ar X iv : h ep - p h / 03 11 07 9 v 1 6 N ov 2 00 3 Electromagnetic Current of a Composed Vector Particle in the

The violation of the rotational symmetry constraint of the matrix elements of the plus component of the vector current, in the Drell-Yan frame, is studied using an analytical and covariant model of a spin-1 composite particle. The contributions from pair diagrams or zero modes, if missed cause the violation of the rotational symmetry. We reanalyze the prescription suggested by Grach and Kondratyuk [Sov. to extract the form factors which can eliminate these contributions in particular models. The light-front description of hadrons [1, 2] in a truncated Fock-space breaks the rotational symmetry, as some rotations are dynamical operators which mixes different components in the Fock-space [3, 4]. The problem to keep the correct rotational properties of a relativistic quantum system, within light-front field theory is difficult to handle, although in principle is solvable, when one is not limited to a Fock-space sector [2]. However, within phenomenological models one is tempted to describe hadrons just with the valence component and calculate observables, in particular the electromagnetic form factors. Thinking on that, one may consider that an analysis with covariant and analytical models, could be useful to give an insight on the properties lost by a description of a composite system in a truncated light-front Fock-space. In that respect, it was studied the rotational symmetry breaking of the plus component of the electromagnetic current (J + = J 0 + J 3) in the Breit-frame respecting the Drell-Yan condition (purely transverse momentum transfer and q + = q 0 + q 3 = 0), using an analytical model for the spin-1 vertex of a two-fermion bound state [3]. Following this work, it was pointed out that pair terms give contributions beyond the valence one, and if ignored, the matrix elements of the current break covariance and the angular condition constraint is not fulfilled [4, 5]. Due to that, different prescriptions to extract the form factors from the microscopic matrix elements, which are calculated only with the valence component of the wave function, do not agree [6]. It was found in a numerical calculation of the ρ-meson electromagnetic form factors considering only the valence contribution [3], that the prescription proposed by [6] to evaluate the form-factors, produced results in agreement with the covariant calculations. Later on, Ref. [5] shown that the above prescription eliminates the pair contributions to the form factors, using only a γ µ structure for the vector meson vertex with the …