Photoproduction of pions in chiral bag models

Threshold pion photoproduction has been calculated for the nucleon, within the framework of chiral bag models. The results of low-energy theorems (LET) are recovered only by taking account of Lorentz ivariance and center-of-mass (CM) correlations of the composite system. In particular the static bag model describes only the Kroll-Ruderman term of the amplitude. The suppression of spurious CM excitations by the concept of effective charges and the use of properly boosted wave functions are necessary in order to obtain also the higher-order contributions as predicted by LET.     

On the coupling constants of pions to nuclei

Pion-nucleus coupling constants are deduced in several representatives cases on the basis of PCAC and experimental information regarding relevant nuclear matrix elements of the axial weak current.     

Quark dynamics and pion-nucleon coupling

In the framework of nonperturbative QCD phenomenology we discuss: (1) The elementary process for the creation of color-singlet q$\text{q}$-pairs inside a hadron. (2) The interaction of the q$\text{q}$-pair with the surrounding quark-gluon medium. An important consequence of these discussions is that meson emission takes place preferentially, if the primary q$\text{q}$-pair is created in the surface region of the hadron. For the case of pseudoscalar coupling we employ PCAC to obtain the coupling of the q$\text{q}$-pair to the pion. The resulting form and coupling strength of the πNN vertex is consistent with the phenomenological OPEP.     

Quark potentials at finite temperatures

We scan the quark-antiquark potential and the meson-meson potential for static quarks in aSU ₃ gluon field from strong coupling to weak coupling. The breakdown of the confinement between quark and antiquark at the phase transition is observed. There is no interaction between pointlike mesons in the whole coupling regime. It is pointed out that the interaction mechanism between two quark clusters can be classified by these two fundamental examples.     

Effect of finite pixel size on optical coupling in QWIPs

We study the optical coupling in quantum well photodetectors, focusing on finite size effects. We introduced a finite-element model of the detector and we show experimentally that the optical coupling efficiency is strongly dependent on the pixel size and that in very small detectors diffraction dominates the grating coupling. A 640 × 512 QWIP focal plane array was characterized to show that the optical response of thinned samples may depend on the substrate thickness noticeably. These results are in much closer agreement with predictions obtained with our model than using standard techniques.     

Quark trapping in the quantised scalar SLAC ‘bag’ model

We build simple ‘bags’ (quarks plus scalars), based on the simplest SLAC Lagrangian, upon the Hartree-Fock vacuum. For relatively weak quark coupling (at least) the self-consistent quantum fluctuations prevent good quark confinement.     

Collision rate of pions and kaons at finite temperature

The energy dependence of the pion and kaon collision rates at finite-temperature are calculated in the approximation that resonances dominate the scattering, as for example, ππ→?→ππ andKπ→K ^*→Kπ. A general formula is derived for the contribution of resonances with arbitrary spin and isospin. Numerical results are given. For very narrow resonances the calculation is performed analytically.     

Quark condensates in non-Abelian fields at finite temperature

The properties of quark condensates in static non-Abelian fields of the chromomagnetic types are investigated. The dependence of the condensate on the field strength and the temperature is discussed. It is shown that the principal term of the asymptotic expansion for all the investigated field types has the form v^–2 in the high-temperature limit. An expansion is obtained for the quantity in the limit of an arbitrary, uniform, weak field for T=0 and at finite temperature.M. V. Lomonosov State University, Moscow. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 39–52, January, 1994.     

Corrections to scaling and finite size effects

Numerical investigations of critical phenomena might lead to different leading correction-to-scaling terms, depending on the type of analysis. We discuss which leading correction-to-scaling behavior is expected for finite system sizes, using two-dimensional percolation as our main example. It turns out that a finite-size scaling from length L to L - 1 is less perturbed by correction-to-scaling terms.     

Long-term fluctuations in globally coupled phase oscillators with general coupling: finite size effects

We investigate the diffusion coefficient of the time integral of the Kuramoto order parameter in globally coupled nonidentical phase oscillators. This coefficient represents the deviation of the time integral of the order parameter from its mean value on the sample average. In other words, this coefficient characterizes long-term fluctuations of the order parameter. For a system of N coupled oscillators, we introduce a statistical quantity D, which denotes the product of N and the diffusion coefficient. We study the scaling law of D with respect to the system size N. In other well-known models such as the Ising model, the scaling property of D is D～O(1) for both coherent and incoherent regimes except for the transition point. In contrast, in the globally coupled phase oscillators, the scaling law of D is different for the coherent and incoherent regimes: D～O(1/N(a)) with a certain constant a>0 in the coherent regime and D～O(1) in the incoherent regime. We demonstrate that these scaling laws hold for several representative coupling schemes.     

Structure functions in nuclei: Quark clusters and size effects

Two of the most interesting ideas to explain the EMC effect, clusters of quarks and confinement size factors, are unable to fully explain the existing data. In this paper we argue that the existence of clusters with different numbers of quarks are in the origin of different size parameters controlling the lowQ ² limit in the QCD evolution and thus we combine the two ideas. As a result we obtain a model for structure functions with two basic effective parameters: the average numberi of nucleons per cluster and the corresponding size parameterR _i . We achieve good agreement with the existing data for a wide range ofQ ²(?2 GeV²) and for all values ofx. Simple parametrizations of the Sea, Gluon and Valence distributions, for pion, nucleon and nuclei, are given. Some applications of the model, in particular to Drell-Yan processes, are discussed.     

Numerical finite size scaling approach to many-body localization

We develop a numerical technique to study Anderson localization in interacting electronic systems. The ground state of the disordered system is calculated with quantum Monte Carlo simulations while the localization properties are extracted from the "Thouless conductance" g, i.e., the curvature of the energy with respect to an Aharonov-Bohm flux. We apply our method to polarized electrons in a two-dimensional system of size L. We recover the well-known universal beta(g)=dlogg/dlogL one parameter scaling function without interaction. Upon switching on the interaction, we find that beta(g) is unchanged while the system flows toward the insulating limit. We conclude that polarized electrons in two dimensions stay in an insulating state in the presence of weak to moderate electron-electron correlations.     

从流夸克质量到组分夸克质量的动力学过渡的研究

在彩虹近似下用Dyson Schwinger 方程研究了禁闭夸克的传播子, 得到了夸克动力学质量随动量变化的关系, 即得到了从流夸克质量到组分夸克质量的动力学过渡.     

ρ-Meson coupling to the nucleon in the chiral bag model

The vector and tensor couplings of the ρ-meson to the nucleon are calculated by assuming the indirect coupling of the ρ-meson to the nucleon, through the pion cloud of the chiral bag model. Qualitative agreement with experiment is found at reasonable bag radii for both versions of the model: the cloudy bag model with pions in the interior, and the standard model with pions excluded from the bag.     

The running coupling at finite temperature

We propose the ensemble averaged running coupling as the meaningful measure of the coupling strength of an equilibrium gas and consider its scaling behaviour. We show that the high temperature limit of the average coupling is obtained by keeping the temperature to the renormalization point ratio,T/μ, fixed. In hot QCD gas this implies asymptotic freedom and a beta function which is independent ofT up to two loops. In the MOM scheme a minimal sensitivity to temperature is obtained with the choice μ=2.6T.     

Linearized holographic isotropization at finite coupling

We study holographic isotropization of an anisotropic homogeneous non-Abelian strongly coupled plasma in the presence of Gauss–Bonnet corrections. It was verified before that one can linearize Einstein’s equations around the final black hole background and simplify the complicated setup. Using this approach, we study the expectation value of the boundary stress tensor. Although we consider small values of the Gauss–Bonnet coupling constant, it is found that finite coupling leads to significant increasing of the thermalization time. By including higher order corrections in linearization, we extend the results to study the effect of the Gauss–Bonnet coupling on the entropy production on the event horizon.