Recent CLAS data on the $pπ^+π^-$ electroproduction off protons at 1.3$<$W$<$1.57 GeV and 0.25$<$$Q^{2}$$<$0.6 GeV$^{2}$ have been analyzed using a meson-baryon phenomenological model. By fitting nine 1-fold differential cross section data for each $W$ and $Q^{2}$ bin, the charged double pion electroproduction mechanisms are identified from their manifestations in the observables. We have extracted the cross sections from amplitudes of each of the considered isobar channels as well as from their coherent sum. We also obtained non-resonant partial wave amplitudes of all contributing isobar channels which could be useful for advancing a complete coupled-channel analysis of all meson electroproduction data.
Photo- and electroproduction of eta, eta-prime mesons on protons are investigated within a relativistic chiral unitary approach based on coupled channels. The s wave potentials for electroproduction and meson-baryon scattering are derived from a chiral effective Lagrangian which includes the eta-prime as an explicit degree of freedom and incorporates important features of the underlying QCD Lagrangian such as the axial U(1) anomaly. The effective potentials are iterated in a Bethe-Salpeter equation and cross sections for eta, eta-prime photo- and electroproduction from protons are obtained. Th…
$γ_{v}NN^*$ transition helicity amplitudes (electrocouplings) of several prominent excited proton states are determined for the first time in independent analyses of $π^+n$, $π^0p$, and $π^+π^-p$ electroproduction off protons. Analysis of $π^+π^-p$ electroproduction has extended considerably information on electrocouplings of high lying N* states, which decay preferentially to the $Nππ$ final states.
We present a gauge invariant approach to photoproduction of mesons on nucleons within a chiral unitary framework. The interaction kernel for meson-baryon scattering is derived from the chiral effective Lagrangian and iterated in a Bethe-Salpeter equation. Within the leading order approximation to the interaction kernel, data on kaon photoproduction from SAPHIR, CLAS and CBELSA/TAPS are analyzed in the threshold region. The importance of gauge invariance and the precision of various approximations in the interaction kernel utilized in earlier works are discussed.
Electroproduction of $π^+$ above the resonance region is analyzed in the Regge model for $π+ρ$ exchanges. Importance of the roles of pion and proton form factors in the process is discussed in comparison with existing models of Kaskulov and Mosel, and of Vrancx and Rychebusch. It is shown that the present model with proton form factor of a simple dipole-type yields a better description of DESY and JLab data over those models for high $Q^2$ and $-t$ region up to 5 GeV$^2$.
We report on a Rosenbluth separation using previously published data by the CLAS collaboration in Hall B, Jefferson Lab for exclusive $π^{0}$ deeply virtual electroproduction (DVEP) from the proton at a mean $Q^{2}$ of $\approx$ 2 (GeV/c)$^{2}$. The central question we address is the applicability of factorization in $π^0$ DVEP at these kinematics. The results of our Rosenbluth separation clearly demonstrate the dominance of the longitudinal contribution to the cross section. The extracted longitudinal and transverse contributions are in agreement with previous data from Hall A at Jefferson La…
Electrocouplings of excited proton states with masses less than 1.8 GeV were determined for the first time from the CLAS data on $π^+π^-p$ electroproduction at photon virtualities $Q^2$ $<$ 1.5 GeV$^2$. Electrocouplings were obtained from a combined fit of all available observables within the framework of a phenomenological reaction model.
In these proceedings we present preliminary $π^{+}π^{-}$ electroproduction cross sections off protons in the kinematical area of 1.4 GeV $< W <$ 1.8 GeV and 0.4 GeV$^{2}$ $< Q^{2} < 1.1$ GeV$^{2}$. Our results extend the kinematical coverage for this exclusive channel with respect to previous measurements. Furthermore, the $π^{+}π^{-}$ electroproduction cross sections were obtained for $Q^2$-bins of much smaller size. The future analysis of this data within the framework of the JLAB-MSU reaction model (JM) will considerably improve our knowledge on the $Q^2$ evolution of the transition $γ_{v}N…
Formalism for radiative correction (RC) calculation in exclusive pion electroproduction on the proton is presented. A FORTRAN code EXCLURAD is developed for the RC procedure. The numerical analysis is done in the kinematics of current Jefferson Lab experiments.
Radiative effects in the electroproduction of photons in polarized ep-scattering are calculated in the leading log approximation and analyzed numerically for kinematical conditions of current measurement at Jefferson Lab. Radiative corrections to the cross sections, their azimuthal distributions and Fourier coefficients are in particular focus. Kinematical regions where the radiative corrections are considerable are identified.
The neutral pion electroproduction on the proton is considered in the framework phenomenological model based on the PCAC, current algebra with hard pions and chiral symmetry breaking interaction. The s- wave differential cross-section of the electroproduction of neutral pions off protons near the threshold is predicted as function photon momentum transfer squared $k^2$ . The s- wave multipoles $ReE_{0+}$ and $ReL_{0+}$ as function of the energy of the $π^0p$- system are given for the small $k^2$ and $% k^2=0.1(GeV/c)^2$. The results are compered with the experiment. It is shown the significant…
We discuss the interpretation of the $ep\to epρ^0$ process in terms of, on the one hand, Generalized Parton Distributions and, on the other hand, an effective hadronic model based on Regge theory.
Experiments at HERA looking for deep-inelastic electroproduction of neutrons in the proton fragmentation region are in progress. They are aimed to measure the pion structure function at small Bjorken $x$. The important condition for such a study is to establish under what kinematical conditions the dominance of the pion-pole graph in the process is guaranteed. We analyse other sources of theleading neutron, in order to figure out the kinematical region where the one-pion exchange dominates.
We report on the first measurement of cross sections for exclusive deeply virtual pion electroproduction off the proton, $e p \to e^\prime n π^+$, above the resonance region at backward pion center-of-mass angles. The $φ^*_π$-dependent cross sections were measured, from which we extracted three combinations of structure functions of the proton. Our results are compatible with calculations based on nucleon-to-pion transition distribution amplitudes (TDAs) and shed new light on nucleon structure.
The charge asymmetry induced by interference of amplitudes with $σ$ and $ρ$ mesons decaying to $π^+π^-$ pair created in the fragmentation region of proton, suggest to be a test of degeneration hypothesis of $ρ$ and $σ$ mesons. Some numerical estimations are given.
The latest results on the structure of the proton and the photon as seen at HERA are reviewed while discussing the question posed in the title of the talk.
We extract the S-matrix element for dipole-proton scattering using the data on diffractive electroproduction of vector mesons at HERA. By considering the full t dependence of this process we are able to reliably unfold the profile of the S-matrix for impact parameter values b>0.3 fm. We show that the results depend only weakly on the choice of the form for the vector meson wave function. We relate this result to the discussion about possible saturation effects at HERA.
We analyze the process $e p \to e p ϕ$ near the threshold within $uud - s\bar s$ cluster model as a probe of the strangeness content of proton. Our consideration is based on the relativistic harmonic oscillator quark model which takes into account the Lorentz-contraction effect of the hadron wavefunctions. We find that the knockout mechanisms are comparable to the diffractive production of vector-meson-dominance model when only (3-5)% strange quark admixture is assumed, which should be compared with (10-20)% of the nonrelativistic quark model prediction. The cross sections of the $uud$- and $s…
The results on the photo- and electroexcitation amplitudes of most nucleon resonances in the mass range up to 2.0 GeV determined from the CLAS experimental data on exclusive $π^+π^-p$ photo-/electroproduction off protons in collaboration between the Jefferson Lab and Moscow State University are presented. The first and only available results on electroexcitation amplitudes from CLAS in a wide range of photon virtualities $Q^2$ $<$ 5.0 GeV$^2$ revealed the nucleon resonance structure as a complex interplay between the inner core of three dressed quarks and external meson-baryon cloud. These res…
The Roper resonance is considered as a mixed state of a three-quark core configuration and a hadron molecular component N+sigma. Based on this ansatz we study electroproduction of the Roper resonance. The strong and electromagnetic couplings induced by the quark core are calculated in the 3P0 model. The contribution of the vector meson cloud to the electromagnetic transition is given in the framework of the VMD model. Results are compared with the recent JLab electroproduction data.
The elementary reaction of kaon exclusive electroproduction on protons has been studied in a broad kinematical range. Data for the calculation of the form factor of the kaon have been taken at different values of the invariant center of mass energy W in the range W=1.8, 1.85, 1.98, 2.08 (GeV), for one value of the transferred 4-momentum Q2=2.35 (GeV/c)2. In this analysis, we calculated sigma-L and sigma-T by using the Rosenbluth separation. Then the electromagnetic kaon form factor was calculated by Chew-Low extrapolation.
Through a determination of the $S$-matrix element for the scattering of a localized colour dipole on a proton, we show that saturation effects can already be sizable at HERA. The saturation scale is found to be around 1-1.5 $GeV^2$ for the highest available energies and for central collisions, and is a decreasing function of the impact parameter.
We review the perturbative QCD formalism of hadronic electromagnetic form factors and the color transparency ratio for quasi-exclusive electroproduction of the proton and pion from nuclear targets. We have completed the first full calculations including all leading order quark subprocesses and integrations over distribution amplitudes, including Sudakov effects. For the case of the proton, the calculated result shows scaling beyond $Q^2=10$ GeV$^2$. The calculation incorporating filtering due to the nuclear medium is cleaner than the corresponding calculation in free space because of attenuati…
The photo- and electroproduction of eta, eta-prime mesons on nucleons are investigated within a relativistic chiral unitary approach based on coupled channels. The s-wave potentials for electroproduction and meson-baryon scattering are derived from a chiral effective Lagrangian which includes the eta-prime as an explicit degree of freedom and incorporates important features of the underlying QCD Lagrangian such as the axial U(1) anomaly. The effective potentials are iterated in a Bethe-Salpeter equation and cross sections for eta, eta-prime photo- and electroproduction from nucleons are obtain…
In view of the major advances achieved by the CLAS experiments in studying the N* electroexcitation amplitudes, as well as further extension of these studies in the experiments with CLAS12, we present an approach for the evaluation of the resonant contributions to inclusive electron scattering off protons. For the first time, the resonant contributions are determined from the experimental results on N* electrocouplings available from the data analyses of exclusive meson electroproduction off protons. This is a useful benchmark for future endeavours on understanding the transition region betwee…
The processes of exclusive diffractive scattering $p+p\to p+p$, $\bar p+p\to\bar p+p$, and $γ^*+p\to V+p$ at high energies are considered in the framework of a unified Regge-eikonal model with a very simple reggeon structure of the eikonal. It is demonstrated that the pomeron trajectory is universal in all reactions and having intercept about 1.31 which could be exctracted explicitly from the data on the proton structure function $F_2(x,Q^2)$. The predictions for the proton-proton cross-sections at LHC energies are given.
In order to get information about the polarized gluon distribution in a proton, we studied the electroproduction of ψ' in polarized electron and polarized proton collisions in the framework of the NRQCD factorization approach. The value of the cross section dΔσ/ dp_T^2 for color-octet mechanism is about 5 times larger than that for color-singlet one, and it might be another test of the color-octet model if the polarized gluon distribution Δg(x) is well known. Furthermore, we found that this reaction is quite effective for testing the model of gluon polarization by measuring the double spin asy…
We present here some arguments to support our suggestion that data on the helicity structure for the hard exclusive electroproduction of $ρ$ mesons at HERA (and in possible future high-energy electron-proton colliders) provide useful information to constrain the $κ$-shape of the unintegrated gluon distribution in the proton.
We evaluate the single target-spin sin(phi_h) and sin(2ϕ_h) azimuthal asymmetries in the semi-inclusive deep inelastic lepton scattering off longitudinally polarized proton target under HERMES kinematic conditions. A good agreement with the HERMES data can be achieved using only the twist-2 distribution and fragmentation functions.
Deeply virtual electroproduction of pion in exclusive reaction $p(e,e'π^+)n$ is studied using a two-component model which includes soft hadronic and hard partonic reaction mechanisms. The results are compared with the experimental data measured at HERMES in the deep inelastic region for values of $Q^2>1$ GeV$^2$ and $W^2 > 10$ GeV$^2$. At forward angles the $π^+$ cross section is longitudinal and is dominated by exchange of Regge poles, with $π$ the dominant trajectory. The off-forward region with $-t>1$ GeV$^2$ is transverse and shows the dominance of partonic subprocesses. An implication of …
The proton, neutron and deuteron structure functions $F_2(x,Q^2)$ and $g_1(x,Q^2)$, measured at intermediate $Q^2$, are analyzed within a Regge approach. This analysis serves to fix the parameters of this scheme which are then used to calculate, in a unified Regge approach, the properties of $ρ^0$ meson electroproduction on the proton and the deuteron. In this way, the double-spin asymmetry observed at HERMES in $ρ^0$ electroproduction on the proton, can be related to the anomalous behavior of the flavor-singlet part of the spin-dependent structure function $g_1(x,Q^2)$ at small $x$.
We investigate coherence length effects and hadron attenuation in lepton scattering off nuclei in the kinematic regime of the HERMES experiment. The elementary electron-nucleon interaction is described within the event generator PYTHIA while a full coupled-channel treatment of the final state interactions is included by means of a BUU transport model. The results of our calculations are in good agreement with the experimentally measured transparency ratio of incoherent rho0 electroproduction off Nitrogen and the multiplicity ratio R_M^h for charged hadrons, pions, kaons, protons and anti-proto…
Transition helicity amplitudes $γ_{v}NN^*$ (electrocouplings) were determined for prominent excited proton states with masses below 1.8 GeV in independent analyses of major meson electroproduction channels: $π^{+}n$, $π^0p$ and $π^+π^-p$. Consistent results on resonance electrocouplings obtained from analyses of these exclusive reactions with different non-resonant contributions demonstrate reliable extraction of these fundamental quantities for states that have significant decays for either $Nπ$ or $Nππ$ channels. Preliminary results on electrocouplings of $N^*$ states with masses above 1.6 G…
We analyze amplitudes for the pion electroproduction on proton derived from Lagrangians based on the local chiral SU(2) x SU(2) symmetries. We show that such amplitudes do contain information on the nucleon axial form factor F_A in both soft and hard pion regimes. This result invalidates recent Haberzettl's claim that the pion electroproduction at threshold cannot be used to extract any information regarding F_A.
The electroproduction of eta and eta' mesons on the proton and the neutron is investigated at tree level within the framework of U(3) chiral perturbation theory. In addition to the Born terms low-lying resonances such as the vector mesons and J^P= 1/2^+, 1/2^- baryon resonances are included explicitly and their contributions are calculated. Results for the separated differential cross sections are presented.
Pion electroproduction off the proton is analyzed in a new framework based on a general parametrization of transition amplitudes, including constraints from gauge invariance and threshold behavior. Data with energies $1.13~{\rm GeV}<W<1.6~{\rm GeV}$ and $Q^2$ below $6~{\rm GeV}^2$ are included. The model is an extension of the latest Jülich-Bonn solution incorporating constraints from pion-induced and photoproduction data. Performing large scale fits ($\sim10^5$ data) we find a set of solutions with $χ^2_{\rm dof}=1.69-1.81$ which allows us to assess the systematic uncertainty of the approach.
We analyze the capability of charm production in deep-inelastic ep scattering at HERA to constrain the gluon distribution g(y,μ^2) of the proton. The dependence of the theoretical predictions for the charm structure function F_2^c on the mass factorization scale μand the charm mass is investigated. F_2^c seems to be well suited for a rather clean and local gluon measurement at small momentum fractions y.
We show that diffractive production of a single neutral pion in photon-induced reactions at high energy is dynamically suppressed due to the approximate chiral symmetry of QCD. These reactions have been proposed as a test of the odderon exchange mechanism. We show that the odderon contribution to the amplitude for such reactions vanishes exactly in the chiral limit. This result is obtained in a nonperturbative framework and by using PCAC relations between the amplitudes for neutral pion and axial vector current production.
A nonrelativistic effective theory to describe the electroproduction reaction of a single pion on the nucleon at leading order in the electromagnetic coupling is constructed. The framework is tailored to accurately describe the cusp generated by the pion and nucleon mass differences. The S- and P- wave multipole amplitudes at two loops for all four reaction channels are provided. As an application, a new low energy theorem is discussed.
Radiative effects in the electroproduction of photons in polarized $ep$-scattering are calculated with the next-to-leading (NLO) accuracy. The contributions of loops and two photon emission were presented in analytical form. The covariant approach of Bardin and Shumeiko was used to extract the infrared divergence. All contributions to the radiative correction were presented in the form of the correction to the leptonic tensor thus allowing for further applications in other experiments, e.g., deep inelastic scattering. The radiative corrections (RC) to the cross sections and polarization asymme…
We perform a calculation of Sivers function in a light-cone SU(6) quark-diquark model with both scalar diquark and vector diquark spectators. We derive the transverse momentum dependent light-cone wave function of the proton by taking into account the Melosh-Wigner rotation. By adopting one-gluon exchange, we obtain a non-vanishing Sivers function of $down$ quark from interference of proton spin amplitudes. We analyze the $\frac{|P_{h\perp}|}{M}$ weighted Sivers asymmetries in $π^+$, $π^-$ and $π^0$ electroproduction off transverse polarized proton target, averaged and not averaged by the kine…
One principal motivation for studying exclusive reactions is that they provide a new class of observables, called off-diagonal parton distributions, for the internal structure of the nucleon. The study of exclusive reactions provides a probe of nucleon structure complementary to purely inclusive studies. The simplest, and possibly the most promising, type of experiment is exclusive electroproduction of pseudoscalar mesons at small t, and at large Q2 and W. We show that using the CLAS spectrometer at JLAB and with beam energies between 4 and 6 GeV, we can obtain good quality electroproduction d…
We present an overview of our efforts to analyze pion-nucleon elastic scattering data, along with data from related photo- and electroproduction reactions, in order to study the baryon spectrum. We then focus on the Delta(1232) resonance. Fits to pion photo- and electroproduction data have been used to extract values for the R_EM = E2/M1 and R_SM = S2/M1 ratios as functions of Q^2. These results are compared to other recent determinations.
We consider pion electroproduction on a proton target close to threshold for Q^2 in the region 1-10 GeV^2. The momentum transfer dependence of the S-wave multipoles at threshold, E_{0+} and L_{0+}, is calculated using light-cone sum rules.
We investigate $ϕ$-meson electroproduction off the proton target, i.e., $γ^* p \to ϕp$, by employing a tree-level effective Lagrangian approach in the kinematical ranges of $Q^2$ = (0$-$4) $\mathrm{GeV}^2$, $W$ = (2$-$5) GeV, and $|t| \leq 2\,\mathrm{GeV}^2$. In addition to the universally accepted Pomeron exchange, we consider various meson exchanges in the $t$ channel with the Regge method. Direct $ϕ$-meson radiations in the $s$- and $u$-channels are also taken into account. We find that the $Q^2$ dependence of the transverse ($σ_{\mathrm{T}}$) and longitudinal ($σ_{\mathrm{L}}$) cross secti…
We show that AdS/QCD generates predictions for the rate of diffractive rho meson electroproduction that are in agreement with data collected at the HERA electron-proton collider.
The backgrounds due to the direct diffractive dissociation of the photon into the $π^+π^-$ as well as $K^+K^-$ pairs to the "elastic" diffractive $ρ'$ and $ϕ$ mesons production in electron-proton collisions are calculated. It is shown that the role of background and background-resonant interference contributions is very important in experimental distribution of $M_{π^+ π^-}$ in the $ρ'$ mass region. The amplitude for the background process is proportional to the $π$-meson - proton or K-meson - proton cross sections. Therefore, describing the HERA data, one can estimate $σ(πp)$ and $σ(Kp)$ at e…
Recent results from phenomenological studies of the CLAS data on 2$π$ electroproduction off proton are presented. The analysis is focused on extracting of N $\to$ $N^{*}$ electromagnetic transition amplitudes from the full data set on unpolarized 1-differential 2$π$ cross-sections.
Exclusive electroproduction of a K or K^* meson on the nucleon can give a Theta^+ pentaquark in the final state. This reaction offers an opportunity to investigate the structure of pentaquark baryons at parton level. We discuss the generalized parton distributions for the N -> Theta^+ transition and give the leading order amplitude for electroproduction in the Bjorken regime.
Elastic virtual photoproduction of charmonia on nucleons is calculated in a parameter free way with the light-cone dipole formalism and the same input: factorization in impact parameters, light-cone wave functions for the photons and the charmonia, and the universal phenomenological dipole cross section which is fitted to other data. The charmonium wave functions are calculated with four known realistic potentials, and two models for the dipole cross section are tested. Very good agreement with data for the cross section of charmonium electroproduction is found in a wide range of $s$ and $Q^2$…
We estimate the leading order amplitudes for exclusive photon and meson electroproduction reactions at large Q^2 in the valence region in terms of skewed quark distributions. As experimental investigations can currently only be envisaged at moderate values of Q^2, we estimate power corrections due to the intrinsic transverse momentum of the partons in the meson wavefunction and in the nucleon. To this aim the skewed parton distribution formalism is generalized so as to include the parton intrinsic transverse momentum dependence. Furthermore, for the meson electroproduction reactions, we calcul…
We analize two reactions: the electroproduction at small x and the Drell-Yan production by protons on nuclei. We show that nuclear shadowing in electroproduction is larger if the virtuality of inter- mediate hadrons is properly taken into account. In the Drell-Yan production, initial state interactions of fast partons are included. With this input, the excess pion contribution improves the description of data for both reactions.
We examine the efficacy of pion exchange models to simultaneously describe leading neutron electroproduction at HERA and the $\bar{d}-\bar{u}$ flavor asymmetry in the proton. A detailed $χ^2$ analysis of the ZEUS and H1 cross sections, when combined with constraints on the pion flux from Drell-Yan data, allows regions of applicability of one-pion exchange to be delineated. The analysis disfavors several models of the pion flux used in the literature, and yields an improved extraction of the pion structure function and its uncertainties at parton momentum fractions in the pion of $4 \times 10^{…
The transition γ_{v}pN^* amplitudes (electrocouplings) for prominent excited nucleon states obtained in a wide area of photon virtualities offer valuable information for the exploration of the N^* structure at different distances and allow us to access the complex dynamics of non-perturbative strong interaction. The current status in the studies of γ_{v}pN^* electrocouplings from the data on exclusive meson electroproduction off protons measured with the CLAS detector at Jefferson Lab is presented. The impact of these results on exploration of the N^* structure is discussed.
We investigate neutral pion electroproduction off protons in the framework of heavy baryon chiral perturbation theory. The chiral expansion of the S--wave multipoles $E_{0+}$ and $L_{0+}$ is carried out to three orders. There appear several undetermined low--energy constants. Three are taken from a recent study of the new TAPS threshold $π^0$ photoproduction data, one is fixed from the proton Dirac form factor and the novel S--wave constants appearing at orders $q^4$ and $q^5$ are determined from a best fit (constrained by resonance exchange) to the recent NIKHEF and MAMI data at photon moment…
We investigate exclusive electroproduction of $π^-Δ^{++}$ within the handbag approach in which the helicity amplitudes factorize into generalized parton distributions (GPDs) and hard partonic subprocesses. We define the $p-Δ$ transversity GPDs while the helicity non-flip GPDs are taken from the literature. For the numerical estimates of observables we utilize large-$N_C$ results in order to relate a few of the $p-Δ$ GPDs to the proton-proton ones and neglect all other GPDs. In the calculation of the twist-2 and twist-3 subprocess amplitudes we take into account quark transverse momenta in comb…
We study the beam single-spin asymmetries $A_{LU}^{\sinϕ_h}$ for charged hadrons produced in semi-inclusive deep inelastic scattering process, by considering the $e H_1^\perp$ term and the $g^\perp D_1$ term simultaneously. Besides the asymmetries for charged pions, for the first time we present the analysis on the asymmetries in the production of charged kaons, protons and antiprotons by longitudinally polarized leptons scattered off unpolarized proton and deuteron targets. In our calculation we use two sets of transverse momentum dependent distributions $g^\perp(x,\bm k_T^2)$ and $e(x,\bm k_…
We present a detailed analysis of rho(0) electroproduction at intermediate energy for quasi-elastic knockout kinematics. The approach is based on an effective Lagrangian which generates exchanges of scalar, pseudoscalar, axial-vector and tensor mesons. The specific role of mesons with different values for J(PC) is analyzed. We show that the pi(0) exchange amplitude and its interference terms with eta(eta') and f1 exchanges dominate in the transverse part of the cross section sigmaT. The main role plays the pi(0) to rho(0) M1 spin transition when coupling the virtual photons to the nucleon. In …
