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in different extensions of the standard model ( sm ) the anomalous interactions can lead to significant modification of the top quark production mechanisms and appearance of the top rare decays  @xcite .
thereby , the precision measurements of the top quark properties and its production mechanisms provide a possibility to obtain an information on new physics beyond the sm . of special interest
it is the study of the top quark anomalous interactions via flavour - changing neutral currents ( fcnc ) .    in the sm
there are no direct fcnc transitions @xmath4 .
only   loop  contributions can make them possible . as a result
such processes are strongly suppressed within the sm : br@xmath5 @xcite .
however , various extensions of the sm could lead to a huge enhancement of fcnc processes  @xcite .
search for the anomalous fcnc interactions of the top quark were performed at the tevatron  @xcite , hera  @xcite , and lep-2  @xcite colliders .
the present constraints on the top quark anomalous couplings in terms of branching ratio ( br ) are presented in the table below .
[ t3 ]    ._the present constraints on top quark anomalous couplings _ [ cols="^,^,^",options="header " , ]     the best constraint on fcnc coupling with a gluon from @xmath3-collision is obtained supposing the interaction with an @xmath6-quark only . when the interaction with a charm quark is also taken into account the constraints on couplings
do not change greatly but the br increases almost twice .
it is explained by the small contribution of the @xmath7-quarks to proton s pdf and appearance of two decay modes of the top - quark .
one also can see that the contribution of @xmath8-boson modifies the results slightly and can be neglected .
finally the derived constraint on the anomalous coupling of the top fcnc interaction with a gluon and the corresponding br are given below :    @xmath9
very recently at the tevatron experiment @xcite better constraint ( @xmath10 times less ) on the @xmath11 , the upper limit on the anomalous coupling of the top quark with a gluon , was obtained .
nevertheless we wish to indicate that the hera results for the single top production provides the way to put the constraints on the anomalous top - quark interactions not only with a photon and @xmath8-boson , but also with a gluon !    moreover ,
such analysis of @xmath3-collision ( e.g. hera data ) allows getting the limits on anomalous coupling of the top quark with a gluon and can be considered as an independent estimation of @xmath11 .
we are grateful to m.  mohammadi najafabadi and o.v .
zenin for useful discussions .
this work was supported in part by the dynasty foundation and by the russian foundation for basic research under grant # 08 - 02 - 91002-c .
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using the constraint on the single top production cross - section obtained at the hera collider , @xmath0 , we evaluate an upper limit on coupling constant of the anomalous top quark interaction with a gluon via flavor - changing neutral current : @xmath1 , br@xmath2     +   + * the constraint on fcnc coupling of the top quark with a gluon from @xmath3  -  collisions * + a.a .  ashimova  mail.ru ] + _ moscow state university , 119992 moscow , russia _ + s.r .  
 slabospitsky  ihep.ru ] + _ state research center + institute for high energy physics , + protvino , moscow region 142281 , russia _