Light-quark Meson Research Articles

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.

Calculation of Quark-antiquark Potential Coefficient and Charge Radius of Light Mesons

Mesons are composed of one quark and one anti quark which are considered to be governed by quantum chromodynamic. In the present paper, it is considered that mesons are made up entirely, at least in principle, of lightquarks (u, d, s). Since mesons consisting of light quarks are relativistic systems, we cannot use the Schrodingerequation, and the theory is rather limited. The light quark (u, d, s) mesons are intrinsically relativistic; becausebinding energies (typically a few hundred MeV) are not small compared to the constituent masses. Thereforethey are based on nonrelativistic quantum mechanics. For the heavy quarks (c, b, t) though, the nonrelativistictheory should work reasonably well. For most part of this paper we restricted our attention to relativistic boundstate of two particles and considered interaction between quark and antiquark, the essential point is that at shortrange we expect a columbic potential and at large distance we have to account for quark confinement, thesimplest is a linear potential and other interaction oscillator potential. Then Dirac equation was solved with thecentral potential analytically and the wave function of system was determined. Finally, by using wavefunction wedetermined the charge radius of p0meson.

Neutral pion production inAu+Aucollisions atsNN=200GeV

The results of mid-rapidity ($0 < y < 0.8$) neutral pion spectra over an extended transverse momentum range ($1 < p_T < 12$ GeV/$c$) in $\sqrt{s_{NN}}$ = 200 GeV Au+Au collisions, measured by the STAR experiment, are presented. The neutral pions are reconstructed from photons measured either by the STAR Barrel Electro-Magnetic Calorimeter (BEMC) or by the Time Projection Chamber (TPC) via tracking of conversion electron-positron pairs. Our measurements are compared to previously published $\pi^{\pm}$ and $\pi^0$ results. The nuclear modification factors $R_{\mathrm{CP}}$ and $R_{\mathrm{AA}}$ of $\pi^0$ are also presented as a function of $p_T$ . In the most central Au+Au collisions, the binary collision scaled $\pi^0$ yield at high $p_T$ is suppressed by a factor of about 5 compared to the expectation from the yield of p+p collisions. Such a large suppression is in agreement with previous observations for light quark mesons and is consistent with the scenario that partons suffer considerable energy loss in the dense medium formed in central nucleus-nucleus collisions at RHIC.

Long-distance behavior of the quark-antiquark static potential. Application to light-quark mesons and heavy quarkonia

Screening effects from sea pairs on the quark-antiquark static potential are analyzed phenomenologically from the light-quark to the heavy-quark meson spectra. From the high excited light-quark meson spectrum, a universal form for the screened static potential is proposed. This potential is then successfully applied to heavy quarkonia. Our results suggest the assignment of $X(4260)$ to the $4s$ state of charmonium and the possible existence of a $5s$ bottomonium resonance around 10748 MeV.

Scalar mesons from [formula omitted] to [formula omitted]: a global view

A new interpretation of the spectrum of scalar mesons from χ b 0 ( 9860 ) down to σ ( 485 ) is suggested in which scalar mesons have a q q ¯ and a four-quark component. The four-quark component becomes increasingly important with decreasing quark mass. The flavor singlet light-quark mesons form the wide background which is interspersed with narrow octet states. There are no states beyond the quark-model classification.

Derivative sources in lattice spectroscopy of excited light-quark mesons

We construct efficient interpolating fields for lattice spectroscopy of mesons by applying covariant derivatives on Jacobi-smeared quark sources. These interpolators are tested in a quenched calculation of excited mesons based on the variational method. We present results for pseudoscalar, scalar, vector and pseudovector mesons.

Vector meson form factors and their quark-mass dependence

The electromagnetic form factors of vector mesons are calculated in an explicitly Poincar\'e covariant formulation, based on the Dyson-Schwinger equations of QCD, that respects electromagnetic current conservation and unambiguously incorporates effects from vector meson poles in the quark-photon vertex. This method incorporates a two-parameter effective interaction, where the parameters are constrained by the experimental values of chiral condensate and ${f}_{\ensuremath{\pi}}$. This approach has successfully described a large amount of light-quark meson experimental data, including ground-state pseudoscalar masses and their electromagnetic form factors and ground-state vector meson masses and strong and electroweak decays. Here we apply it to predict the electromagnetic properties of vector mesons. The results for the static properties of the \ensuremath{\rho} meson are as follows: charge radius $\ensuremath{\langle}{r}_{\ensuremath{\rho}}^{2}\ensuremath{\rangle}=0.54\phantom{\rule{0.3em}{0ex}}{\mathrm{fm}}^{2}$, magnetic moment $\ensuremath{\mu}=2.01$, and quadrupole moment $\mathcal{Q}=\ensuremath{-}0.41$. We investigate the quark-mass dependence of these static properties and find that our results at the charm quark mass are in agreement with recent lattice simulations. The charge radius decreases with increasing quark mass, but the magnetic moment is almost independent of the quark mass.

Hadron spectroscopy: theory and experiment

Many new results on hadron spectra have been appearing in the past few years thanks to improved experimental techniques and searches in new channels. New theoretical techniques including refined methods of lattice QCD have kept pace with these developments. Much has been learned about states made of both light (u, d and s) and heavy (c, b) quarks. The present review treats light-quark mesons, glueballs, hybrids, particles with a single c or b quark, charmonium and bottomonium states. Some prospects for further study are noted.

Experimental Status of Exotic Mesons and the GlueX Experiment

One of the unanswered and most fundamental questions in physics regards the nature of the confinement mechanism of quarks and gluons in QCD. Exotic hybrid mesons manifest gluonic degrees of freedom and their spectroscopy will provide the data necessary to test assumptions in lattice QCD and the specific phenomenology leading to confinement. Within the past two decades a number of experiments have put forth tantalizing evidence for the existence of exotic hybrid mesons in the mass range below 2 GeV. 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.

Quark-antiquark states and their radiative transitions in terms of the spectral integral equation: Light mesons

We continue the investigation of mesons in terms of the spectral integral equation initiated before for the $$b\bar b$$ and $$c\bar c$$ systems; we consider the light-quark (u, d, s) mesons with masses M ≤ 3 GeV. The calculations have been performed for the mesons lying on linear trajectories in the (n, M 2) planes, where n is the radial quantum number. Our consideration relates to the $$q\bar q$$ states with one component in the flavor space, with the quark and antiquark masses equal to each other, such as π(0−+), ρ(1−−), ω(1−−), ϕ(1−−), a 0(0++), a 1(1++), a 2(2++), b 1(1+−), f 2(2++), π 2(2−+), ρ 3(3−−), ω 3(3−−), ϕ 3(3−−), π 4(4−+) at n ≤ 6. We obtained the wave functions and mass values of mesons lying on these trajectories. The corresponding trajectories are linear, in agreement with data. We have calculated the two-photon decays π, a 0(980), a 2(1320), f 2(1285), f 2(1525) and radiative transitions ρ, ω → γπ, which agree qualitatively with the experiment. On this basis, we extract the singular part of the interaction amplitude, which corresponds to the so-called “confinement interaction.” The description of the data requires the presence of the strong t-channel singularities for both scalar and vector exchanges.

High transverse momentum η meson production inp+p,d+Au, and Au+Au collisions atsNN=200GeV

Inclusive transverse momentum spectra of η mesons in the range pT≈2–12 GeV/c have been measured at midrapidity (|η|<0.35) by the PHENIX experiment at RHIC in p+p,d+Au, and Au+Au collisions at sNN=200 GeV. The η mesons are reconstructed through their η→γ γ channel for the three colliding systems as well as through the η→π0π+π− decay mode in p+p and d+Au collisions. The nuclear modification factor in d+Au collisions, RdAu(pT)≈1.0–1.1, suggests at most only modest pT broadening (“Cronin enhancement”). In central Au+Au reactions, the η yields are significantly suppressed, with RAuAu(pT)≈0.2. The ratio of η to π0 yields is approximately constant as a function of pT for the three colliding systems in agreement with the high-pT world average of Rη/π0≈0.5 in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions for a wide range of center-of-mass energies (sNN≈3–1800 GeV) as well as, for high scaled momentum xp, in e+e− annihilations at s=91.2 GeV. These results are consistent with a scenario where high-pT η production in nuclear collisions at the Relativistic Heavy Ion Collider is largely unaffected by initial-state effects but where light-quark mesons (π0,η) are equally suppressed due to final-state interactions of the parent partons in the dense medium produced in Au+Au reactions.22 MoreReceived 6 November 2006DOI:https://doi.org/10.1103/PhysRevC.75.024909©2007 American Physical Society

Hadronic Spectra and Light-Front Wave Functions in Holographic QCD

We show how the string amplitude phi(z) defined on the fifth dimension in AdS5 space can be precisely mapped to the light-front wave functions of hadrons in physical space-time. We find an exact correspondence between the holographic variable z and an impact variable zeta, which represents the measure of transverse separation of the constituents within the hadrons. In addition, we derive effective four dimensional Schrödinger equations for the bound states of massless quarks and gluons which exactly reproduce the anti-de Sitter conformal field theory results and give a realistic description of the light-quark meson and baryon spectrum as well as the form factors for space-like Q2. Only one parameter which sets the mass scale, lambda(QCD), is introduced.

MATCHING MESON RESONANCES TO OPE IN QCD

We investigate the possible corrections to the linear Regge trajectories for the light-quark meson sector by matching two-point correlators of quark currents to the Operator Product Expansion. We find that the allowed modifications to the linear behavior must decrease rapidly with the principal quantum number. After fitting the lightest states in each channel and certain low-energy constants the whole spectrum for meson masses and residues is obtained in a satisfactory agreement with phenomenology. The perturbative corrections to our results are discussed.

Applications of AdS/CFT Duality to QCD

Even though quantum chromodynamics is a broken conformal theory, the AdS/CFT correspondence has led to important insights into the properties of QCD. For example, as shown by Polchinski and Strassler, dimensional counting rules for the power-law falloff of hadron scattering amplitudes follow from dual holographic models with conformal behavior at short distances and confinement at large distances. We find that one also obtains a remarkable representation of the entire light-quark meson and baryon spectrum, including all orbital excitations, based on only one mass parameter. We also show how hadron light-front wavefunctions and hadron form factors in both the space-like and time-like regions can be predicted.

The instanton molecule liquid and ‘sticky molasses’ above Tc

The main objective of this work is to explore the evolution in the structure of the quark–antiquark bound states in going down in the chirally restored phase from the so-called zero binding points Tzb to the QCD critical temperature Tc at which the Nambu–Goldstone and Wigner–Weyl modes meet. In doing this, we adopt the idea recently introduced by Shuryak and Zahed for charmed , light-quark mesons π, σ, ρ, A1 and gluons that at Tzb, the quark–antiquark scattering length goes through ∞ at which conformal invariance is restored, thereby transforming the matter into a near-perfect fluid behaving hydrodynamically, as found at RHIC. We show that the binding of these states is accomplished by the combination of (i) the colour Coulomb interaction, (ii) the relativistic effects and (iii) the interaction induced by the instanton–anti-instanton molecules. The spin–spin forces turned out to be small. While near Tzb all mesons are large-size non-relativistic objects bound by the Coulomb attraction, near Tc they get much more tightly bound, with many-body collective interactions becoming important and making the σ and π masses approach zero (in the chiral limit). The wavefunction at the origin grows strongly with binding, and the near-local four-Fermi interactions induced by the instanton molecules play an increasingly more important role as the temperature moves downward towards Tc.

Spin-orbit and tensor forces in heavy-quark light-quark mesons: Implications of the newDsstate at 2.32 GeV

We consider the spectroscopy of heavy-quark light-quark mesons with a simple model based on the non-relativistic reduction of vector and scalar exchange between fermions. Four forces are induced: the spin-orbit forces on the light and heavy quark spins, the tensor force, and a spin-spin force. If the vector force is Coulombic, the spin-spin force is a contact interaction, and the tensor force and spin-orbit force on the heavy quark to order $1/m_1m_2$ are directly proportional. As a result, just two independent parameters characterize these perturbations. The measurement of the masses of three p-wave states suffices to predict the mass of the fourth. This technique is applied to the $D_s$ system, where the newly discovered state at 2.32 GeV provides the third measured level, and to the $D$ system. The mixing of the two $J^P=1^+$ p-wave states is reflected in their widths and provides additional constraints. The resulting picture is at odds with previous expectations and raises new puzzles.

SPECTROSCOPY AND CONFINEMENT

In this talk I progress semi-historically from the potential model as applied to heavy onia states to the QCD string dynamics of light quark mesons. The virtues and problems of the potential model are reviewed. Some of the difficulties are solved by the introduction of QCD string confinement. The method of solving string models numerically is outlined and some recent analytic results are discussed. We end by exploring some consequences of the model and how the observed light quark mesons can be classified simply.

Hybrid meson decay from the lattice

We discuss the allowed decays of a hybrid meson in the heavy quark limit. We deduce that an important decay will be into a heavy quark non-hybrid state and a light quark meson, in other words, the de-excitation of an excited gluonic string by emission of a light quark-antiquark pair. We discuss the study of hadronic decays from the lattice in the heavy quark limit and apply this approach to explore the transitions from a spin-exotic hybrid to $\chi_b \eta$ and $\chi_b S$ where $S$ is a scalar meson. We obtain a signal for the transition emitting a scalar meson and we discuss the phenomenological implications.

Light hadron spectroscopy with two flavors of dynamical quarks on the lattice

We present results of a numerical calculation of lattice QCD with two degenerate flavors of dynamical quarks, identified with up and down quarks, and with a strange quark treated in the quenched approximation. The lattice action and simulation parameters are chosen with a view to carrying out an extrapolation to the continuum limit as well as chiral extrapolations. Gauge configurations are generated with a renormalization-group improved gauge action and a mean field improved clover quark action at three values of $\beta$ and four sea quark masses. The sizes of lattice are chosen so that the physical spatial size is kept constant. Hadron masses, light quark masses and meson decay constants are measured at five valence quark masses. We also carry out complementary quenched simulations with the same improved actions. The quenched spectrum from this analysis agrees well in the continuum limit with the one of our earlier work using the standard action. We find the two-flavor full QCD meson masses in the continuum limit to be much closer to experimental meson masses than those from quenched QCD. We take these results as manifestations of sea quark effects in two-flavor full QCD. For baryon masses full QCD values for strange baryons are in agreement with experiment, while they differ increasingly with decreasing strange quark content, resulting in a nucleon mass higher than experiment. The pattern suggests finite size effects as a possible origin for this deviation. For light quark masses in the continuum limit we obtain values which are reduced by about 25% compared to the values in quenched QCD. We also present results for decay constants where large scaling violations obstruct a continuum extrapolation. Need for a non-perturbative estimate of renormalization factors is discussed.

Light-quark mesons and four-quark condensates at finite temperature

We propose an analogue of the familiar gap equation for the case of four-quark condensates at finite temperature. The condensates of interest correspond to scalar, vector, pseudoscalar, axial vector, and tensor Dirac structures. Working with correlators at zero chemical potential without factorization, we arrive at coupled equations for these four-quark condensates and the masses of certain light-quark mesons. We study the temperature dependence of the four-quark condensates and masses; in one of our models, factorization of the four-quark condensates is shown to be increasingly violated as the temperature is increased toward ${T}_{c}.$ The ${2}^{++}$ tensor mesons ${a}_{2}{(1320)\ensuremath{-}f}_{2}(1270)$ are identified as especially sensitive probes of the four-quark condensates.