Hadron Phenomenology Research Articles

Relativistic chiral Hartree-Fock description of nuclear matter with constraints from nucleon structure and confinement

We present a relativistic chiral effective theory for symmetric and asymmetric nuclear matter taken in the Hartree-Fock scheme. The nuclear binding is ensured by a background chiral invariant scalar field associated with the radial fluctuations of the chiral quark condensate. Nuclear matter saturation is obtained once the scalar response of the nucleon-generating three-body repulsive forces is incorporated. For those parameters related to the scalar sector and quark confinement mechanism inside the nucleon we make use of an analysis of lattice results on nucleon mass evolution with quark mass. The other parameters are constrained as much as possible by standard hadron and nuclear phenomenology. Special attention is paid to the treatment of the propagation of the scalar fluctuations. The rearrangement terms associated with in-medium modified mass and coupling constants are explicitly included to satisfy the Hugenholtz-Van Hove theorem. We point out the important role of the tensor piece of the rho exchange Fock term to reproduce the asymmetry energy of nuclear matter. We also discuss the isospin dependence of the Landau nucleon effective mass.

Hadron properties and Dyson–Schwinger equations

An overview of the theory and phenomenology of hadrons and QCD is provided from a Dyson–Schwinger equation viewpoint. Following a discussion of the definition and realization of light-quark confinement, the nonperturbative nature of the running mass in QCD and inferences from the gap equation relating to the radius of convergence for expansions of observables in the current-quark mass are described. Some exact results for pseudoscalar mesons are also highlighted, with details relating to the U A ( 1 ) problem, and calculated masses of the lightest J = 0 , 1 states are discussed. Studies of nucleon properties are recapitulated upon and illustrated: through a comparison of the ln -weighted ratios of Pauli and Dirac form factors for the neutron and proton; and a perspective on the contribution of quark orbital angular momentum to the spin of a nucleon at rest. Comments on prospects for the future of the study of quarks in hadrons and nuclei round out the contribution.

Chiral symmetry and the string description of excited hadrons

A large symmetry group is perhaps experimentally observed in excited hadrons which includes the chiral group $U(2{)}_{L}\ifmmode\times\else\texttimes\fi{}U(2{)}_{R}$ as a subgroup. To possess this large symmetry a dynamical model for excited hadrons, presumably a string model, should explain formation of chiral multiplets and, at the same time, predict coinciding slopes of the angular and radial Regge trajectories. This is possible only if both the dynamics of the string and the chirality of the quarks at the ends of the string are considered together. We construct a model-independent unitary transformation from the relativistic chiral basis to the ${^{2S+1}L_{J}}$ basis, commonly used in hadronic phenomenology as well as in the string models, and demonstrate that a hadron belonging to the given chiral representation is a fixed superposition of the basis vectors with different $L$'s. Thus the description of highly excited hadron in terms of a fixed $L$ is not compatible with chiral symmetry and has to be disregarded in favor of the description in terms of the total hadron spin $J$. Therefore, dynamics of the string must deliver the principal quantum number $\ensuremath{\sim}n+J$, in order chiral multiplets with different spins to become degenerate, as required by the large symmetry group.

Theory and phenomenology of vector mesons in medium

Electromagnetic probes promise to be direct messengers of (spectral properties of) hot and dense matter formed in heavy-ion collisions, even at soft momentum transfers essential for characterizing possible phase transitions. We examine how far we have progressed towards this goal by highlighting recent developments, and trying to establish connections between lattice QCD, effective hadronic models and phenomenology of dilepton production.

Theory and phenomenology of hadrons

This Dyson-Schwinger equation (DSE) aperçu highlights recent applications to mesons. It reports features of, and results for, pseudoscalar and scalar bound-state residues in vacuum polarisations, and exhibits how a restoration of chiral symmetry in meson trajectories could be manifest in a relationship between them. It also touches on nucleon studies, emphasising the importance of both scalar and axial-vector diquark correlations, and reporting the calculation of μnGEn(Q2)/GMn(Q2). The value of respecting symmetries, including Poincaré covariance, is stressed.

New developments in treacherous points of light-front dynamics

Light-front dynamics(LFD) plays an important role in hadron phenomenology. The last few years, however, it has been emphasized that treacherous points such as zero-mode contributions should be taken into account for successful LFD applications to hadron phenomenology. We discuss some examples of treacherous points and present new progress made the last few years to handle them correctly.

Phenomenology of infrared finite gluon propagator and coupling constant

We report on some recent solutions of the Dyson-Schwinger equations for the infrared behavior of the gluon propagator and coupling constant, discussing their differences and proposing that these different behaviors can be tested through hadronic phenomenology. We discuss which kind of phenomenological tests can be applied to the gluon propagator and coupling constant, how sensitive they are to the infrared region of momenta and what specific solution is preferred by the experimental data.

Revealing treacherous points for successful light-front phenomenological applications

Light-front dynamics(LFD) plays an important role in hadron phenomenology as evidenced from recent development of generalized parton distributions and other quantities involving hadrons. For successful LFD applications to hadron phenomenology, however, treacherous points such as zero-mode contributions should be taken into account. For a concrete example of zero-mode contribution, we present Standard Model analysis of vector anomaly in the CP-even form factors of W^{\pm} gauge bosons. Main distinguished features of LFD is discussed in comparison with other Hamiltonian dynamics. We also present a power counting method to correctly pin down which hadron form factors receive the zero-mode contribution and which ones do not. Indications from our analysis to hadron phenomenology are discussed.

Dynamical study ofQQ–u¯d¯mesons

It has been recently conjectured by Selem and Wilczek 1 the existence of a ss-[ud] meson due to strong correlations between the two light antiquarks. We make a detailed study of this system within a dynamical quark model which has proven to be successful in reproducing the most important features of low-energy hadron phenomenology. Our results, obtained within a parameter-free calculation, show that the antidiquark component of the ssud system indeed entails the stronger attraction, and drives its energy much lower than the N{xi} threshold, but still above the K{sup 0}K*{sup -} or K*{sup 0}K{sup -} thresholds. We have also studied the ccud and bbud systems. Exotic mesons are only expected to exist in the limit of large mass for the two-quark subsystem, bbud, since the calculated mass is below the B{sup 0}B*{sup -} or B*{sup 0}B{sup -} thresholds.

Functional Integral Approaches to the Bosonization of Effective Multi-Quark Interactions with UA(1) Breaking

Low energy hadron phenomenology involving the (u,d,s) quarks is often ap- proached through effective multi-quark Lagrangians with the symmetries of QCD. A very successful approach consists in taking the four-quark Nambu-Jona-Lasinio Lagrangian with the chiral UL(3)◊UR(3) symmetry in the massless limit, combined with the UA(1) breaking six-quark flavour determinant interaction of 't Hooft. We review the present status and some very recent developments related to the functional integration over the cubic term in auxi- liary mesonic variables that one introduces to bosonize the system. Various approaches for handling this functional, which cannot be integrated exactly, are discussed: the stationary phase approximation, the perturbative expansion, the loop expansion, their interrelation and importance for the evaluation of the effective action. The intricate group structure rules out the method of Airy's integral. The problem of the instability of the vacuum is stated and a solution given by including eight-quark interactions.

Hadron Phenomenology from Quenched Overlap Fermions

We compute hadron masses and nucleon matrix elements in quenched QCD using overlap fermions.

Semiperturbative construction for the quark-gluon vertex

We construct a model for the quark-gluon vertex of Landau gauge QCD. This is of twofold interest: on the one hand the quark-gluon interaction is at the heart of quark confinement, on the other hand it is a central element in hadron phenomenology based on QCD Greens functions. We employ the non-Abelian one-loop diagram in perturbation theory, which is of order N c . As a novelty we replace the tree-level quark and gluon propagators in this diagram by their dressed counterparts solving the Dyson-Schwinger equations. The N c -suppressed Abelian diagram is an order of magnitude smaller in various kinematics. We also study the effect of ghost dressing factors on the vertex obtaining a construction in good agreement with recent low-momentum lattice calculations.

Coincident GRB neutrino flux predictions: Implications for experimental UHE neutrino physics

In the hadronic fireball phenomenology of Gamma Ray Bursts (GRBs), it is expected that the observed photons are accompanied by UHE neutrinos, which have not been observed yet. It is one of the challenges of experimental UHE neutrino astrophysics to look for a signal from GRBs. In this paper, the differences between a search for a diffuse signal and an examination of a source sample given by e.g. BATSE will be analyzed. Since redshift information is needed to determine the correct energy spectrum, long duration bursts with redshifts from different estimate methods will be used. We will start with an overview of the current understanding of GRB neutrino physics and will then use this knowledge to make predictions for a coincidence flux and a corresponding diffuse flux. It can be shown that shape and normalization of the spectrum is highly dependent on the set of bursts used and that individual bursts can determine the total spectrum.

New Results in Light-Front Phenomenology

The light-front quantization of gauge theories such as QCD in light-cone gauge provides a frame-independent wavefunction representation of relativistic bound states, simple forms for current matrix elements, explicit unitarity, and a trivial vacuum. 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 and to define a kinematical definition of angular momentum. 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 analyses 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.

Remarks on diquarks, strong binding, and a large hidden QCD scale

We present arguments regarding diquarks possible role in low-energy hadron phenomenology that escaped theorists' attention so far. Good diquarks, i.e. the $0^{+}$ states of two quarks, are argued to have a two-component structure with one of the components peaking at distances several times shorter than a typical hadron size (a short-range core). This can play a role in solving two old puzzles of the 't Hooft 1/N expansion: strong quark mass dependence of the vacuum energy density and strong violations of the Okubo-Zweig-Iizuka (OZI) rule in the quark-antiquark $0^\pm$ channels. In both cases empiric data defy 't Hooft's 1/N suppression. If good diquarks play a role at an intermediate energy scale they ruin 't Hoofts planarity because of their mixed-flavor composition. This new scale associated with the good diquarks may be related to a numerically large scale discovered in [V. Novikov, M. Shifman, A. Vainshtein and V. Zakharov, Nucl. Phys. B 191, 301 (1981)] in a number of phenomena mostly related to vacuum quantum numbers and $0^\pm$ glueball channels. If SU(3)$_{\rm color}$ of bona fide QCD is replaced by SU(2)$_{\rm color}$, diquarks become well-defined gauge invariant objects. Moreover, there is an exact symmetry relating them to pions. In this limit predictions regarding good diquarks are iron-clad. If passage from SU(2)$_{\rm color}$ to SU(3)$_{\rm color}$ does not lead to dramatic disturbances, these predictions remain qualitatively valid in bona fide QCD.

Novel aspects of QCD in leptoproduction

I review several topics in electroproduction which test fundamental aspects of QCD. These include the role of final-state interactions in producing diffractive leptoproduction processes, the shadowing of nuclear structure functions, and target-spin asymmetries. The antishadowing of nuclear structure functions is shown to be quark-flavor specific, suggesting that some part of the anomalous NuTeV result for $\sin^2\theta_W$ could be due to the non-universality of nuclear antishadowing for charged and neutral currents. I also discuss the physics of the heavy-quark sea, hidden color in nuclear wavefunctions, and evidence for color transparency for nuclear processes. The AdS/CFT correspondence connecting superstring theory to superconformal gauge theory has important implications for hadron phenomenology in the conformal limit, including an all-orders demonstration of counting rules for hard exclusive processes, as well as determining essential aspects of hadronic light-front wavefunctions.

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.

Pseudoscalarqq¯mesons and effective QCD coupling enhanced by⟨A2⟩condensate

Recent developments provided evidence that the dimension 2 gluon condensate $⟨{A}^{2}⟩$ is important for the nonperturbative regime of Yang-Mills theories (quantized in the Landau gauge). We show that it may be relevant for the Dyson-Schwinger approach to QCD. In order that this approach leads to a successful hadronic phenomenology, an enhancement of the effective quark-gluon interaction seems to be needed at intermediate (${p}^{2}\ensuremath{\sim}0.5\text{ }\text{ }{\mathrm{G}\mathrm{e}\mathrm{V}}^{2}$) momenta. It is shown that the gluon condensate $⟨{A}^{2}⟩$ provides such an enhancement. It is also shown that the resulting effective strong running coupling leads to the sufficiently strong dynamical chiral symmetry breaking and successful phenomenology at least in the light sector of pseudoscalar mesons.

Hadron physics: some perspectives

Hadron phenomenology is a long standing research field in Mexico. In this article I present a short overview of the phenomena and their relevance. Special attention is paid to topics in which I have being involved. The perspective is intended to show to the freshman the still open possibilities to explore, rather than a historical one.

Quark spectra and light hadron phenomenology from overlap fermions with improved gauge field action

We present first results from a simulation of quenched overlap fermions with improved gauge field action. Among the quantities we study are the spectral properties of the overlap operator, the chiral condensate and topological charge, quark and hadron masses, and selected nucleon matrix elements. To make contact with continuum physics, we compute the renormalization constants of quark bilinear operators in perturbation theory and beyond.