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within the nucleon excited states ( @xmath5 ) the @xmath0 resonance , also known as roper , plays a special role . contrarily to the @xmath6 and other nucleon excitations , the roper was not identified as a bump in a reaction cross - section but was instead found in the analysis of phase - shifts  @xcite . nowadays there is evidence that the roper should be identified as the first radial excitation of the nucleon quark core , although meson excitations are also important for the internal structure .    calculations based on valence quark degrees of freedom are consistent with the @xmath1 transition form factors for large @xmath2 ( @xmath7 gev@xmath4 )  @xcite . however , estimates based exclusively on quark degrees of freedom fail to describe the small @xmath2 data ( @xmath8 gev@xmath4 )  @xcite . the gap between valence quark models and the data at low @xmath2 has been interpreted as the manifestation of the meson cloud effects  @xcite . when the meson cloud contributions are included in quark models the estimates approach the data  @xcite . besides , the calculations based on dynamical coupled - channel reaction models , where the baryon excitations are described as baryon - meson states with extended baryon cores  @xcite , corroborates the importance of the meson cloud effects . in those models the mass associated with the roper bare core is about 1.7 gev . only when the meson cloud dressing is considered the roper mass is reduced to the experimental value  @xcite . the interpretation of the roper as a radial excitation of the nucleon combined with a dynamical meson cloud dressing solves the long standing problem of the roper mass in the context of a quark model  @xcite . the roper decay widths into @xmath9 , @xmath10 and @xmath11 are large comparative to other @xmath5 decays . those decay widths are also difficult to explain in the context of a quark model . the meson cloud dressing helps to explain the @xmath12 width , where the contributions associated with baryon - meson - meson states play an important role  @xcite . overall the recent developments in the study of the roper electromagnetic structure point to the picture of a radial excitation of the nucleon surrounded by a cloud of mesons  @xcite . there is therefore a strong motivation to study the roper internal structure and to disentangle the effects of the valence quark component from the meson cloud component . in particular , one can use the knowledge of the baryon core structure to infer the contribution due to the meson cloud . this procedure was used in previous works based on different frameworks for the baryon core  @xcite .    in the case of the nucleon the valence quark degrees of freedom produce the dominant effect in the elastic form factors . the meson cloud contribution to the nucleon wave function is estimated to be of the order of a few percent  @xcite . as for the roper , the meson cloud seems to play a more prominent role . we conclude at the end that the valence quark degrees of freedom provide a very good description of the data , but the meson cloud contribution are important below 1.5 gev@xmath4 , particularly for the dirac form factor .    in the present work we propose a new framework to analyze the role of the valence quarks and the meson cloud in the roper . we use llight - front holography to estimate the leading order ( lowest fock state ) contribution to the @xmath1 transition form factors . since the leading order calculation includes only pure valence quark effects in the baryon wave functions ( @xmath13 contributions ) , there is no contribution from the meson cloud . in those conditions the gap between the calculations and the data must be essentially the consequence of the meson cloud effects .    the light - front ( lf ) formalism is particularly appropriate to study hadron systems , ruled by qcd , and to describe the hadronic structure in terms of the constituents  @xcite . the lf wave functions ( lfwf ) are relativistic and frame independent  @xcite . the connection between lf quantization of qcd and anti - de - sitter conformal field theory ( ads / cft ) leads to light - front holography  @xcite . lf holography have been used to study the structure of hadron properties , such as the hadron mass spectrum , parton distribution functions , meson and baryon form factors etc . in particular , the formalism was recently applied to the study of the nucleon  @xcite and roper  @xcite electromagnetic structure . an important advantage of the lf formalism applied to hadronic physics is the systematic expansion of the wave functions into fock states with different number of constituents  @xcite . in the case of the baryons the leading order contributions is restricted to the three - valence quark configuration . although the restriction to the lowest order fock state ( three valence - quark system ) may look as a rough simulation of the real world , it may provide an excellent first approximation when the confinement is included in the lfwf , defined at the light - front time  @xcite . in those conditions non - perturbative physics is effectively taken into account by lf holography  @xcite .    under the assumption that lf holography can describe accurately the large-@xmath2 region dominated by valence quark degrees of freedom , we calibrate the free parameters of the model by the available data above a given threshold @xmath14 ( @xmath152.5 gev@xmath4 ) . this procedure differs from the previous studies where the free parameters , associated with the nucleon anomalous magnetic moments were fixed at @xmath16 , where the meson cloud contamination is expected to be stronger . once the free parameters are fixed by the nucleon data , one uses the model to calculate the @xmath1 transition form factors . since no parameter is adjusted by the roper data , our calculations are true predictions for the @xmath1 transition form factors . this article is organized as follows : in sec . [ secholography ] we discuss lf holography . in the following section ( sec .  [ secnucleon ] ) , we review the results for the nucleon form factors and fix the parameters by the nucleon data . our results for the valence quark contribution to the @xmath1 transition form factors and their discussion are presented in sec . [ secroper ] . the outlook and conclusions are presented in sec . [ secconclusions ] . it is by now well known that string theory or gravity in anti - de - sitter ( ads ) space can provide a description of some lower - dimensional ( conformal ) gauge theories ( cft ) at strong coupling  @xcite . this ads / cft correspondence , or holography , can , with certain important restrictions , be applied to qcd - like theories , e.g.  @xcite . it remains an open question , however , whether the _ top - down _ string theory methods are practical enough to model qcd itself . instead it proved efficient to apply _ bottom - up _ approaches to qcd  @xcite . such approaches are motivated phenomenologically by the string models , but lack a first - principle justification . among those , the approach of lf holography is based on a comparison of the results of the lf quantization of qcd  @xcite with the predictions of holographic ( ads / cft ) models  @xcite . light - front quantization provides a powerful tool to study systems ruled by microscopic qcd dynamics . it starts with introducing a special ( light - front ) parametrization of the hamiltonian , which consequently acts on a hilbert space of the lfwf spanned by partonic fock states associated with quarks and gluons  @xcite . using lf parametrization one can separate the relative and the overall kinematics of partons obtaining wave functions @xmath17 where @xmath18 is the fraction of momentum of the parton @xmath19 , @xmath20 is the transverse momentum and @xmath21 is the number of components of the fock state . functions @xmath17 define the probability of the given @xmath21-parton fock state . in the massless quark limit , considering the 2d fourier transform from @xmath22 to the transverse impact variable @xmath23 , one passes to a wave function defined in the transverse impact space , @xmath24  @xcite .    in the semiclassical approximation , when the quark loops are neglected and the quarks are massless , one can replace the dependence on the variables @xmath25 and @xmath23 in @xmath26 by dependence on a single parameter @xmath27 , except for an overall factor @xmath28  @xcite . the variable @xmath29 measures the separation between the active quark and the remaining spectator partons . the key observation of lf holography is the equivalence of characteristic lfwfs @xmath30 and wave functions of matter fields in five - dimensional anti - de - sitter ( ads@xmath31 ) space , assuming that one identifies @xmath29 with the radial coordinate of ads@xmath31  @xcite . the residual @xmath25 dependent factor @xmath32 is not relevant in the holographic correspondence , since its effect can be absorbed in the normalization factors in the 5d theory calculation of matrix elements . the full lfwf is an expansion over @xmath21-parton fock states @xmath33 with coefficients @xmath30 . in the standard terminology those are the states of a given _ twist _ @xmath34 . the purpose of this work is to understand the leading twist contribution ( @xmath35 ) , which corresponds to the valence quark approximation to certain qcd processes .    to study the electromagnetic properties of the nucleon excitations we note that a three - quark system ( @xmath35 ) , as the nucleon and the nucleon radial excitations can be regarded as two - body systems with an active quark and a spectator cluster with masses ( eigenvalues ) and wave functions determined by the lf wave equation  @xcite . in the bottom - up holographic approach to qcd the nucleon and the nucleon excitations can be introduced as fermion fields in five - dimensional ads space  @xcite . to define this curved space one requires the metric tensor @xmath36 , which can be conveniently introduced via line element @xmath37 in the poincar coordinates : [ eqmetric ] ds^2  =  ( _ dx^dx^- dz^2 ) , where @xmath38 is the 4d minkowski metric tensor and @xmath39 is a parameter called ads radius . the latter sets the scale for the space s curvature . ads@xmath31 radial coordinate @xmath40 is to be identified with the lf parameter @xmath29 . from the metric one can define the dirac operator @xmath41 , which we write as = e^m_a^a ( _ m + _ m^ab[^a,^b ] ) . here @xmath42 , @xmath43 is the standard set of five dirac gamma matrices and we choose to represent in the chiral basis . frame @xmath44 and spin - connection @xmath45 tensor fields can be computed from the metric tensor . for completeness we summarize the explicit formulas here e_a^m ^a & = & ( ^ , - i _ 5 ) , [ eqframe ] + _ m^ab & = & ( ^az_m^b-^bz_m^a ) it is also understood that @xmath46    substantiating the above claim , the kinematic properties of nucleons , _ e.g. _ the spectrum , can be described by a theory of a massive fermion in ads@xmath31 : [ eqaction ] s_n & = & d^4 x d z   ( -- ) , where @xmath47 is the 5d fermion mass to be fixed below . the function @xmath48 is an effective scalar potential , whose role is to introduce the complex dynamics of qcd , essentially the confinement . when @xmath49 , the equations of motion of the 5d theory can be cast in the form corresponding to the lf schrdinger equation in the conformal limit  @xcite . a phenomenologically compelling choice of the potential is  @xcite [ potential ] = ^2 z^2 . the dimensionful parameter @xmath50 breaks conformal symmetry and introduces a mass scale , which determines the meson and baryon spectrum  @xcite . it can be thus related to @xmath51 . a conventional way to introduce this potential is through the dilaton field background  @xcite . this procedure makes the connection with top - down ads / cft constructions . the corresponding bottom - up case is dubbed the soft - wall model .    in four dimensions the four - component dirac spinor describes two independent chirality modes . although five - dimensional dirac spinor @xmath52 equally has four components , it will correspond to a nucleon with a specific single choice of chirality . this is a consequence of boundary conditions necessary in the holographic approach  @xcite . in order to preserve cp invariance , the second chirality mode is introduced similarly as eq . ( [ eqaction ] ) , but with an opposite sign in front of @xmath47 and @xmath53 , _ e.g. _  @xcite . for simplicity of presentation we only review one chirality mode here .      in order to solve the dirac equations derived from action  ( [ eqaction ] ) the fermion field ( spin 1/2 and positive parity ) @xmath52 is decomposed into its left- and right- chirality components : ( x , z ) = _ l(x , z ) + _ r(x , z ) , where @xmath54 . the solution can then be found via separation of variables , assuming a plane wave dependence on the 4d spacetime coordinates _ l / r(x , z )  =  _ l / r(x)z^2f_l / r(z ) , where left and right weyl spinors @xmath55 satisfy the 4d dirac equation with mass @xmath56 . we also pull out the @xmath57 factor for convenience . in the end , one is left with a pair of coupled equations for scalar _ profile _ functions @xmath58 : f_l / r ( z ) = m f_r / l(z ) . [ eqholog1 ]    the equations must be solved with an appropriate choice of boundary conditions . first , holography requires regularity of the wave function @xmath59 as @xmath60 . for @xmath61 there are two linearly independent solutions @xmath62 . in holography one usually chooses to call one of those solutions the source and another one the vev of the operator dual to the bulk ( 5d ) field @xmath52  in this case baryon interpolating operator . for the problem in question it only makes sense to choose the vev solution as @xmath63 and the source @xmath64 . the opposite choice , source @xmath65 and vev @xmath66 leads to a non - physical spectrum . note that for the opposite chirality case one changes @xmath67 , and the two choices of the @xmath61 boundary conditions are interchanged , with the physical one being @xmath65 and @xmath68 . after fixing the boundary conditions it is standard to rewrite system ( [ eqholog1 ] ) as a second order schrdinger - like differential equation for either @xmath69 , or @xmath70 . since we are solving the boundary problem , any solution can be expanded over an eigenfunction basis , labeled by an integer @xmath21 . equation @xmath71 has a set of eigenfunctions @xmath72 expressed in terms of the generalized laguerre polynomials @xmath73 : [ eqfr ] f_r , n(z )  z^m e^- ^2z^2/2l_n^m-1/2(^2z^2 ) . the second component @xmath70 can now be calculated using eqs . ( [ eqholog1 ] ) : [ eqfl ] f_l , n(z )  z^m+1 e^-^2z^2/2l_n^m+1/2(^2z^2 ) . these eigenfunctions correspond to eigenvalues given by m_n^2= 4 ^2 ( n + m + ) . [ leigenvalues ] we recall that in the previous expressions @xmath74 . the eigenvalues provide the result for the 4d spectrum of nucleon excitations with radial excitation quantum number @xmath21 and angular momentum quantum number linear in the parameter @xmath75 , which we will fix below . the above choice of the potential  ( [ potential ] ) leads to the spectrum consistent with regge trajectories  @xcite . summing up , the full solution for the 4d positive chirality mode reads [ poschirmod ] _ n^+(x , z)= z^2 (    c f_l , n(z)_n(x ) + f_r , n(z ) _ n ( x ) +    ) , where @xmath76 are two component spinors related with the weyl spinors : @xmath77 and @xmath78 . the negative chirality modes is obtained via an appropriate change of signs and exchange of the solutions for @xmath79  @xcite : [ negchirmod ] _ n^-(x , z)= z^2 (    c f_r , n(z)_n(x ) + -f_l , n(z)_n(x ) +    ) .    using the component associated with the index @xmath21 we can write the fermion fields with positive and negative parities as ^(x , z ) = _ n _ n^(x , z ) .      in hadron physics , information on the structure of nucleon resonances is contained in the matrix elements of interaction currents between the initial and final hadrons , including the nucleon and the nucleon excitations ( baryons ) . in holography , 4d operators of conserved currents @xmath80 are described by massless gauge fields @xmath81 in ads@xmath82 space . interacting bulk baryon fields @xmath52 couple to the gauge fields , which is a dual holographic description of the coupling of baryons to conserved currents . matrix elements of the currents , can be computed from the interaction terms , which are , schematically , overlaps of the bulk fields with the source gauge field  @xcite : [ sint ] s_int = d^4 x d z |(x , z ) ( x , z ) ( x , z).here @xmath83 encodes a coupling of field @xmath81 to fermions . we consider in particular the decomposition ( x , z ) = _ 0(x , z ) + _ 1(x , z ) + _ 2(x , z ) , [ eqvtotal ] as discussed next . for simplicity , we only present the interaction for positive chirality . for the opposite chirality case one has to adjust the signs in the above expression to preserve the cp invariance  @xcite . the term @xmath84 is representing the minimal dirac coupling given by  @xcite _ 0(x , z ) = q ^m v_m ( x , z ) , where @xmath81 could be either the abelian dual of the electromagnetic current , or the non - abelian isovector current . following the parametrization of  @xcite @xmath85 , the nucleon charge ( @xmath86 ) , for the elastic electromagnetic transitions , and @xmath87 for the transition between the nucleon and higher mass @xmath88 resonances ( @xmath89 is the pauli isospin operator ) . minimal _ coupling only yields the dirac form factor . to produce the pauli form factor more input from the holographic side is necessary . abidin and carlson  @xcite proposed to consider a non - minimal extension of the coupling @xmath90 , adding a term [ v1 ] _ 1 ( x , z )  =   _ n [ ^m,^n]v_mn(x , z ) , [ eqv1 ] where @xmath91 $ ] and one can consider different couplings @xmath92 and @xmath93 for the isoscalar and isovector parts respectively [ @xmath94 . as one may expect from its lorentz structure , this term yields the pauli form factors , as well as a correction to the dirac form factors . moreover , since the non - minimal term contains derivatives of the field @xmath81 and extra powers of @xmath40 , the pauli form factor turns out to be subleading with respect to the dirac form factor at large momentum transfer , as expected , while the correction to the dirac form factor is of the same order . the effect of the term @xmath95 on the nucleon form factors is studied in refs . we provide additional details in what follows .    more non - minimal couplings can be considered . in refs . @xcite the following non - minimal ( isovector ) term was suggested [ v2 ] _ 2 ( x , z )  = g_v _ 3 ^m^5v_m ( x , z ) . where @xmath96 is a coupling constant . this coupling is consistent with gauge invariance and discrete symmetries , that can be added to the 5d action to improve the fitting of experimental data  @xcite . one first observation about this term is that it naively breaks cp in 4d . however , this is not the case , since 5d fermion @xmath52 describes only one 4d chirality . adding a similar interaction term for the opposite chirality , with the opposite sign of coupling , ensures 4d cp invariance . a more serious issue is that eq . ( [ v2 ] ) breaks 5d covariance . adding such term requires an implicit background 5d vector field . such background fields are more common in the higher - dimensional top - down holographic models with flux compactifications . here we assume that such a flux compactification exists and we assess its effect on the observable form factors . we further comment on the general structure of the term  ( [ sint ] ) in ref . @xcite . in order to calculate transition matrix elements of the currents using eq . ( [ sint ] ) one needs to specify the 5d gauge field @xmath97 . similarly to fermions , a bosonic gauge field dual to the isovector or isoscalar current is introduced by a 5d lagrangian . specifically , one needs to solve equations following from the action s_j  =  -d^4 x d z trv_mnv^mn , subject to appropriate boundary conditions . these boundary conditions differ from the boundary conditions for the fields associated with the nucleon and the nucleon excitations characterized by the quantum numbers @xmath21 and @xmath75 , since they have to be of the _ source _ type at @xmath98 ( the leading solution as opposed to the subleading vev - type ) . the regularity condition requires @xmath81 to vanish for @xmath99 .    in what follows we will be interested in the abelian electromagnetic current , so the non - abelian details of this discussion can be omitted . altogether , the solution can be cast in the form  @xcite [ fourier ] v_(x , z )  =  e^-i q x _ ( q)v(-q^2,z ) , where @xmath100 is the photon polarization vector , and [ vpropagator ] v(q^2,z)= ^2 z^2 _ 0 ^ 1 x^ e^- , with @xmath101 . in the limit @xmath102 , one has @xmath103 .    substituting eqs . ( [ fourier ] ) and ( [ vpropagator ] ) together with the modes  ( [ poschirmod ] ) and  ( [ negchirmod ] ) into the interaction lagrangian  ( [ sint ] ) yields matrix elements of the current up to some normalization terms , from which form factors are read off directly . we present the results in sec . [ secnucleon ] for the nucleon and in sec . [ secroper ] for the roper .      in this section we specify the details of the holographic model and relating it to the observable spectrum of the nucleon radial excitations and the @xmath104 mesons .    first , the nucleon ( @xmath105 ) and the roper ( @xmath106 ) are the states with radial quantum numbers @xmath107 and angular momentum @xmath108 . any of these states must be regarded as a superposition of the given twist ( number of partons ) fock states , so we assume that the 5d mass @xmath47 is encoding both the angular momentum @xmath109 and the twist @xmath110 , @xmath111 . a way to fix the relation for @xmath108 is to analyze the large @xmath2 scaling of @xmath72 and consequently the form factors . as discussed later , the choice @xmath112 yields the correct falloff estimated by perturbative qcd ( pqcd )  @xcite . consequently , the spectrum  ( [ leigenvalues ] ) takes the following form  @xcite , m_nn= 2 . [ eqmassn ] thus , holography provides an estimate for the spectrum of the nucleon radial excitations in terms of a single scale parameter @xmath50 . similarly , one obtains the spectrum of mesons and baryons with different angular momenta and parity  @xcite . the @xmath104 meson ( mass @xmath113 ) is a traditional reference for the hadron states . in lf holography we can write @xmath114 . the nucleon mass , for example , approximately satisfies @xmath115 . if we assume that the nucleon mass is primarily composed of the leading twist @xmath35 contribution , @xmath116 , then the phenomenological value of @xmath50 is fixed at [ kappa ]  =  ~_qcd . the leading twist estimate of the masses of nucleon radial radial excitations is now given by @xmath117 , @xmath118 . this yields @xmath119 for the roper mass in leading twist approximation ( @xmath35 ) . it is worth mentioning that the lf estimate of the roper mass , @xmath120 , is not so accurate as for the nucleon and the @xmath104 meson . this is consistent with a general expectation that leading twist approximation is not so accurate for baryons as it is for mesons . in particular for the roper , there are indications that the baryon - meson - meson corrections of the twist order @xmath121 are important  @xcite . in the next subsection we collect further comments on the spectrum and other issues of lf holography . the spectrum of the hadrons , as predicted by lf holographic approach has further issues . a similar approach to mesons yields similar regge - behaved spectra as the one given by eq . ( [ eqmassn ] ) . for the @xmath104 meson family one gets @xmath122 for @xmath123  @xcite . brodsky and teramond argue , however , that those twist-2 mass poles should be shifted to their physical values , suggesting that @xmath124  @xcite . these formula gives a very good agreement with the vector meson masses for @xmath125 gev  @xcite , as in eq . ( [ kappa ] ) , provided that @xmath126 mev . similar values were used in refs . @xcite . the scaling dimension @xmath127 is defined by the behavior of the wave function in the limit @xmath102 : @xmath128  @xcite . the holographic approach relates the 5d mass parameter @xmath75 with the scaling dimension @xmath127 of the hadron interpolating operator . for example , for the vector field this relation is  =  2 + . for the massless vector field @xmath81 above , this infers @xmath129 , the correct scaling dimension of a current operator  @xcite . for fermions however the relation is = 2 + |m| . in the leading twist this yields @xmath130 for the nucleon / roper interpolating operator , in contrast to the scaling dimension @xmath131 of the operator @xmath132 , composed of three quark fields . this discrepancy is even more pronounced if one applies the identification @xmath133 used in ref . indeed , comparing eqs . ( [ eqfr ] ) and  ( [ leigenvalues ] ) with eqs . ( 5.34 ) and  ( 5.38 ) of ref . @xcite one ends up with the relation @xmath134 , which implies @xmath135 .    in qcd the ( anomalous ) scaling dimensions of the interpolating operators are not well - defined , because of the logarithmic running of the gauge coupling . in a putative conformal theory , which would approximate the slow running of the coupling , the anomalous scaling dimensions would exist and correspond to a dual gravity theory of the type studied here . in particular , the massless vector field of a gravity dual would correspond to @xmath129 conserved symmetry current . in the meantime we have to accept that the correct choice of the operator dimension leads to unpleasant effects like incorrect scaling of the form factors for very large @xmath2 . one can argue , in principle , that predictions of holographic models are generally valid in the strongly coupled regime of a theory , thus the pqcd prediction should not be compared with the outcome of the holographic analysis . we believe , however , that this argument does not quite apply to the scaling of the form factors , since the pqcd prediction is rather based on the assumption of a particular partonic structure of the hadron , which is not exactly the full perturbative quark picture  @xcite . we hope that some more advanced holographic model can resolve the issues mentioned above . in particular , top - down models , as more complex ones , naturally share more intricate details with qcd . a prototypical example would be ( the glueball sector of ) the klebanov - strassler theory  @xcite . albeit supersymmetric , this theory encodes a logarithmically running gauge coupling , thus , extracting dimensions of the operators in this theory requires removing the logarithmic scale dependence ( _ e.g. _  @xcite ) . moreover , in this theory , the states with the same quantum numbers tend to mix with each other . therefore the mass eigenstates are superpositions of states with different twist . mixing has effect of changing the resulting spectrum as well as shifting the values of the dimensions  @xcite . the transition current @xmath136 between two nucleon states ( elastic transition ) can be expressed , omitting the asymptotic spin states ( spinors ) and the electric charge @xmath137 , as  @xcite j^= f_1n ( q^2 ) ^+ f_2n ( q^2 ) , [ eqnucleoncur ] where @xmath138 ( @xmath139 ) define , respectively , the dirac and pauli form factors of the proton ( @xmath140 ) and neutron ( @xmath141 ) . in the lf formalism , the two form factors appear in a spin - nonflip ( @xmath142 ) and a spin - flip ( @xmath143 ) transitions  @xcite .    the calculation of the overlaps  ( [ sint ] ) in the holographic model with the interaction ( [ eqvtotal ] ) , where one replaces @xmath52 and @xmath144 by the appropriate initial and final nucleon state modes , in this case the nucleon modes @xmath105 . the leading twist case ( @xmath35 ) , the dirac form factor is determined by the functions @xmath145 and @xmath146 , and the pauli form factor is determined by overlap of the @xmath147 and @xmath148 components  @xcite . the final expressions for the form factors are then  @xcite : f_1n & = & e_n + + & & g_v _ n + + & & _ n , [ eqf1n ] + f_2n & = & _ n , [ eqf2n ] where @xmath149 and @xmath50 is the holographic scale discussed in the previous section . in eqs . ( [ eqf1n])-([eqf2n ] ) , @xmath150 ( @xmath151 , @xmath152 ) and @xmath153 take different values for proton and neutron . in eq . ( [ eqf2n ] ) we can replace @xmath154 by unity , if @xmath155 is a good approximation for the nucleon mass .    in an exact @xmath156-flavor model one has @xmath157 . these parameters can be determined by the proton and neutron bare anomalous magnetic moment @xmath158 and @xmath159 ( we use the upper index @xmath160 to indicate the bare values ) . in a model with no meson cloud one can write @xmath161 and @xmath162 , assuming @xmath163 . the holographic results  ( [ eqf1n])-([eqf2n ] ) yield the correct perturbative qcd behavior for the nucleon electromagnetic form factors : @xmath164 and @xmath165 , as consequence of @xmath166 . for a more detailed discussion see refs . @xcite .    as previously discussed the non - minimal couplings @xmath153 and @xmath96 are included phenomenologically . as can be seen from eqs . ( [ eqf1n])-([eqf2n ] ) , the minimal coupling only produces the dirac form factor . the non - minimal couplings generate a non - zero @xmath143 and adds an extra contribution to the dirac form factor ( proportional to @xmath153 )  @xcite . relations of the type  ( [ eqf1n])-([eqf2n ] ) were derived for the first time in ref . @xcite for the case @xmath167 .    to determine the values of @xmath96 and @xmath153 we performed a fit to the data @xmath168 where @xmath169 is the threshold of the data included in the fit . we considered the cases @xmath170 , 2.0 and 2.5 gev@xmath4 , since the meson cloud effects are in principle suppressed for large @xmath2 , but we do not know a priory the threshold of the suppression . we use the data from refs . check refs . @xcite for a detailed description of the database .    from the fits , we conclude that the results for the neutron electric form factor @xmath171 are very sensitive to the value of @xmath169 . if the low @xmath2 is included in the fit ( @xmath172 1.0 gev@xmath4 ) the best fit favors a negative @xmath171 for low @xmath2 , in conflict with the measured data . this result is very pertinent , because it is known that meson cloud effects , in particular the pion cloud , is very important for the description of the @xmath171 at low @xmath2 . since the pion cloud contributions are not included in the parametrization of eqs . ( [ eqf1n])-([eqf2n ] ) , it is not surprising to see that the fit with @xmath173 gev@xmath4 fails to describe the @xmath171 data . it can be surprising , however , to note that the best description of the nucleon data occurs for @xmath174 gev@xmath4 in the fits with @xmath175 and 2.5 gev@xmath4 , when only a few data points for @xmath171 are considered . one then concludes that the shape of @xmath171 is determined by the form factors @xmath176 , @xmath177 and @xmath178 . this result shows the consistence of the fitting procedure . the parameters obtained for the fits with @xmath170 , 2.0 and 2.5 gev@xmath4 are presented in table  [ tabletub ] . we can conclude that the parameters are sensitive to the data included in the fit . in particular , @xmath96 depends strongly of the threshold @xmath169 . @xmath96 , @xmath179 and @xmath180 obtained by the fit of the nucleon form factor data with @xmath14 . [ cols="^,^,^,^",options="header " , ]     the results obtained for the nucleon electric ( @xmath181 ) and magnetic ( @xmath182 form factors using the parameters from table  [ tabletub ] are presented in fig .  [ figgegm ] . in the figure , we include a band to represent the interval between the model with @xmath173 gev@xmath4 and @xmath183 gev@xmath4 . in the calculations we used @xmath184 gev , in order to have a good description of the @xmath104 mesons and nucleon masses .    in general the estimates from different parametrizations are very close , except , for the neutron electric form factor . the function @xmath171 is in fact very sensitive to the parameters @xmath96 , @xmath179 and @xmath180 . in the graph of @xmath171 the lower limit corresponds to @xmath173 gev@xmath4 and the upper limit to the case @xmath185 gev@xmath4 . the intermediate case ( @xmath175 gev@xmath4 ) is included in order to emphasize the strong dependence of @xmath171 with the threshold @xmath169 used in the fit , and it is represented by the dashed - line .    concerning the remaining form factors , one can notice an overestimation of the low @xmath2 data ( @xmath186 gev@xmath4 ) for @xmath177 and @xmath187 . these results may be interpreted as the manifestation of the pion cloud effects , not included in the formalism associated with eqs . ( [ eqf1n])-([eqf2n ] ) .    in the past @xcite the parameters @xmath153 and @xmath96 have been fixed by the static properties of the nucleon , such as the anomalous magnetic moments and the nucleon electric charge radius . since those observables depend on the meson cloud , we choose in this work to fix the parameters in a region where the meson cloud is significantly reduced , in order to obtain a more accurate estimate of the bare parameters . in the recent work the brodsky - teramond model for the nucleon  @xcite was improved with the inclusion of the explicit pion cloud contributions ( @xmath188 states ) with a few adjustable parameters . it was shown that , indeed , the inclusion of the pion cloud contribution ( @xmath189 ) is fundamental for an accurate description of @xmath171 .    in order to understand the role of the valence quark degrees of freedom we restrict the present study to the the leading twist contribution ( @xmath35 ) . once fixed the parameters of the model ( bare parameters ) one can use those parameters to calculate the valence quark contributions for the @xmath1 transition form factors . we consider now the @xmath1 transition form factors . again omitting the spinors of the nucleon and the roper ( and the electric charge @xmath137 ) , the transition current @xmath136 can be expressed as  @xcite j^= f_1n^(q^2 ) ( ^- ) + f_2n^(q^2 ) , + [ eqropercur ] where @xmath190 and @xmath191 ( @xmath86 ) are the dirac and pauli transition form factors , respectively .    using the holographic wave functions for the nucleon and the roper combined with the interaction ( [ eqvtotal ] ) , one concludes that the dirac and pauli form factors associated with the @xmath1 in leading twist  @xcite are f_1n^&= & _ n + + & & g_v _ n + + & & _ n , [ eqf1r ] + f_2n^&= & _ n ( ) ^2 + & & , [ eqf2r ] where @xmath192 , @xmath193 , @xmath194 , @xmath195 and @xmath196 .    in eq . ( [ eqf2r ] ) we can replace @xmath197 by unity if @xmath198 is a good approximation to the roper mass . in comparison with ref . @xcite we include an extra factor @xmath199 in order to be consistent with the more usual definition of the transition form factors  ( [ eqropercur ] ) . the results for the nucleon to roper transition form factors associated with the parameters @xmath96 , @xmath179 and @xmath180 discussed previously ( table  [ tabletub ] ) , are presented in fig . [ figroper ] , for the proton target ( @xmath140 ) . as for the nucleon we use @xmath200 gev . the data presented here are those from clas / jefferson lab for single pion production  @xcite and double pion production  @xcite , and it is collected in the database  @xcite . we do not discuss the results for the neutron target ( @xmath141 ) because they are restricted to the photon point .    in fig . [ figroper ] , we can see that , contrary to the nucleon case , the results are almost insensitive to the variation of parameters , therefore we consider only the upper and lower cases ( @xmath173 and 2.5 gev@xmath4 ) , delimited by the band . in the figure one can see that the holographic approach based on eqs . ( [ eqf1r])-([eqf2r ] ) in the leading twist approximation gives a very good description of the large @xmath2 data ( @xmath201 gev@xmath4 ) in the range @xmath202, ... ,1.57 , @xmath203, ... ,0.42 and @xmath204, ... ,@xmath205 .    in the above holographic approach one obtains a clear estimate of the valence quark contributions based on framework that includes only one pre - defined parameter : the mass scale @xmath50 . since the coefficients @xmath96 , @xmath179 , @xmath180 are determined by the nucleon form factors , one can consider this estimate of the valence quark contributions for the nucleon to roper form factors as a parameter - free prediction .    the present results for @xmath206 and @xmath207 are consistent with estimates of the form factors based on different frameworks , such as the results of refs . in particular , the present model and the results of the covariant quark model from refs . @xcite are very close for @xmath208 gev@xmath4 .    the original holographic calculation of the nucleon to roper transition form factors was done in ref . @xcite . in that work higher fock states ( twist 4 and 5 contributions ) were also taken into account . however the effective contribution of the higher fock states was determined by the equation for the roper mass , and not by the physics associated with the form factors ( dominance of the leading twist contributions for large @xmath2 ) . in addition , the overlap between the nucleon and roper states was determined in ref . @xcite using coefficients adjusted to the form factor data .    in the present work we rather prefer to concentrate on deriving a clean estimate of the valence quark contribution using an alternative method of fixing the parameters of the model , instead of trying to estimate the quark - antiquark and gluon contributions using lf holography since we do not consider higher twist contributions , the overlap between the states is just the result of the overlap between the valence quark states of both baryons with no unknown coefficients . although we conclude that the leading twist calculation is a good approximation for the form factors , one still notes that it may not be sufficient for a satisfactory estimate for the mass of the roper . different estimates of the mass suggest , in fact , that higher fock states are crucial for the explanation of the experimental value  @xcite .    in fig . [ figroper ] , we also present the estimate of the lf holographic model from teramond and brodsky for @xmath206  @xcite . in their formulation , the dirac form factor is expressed by an analytic function dependent on the @xmath104 meson masses  @xcite . the closeness between results is a consequence of the use of the holographic masses instead of the physical masses . we note at this point , that , the relations ( [ eqf1r])-([eqf2r ] ) are also analytic parametrization of the roper form factors , since the form factors are expressed in terms of functions of @xmath209 , using @xmath210 . we leave however the comparison between analytic parametrization of the roper form factors to a separated work  @xcite .    in the graph for @xmath206 one can notice a deviation between our estimate and the data for @xmath186 gev@xmath4 . this result can be interpreted as the consequence of the meson cloud contributions not included in our leading twist analysis . similar results were obtained in independent works  @xcite . sizable contributions of the meson cloud effects were found in refs . @xcite .    as for the results for @xmath207 , one can note that the parametrization based on the lf holography gives negative values for small @xmath2 , and are close to the data . in particular we estimate the change of sign in @xmath207 for @xmath211 gev@xmath4 . the results for @xmath207 suggest that contrarily to the form factor @xmath206 , where the meson cloud are sizable , for @xmath207 the meson cloud contribution are very small at low @xmath2 . to the best of our knowledge , it is the first time that this fact was observed in the context of a quark model . there is , nevertheless , a discrepancy between the model and the data in the region between 0.9 and 1.5 gev@xmath4 .    concerning the @xmath207 data , one can note that in the region gev@xmath4 there are only two datapoints associated the two pion electroduction clas data  @xcite . it is important to check if the new data associated with the one pion production confirms the trend from ref . @xcite . at the moment one can conclude that the holographic model gives a very good description of the @xmath207 data except for three data points in the range 0.91.5 gev@xmath4 ( underestimation of the data in about 60% ) .      summarizing the results presented here for the nucleon and roper form factors , one can conclude that the leading twist approximation provides a very good estimate for both nucleon and roper form factors . this result is consistent with the lf formalism , because in the drell - yan - west frame the contributions from higher fock states are small  @xcite . it is expected however that the meson cloud effects provide significant contributions to the @xmath212 transition form factors for some nucleon excitations @xmath5 for small @xmath2  @xcite .    in the present work , the meson cloud effect is observed in particular for the form factors of the roper : @xmath206 , and in the region 0.91.5 gev@xmath4 for @xmath207 . those meson cloud corrections come from higher twist contributions to the light - front wave functions . for higher mass resonances it is expected that higher twist corrections also give important contributions to the transition form factors at low @xmath2 . in that case lf holography can be used to estimate mainly the contribution of the quark core . although the estimate for low @xmath2 may appear rough , since the calculation is based on massless quarks ( and the physical quarks have mass ) , for large @xmath2 the estimate is expected to be accurate due to the dominance of the valence quark degrees of freedom .    in the future one can use holography to estimate transition form factors in the leading twist approximation for other @xmath5 states . those estimates are expected to be accurate at large @xmath2 , since they are based on valence quark degrees of freedom . at low @xmath2 lf holography may fail in leading twist , since the framework provides only the valence quark contributions to the form factors . that information can be , however , very useful to understand the role of the meson cloud contributions . on one hand , we can use the comparison with the data to estimate the effect of the meson cloud contribution . on the other hand , estimates of the bare core can also be used as input to dynamical coupled - channel reaction models in the parametrization of baryon bare core  @xcite . it is worth mentioning that some authors interpret the roper as a dynamically - generated baryon - meson resonance , without an explicit reference to three - quark systems , except for the nucleon and the @xmath6  @xcite . as far as we know , there are no calculations of @xmath1 transition form factors for intermediate and large @xmath2 , based on dynamically - generated resonance models for the roper . future lattice qcd simulations can help to understand the role of the baryon - meson contribution for the transition form factors at low @xmath2  @xcite . in the present work we apply the lf holography in the soft - wall approximation to the study of the electromagnetic structure of the nucleon and nucleon excitations . more specifically we study the nucleon and @xmath1 transition form factors in the leading twist approximation .    since in the leading twist approximation the transition form factors are determined by the valence quark degrees of freedom , in the present approach we estimate the contribution of valence quarks to the electromagnetic form factors . the light - front wave functions are determined by an appropriate choice of boundary conditions in the 5d space and by the expected pqcd falloff for the dirac and pauli form factors ( bottom - up approach to qcd ) . the light - front holography in the soft - wall version was used previously in the study of the nucleon elastic form factors , in leading twist and higher orders . since in those works the couplings are adjusted to the dressed couplings ( empirical anomalous magnetic moments ) they may describe well the low @xmath2 data , but fail in the description of the large-@xmath2 data . in the present work we fix the free parameters of the model associated with the bare couplings using the large @xmath2 data for the nucleon , where the effect of the meson cloud contributions is significantly reduced . our expressions for the electromagnetic form factors depend only of the holographic mass scale @xmath50 and of three bare couplings : @xmath96 , @xmath179 and @xmath180 . once determined the bare couplings by the nucleon data , the model is used to predict the @xmath1 transition form factors . our results for the @xmath1 transition form factors compare well with the empirical data . for the dirac form factor , the deviation observed for @xmath213 gev@xmath4 is compatible with the interpretation that meson cloud contributions are important in that region . as for the pauli form factor , our estimate is very close to the data , both at low and at large @xmath2 , except for 3 datapoints in the range 0.91.5 gev@xmath4 . the result at low @xmath2 , suggests that the meson cloud contributions for @xmath207 are small . as far as we know this is the first time that this effect is observed . the method used in the present work for the roper can in the future be extended for higher mass nucleon excitations . the use of the light - front holography in leading twist provides then a natural method to estimate the valence quark contributions for the transition form factors . the effect of the meson cloud can then be estimated from the comparison with the experimental data . theoretical estimates of the bare core contributions are very important for the study of the baryon - meson reactions and nucleon electroproduction reactions . the results from light - front holography may be used as input to dynamical coupled - channel reaction models in the theoretical study of those reactions . g.  r.  thanks valery lyubovitskij for useful clarifications about the calculation of the transition form factors . this work was supported by the universidade federal do rio grande do norte / ministrio da educao ( ufrn / mec ) . the work of d.  m. was also partially supported by the rfbr grant 16 - 01 - 00291 . i.  t.  obukhovsky , a.  faessler , d.  k.  fedorov , t.  gutsche and v.  e.  lyubovitskij , phys . d * 84 * , 014004 ( 2011 ) [ arxiv:1104.0957 [ hep - 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the structure of the nucleon and the first radial excitation of the nucleon , the roper , @xmath0 , is studied within the formalism of light - front holography . 
 the nucleon elastic form factors and @xmath1 transition form factors are calculated under the assumption of the dominance of the valence quark degrees of freedom . 
 contrary to the previous studies , the bare parameters of the model associated with the valence quark are fixed by the empirical data for large momentum transfer ( @xmath2 ) assuming that the corrections to the three - quark picture ( meson cloud contributions ) are suppressed . 
 the @xmath1 transition form factors are then calculated without any adjustable parameters . 
 our estimates are compared with results from models based on valence quarks and others . 
 the model compares well with the @xmath1 transition form factor data , suggesting that meson cloud effects are not large , except in the region @xmath3 gev@xmath4 . 
 in particular , the meson cloud contributions for the pauli form factor are small .