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it has been proposed by magahiz et al .
@xcite to observe the reaction @xmath2 with the clas large acceptance spectrometer at cebaf to gain insight into strangeness electroproduction of nuclei .
if the longitudinal and transverse part of the differential cross section could be separated , the t - channel reaction would allow the measurement of the @xmath0 electromagnetic form factor which due to the mass difference between the strange and down quarks does non vanish .
in view of this proposed experiment predictions for the @xmath0 form factor have been recently published and it commonly turned out that due to the accessible values of momentum transfer up to few @xmath3 a covariant description of the underlying dynamics is mandatory .
cardarelli et al .
investigated a relativistic constituent quark model based on the light front formalism @xcite .
therein they made use of an interaction kernel motivated by an effective @xmath1 - hamiltonian which has been developed by godfrey and isgur @xcite . in another paper buck
, williams and ito @xcite calculated the @xmath4 and @xmath5 form factors by employing a model described in @xcite , with a separable ansatz including symmetry breaking effects .
in two previous papers @xcite we presented a covariant quark model based on the salpeter equation and used it to compute ( transition ) form factors between the light pseudoscalar and vector mesons @xcite .
this brief report shall serve as an extension of the latter to the neutral strange meson .
starting from the bethe - salpeter equation , we use a @xmath1 - interaction assumed instantaneous in the rest frame of the bound state and free effective quark propagators to arrive at the salpeter equation , which is expressed as an eigenvalue problem for the bound state mass and solved numerically @xcite .
in addition to the calculation of mass spectra we have presented there a method to reconstruct the four - dimensional bethe - salpeter amplitude from the equal - time salpeter amplitudes .    in our model the interaction
consists of a confining potential which is linearly rising in coordinate space , and an instanton - induced interaction derived by ( see @xcite and references therein ) as a possible solution of the u@xmath6(1)-problem .
we would like to emphasize that the potential and mass parameters used in our model have been fixed in @xcite to obtain a reasonable agreement with the experimental mass spectrum of the low lying pseudoscalar and vector mesons as well as the leptonic @xmath4 and @xmath7 decay widths .
the electromagnetic form factors as well as the other decay widths ( e.g. @xmath8 ) are consistently obtained by employing the mandelstam formalism to the formerly calculated bethe - salpeter amplitudes .
no additional parameter is used to calculate the current matrix elements . in lowest order
we find for the electromagnetic current coupling to the quark : @xmath9 \gamma_{\mu }    \chi_p(x , y )    \right\}\end{aligned}\ ] ] which is formally analogous to the results obtained by buck et al .
@xcite except for the inner structure of the amputated bethe - salpeter amplitude , which in our model contains in general eight amplitudes for fixed spin and parity @xcite and thus reflects the full dirac structure of the @xmath1 - system .
our results for the @xmath0 form factor @xmath10 are shown in fig .
[ abb1 ] and [ abb2 ] for small and large momentum transfer ( in fig .
[ abb2 ] we plotted @xmath11 ) .
our calculation agrees remarkably well with the prediction of buck et al .
@xcite , where the parameter have been fixed to reproduce the @xmath12 and @xmath13 charge radii and decay constants . however , our maximum of @xmath11 appears at a smaller momentum transfer of approximately 2 gev@xmath14 .    as our calculation has been performed in the framework of a covariant quark model which includes confinement , and therefore is able to describe not only the masses and decay properties of the pseudoscalars but also of the vector mesons , the @xmath0 form factor calculation is put on a more general basis than in the work of buck et al .
@xcite , although our results do not differ significantly .    the @xmath0 charge radius , as has been discussed e.g.  in @xcite , is most sensitive to the mass difference between the strange and down quark mass .
we have estimated the charge radius by a least - square fit of a quadratic function to our form factor below 0.1 gev@xmath14 and studied its dependence on the differences of the constituent quark masses @xmath15 keeping the sum of them fixed to our original value @xmath16  @xcite .
the results plotted in fig .
[ abb3 ] indeed shows a strong dependence on the quark mass difference , as long as it is smaller than 250 mev .
an experimental measurement of the @xmath0 charge radius therefore would be an interesting opportunity to determine the difference between strange and nonstrange constituent quark mass , alternatively to estimates from purely spectroscopic quark model calculations .
r.a.magahiz et al . , letter of intent submitted to cebaf pac 9 , available on www at http://www.phys.cmu.edu/hyperons/homepage.html .
f.cardarelli et al .
, preprint nucl - th/9507038 .
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c.j.burden , c.d.roberts , m.j.thomson , nucl - th/9511012 ( 1995 ) .
the electromagnetic form factor for the @xmath0 is calculated in a covariant formulation of the salpeter equation for @xmath1 - bound states , which has been presented recently for the mass spectrum , decay properties and form factors of the light pseudoscalar and vector mesons . 
 the @xmath0 charge radius dependence on the difference between strange and down constituent quark mass is discussed .    
 = 0.45