Gluonic Degrees Of Freedom Research Articles

Exotics at PANDA

One highlight in the field of hadron spectroscopy is the recent discovery of unexpected charmonium-like X, Y, Z resonances. The nature of most of these states is still controversial and emphasizes the discussion on exotic states with gluonic degrees of freedom. The PANDA experiment, a core project of the future Facility for Antiproton and Ion Research (FAIR) at GSI in Darmstadt, will investigate antiproton annihilations with the aim to explore fundamental questions in the nonperturbative region of QCD. The PANDA detector, equipped with a fixed target and provided with a high quality antiproton beam in the momentum range between 1.5 and 15 GeV/c, is optimized for high precision measurements of hadrons with masses up to 5 GeV/c2 and in particular of resonances in the charmonium and open charm region. Due to the gluon-rich antiproton annihilation process, it is expected to observe exotic states and therefore to answer fundamental questions in this field. The physics program for the search for exotics is outlined and detailed feasibility studies are presented. Furthermore, strategies for the unambiguous determination of the quantum numbers of these states are discussed.

Quark Confinement Mechanism and the Scale Λ QCD

The confinement mechanism proposed earlier by the author is applied to problem of arising the so-called scale Λ QCD within the framework of QCD. The natural physical assumption consists of that 1/Λ QCD ∼ 〈r〉 where 〈r〉 is a characteristic size of hadron (radius of confinement). The above confinement mechanism allows us to calculate 〈r〉 for mesons in terms of quark and gluonic degrees of freedom and this permits to conclude that Λ QCD should slightly change from hadron to hadron.

Recent Results from PHENIX Experiment at RHIC: Exploring the QCD Medium

We review some important results from the PHENIX experiment at RHIC. They were obtained in a unique environment for studying QCD bulk matter at temperatures and densities that sur- pass the limits where hadrons exist as individual entities, so raising to prominence the quark- gluon degrees of freedom. We present measurements of nuclear modification factors for neutral pions, light favors (strange hadrons), direct-photons and non-photonic electrons from decays of particles carrying charm or beauty quarks. We interpret the large suppression of hadron produc- tion at high transverse momenta as resulting from a large energy loss by the precursor parton on its path through the dense matter, primarily driven by gluon radiation. This dense QCD matter responds to energy loss in a pattern consistent with that expected from a hydrodynamic fluid. Further, its elliptic flow measurements suggest that the hadronization of bulk partonic matter exhibits collectivity with effective partonic degrees of freedom. The results are shown as a function of transverse momentum, centrality in different collision systems and energies.

Mixing angle of hadrons in QCD: A new view

A new method for calculation of the mixing angle between the hadrons within QCD sum rules is proposed. In this method, the mixing is expressed in terms of quark and gluon degrees of freedom. As an application, the detailed calculation of the mixing angle between heavy cascade baryons Xi_Q and Xi_Q', Q=c,~b is presented and it is found that the mixing angle between Xi_b (Xi_c) and Xi_b' (Xi_c') is given by theta_b = 6.4^\circ \pm 1.8 ^\circ (theta_c = 5.5^\circ \pm 1.8^\circ).

Shear viscosity in a perturbative quark–gluon plasma

Among the key features of hot and dense QCD matter produced in ultra-relativistic heavy-ion collisions at RHIC is its very low shear viscosity, indicative of the properties of a near-ideal fluid, and a large opacity demonstrated by jet energy loss measurements. In this work, we utilize a microscopic transport model based on the Boltzmann equation with quark and gluon degrees of freedom and cross sections calculated from perturbative quantum chromodynamics to simulate an ideal quark–gluon plasma in full thermal and chemical equilibrium. We then use the Kubo formalism to calculate the shear viscosity to entropy-density ratio of the medium as a function of temperature and system composition. One of our key results is that the shear viscosity over entropy-density ratio η/s becomes invariant to the chemical composition of the system when plotted as a function of energy-density instead of temperature.

Experimental Projects for Spectroscopy of Pionic Atoms and N*(1535) in Nuclei

Chiral symmetry and its spontaneous breakdown play an important role as a link between QCD and nuclear physics in the lowest energy region of strong interaction where the quark and gluon degrees of freedom are hidden and pseudo-scalar mesons of massless Nambu-Goldstone bosons appear.1) Chiral condensate 〈qq〉 is known to parametrize the symmetry breaking, and is theoretically calculated on the plane of temperature and density. The theory tells that beyond certain energy density, the chiral condensate is expected to vanish and the symmetry restores.2) There are several experimental approaches to understand the chiral dynamics especially on its relation to the mechanism of hadron mass generation. One approach utilizes head-on collisions of heavy ions with relativistically high energies in order to reach and explore extremely high temperature region.3) The other takes examination of hadrons in nuclear matter which is regarded as extremely high density material.4)–8) The above theory predicts 30% reduction of the chiral condensate at the normal nuclear density to that in vacuum.9)–11) This article briefly describes two experimental projects to study the chiral dynamics both in the meson and in the baryon sectors,12) namely, “Precision spectroscopy of pionic atoms”13) and “N∗(1535) production and its in-medium spectroscopy”.14) The former pursues high precision systematic spectroscopy of deeply bound pionic atoms∗) in the RI beam factory in RIKEN. The systematic spectroscopy will serve unique and high quality information on the in-medium strong interaction strength between the pion and nuclei, which improves the precision of the quantitative estimation of the chiral condensate at the normal nuclear density.5) The latter, “N∗(1535) production and its in-medium spectroscopy”, has its own particular interest.14),15) According to the theories, N∗(1535) is a candidate of chiral partner of nucleons,12),16),17) and thus, the mass difference of the N∗(1535) and nucleon is reduced as the chiral symmetry restores. In-medium N∗(1535) spectroscopy will provide strong evidence of the hadron mass generation through direct measurement of the mass difference through possible level crossing between N∗(1535)-hole and η modes in the nuclear medium.18),19)

Gluon parton density in the chiral quark model

Dynamical chiral symmetry breaking is one of the most important properties of low energy QCD. The breaking pattern has a profound impact on phenomenological quantities. Here we extend the idea of the meson cloud approach in the chiral quark model to include a gluon cloud in order to model the parton densities in the nucleon, based on the constituent quark framework. To obtain parton densities, including the gluon density in the constituent quark approach , we use the splitting functions of a quark to quark-meson and quark-gluon at low Q 2 values. The parton densities at high energies can be obtained, using the DGLAP evolution equations. A good agreement with that expected is computed for the fraction of the total momentum of the proton which is carried by gluon. The chiral quark model (χQM) is used to study the flavor structure of the constituents quark and the nucleon within the conventional mesonic cloud picture. Using this model, the effects of SU (3) f symmetry breaking can be discussed . The implications of the Gottfired sum rule (GSR) violation on the Δ-n mass splitting is also considered. At a low-resolution scale energy the constituent quark picture is successfully described hadron structure function. The sea quark and gluon degrees of freedom are assumed to be absorbed into constituent quarks as quasi-particles [1, 2]. A relation between the two regimes of hadron structure function description; i.e. the chiral quark and the constituent quark models, has a considerable significance which has been investigated widely in literature, and attracts much more attention in recent years [3].

MOMENTS OF SPIN STRUCTURE FUNCTIONS: SUM RULES AND POLARIZABILITIES

Nucleon structure study is one of the most important research areas in modern physics and has challenged us for decades. Spin has played an essential role and often brought surprises and puzzles to the investigation of the nucleon structure and the strong interaction. New experimental data on nucleon spin structure at low to intermediate momentum transfers combined with existing high momentum transfer data offer a comprehensive picture in the strong region of the interaction and of the transition region from the strong to the asymptotic-free region. Insight into some aspects of the theory for the strong interaction, Quantum Chromodynamics (QCD), is gained by exploring lower moments of spin structure functions and their corresponding sum rules (i.e., the Bjorken, Burkhardt–Cottingham, Gerasimov–Drell–Hearn (GDH), and the generalized GDH). These moments are expressed in terms of an operator-product expansion using quark and gluon degrees of freedom at moderately large momentum transfers. The higher-twist contributions have been examined through the evolution of these moments as the momentum transfer varies from higher to lower values. Furthermore, QCD-inspired low-energy effective theories, which explicitly include chiral symmetry breaking, are tested at low momentum transfers. The validity of these theories is further examined as the momentum transfer increases to moderate values. It is found that chiral perturbation theory calculations agree reasonably well with the first moment of the spin structure function g1 at low momentum transfer of 0.05–0.1 GeV2 but fail to reproduce some of the higher moments, noticeably, the neutron data in the case of the generalized polarizability δLT. The Burkhardt–Cottingham sum rule has been verified with good accuracy in a wide range of Q2 assuming that no singular behavior of the structure functions is present at very high excitation energies.

QCD confinement and the GlueX experiment at Jefferson Lab

An understanding of the confinement mechanism in QCD requires a detailed mapping of the spectrum of hybrid mesons. Understanding confinement means understanding the role of gluons and it is in hybrid mesons that the gluonic degrees of freedom are manifest. High statistics searches for such states with π and p beams have resulted in some tantalizing signals. There is good reason to expect beams of photons to yield hybrid mesons with JPC quantum numbers not possible within the conventional picture of mesons as qq̄ bound states. Meager data currently exist on the photoproduction of light quark mesons. This talk represents an overview of the available data and what has been learned. In looking toward the future, the GlueX experiment at Jefferson Laboratory represents a new initiative that will perform detailed spectroscopy of the light-quark meson spectrum. This experiment and its capabilities will be reviewed.

Diffractive upsilon photo-production off the proton in the QCD inspired model

Based on the generalized vector meson dominance model in QCD, we study the photoproduction of vector meson ϒ off the proton by use of the QCD-inspired model in which the contributions from quark-quark, gluon-gluon and quark-gluon interference term to observable are taken into consideration. Calculations are performed for total cross section σ tot, differential cross section dσ/dt, ratio of the real part to imaginary part of forward scattering amplitude ρ and nuclear slop parameter function β. We analyze the individual contributions from quark gluon degrees of freedom and the QCD Odderon to the total cross section σ tot (s), differential cross section dσ/dt, ratio of the real part to imaginary part of the forward scattering amplitude ρ, and nuclear slop parameter function β. The mediators of interactions between projectiles (the quark and antiquark pair fluctuated from the real photon) and the proton target (three quark system) are the tensor Glueball and Odderon instead of using the usual Pomeron exchange. The theoretical predictions for σ tot(s) are consistent with the experimental data within error bars of the data. The data for dσ/dt, β and ρ are urgently needed. The gluon-gluon interaction makes a significant contribution to the observables while the Odderon contribution is negligibly small. Therefore, we may conclude that it is impossible to find the QCD Odderon in the γ+p → ϒ+p process as suggested before.

Deeply virtual Compton scattering and generalized parton distributions at CLAS⋆

The exclusive electroproduction of real photons and mesons at high momentum transfer allows us to access the Generalized Parton Distributions (GPDs). The formalism of the GPDs provides a unified description of the hadronic structure in terms of quark and gluonic degrees of freedom. In particular, the Deeply Virtual Compton Scattering (DVCS), ep → e′p′γ , is one of the key reactions to determine the GPDs experimentally, as it is the simplest process that can be described in terms of GPDs. A dedicated experiment to study DVCS has been carried out in Hall B at Jefferson Lab. Beam-spin asymmetries, resulting from the interference of the Bethe-Heitler process and DVCS have been extracted over the widest kinematic range ever accessed for this reaction ( 1.2 < Q2 < 3.7 (GeV/c2, 0.09 < - t < 1.3 (GeV/c2, 0.13 < xB < 0.46 . In this paper, the results obtained experimentally are shown and compared to GPD parametrizations.

Heavy quarkonium hybrids from Coulomb gauge QCD

Using the nonrelativisitc reduction of Coulomb gauge QCD we compute a spectrum of the low mass hybrid mesons containing a heavy quark-antiquark pair. The gluon degrees of freedom are treated in the mean field approximation calibrated to the gluelump spectrum. We discuss the role of the non-Abelian nature of the QCD Coulomb interaction in the ordering of the spin-parity levels.

Quark and Gluon Degrees of Freedom in High-Energy Heavy Ion Collisions

I discuss some recent progress in our understanding of high energy nuclear collisions. I will focus on two topics which I was lucky to co-pioneer in the recent past. One is recombination of quarks and its interpretation as a signal for deconfinement, the second is electromagnetic radiation from jets passing through a quark gluon plasma. This talk was given during the award ceremony for the 2007 IUPAP Young Scientist Award.

GAUSSIAN CONFINEMENT IN A JKJ DECAY MODEL

In microscopic decay models, one attempts to describe hadron strong decays in terms of quark and gluon degrees of freedom. We begin by assuming that strong decays are driven by the same interquark Hamiltonian which determines the spectrum, and that it incorporates gaussian confinement. An A → BC decay matrix element of the JKJ Hamiltonian involves a pair-production current matrix elements times a scatering matrix element. Diagrammatically this corresponds to an interaction between an initial line and produced pair.

Excited flux tube from q¯g hybrid mesons

Abstract.In the framework of quark models, hybrid mesons are either seen as two-body q¯ systems with an excited flux tube connecting the quark to the antiquark or as three-body q¯g systems including a constituent gluon. In this work we show that, starting from the three-body wave function of the q¯g hybrid meson in which the gluonic degrees of freedom are averaged, the excited flux tube picture emerges as an equivalent q¯ potential. This equivalence between the excited flux tube and the constituent-gluon approach is confirmed for heavy hybrid mesons but, for the first time, it is shown to hold in the light sector too, provided the contribution of the quark dynamics is correctly taken into account.

Evidence from identified particles for active quark and gluon degrees of freedom

Measurements of intermediate pT (1.5 < pT < 5.0 GeV/c) identified particle distributions in heavy-ion collisions at SPS and RHIC energies display striking dependences on the number of constituent quarks in the corresponding hadron. One finds that elliptic flow at intermediate pT follows a constituent quark scaling law as predicted by models of hadron formation through coalescence. In addition, baryon production is also found to increase with event multiplicity much faster than meson production. The rate of increase is similar for all baryons and seemingly independent of mass. This indicates that the number of constituent quarks determines the multiplicity dependence of identified hadron production at intermediate pT. We review these measurements and interpret the experimental findings.

Some recent research highlights from the CSSM

In this article we provide an overview of some of the recent research highlights from the CSSM. We present new results of our studies into the Θ + pentaquark state from lattice QCD considering both spin-1/2 and spin-3/2 states. We continue our exploration of exotic states with a study of exotic meson states using hybrid meson interpolators with explicit gluonic degrees of freedom. To investigate scaling, we compare the behaviour of the quark mass function M ( q 2 ) , and wave-function renormalization function Z ( q 2 ) , in dynamical lattice QCD, for a variety of bare quark masses, on two lattices with lattice spacing of 0.090 and 0.125 fm, but similar physical volumes.

SUM RULES AND MOMENTS OF THE NUCLEON SPIN STRUCTURE FUNCTIONS

The nucleon has been used as a laboratory to investigate its own spin structure and quantum chromodynamics. New experimental data on nucleon spin structure at low to intermediate momentum transfers combined with existing high momentum transfer data offer a comprehensive picture of the transition region from the confinement regime of the theory to its asymptotic freedom regime. Insight for some aspects of the theory is gained by exploring lower moments of spin structure functions and their corresponding sum rules (i.e. the Gerasimov–Drell–Hearn, Bjorken and Burkhardt–Cottingham). These moments are expressed in terms of an operator-product expansion using quark and gluon degrees of freedom at moderately large momentum transfers. The sum rules are verified to good accuracy assuming that no singular behavior of the structure functions is present at very high excitation energies. The higher-twist contributions have been examined through the moments evolution as the momentum transfer varies from higher to lower values. Furthermore, QCD-inspired low-energy effective theories, which explicitly include chiral symmetry breaking, are tested at low momentum transfers. The validity of these theories is further examined as the momentum transfer increases to moderate values. It is found that chiral perturbation calculations agree reasonably well with the first moment of the spin structure function g1 at momentum transfer of 0.1 GeV2 but fail to reproduce the neutron data in the case of the generalized polarizability δLT.

Applications of light-front QCD

Light-front Fock state wavefunctions encode the bound state properties of hadrons in terms of their quark and gluon degrees of freedom at the amplitude level. The freedom to choose the light-like quantization four-vector provides an explicitly covariant formulation of light-front quantization and can be used to determine the analytic structure of light-front wave functions. The AdS/CFT correspondence of large N_C supergravity theory in higher-dimensional anti-de Sitter space with supersymmetric QCD in 4-dimensional space-time has interesting implications for hadron phenomenology in the conformal limit, including an all-orders demonstration of counting rules for exclusive processes. String/gauge duality also predicts the QCD power-law behavior of light-front Fock-state hadronic wavefunctions with arbitrary orbital angular momentum at high momentum transfer. The form of these near-conformal wavefunctions can be used as an initial ansatz for a variational treatment of the light-front QCD Hamiltonian. I also briefly review recent work which shows that some leading-twist phenomena such as the diffractive component of deep inelastic scattering, single spin asymmetries, nuclear shadowing and antishadowing cannot be computed from the LFWFs of hadrons in isolation.

A NEW SIGNATURE FOR GLUEBALLS

In this work we use a mapping technique to derive in the context of a constituent gluon model an effective Hamiltonian that involves explicit gluon degrees of freedom. We study glueballs with two gluons using the Fock–Tani formalism.