Low-energy QCD Research Articles

Low-energy QCD and hadronic structure

Recent developments and selected topics in low-energy QCD are summarized, from chiral effective field theory to systems with strange and charm quarks, from lattice QCD to precision experiments.

η and η′ physics at BES-III

Decays of both η and η′ provide very useful information in our understanding of low-energy QCD, and experimental signatures for these decays would be extremely helpful at BES-III. The rare decays of the η and η′ mesons could serve as a low-energy test of the standard model and beyond. The sensitivities of the measurements of η and η′ decays are discussed at BES-III, in which the η and η′ mesons are produced in the ψ decays.

The KLOE-2 project

By the end of 2009 the KLOE-2 detector is expected to start data taking at the improved DAϕNE ψ-factory of the Laboratori Nazionali di Frascati of INFN. The KLOE-2 physics program is wide, spanning from studies on neutral kaon quantum interferometry, to precise tests of lepton flavour violation, to low energy QCD studies.In this paper, the status of the project is described. Some more attention is given to the contribution of KLOE-2 to the study of discrete symmetries conservation/violation.

Relativistic models for nuclear structure and low-energy QCD phenomenology

In this contribution we will report about developments on a recently proposed relativistic nuclear energy density functional based on the characteristic features of low-energy QCD and its symmetry breaking pattern.

PION AND KAON STRUCTURES FROM THE INSTANTON VACUUM

In this presentation, we briefly review recent investigations on pion and kaon structures from the instanton vacuum. Starting from the low-energy QCD partition function, we have computed the Gasser-Leutwyler low-energy constants, electromagnetic form factors of the pion and kaon, and semileptonic decay form factors of the kaon. The results are in good agreement with the experimental and empirical data.

Global aspects of the scalar meson puzzle

A generalized linear sigma model for low-energy QCD is employed to study the quark structure of eight low-lying scalar isomultiplets as well as eight low-lying pseudoscalar isomultiplets. The model, building on earlier work, assumes the possible mixing of quark antiquark states with others made of two quarks and two antiquarks. No a priori assumption is made about the quark contents of the states, which emerge as predictions. An amusing and contrasting pattern for the quark structure is found; the lighter conventional pseudoscalars are, as expected, primarily of two-quark type whereas the lighter scalars have very large four-quark admixtures. The new feature of the present paper compared to earlier ones in this series involves the somewhat subtle and complicated effects of SU(3) flavor breaking. They do not alter the general pattern of two-quark vs four-quark mixing obtained in the SU(3)symmetric case but, of course, give a more detailed picture.

About the possibility of five-dimensional effective theories for low-energy QCD

The AdS/QCD models suggest the interesting idea that the effective theory of low-energy QCD may be formulated as a 5-dimensional field theory in the weak coupling regime in which the fifth coordinate plays a role of inverse energy scale. Taking the point of view that this is just an efficient parameterization of the non-perturbative dynamics of strong interactions, we discuss on a qualitative level an alternative possibility for a simpler 5-dimensional parameterization of the main phenomena in the low-energy QCD. We propose to interpret the effect of chiral symmetry breaking as the effective appearance of a compactified extra dimension with the radius of the order of the inverse scale of chiral symmetry breaking. Following some heuristic arguments two dual scenarios for the emergence of the excited light mesons are introduced. In the first scenario, the meson resonances are interpreted as the effects of Kaluza–Klein excitations of quarks inside mesons, in the second one as the formation of gluon strings wound around the compactified dimension an appropriate number of times. Matching of these scenarios permits us to express the slope of radial Regge trajectories through the order parameters of the chiral symmetry breaking, with the compactification radius being excluded. This example shows qualitatively that the extra dimension may play an auxiliary role providing a short way for deriving new relations.

Effective field theories for the heavy quarkonium

We briefly review how nonrelativistic effective field theories give us a definition of the QCD potentials and a coherent field-theory-derived quantum-mechanical scheme to calculate the properties of bound states made by two or more heavy quarks. In this framework heavy quarkonium properties depend only on the QCD parameters (quark masses and α s ) and nonpotential corrections are systematically accounted for. The relation between the form of the nonperturbative potentials and the low-energy QCD dynamics is also discussed.

Physics program at the DAΦNE collider upgraded in luminosity

We review the physics program at the Frascati e + e − collider, whose upgrade in luminosity is currently underway, giving a summary of the proposals for experiments at the upgraded collider, and an outlook on the improvement of the measurements of interest for precision tests of the Standard Model, CP and CPT symmetries, and low-energy QCD interactions.

A holographic QCD and excited baryons from string theory

We study baryons of arbitrary isospin in a stringy holographic QCD model. In this D4-D8 holographic setting, the flavor symmetry is promoted to a gauge symmetry in the bulk, and produces, as KK modes of the gauge field, pions and spin one mesons of low energy QCD. Baryons of arbitrary isospins are represented as instanton solitons with isospin and spin quantum numbers locked, in a manner similar to the Skyrmion model. The soliton picture leads to a natural effective field theory of arbitrary baryons interacting with mesons. Couplings of baryons to axial mesons, including pions, are dominated in the large $N_c$ limit by a direct coupling to the flavor field strength in five dimensions. We delineate the relevant couplings and determine their strengths. This work generalizes part of hep-th/0701276 and arXiv:0705.2632 [hep-th] to all excited baryons. Due to technical difficulties in introducing relativistic higher spin fields, we perform all computations in the nonrelativistic regime, which suffices for the leading $N_c$ predictions.

Nuclear energy density functionals constrained by low-energy QCD

A microscopic framework of nuclear energy density functionals is reviewed, which establishes a direct relation between low-energy QCD and nuclear structure, synthesizing effective field theory methods and principles of density functional theory. Guided by two closely related features of QCD in the low-energy limit: a) in-medium changes of vacuum condensates, and b) spontaneous breaking of chiral symmetry; a relativistic energy density functional is developed and applied in studies of ground-state properties of spherical and deformed nuclei.

Two chiral nonet model with massless quarks

We present a detailed study of a linear sigma model containing one chiral nonet transforming under $\mathrm{U}(1{)}_{\mathrm{A}}$ as a quark-antiquark composite and another chiral nonet transforming as a diquark-antidiquark composite (or, equivalently from a symmetry point of view, as a two meson molecule). The model provides an intuitive explanation of a current puzzle in low energy QCD: Recent work has suggested the existence of a lighter than 1 GeV nonet of scalar mesons which behave like four quark composites. On the other hand, the validity of a spontaneously broken chiral symmetric description would suggest that these states be chiral partners of the light pseudoscalar mesons, which are two quark composites. The model solves the problem by starting with the two chiral nonets mentioned and allowing them to mix with each other. The input of physical masses in the SU(3) invariant limit for two scalar octets and an excited pion octet results in a mixing pattern wherein the light scalars have a large four quark content while the light pseudoscalars have a large two quark content. One light isosinglet scalar is exceptionally light. In addition, the pion pion scattering is also studied and the current algebra theorem is verified for massless pions which contain some four quark admixture.

Hypernuclei and in-medium chiral dynamics

A recently introduced relativistic nuclear energy density functional, constrained by features of low-energy QCD, is extended to describe the structure of hypernuclei. The density-dependent mean field and the spin-orbit potential of a Λ-hyperon in a nucleus, are consistently calculated using the SU(3) extension of in-medium chiral perturbation theory. The leading long-range ΛN interaction arises from kaon-exchange and 2π-exchange with a Σ-hyperon in the intermediate state. Scalar and vector mean fields, originating from in-medium changes of the quark condensates, produce a sizeable short-range spin-orbit interaction. The model, when applied to oxygen as a test case, provides a natural explanation for the smallness of the effective Λ spin-orbit potential: an almost complete cancellation between the background contributions (scalar and vector) and the long-range terms generated by two-pion exchange.

The theory of nuclear forces: Is the never-ending story coming to an end?

I review the recent progress in our understanding of nuclear forces in terms of an effective field theory (EFT) for low-energy QCD and put this progress into historical perspective. This is followed by an assessment of the current status of EFT based nuclear potentials. In concluding, I will summarize some unresolved issues.

Chiral pion–nucleon dynamics in finite nuclei: Spin–isospin excitations

The nuclear density functional framework, based on chiral dynamics and the symmetry breaking pattern of low-energy QCD, is extended to the description of collective nuclear excitations. Starting from the relativistic point-coupling Lagrangian introduced in [P. Finelli, N. Kaiser, D. Vretenar, W. Weise, Nucl. Phys. A 770 (2006) 1], the proton–neutron (quasiparticle) random phase approximation is formulated and applied to investigate the role of chiral pion–nucleon dynamics in excitation modes involving spin and isospin degrees of freedom, e.g. isobaric analog states and Gamow–Teller resonances.

Lattice QCD and nuclear physics

A steady stream of developments in lattice QCD have made it possible today to begin to address the question of how nuclear physics emerges from the underlying theory of strong interactions. A central role in this understanding play both the effective-field theory description of nuclear forces and the ability to perform accurate non-perturbative calculations in low-energy QCD. Here I present some recent results that attempt to extract important low-energy constants of the effective-field theory of nuclear forces from lattice QCD.

String theoretic QCD axion with stabilized saxion and the pattern of supersymmetry breaking

String theoretic axion is a prime candidate for the QCD axion solving the strong CP problem. For a successful realization of the QCD axion in string theory, one needs to stabilize moduli including the scalar partner (saxion) of the QCD axion, while keeping the QCD axion unfixed until the low energy QCD instanton effects are turned on. We note that a simple generalization of KKLT moduli stabilization provides such set-up realizing the axion solution to the strong CP problem. Although some details of moduli stabilization are different from the original KKLT scenario, this set-up leads to the mirage mediation pattern of soft SUSY breaking terms as in the KKLT case, preserving flavor and CP as a consequence of approximate scaling and axionic shift symmetries. The set-up also gives an interesting pattern of moduli masses which might avoid the cosmological moduli, gravitino and axion problems.

Chiral symmetry breaking from five dimensional spaces

Based on the AdS/CFT correspondence we study the breaking of the chiral symmetry in QCD using a simple five dimensional model. The model gives definite predictions for the spectrum of vector mesons, their decay constants and interactions as a function of one parameter related to the quark condensate. We calculate the coefficients Li of the low-energy QCD chiral lagrangian, as well as other physical quantities for the pions. All the predictions are shown to be in good agreement with the experimental data. We also show that they are robust under modifications of the 5D metric in the IR, and that some of them arise as a consequence of the higher-dimensional gauge symmetry. For example, at the tree-level, we find M ρ ≃ 3 g ρ π π F π , F ρ ≃ 3 F π and BR ( a 1 → π γ ) = 0 .

Description of spin and isospin collective excitations with a nuclear energy density functional constrained by low-energy QCD

We have recently formulated a novel approach to nuclear DFT, emphasizing links between low-energy QCD, its symmetry breaking pattern, and the nuclear many-body dynamics. Promising results have been obtained in the description of ground-state observables over a wide range of spherical and deformed nuclei. In particular, we have shown that chiral pionic fluctuations with explicit Δ(1232) excitations account for the isovector dependent part of the nuclear interaction. The relativistic random phase approximation, based on the equivalent point-coupling model with density dependent contact interactions, has been developed and applied in studies of low-energy collective excitations. Especially important is the investigation of the role of chiral pion-nucleon dynamics in excitation modes which include spin and isospin degrees of freedom, e.g. isobaric analog states and Gamow Teller resonances.

Yukawa's Pion, Low-Energy QCD and Nuclear Chiral Dynamics

A survey is given of the evolution from Yukawa's early work, via the understanding of the pion as a Nambu-Goldstone boson of spontaneously broken chiral symmetry in QCD, to modern developments in the theory of the nucleus based on the chiral effective field theory representing QCD in its low-energy limit.