Electromagnetic structure of vector mesons

Electromagnetic structure of the complete nonet of vector mesons (ρ0, ρ, ρ−, ω, φ,K∗0,K∗+, K̄∗0,K∗−) is investigated in the framework of the Unitary and Analytic model and insufficient experimental information on it is discussed.


Introduction
According to the SU(3) classification of hadrons the spin 1 − particles are included in a vector meson nonet.The main purpose of our contribution is to outline a description of the electromagnetic (EM) structure of vector mesons within the framework of the universal Unitary and Analytic (U&A) model with respect to the known theoretical constrains and an insufficient experimental information.
In general the EM structure of all vector particles, owing to the spin value 1, is completely described by three independent EM form factors (FFs).
Since the neutral vector mesons ρ 0 , ω, φ from the nonet under consideration are self-conjugate particles obeying the crossing relation, their EM FFs are identically equal to zero in the whole region of definition.As a result, further only the EM structure of ρ + , ρ − , K * 0 , K * + , K * 0 , K * − vector mesons will be investigated.

Asymptotic behavior of vector meson structure functions
According to the quark model [1] the vector meson structure function A(q 2 ) has the asymptotic behavior where t = q 2 and A(t) is obtained from the differential cross section of the scattering (or e + e − annihilation) process of electron on a vector meson by the Rosenbluth separation.It can be decomposed into Sachs EM FFs Moreover it has been demonstrated [2] that at high q 2 the spin 1 particle EM FFs fulfill the following relations a e-mail: cyril.adamuscin@savba.skThe latter two equations (1 and 2) imply that EM FFs of vector mesons have the following asymptotic behaviors 3 Electromagnetic structure of the ρ + , ρ − vector mesons Due to the conservation of G-parity one can saturate γ * ρ + ρ − vertex only with ρ meson and its excitations.
In order to satisfy asymptotic behavior given by (3) one needs to saturate "advanced" VMD model [3] with at least two neutral resonances ρ, ρ f v is ratio of the corresponding coupling constants.Now we apply U&A transformations to be explained in [4] and one obtains the U&A parametrizations of the charged ρ EM FFs The values of two unknown form factor normalizations G ρ M (0), G ρ Q (0) have been predicted within the light front theory [5].The parametrizations (6) contain also four free parameters a ρ Cρ , t ρ inC , t ρ inM , t ρ inQ to be ratio of the coupling constants and three effective thresholds, which can be found by a comparison of ( 6) with a few experimental points on ρ meson helicity amplitudes [6].

EM structure of the K * (892) vector mesons
The K * (892) meson represents isodublet of particles K * (892) + , K * (892) 0 and similarly to the nucleon EM FFs, its EM FFs can be decomposed into the isoscalar (S) and isovector (V) parts as follows EPJ Web of C onferences The asymptotic behavior is alike the charged ρ meson case given by the conditions (3).To satisfy them one needs to saturate isoscalar EM FFs by two isoscalar vector mesons ω, φ and isovector EM form factors by two isovector vector mesons ρ, ρ .After performing the same procedure as for the charged ρ meson we have constructed U&A model of the K * (892) meson EM structure Isoscalar parts: Isovector parts: It has eight free parameters a K * V Cρ , a K * S Cω , t K * V inC , t K * V inM , t K * V inQ , t K * S inC , t K * S inM , t K * S inQ and four unknown FF normalizations G K * 0 M (0), G K * 0 Q (0), G K * + M (0), G K * + Q (0).Unlike the ρ meson case there are no corresponding experimental measurements on K * (892) meson EM form factors, therefore we can not carry out any comparison of the theoretical model with data and to determine those twelve free parameters.