Expansion For Quantum Chromodynamics Research Articles

U-spin predictions of the transition magnetic moments of the electromagnetic decay of the Σ*(1385) baryons

The transition magnetic moments for decuplet-to-octet baryon electromagnetic decays are calculated from the CLAS experimental results and are compared with calculations to first order in the 1/Nc expansion of quantum chromodynamics (QCD) and new U-spin predictions. Using the U-spin predictions for the Σ*0 → Σ0γ and Σ*+ → Σ+γ decays, the SU(3)-forbidden transition Σ*− → Σ−γ is obtained. In addition, the doubly strange baryon radiative decay Ξ*0 → Ξγ is predicted using U-spin.

On the Large Nc Expansion in Quantum Chromodynamics

I discuss methods based on the large Nc expansion to study nonperturbative aspects of quantum chromodynamics, the theory of the strong force. I apply these methods to the analysis of weak decay processes and the nonperturbative computation of the weak matrix elements needed for a complete evaluation of these decays in the Standard Model of elementary particle physics.

Gluon propagator in a new perturbative expansion of Quantum Chromodynamics with a non-perturbative background

Using a new perturbative expansion method in Quantum Chromodynamics with a non-perturbative gluon background, the gluon propagator is calculated up to the one-loop level, and renormalized in the modified minimal subtraction scheme. The resultant renormalization constants of the quantum gluon field and the gauge parameter receive a non-perturbative contribution coming from the gluon condensate besides the usual perturbative one, respectively.

Nonperturbative expansion in quantum chromodynamics and the inclusive decay of the τ lepton

The nonperturbative method in quantum chromodynamics based on a new small expansion parameter is applied to the description of the semi-lepton inclusive decay of the τ lepton. The contribution of renormalon chains are taken into account with a special representation for the effective current coupling constant. With calculations to O(a5), we show that the renormalon contribution is significant when the value of the strong coupling constant from the experimental data on τ decay is included. Evolution of the value obtained for the coupling constant to the scale of the Z boson leads to good agreement with the available experimental data for the RZ ratio.

Nonperturbative expansion in QCD

The purpose of this article is to construct the nonperturbative expansion in quantum chromodynamics using a new small parameter and apply it to the investigation of the connection between nonperturbative and perturbative regimes of the effective coupling constant. We calculate the nonperturbative renormalization group β-function and discuss the properties of the series convergence using the two-loop approximation in this method. Based on the information from meson spectroscopy we derive the effective coupling constant in the perturbative region.

New expansion in QCD

We formulate a systematic, nonperturbative expansion in quantum chromodynamics using a new small parameter. The quark-gluon running coupling constant, quark masses and static quark-antiquark potential are constructed.

Multipole expansion in quantum chromodynamics and the radiative decaysJψ→γ+ηandJψ→γ+π0

A generalized formalism of QCD multipole expansion including constituent-quark-field quantization is proposed. It can be applied to study soft-gluon emission processes of heavy-quark-antiquark systems with quark-flavor changing or quark pair annihilation. As an application, the radiative decays $\frac{J}{\ensuremath{\psi}}\ensuremath{\rightarrow}\ensuremath{\gamma}+\ensuremath{\eta}$ and $\frac{J}{\ensuremath{\psi}}\ensuremath{\rightarrow}\ensuremath{\gamma}+{\ensuremath{\pi}}^{0}$ are studied in this framework. It is shown that $\frac{J}{\ensuremath{\psi}}\ensuremath{\rightarrow}\ensuremath{\gamma}+\ensuremath{\eta}$ is dominated by a vector-meson-dominance mechanism in which the vector mesons are $n^{3}S_{1}$ states of $c\overline{c}$ and the calculated rate is in agreement with the experiment. The decay $\frac{J}{\ensuremath{\psi}}\ensuremath{\rightarrow}\ensuremath{\gamma}+{\ensuremath{\pi}}^{0}$ is dominated by a different vector-meson-dominance mechanism in which the vector meson is ${\ensuremath{\rho}}^{0}$.

Skyrmions and effective Lagrangians

The need to model quantum chromodynamics (QCD) at low energy is emphasized. An outline of the1/NC expansion of QCD, for large NC, shows the deep link between the Skyrme effective Lagrangian and QCD. The Skyrme model, built from the nonlinear σ model plus a stabilizer term related to vector dominance, is briefly described. The model satisfies like QCD chiral symmetry. We illustrate how the gauged Wess–Zumino action demonstrates that the topological current can be identified with the baryon current. We recall how one can show that the topological soliton is a fermion for odd NC. An example of an effective Lagrangian, built from π and low-mass vector mesons, ω, ρ, and A1 fields, is given. It describes rather well low-energy meson and baryon physics. Predictions of effective Lagrangians of the Skyrme type on meson–meson, meson–baryon, and baryon–baryon scatterings at low energy are depicted. A two-phase chiral symmetric model, the chiral bag, is introduced. It contains an inner core of confined quarks and gluons surrounded by meson fields in the topological configuration of a Skyrmion. It can describe nuclei from the low- to the high-energy range.

Yukawa theories as effective theories of quantum chromodynamics for a large number of colors

An effective theory for low-energy nuclear interactions is proposed, based on results obtained from the 1/${N}_{c}$ expansion of quantum chromodynamics. The Lagrangian is local in the meson sector, but in the baryon sector it is nonlocal both in the meson-baryon Yukawa coupling and in the baryon propagators. It satisfies an important consequence of the 1/${N}_{c}$ expansion, the suppression of baryon loops. Our findings are then shown to support the traditional approaches in nuclear physics and, more especially, the relativistic nuclear many-body theories at baryon-tree level.

Weak decay amplitudes in large N c QCD

A systematic analysis of nonleptonic decay amplitudes is presented using the large N c expansion of quantum chromodynamics. In the K-meson system, this analysis is applied to the calculation of the weak decay amplitudes, weak mixing and CP violation.

Theoretical Investigation of the Isospin Violating Hadronic Transitions ψ′→ψ(11P1)+π° and T′′→T(11P1)+π° ††Supported by the Science Fund of the Chinese Academy of Sciences

The isospin violating processes ψ′→ψ(11P1)+π° and T′′→T(11P1)+π° are investigated in the framework of the multipole expansion in quantum chromodynamics. Using the rate of ψ′→ψ+π° as an input to determine the isospin violating vertex, we obtain the following branching ratios for the processesfor . These are too small to be detected in the present experiments.

U(1) problem: Current algebra and theθvacuum

The effective Lagrangian which gives a full solution of the U(1) problem is obtained from the 1/N expansion of quantum chromodynamics. This Lagrangian satisfies all the anomalous n-point Ward identities for arbitrary q/sup 2/, and includes chiral and SU(3) breaking. The relationship between the eta' mass and the theta dependence of the vacuum energy in the absence of quarks and the form of the strong CP violation in the effective-Lagrangian framework are deduced.

Operator expansion in quantum chromodynamics beyond perturbation theory

We discuss the status of the operator expansion at short distances within the framework of non-perturbative QCD. The question of instanton effects is investigated in various aspects. Consistency between instanton calculations and the general operator expansion requires a very special relation between the expansion coefficients. We check, by considering particular examples, that this relation does exist.

Quantum-chromodynamic estimates for heavy-particle production

The associated production of hadrons containing heavy quarks is studied in the framework of a model based on quark-gluon color gauge field theory [quantum chromodynamics (QCD)]. We assume that the dominant mechanism for the production of heavy quarks in real and virtual photon beams is $\ensuremath{\gamma}({Q}^{2})V\ensuremath{\rightarrow}c\overline{c}$ where $V$ denotes a vector gluon and $c$ an arbitrary heavy quark. For $\ensuremath{\pi}$, $p$, and $\overline{p}$ beams we consider the mechanisms $\mathrm{VV}\ensuremath{\rightarrow}c\overline{c}$ and $q\overline{q}\ensuremath{\rightarrow}c\overline{c}$. The cross sections for the internal subprocesses are calculated at lowest order in the perturbation expansion for QCD. We include a brief discussion of higher-order corrections to our calculation.

Hadron scattering in two-dimensional QCD: (II). Second-order calculations, multi-Regge and inclusive reactions

Hadron scattering is studied in higher orders of the 1/ N c expansion for quantum chromodynamics in two dimensions. In order 1/ N c 2, individual pomeron-like cylinder graphs give a forward amplitude growing as s, but cancellations in their sum remove this embryonic pomeron. The cancellations reflect colour gauge invariance in a way specific to two dimensions. Otherwise, 1/ N c 2 diagrams yield the expected “Regge” pole renormalizations and “cut” singularities. Factorizing multi-Regge behaviour is demonstrated for higher point functions. Final-state hadron distributions in hadronic and deep inelastic collisions are closely related.

Demythification of electroproduction local duality and precocious scaling

In an effort to develop a quantitative check of asymptotically free color-gauge theories, we analyze the logarithmic corrections to ξ-scaling coming from anomalous dimensions and coefficient functions of twist-two operators and compare with electroproduction data for 1 ⩽ Q 2 ⩽ 16 GeV 2. Excellent agreement is obtained using g 2(2 GeV) 4π 2 = 0.17 for the effective quark-gluon coupling in the color-gauge theory. Effects of higher-twist operators are suppressed by powers of M 0 2 Q 2 . We use data from the resonance region to show M 0 ⋍ 400 MeV , in agreement with theoretical expectations. Our fit to νW 2 in the scaling region also describes the resonance region in the sense of Bloom-gilman local duality. We show that local duality is a consequence of the moment predictions obtained from the operator-product expansion in quantum chromodynamics. We resolve a paradox associated with local duality and spin-zero targets. Present measurements of R = σ L σ T at large x and Q 2 are systematically higher than our predictions.