Spin-1 Particle in the Light-Front Approach

The electromagnetic current of spin-1 composite particles does not transform properly under rotations if only the valence contribution is considered in the light-front model. In particular, the plus component of the current, evaluated only for the valence component of the wave function, in the Drell-Yan frame violates rotational symmetry, which does not allow a unique calculation of the electromagnetic form-factors. The prescription suggested by Grach and Kondratyuk [Sov. J. Nucl. Phys. 38, 198 (1984)] to extract the form factors from the plus component of the current, eliminates contributions from pair diagrams or zero modes, which if not evaluated properly cause the violation of the rotational symmetry. We address this problem in an analytical and covariant model of a spin-1 composite particle.Comment: To appear Brazilian Journal of Physics (2004), 4 pages, no figures. Use multicols.st

Experimental response of RC columns built with plain bars under unidirectional cyclic loading

A large number of existing reinforced concrete (RC) buildings structures were designed and built before mid-70’s, when the reinforcing bars had plain surface and prior to the enforcement of the modern seismic-oriented design philosophies. This paper describes a series of unidirectional cyclic tests performed on seven full-scale columns built with plain reinforcing bars, without adequate reinforcement detailing for seismic demands. The specimens have different reinforcing steel details and different cross sections. A further monotonic test was also carried out for one of the specimens and an additional column, built with deformed bars, was cyclically tested for comparison with the results for the specimens with plain bars. The main experimental results are presented and discussed. The influence of bond properties on the column behaviour is evidenced by differences observed between the cyclic response of similar specimens with plain and deformed bars. The influence of reinforcement amount and displacement history on the column response is also investigated

Light-Front projection of spin-1 electromagnetic current and zero-modes

The issue of the contribution of zero-modes to the light-front projection of the electromagnetic current of phenomenological models of vector particles vertices is addressed in the Drell-Yan frame. Our analytical model of the Bethe-Salpeter amplitude of a spin-1 fermion-antifermion composite state gives a physically motivated light-front wave function symmetric by the exchange of the fermion and antifermion, as in the $\rho$-meson case. We found that among the four independent matrix elements of the plus component in the light-front helicity basis only the $0\to 0$ one carries zero mode contributions. Our derivation generalizes to symmetric models, important for applications, the above conclusion found for a simplified non-symmetrical form of the spin-1 Bethe-Salpeter amplitude with photon-fermion point-like coupling and also for a smeared fermion-photon vertex model.Comment: Use elservier style. 14 page

Cyclic response of RC beam-column joints reinforced with plain bars: an experimental testing campaign

Existing reinforced concrete (RC) buildings constructed until the mid-70’s, with plain reinforcing bars, are expected to behave poorly when subjected to earthquake actions. This paper describes an experimental program designed to investigate the influence of poor detailing on the cyclic behaviour of RC beam-column joint elements. Cyclic tests were performed on five interior and five exterior full-scale beam-column joints with different detailing characteristics and reinforced with plain bars. An additional joint of each type was built with deformed bars for an evaluation of the influence of bond properties on the cyclic response of the structural element. The force-displacement global response, energy dissipation, equivalent damping and damage behaviour of the joints was investigated and the main results are presented and discussed. The experimental results indicate that the bond-slip mechanism has significantly influenced the cyclic response of the beam-column joints. The specimens built with plain bars showed lower energy dissipation, stiffness and equivalent damping

Light-front model of the kaon electromagnetic current

The electromagnetic form factor is extracted from both components of the electromagnetic current: J(plus) and J(minus) with a pseudo-scalar coupling of the quarks to the kaon. In the case of J(plus) there is no pair term contribution in the Drell-Yan frame. However, J(minus) component of the electromagnetic current the pair term contribution is different from zero and is necessary include it to preserve the rotational symmetry of the current.Comment: 4 pages, 1 figure, uses World Scientific style file. To be published by World Scientific in the proceedings of the "VIII International Workshop on Hadron Physics,(HADRONS 2002)", Bento Goncalves, RS, Brazi

Nonlinear modeling of the cyclic response of RC columns

Cyclic load reversals (like those induced by earthquakes) result in accelerated bond degradation, leading to significant bar slippage. The bond-slip mechanism is reported to be one of the most common causes of damage and even collapse of existing RC structures subjected to earthquake loading. RC structures with plain reinforcing bars, designed and built prior to the enforcement of the modern seismic-oriented design philosophies, are particularly sensitive to bond degradation. However, perfect bond conditions are typically assumed in the numerical analysis of RC structures. This paper describes the numerical modeling of the cyclic response of two RC columns, one built with deformed bars and the other with plain bars and structural detailing similar to that typically adopted in pre-1970s structures. For each column, different modeling strategies to simulate the column response were tested. Models were built using the OpenSees and the SeismoStruct platforms, and calibrated with the available tests results. Within each platform, different types of nonlinear elements were used to represent the columns. Bond-slip effects were included in the OpenSees models resorting to a simple modeling strategy. The models and the parameters adopted are presented and discussed. Comparison is established between the most relevant experimental results and the corresponding results provided by the numerical models. Conclusions are drawn about the capacity of the tested models to simulate the columns response and about the influence of considering or not considering the effects of bars slippage

Relativistic dynamics of Qqq systems

The bound state of constituent quarks forming a $Qqq$ composite baryon is investigated in a QCD-inspired effective light-front model. The light-front Faddeev equations are derived and solved numerically. The masses of the spin 1/2 low-lying states of the nucleon, $\Lambda^0$, $\Lambda^+_c$ and $\Lambda^0_b$ are found and compared to the experimental data. The data is qualitatively described with a flavor independent effective interaction.Comment: 18 pages, include 6 figures, 2 eps figures, use latex, revtex. To appear Phys. Rev D (2002