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one of the challenges in the modern physics is the description of the internal structure of the baryons and mesons . the electromagnetic structure of the nucleon @xmath1 and the nucleon resonances @xmath2 can be accessed through the @xmath0 reactions , which depend of the ( photon ) momentum transfer squared @xmath13  @xcite . the data associated with those transitions are represented in terms of helicity amplitudes and have been collected in the recent years at jefferson lab , with increasing @xmath13  @xcite . the new data demands the development of theoretical models based on the underlying structure of quarks and quark - antiquark states ( mesons )  @xcite . those models may be used to guide future experiments as the ones planned for the jlab12 gev upgrade , particularly for resonances in the second and third resonance region [ energy @xmath14@xmath15 gev ] ( see figure  [ figsigmaw ] )  @xcite . in that region there are several resonances @xmath2 from the multiplet @xmath4 $ ] of @xmath16 , characterized by a negative parity  @xcite . according with the single quark transition model ( sqtm ) , when the electromagnetic interaction is the result of the photon coupling with just one quark , the helicity amplitudes of the @xmath4 $ ] members depend only on three independent functions of @xmath13 : @xmath17 and @xmath18  @xcite . in this work we use the covariant spectator quark model  @xcite developed for the @xmath19 and @xmath20 transitions , also members of @xmath4 $ ] , to calculate those functions  @xcite . since the covariant spectator quark model breaks the @xmath21-flavor symmetry , we restrict our study to reactions with proton targets ( average on the sqtm coefficients )  @xcite . later on , with the knowledge of the functions @xmath17 , and @xmath18 we predict the helicity amplitudes for transitions associated with the remaining members of the multiplet @xmath4 $ ]  @xcite . representation of the @xmath22 cross section . the graph define the 3 resonance regions . the vertical lines represent resonant states described by the covariant spectator quark model . at red we indicate the states studded in this work . at blue are the states used as input.,width=362 ] the covariant spectator quark model is based on the formalism of the covariant spectator theory  @xcite . in the covariant spectator quark model , baryons are treated as three - quark systems . the baryon wave functions are derived from the quark states according with the @xmath23 symmetry group . a quark is off - mass - shell , and free to interact with the photon fields , and other two quarks are on - mass - shell  @xcite . integrating over the quark - pair degrees of freedom we reduce the baryon to a quark - diquark system , where the diquark can be represented as an on - mass - shell spectator particle with an effective mass @xmath24  @xcite . the electromagnetic interaction with the baryons is described by the photon coupling with the constituent quarks in the relativistic impulse approximation . the quark electromagnetic structure is represented in terms of the quark form factors parametrized by a vector meson dominance mechanism  @xcite . the parametrization of the quark current was calibrated in the studies of the nucleon form factors data  @xcite , by the lattice qcd data for the decuplet baryon  @xcite , and encodes effectively the gluon and quark - antiquark substructure of the constituent quarks . the quark current has the general form  @xcite j_q^(q^2 ) = j_1(q^2 ) ^+ j_2(q^2 ) , [ eqjq ] where @xmath25 is the nucleon mass and @xmath26 @xmath27 are the dirac and pauli quark form factors . in the @xmath21-flavor sector the functions @xmath26 can also be decomposed into the isoscalar ( @xmath28 ) and the isovector ( @xmath29 ) components : @xmath30 , where @xmath31 acts on the isospin states of baryons ( nucleon or resonance ) . the details can be found in  @xcite . when the nucleon wave function ( @xmath32 ) and the resonance wave function ( @xmath33 ) are both expressed in terms of the single quark and quark - pair states , the transition current in impulse approximation as can be written  @xcite j^= 3 _ _ k |_r ( p_+,k ) j_q^_n(p_-,k ) , [ eqjmu ] where @xmath34 , and @xmath35 are the nucleon , the resonance , and the diquark momenta respectively . in the previous equation the index @xmath36 labels the possible states of the intermediate diquark polarizations , the factor 3 takes account of the contributions from the other quark pairs by the symmetry , and the integration symbol represents the covariant integration over the diquark on - mass - shell momentum . in the study of inelastic reactions we replace @xmath37 in equation  ( [ eqjq ] ) . this procedure ensures the conservation of the transition current and it is equivalent to the use of the landau prescription  @xcite . using equation  ( [ eqjmu ] ) , we can express the transition current in terms of the quark electromagnetic form factor @xmath38 ( @xmath39 ) and the radial wave functions @xmath40 and @xmath41  @xcite . the radial wave functions are scalar functions that depend on the baryon ( @xmath42 ) and diquark ( @xmath35 ) momenta and parametrize the momentum distributions of the quark - diquark systems . from the transition current we can extract the form factors and the helicity transition amplitudes , defined in the rest frame of the resonance ( final state ) , for the reaction under study  @xcite . there are however some processes such as the meson exchanged between the different quarks inside the baryon , which can not be reduced to simple diagrams with quark dressing . those processes are regarded as arising from a meson exchanged between the different quarks inside the baryon and can be classified as meson cloud corrections to the hadronic reactions  @xcite . the covariant spectator quark model was used already in the study of several nucleon excitations including isospin 1/2 systems @xmath43  @xcite and the isospin 3/2 systems  @xcite . the model generalized to the @xmath44-flavor sector was also used to study the octet and decuplet baryons as well as transitions between baryons with strange quarks  @xcite . in figure  [ figsigmaw ] the position of the nucleon excitations are represented and compared with the bumps of the cross sections . based on the parametrization of the quark current ( [ eqjq ] ) in term of the vector meson dominance mechanism , the model was extended to the lattice qcd regime ( heavy pions and no meson cloud )  @xcite , to the nuclear medium  @xcite and to the timelike regime  @xcite . the model was also used to study the nucleon deep inelastic scattering  @xcite and the axial structure of the octet baryon  @xcite . for the study of the states @xmath45 ( @xmath47 ) and @xmath46 ( @xmath48 ) it is necessary to specify the shape of the radial wave function of the nucleon and resonant states . the radial wave function can be represented using the dimensionless variable @xmath49 , where @xmath50 is the mass of the baryon @xmath51 . we choose in particular _ n(p , k)= , _ r(p , k)= , [ eqpsiradial ] where @xmath52 are normalization constants , @xmath53 and @xmath54 are momentum range parameters in units @xmath55 . we use @xmath56 for @xmath45 and @xmath57 for @xmath46 . those parameters are fixed by a fit to the the large @xmath13 data  @xcite . the coefficient @xmath58 is determined by an orthogonality condition between the nucleon and the state @xmath59 . in the following we will use also the spectroscopic notation to represent the states @xmath45 ( or @xmath60 ) and @xmath45 ( or @xmath61 ) . in addition we use @xmath62 and @xmath63 to represent the @xmath60 and @xmath61 masses , respectively .      from the study of  @xcite we conclude that we can write the amplitude @xmath7 for the @xmath45 state in terms of the dirac transition form factor ( @xmath64 ) . the final result is then a_1/2 = - f_s ( f_1 + + 2 f_1- _ 3 ) i_s11 _ s , _ s11 ( q^2)= _ k _ s11(p_+,k ) _ n(p_-,k ) , [ eqa12-s11 ] where @xmath65 and @xmath66 is a covariant integral calculated on the @xmath60 rest frame . the result ( [ eqa12-s11 ] ) is valid for large @xmath13 since only valence quark effects are considered and it is the consequence of the observation that the pauli transition form factor ( @xmath67 ) vanishes for @xmath68 gev@xmath69 , which is interpreted as the consequence of the cancellation between valence quark and meson cloud effects  @xcite . in this work we updated the model from  @xcite using the radial wave function @xmath41 given by equation ( [ eqpsiradial ] ) , in order to ensure the exact orthogonality between nucleon and @xmath45 states . in the process we introduce a new parameter ( @xmath70 ) that is adjusted by the large @xmath13 data ( no meson cloud ) @xcite .      the model for from  @xcite can be used to calculate the electromagnetic transition form factors for the @xmath71 transition , including the magnetic dipole @xmath72 and the electric quadrupole @xmath73 form factors , based in the effects of the valence quarks . one obtain then @xmath74 , where g_m= r(f_1 + + 2 f_1- _ 3 ) i_d13 + ( f_2 + + 2 f_2- _ 3 ) i_d13 . in the equations @xmath75 , and @xmath76 is the new invariant integral defined at the resonance rest frame . the result @xmath77 is interesting , since it is consistent with the expected qcd behavior for large @xmath13 , but it is inconsistent with the data at low @xmath13 , that shows a significant magnitude for the amplitude @xmath78 , near @xmath79  @xcite . in general , quark models predict small contributions for @xmath8 ( 2040% )  @xcite . there are however indications that the effects of the meson cloud contribution dominate the amplitude , as supported by the calculation from ebac at jefferson lab  @xcite . based on that information we represent the helicity amplitudes as a_1/2= f_d g_m + f_d g_4^ , a_3/2= f_d g_4^ , [ eqamps - d13 ] where @xmath80 . in the equations  ( [ eqamps - d13 ] ) , @xmath81 is a function that is not determined by the covariant spectator quark model ( that predicts @xmath82 ) and parametrize the amplitude @xmath8 assuming the dominance of the pion / meson cloud effects . the function @xmath81 is fitted to the data with a model inspired on the pion and meson cloud contributions for the @xmath83 transition  @xcite . the results from the amplitudes @xmath7 ( valence quark ) and @xmath8 ( meson cloud ) are presented on figure  [ fign1520 ] . results for the resonance @xmath46 . the amplitude @xmath8 the result of meson cloud effects . was no contributions from valence quarks . data from clas  @xcite.,title="fig:",width=192 ]   results for the resonance @xmath46 . the amplitude @xmath8 the result of meson cloud effects . was no contributions from valence quarks . data from clas  @xcite.,title="fig:",width=192 ] the combination of the wave functions of a baryon ( three - quark system ) given by @xmath16 group and the description of electromagnetic interaction in impulse approximation leads to the so - called single quark transition model @xcite . in this context _ single _ means that only one quark couples with the photon . in these conditions the sqtm can be used to parametrize the transition current between two multiplets , in an operational form that includes only four independent terms , with coefficients exclusively dependent of @xmath13 . in particular , the sqtm can be used to parametrize the @xmath0 transitions , where @xmath2 is a @xmath1 ( isospin 1/2 ) or a @xmath84 ( isospin 3/2 ) state from the @xmath4 $ ] multiplet , in terms on three independent functions of @xmath13 : @xmath17 , and @xmath18 @xcite . the relations between the functions @xmath17 , and @xmath18 and the amplitudes are presented in the table  [ tableamp ] . using the results for the @xmath85 and @xmath71 amplitudes , respectively @xmath86 , @xmath87 , and @xmath88 in the spectroscopic notation , we can write & & a= 2 + a_1/2^d13 + a_3/2^d13 , b= 2 - 2 a_1/2^d13 , + & & c= - 2 - a_1/2^d13 + a_3/2^d13 . [ eqabc ]    .[tableamp ] amplitudes @xmath7 and @xmath8 estimated by sqtm for the proton targets ( @xmath89 )  @xcite . the angle @xmath90 is the mixing angle associated with the @xmath91 states ( @xmath92 ) . the angle @xmath93 is the mixing angle associated with the @xmath94 states ( @xmath95 ) . [ cols="^,^,^",options="header " , ]     another interesting prediction relatively to the helicity amplitudes of baryons with negative parity is the correlation between the amplitudes @xmath7 and @xmath96 associated with the @xmath97 transition . the consequence of the experimental result , @xmath98 , observed for @xmath68 gev@xmath69 is that we can write in that regime @xmath99 , where @xmath100  @xcite . the correlation between the 2 amplitudes is shown on figure [ figscaling ] . we combine the frameworks of the covariant spectator quark model and the single quark transition model in order to make predictions for the helicity amplitudes associated with negative parity resonances in the region of masses @xmath101 1.51.8 gev . the predictions are expected to be valid for @xmath102 gev@xmath69 . simple parametrizations for the amplitudes are calculated to facilitate the comparison with future experiments for @xmath103 gev@xmath69 . contrarily to what it was expected , in some transitions , like for the resonances @xmath45 and @xmath104 , the valence quark effects are not dominant in the region @xmath105 2 gev@xmath69 . the author thanks kumar gupta for comments and suggestions . the author was supported by the brazilian ministry of science , technology and innovation ( mcti - brazil ) . 35                                        ramalho g , pea m t and gross f 2008 _ eur . phys .  j.  a _ * 36 * , 329 ; _ phys .  rev .  d _ * 78 * , 114017 ; ramalho g , pea m t and gross f 2010 _ phys .  rev . d _ * 81 * , 113011 ; ramalho g , pea m t and stadler a 2012 _ phys .  rev . d _ * 86 * , 093022 .
we present a model for the @xmath0 helicity amplitudes , where @xmath1 is the nucleon and @xmath2 is a negative parity nucleon excitation , member of the @xmath3-multiplet @xmath4 $ ] . 
 the model combines the results from the single quark transition model for the helicity amplitudes with the results of the covariant spectator quark model for the @xmath5 and @xmath6 transitions . with the knowledge of the amplitudes @xmath7 and @xmath8 for those transitions we calculate three independent coefficients defined by the single quark transition model and make predictions for the helicity amplitudes associated with the @xmath9 , @xmath10 , @xmath11 , and @xmath12 transitions . in order to facilitate the comparison with future experimental data at high @xmath13 
 , we provide also simple parametrizations for the amplitudes , compatible with the expected falloff at high @xmath13 .