explicitly into account through a microscopic, QCD-inspired, quark-antiquark pair creation mechanism.

In-medium decay widths of hidden and open charm vector mesons in a field theoretic model for composite hadrons

We calculate the decay widths of the charmonium states, J/ψ, ψ(3686) and ψ(3770), to [Formula: see text] pairs, as well as the decay width of D* → Dπ, in isospin asymmetric strange hadronic matter, using a field theoretical model for composite hadrons with quark constituents. For this purpose, we use the quark–antiquark pair creation term of the free Dirac Hamiltonian written in terms of the constituent quark field operators, and use explicit charmonium, D, [Formula: see text], D* and π states to evaluate the matrix elements for the charmonium as well as D* decay amplitudes. The medium modifications of the partial decay widths of charmonium to [Formula: see text] pair, arising from the mass modifications of the [Formula: see text] and the charmonium states calculated in a chiral effective model, are also included. The results of the present investigations are then compared with the decay widths computed earlier, in a model using light quark pair creation in 3P0 state. As in 3P0 model, the decay amplitude in the present model is multiplied with a strength parameter for the light quark pair creation, which is fitted from the observed vacuum decay width. The effects of the isospin asymmetry, the strangeness fraction of the hadronic matter on the masses of the charmonium states and [Formula: see text] mesons and hence on the decay widths, have also been studied. The isospin asymmetry effect is observed to be dominant for high densities, leading to appreciable difference in the decay channels of the charmonium to D+ D- and [Formula: see text] pairs. The decay width of D* → Dπ in the hadronic matter has also been calculated within the composite quark model in the present work, accounting for the medium modifications of the D and D* masses. The density modifications of the charmonium states and D(D*) mesons, which are observed to be appreciable at high densities, will be of relevance in the compressed baryonic matter (CBM) experiments at the future facility of FAIR, GSI, where charmed hadrons will be produced by annihilation of antiprotons on nuclei. The interactions of the charmonium states and D(D*) with the nuclear medium could lead to the possibility of the formation of exotic bound states of the nuclei with the (excited) charmonium states as well as with D(D*) mesons.

Electromagnetic instability in holographic QCD

Using the AdS/CFT correspondence, we calculate the vacuum decay rate for the Schwinger effect in confining large $N_{c}$ gauge theories. The instability is induced by the quark antiquark pair creation triggered by strong electromagnetic fields. The decay rate is obtained as the imaginary part of the Euler-Heisenberg effective Lagrangian evaluated from the D-brane action with a constant electromagnetic field in holographic QCD models such as the Sakai-Sugimoto model and the deformed Sakai-Sugimoto model. The decay rate is found to increase with the magnetic field parallel to the electric field, while it decreases with the magnetic field perpendicular to the electric field. We discuss generic features of a critical electric field as a function of the magnetic field and the QCD string tension in the Sakai-Sugimoto model.

Electric field quench in AdS/CFT

An electric field quench, a suddenly applied electric field, can induce nontrivial dynamics in confining systems which may lead to thermalization as well as a deconfinement transition. In order to analyze this nonequilibrium transitions,we use the AdS/CFT correspondence for $\mathcal{N}=2$ supersymmetric QCD that has a confining meson sector. We find that the electric field quench causes the deconfinement transition even when the magnitude of the applied electric field is smaller than the critical value for the static case (which is the QCD Schwinger limit for quark-antiquark pair creation). The time dependence is crucial for this phenomenon, and the gravity dual explains it as an oscillation of a D-brane in the bulk AdS spacetime. Interestingly, the deconfinement time takes only discrete values as a function of the magnitude of the electric field. We advocate that the new deconfinement phenomenon is analogous to the exciton Mott transition.

TheX(3872) as a charmonium state plus an extra component due to continuum coupling effects

We discuss the results of an unquenched quark model (UQM) calculation of the self energy corrections to charmonium spectrum due to the coupling to the meson-meson continuum. In our approach, the effects of quark-antiquark pairs are introduced explicitly into the QM through a 3 P 0 quark-antiquark pair-creation mechanism. Our results are fitted to the experimental data, so that the sum of the bare and self energies gives the values of the physical masses of the states of interest. Our results for the charmonium spectrum are used to discuss the nature of the X (3872) resonance.

Mixing of the low-lying three- and five-quarkΩstates with negative parity

Mixing of the low-1/2^{-}$ and $3/2^{-}$ is investigated, employing an instanton-induced quark-antiquark pair creation model, which precludes transitions between $s^{3}$ and $s^{4}\bar{s}$ configurations. Models with hyperfine interactions between quarks of three different kinds, namely, one-gluon-exchange, Goldstone-boson-exchange (GBE) and an instanton-induced interaction (INS) are called OGE, GBE and INS models, respectively. Numerical results show that the instanton-induced pair creation causes strong mixing between the three- and five-quark configurations with spin 3/2, and that this mixing decreases the energy of the lowest spin 3/2 states in all three different hyperfine interaction models to $\sim 1750\pm50$ MeV. On the other hand, transition couplings between $s^3$ and $s^{3}q\bar{q}$ states with spin 1/2 caused by instanton-induced $q\bar{q}$ creation is very small and the resulting mixing of three- and five-quark configurations in the OGE and INS models is negligible, while the mixing of the spin 1/2 states in GBE model is not, but effects of this mixing on energies of mixed states are also very small. Accordingly, the lowest $\Omega$ states with negative parity in all three hyperfine interactions models have spin 3/2.

Hyperon-Meson Vertices and Hyperon-Nucleon Interactions on Quark Model

In this paper, we introduce two parts of work: the hyperon-meson vertex structure from the one-gluon-exchange quark-antiquark pair creation model and the hyperon-nucleon interactions on a modified quark potential model. For both parts some significant results have been obtained.

Strangeness of the proton

The contribution of strange quarks to the radius and the magnetic moment of the proton is discussed in the framework of the unquenched quark model in which the effects of the ss̄ pairs (strange sea) are taken into account through a 3P0 quark-antiquark pair creation mechanism. The results for the the strange magnetic moment and the strangeness radius of the proton are found to be compatible with zero, in agreement with the latest experimental results from parity-violating electron scattering experiments and recent lattice calculations.

Bottomonium self-energies due to the coupling to the meson-meson continuum

We present an unquenched quark model calculation of the mass shifts of $1S$, $2S$, and $1P$ bottomonium states due to the coupling to the meson-meson continuum. In our approach, the effects of the $q\overline{q}$ pairs are taken explicitly into account through a microscopic, QCD-inspired, quark-antiquark pair creation mechanism.

Ss¯sea pair contribution to electromagnetic observables of the proton in the unquenched quark model

We present an unquenched quark model calculation of the $s\overline{s}$ sea pairs' contribution to the magnetic moment of the proton (the strange magnetic moment) and its charge radius (the strange radius). In our approach the effects of the $s\overline{s}$ pairs are taken explicitly into account through a microscopic, QCD-inspired, quark-antiquark pair creation mechanism. Our results for the two ``strangeness'' observables are compatible with the latest experimental results and recent lattice calculations.