The CLAS detector at Jefferson Lab has provided major part of all available worldwide data on exclusive meson electroproduction off protons in the resonance region. [4,5,6,7]. The channels , , , , and were studied with nearly complete coverage of the final hadron phase space . All measured observables can be found in the CLAS Physics Data Base .
The future studies in the experiments with the CLAS12 detector at JLAB will allow to study electrocouplings of nucleon resonances at the unexplored distance scales. The kinematics regions at very low (0.05 GeV GeV ) and high photon virtualities (5.0 GeV GeV ) will become accessible. The expected results can shed light on the most important open problems of the Standard Model: the nature of more than 98% of hadron mass, quark-gluon confinement, and description of the excited nucleon state structure in QCD [1,2], as well as allowing a search for the new states of hadron matter  predicted in QCD.
2. Resonance electrocouplings from the CLAS data
Photo- and electroexcitation of nucleon resonance are described by two transverse , ) and a longitudinal ) electrocoupling amplitudes, which give information about the structure of nucleon resonance. Most of the results on electrocouplings have been extracted from independent analyses of , , and electroproduction off protons. Differential cross sections and polarization observables were obtained with CLAS at GeV and GeV 6.0 GeV .
One-pion data were analyzed in the context of two essentially different approaches: a unitary isobar model (UIM) and dispersion relations (DR) [10,11]. UIM describes the resonance part of the electroproduction amplitude as a electroexcitations in the -channel. The non resonant part is a superposition of reggeized - and - in the -channel plus non-resonant Born terms. DR approach relates the real and imaginary parts of the invariant amplitudes describing the electroproduction. Consistent and reasonable description of the one-pion cross sections were obtained in both approaches at 1.7 GeV and GeV .
Two-pion electroproduction data were analyses in the framework of the JM reaction model [12,13]. This model includes the formation of , , , , , and in the intermediate state as well as direct production of without formation of unstable hadrons in the intermediate state. All well established resonances with masses below 2 GeV were included in the resonant amplitudes of and sub-channels in the framework of the unitarized Breit-Wigner ansatz . The JM model provides successful description of the electroproduction differential cross sections at GeV and 0.2 GeV GeV and the preliminary photoproduction data at 2 GeV. The achieved quality of the CLAS data description allows us determine both resonance electrocouplings and , and decay widths from the fit of experimental data to the nine single differential cross section.
Resonance electrocouplings were obtained from CLAS data in the exclusive channels: at GeV in the mass range up to 1.7 GeV, at GeV in the mass range up to 1.6 GeV, and at GeV . Photocouplings and and hadronic decay widths of all well established resonances in the mass range from 1.6 GeV to 2.0 GeV that decay preferentially into the final states were extracted. Photocouplings extracted from the photoproduction are consistent with the results of RPP , where photocouplings were obtained from analyses of photoproduction.
The studies of the and resonances with the CLAS detector [15,12] gave information on their electrocouplings in the range from 0.25 GeV to 5.0 GeV . The low-lying resonances , , , and are the most explored excited nucleon states. Furthermore, electrocouplings for the high-lying , , and states have recently been determined for the first time from the CLAS data at 1.5 GeV 4.5 GeV .
Fig. 1 shows electrocouplings for , , and together with the preliminary results on the and electrocouplings from the CLAS electroproduction off protons at 0.5 GeV GeV . Consistent results for the pN* electrocouplings of and determined in independent analyses of the electroproduction channels, and demonstrates reliability of the extracted quantities, since these channels have quite different background contributions.
Analyses of the CLAS results strongly suggest that the structure of nucleon resonances for GeV is determined by a complex interaction between the inner core of three dressed quarks and the external meson-baryon cloud which depends on the quantum numbers of the excited nucleon state.
3. Extrapolation of the integrated cross section at 5 GeV
The maximal achievable value will be extended to 12 GeV in the CLAS12 detector. The knowledge of the approximate cross sections of the meson electroproduction off protons are required to evaluate the CLAS12 detector efficiency. The efficiency will be evaluated by the method of Monte-Carlo. It requires an event generators based on the realistic cross sections, while the experimental data are not available at GeV . The procedure was developed to extrapolate the cross section from the region 5.0 GeV , where the CLAS data are available, into the region GeV 12.0 GeV for the electroproduction channels , , and .
The procedure is based on the extrapolation of the contribution of the exclusive channels to the structure functions . Structure functions and can be calculated from the transversal and longitudinal components of the inclusive cross section as
(1) where is a mass of proton, is tranfered energy, and . The contribution of the exclusive channel ( ) to the structure functions ( and ) is calculated when using exclusive cross sections and in (1). Operator Product Expansion approximation predicts Q -evolution of the momenta of the inclusive structure functions and . We assumed that the same approximation can be applied to the structure functions as well as to the contribution of the exclusive channels to the structure function at . Thus, and can be parameterized as
(2) where are parameters. We limited ourselves to three parameters , , and for every channel and they were determined from the fit of and to the experimental data at GeV for the channels , , and . Fitting procedure was applied in each bin of independently requiring the ratios to be from 0 to 1. Then and were extrapolated into the region GeV 12.0 GeV according to (2) with the parameters , , and determined from the fit. An example of the interpolated and extrapolated is shown in Fig. 2. The extrapolated cross section was calculated starting from extrapolated and . Right plot of Fig. 2 demonstrates an example of the extrapolated integrated cross section The shape of the differential cross sections at GeV were assumed to be the same it is at GeV .
High quality meson electroproduction data from CLAS allowed to determine the electrocouplings of most well-established resonances with the masses below 1.8 GeV from analyses of the , , and electroproduction channels. CLAS data showed the structure of excited nucleon states as a complex interaction between inner core of three dressed quarks and external meson-baryon cloud. After the 12 GeV upgrade, CLAS12 will be be able of obtaining electrocouplings of all prominent resonance at still unexplored ranges of low photon virtualities down to 0.05 GeV and highest photon virtualities ever from 5.0 GeV to 12 GeV . The expected results will allow us to search for new states of baryon matter. Integrated cross sections from the CLAS data for the reactions , , and were extrapolated into the region GeV , which will be accessible by the CLAS12 detector. These cross sections will be used to evaluate the CLAS12 detector efficiency by the Monte-Carlo method.