We consider backward vector meson exclusive electroproduction off nucleons in the framework of collinear QCD factorization. Nucleon to vector meson transition distribution amplitudes arise as building blocks for the corresponding factorized amplitudes. In the near-backward kinematics, the suggested factorization mechanism results in the dominance of the transverse cross section of vector meson production ($σ_T \gg σ_L$) and in the characteristic $1/Q^8$-scaling behavior of the cross section. We evaluate nucleon to vector meson TDAs in the cross-channel nucleon exchange model and present estimates of the differential cross section for backward $ρ^0$, $ω$ and $ϕ$ meson production off protons. The resulting cross sections are shown to be measurable in the forthcoming JLab@12 GeV experiments.
We discuss the relevance of studying single transverse-spin asymmetry in hard-exclusive meson electroproduction in the backward region. Such an asymmetry could help us discriminate between contributions from a soft baryon exchange in the u-channel and a hard parton-induced scattering.
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.
Nucleon to meson transition distribution amplitudes (TDAs), non-diagonal matrix elements of nonlocal three quark operators between a nucleon and a meson states, arise within the collinear factorized description of hard exclusive electroproduction of mesons off nucleons in the backward direction. Below we address the problem of modelling pion to nucleon TDAs. We suggest a factorized Ansatz for quadruple distributions with input from the soft pion theorem for pi N TDAs. In order to satisfy the polynomiality property in its full form the spectral representation is complemented with a D-term like …
We use general relations between the Transition Distribution Amplitudes (TDAs), entering the description of the p -> pi0 transition, and the proton Distribution Amplitudes (DAs) in the soft-pion limit to estimate the size of the amplitude for backward electroproduction of pi0 at large Q2.
Baryon to meson transition distribution amplitudes (TDAs), non-diagonal matrix elements of the nonlocal three quark operator between a nucleon and a meson state, extend the concept of generalized parton distributions. These non-perturbative objects which encode the information on three quark correlations inside the nucleon may be accessed experimentally in backward meson electroproduction reactions. We suggest a general framework for modelling nucleon to pion (pi N) TDAs employing the spectral representation for pi N TDAs in terms of quadruple distributions. The factorized Ansatz for quadruple…
We study the scaling regime of pion electroproduction in the backward region, eN->e'N'pi. We compute the leading-twist amplitude in the kinematical region, where it factorises into a short-distance matrix element and long-distance dominated nucleon Distribution Amplitudes and nucleon to pion Transition Distribution Amplitudes. Using the chiral limit of the latter, we obtain a first estimate of the cross section, which may be experimentally studied at JLab or Hermes.
The extension of the concept of generalized parton distributions leads to the introduction of baryon to meson transition distribution amplitudes (TDAs), non-diagonal matrix elements of the nonlocal three quark operator between a nucleon and a meson state. We present a general framework for modelling nucleon to pion ($πN$) TDAs. Our main tool is the spectral representation for πN TDAs in terms of quadruple distributions. We propose a factorized Ansatz for quadruple distributions with input from the soft-pion theorem for πN TDAs. The spectral representation is complemented with a D-term like con…
$γ_{v}NN^*$ 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.
Backward-angle meson electroproduction above the resonance region, which was previously ignored, is anticipated to offer unique access to the three quark plus sea component of the nucleon wave function. In this letter, we present the first complete separation of the four electromagnetic structure functions above the resonance region in exclusive omega electroproduction off the proton, e + p -> e' + p + omega, at central Q^2 values of 1.60, 2.45 GeV^2 , at W = 2.21 GeV. The results of our pioneering -u ~ -u min study demonstrate the existence of a unanticipated backward-angle cross section peak…
Electroproduction of two mesons well separated in rapidity allows the first feasible selective access to chiral-odd transversity GPDs provided one of these mesons is a transversely polarized vector meson rho_T.
We analyze light meson electroproduction within the handbag model. We study cross sections and spin asymmetries for various mesons. The essential role of the transversity $\tilde H_T$ and $\tilde E_T$ GPDs in electroproduction of pseudoscalar mesons is found. Our results are in good agrement with experiment.
It is reported on an analysis of electroproduction of light mesons at small Bjorken-x within the handbag approach. The partonic subprocesses, meson electroproduction off quarks or gluons, are calculated within the modified perturbative approach in which quark transverse momenta are retained. The soft hadronic matrix elements, generalized parton distributions, are constructed by means of double distributions. The constraints from parton distributions and sum rules are taken into account. Various moments of these generalized parton distributions are compared to recent results from lattice gauge …
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.
We discuss the recent data of exclusive $π^0$ (and $π^+$) electroproduction on the proton obtained by the CLAS collaboration at Jefferson Lab. It is observed that the cross sections, which have been decomposed in $σ_T +εσ_L$, $σ_{TT}$ and $σ_{LT}$ structure functions, are dominated by transverse amplitude contributions. The data can be interpreted in the Generalized Parton Distribution formalism provided that one includes helicity-flip transversity GPDs.
We develop a non-perturbative analysis of the electro-production of heavy vector mesons ($ϕ$, $J/Ψ$) from threshold to high energy. We use the holographic construction with bulk confinement enforced through a soft wall. Using Witten diagrams, we evaluate the pertinent cross sections for heavy vector mesons ($ϕ$, $J/Ψ$) production and study their dependence on both the incoming virtual photon polarization as well as the outgoing polarization of the heavy meson. Our results for $J/Ψ$ electro-production compares well with the available HERA data at low and intermediate $Q^2$, and for a wide range…
At large virtuality $Q^2$, the coupling to the $ρ$ meson production channels provides us with a natural explanation of the surprisingly large cross section of the $ω$, as well as the $π^+$, meson electroproduction recently measured at backward angles, without destroying the good agreement between the Regge pole model and the data at the real photon point. Together with elastic re-scattering of the outgoing meson it also provides us with a way to explain why the node, that appears at $u\sim -0.15$ GeV$^2$ at the real photon point, disappears at moderate virtuality $Q^2$. Predictions are given f…
It is reported on a global analysis of hard vector-meson electroproduction which is based on the handbag factorization. The generalized parton distributions are constructed from their forward limits with the help of double distributions and the partonic subprocesses are calculated within the modified perturbative approach.
Transversity generalized parton distributions (GPDs) can be probed in diffractive electro- and photoproduction of two vector mesons on a hadron in kinematics where the two vector mesons are separated by a large rapidity gap. We report on calculations for this process in the case of coherent $ρ^0-ω$ meson production on a deuteron target. Our cross section results show that an electron-ion collider with deuteron beams and forward detectors could probe deuteron transversity GPDs.
We analyze light meson electroproduction in the intermediate energy range where the quark contributions are essential. Our calculations are carried out on the basis of the handbag approach. We study cross sections and spin asymmetries for various vector and pseudoscalar mesons. Our results are in good agrement with COMPASS and HERMES experiments.
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…
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.
Relying on the collinear factorization approach, we demonstrate that H1 and ZEUS measurements of exclusive light vector meson and photon electroproduction cross sections can be simultaneously described for photon virtualities of $Q >2 GeV$. Our findings reveal that quark exchanges are important in this small $x_{Bj}$ region and that in leading order approximation the gluonic skewness ratio is much smaller than one.
We study light vector meson electroproduction at small $x$ within the generalized parton distributions (GPDs) model. The modified perturbative approach is used, where the quark transverse degrees of freedom in the vector meson wave function and hard subprocess are considered. Our results on the cross section and spin observables are in good agreement with experiment
We study universality in the electroproduction of vector mesons using a unified nonperturbative approach which has already proved to reproduce extremely well the available experimental data. In this framework, after the extraction of factors that are specific of each vector meson, we arrive at a reduced integrated elastic cross section which is universal. Our calculations suggest a finite infrared behavior for the strong coupling constant.
We analyze electroproduction of light vector meson at small Bjorken $x$ within the generalized parton distribution (GPD) approach. Calculation is based on the modified perturbative approach, where the quark transverse degrees of freedom in the hard subprocess are considered. Our results on the cross section are in fair agreement with experiment from HERMES to HERA energies.
It is reported on an analysis of electroproduction of light vector mesons at small Bjorken-x within the handbag approach. The partonic subprocesses, meson electroproduction off quarks or gluons, are calculated within the modified perturbative approach in which quark transverse momenta are retained. The soft hadronic matrix elements, generalized parton distributions, are constructed from the CTEQ6 parton distribution functions by means of double distributions. The cross sections for longitudinal polarized virtual photons evaluated from this approach, are in very good agreement with experiment i…
We determine the magnetic dipole moment of the rho meson using preliminary data from the BaBar Collaboration for the $e^+ e^- \to π^+ π^- 2 π^0$ process, in the center of mass energy range from 0.9 to 2.2 GeV. We describe the $γ^* \to 4π$ vertex using a vector meson dominance model, including the intermediate resonance contributions relevant at these energies. We find that $μ_ρ= 2.1 \pm 0.5$ in $e/2 m_ρ$ units.
It is shown that birefringence effect influences on production of particles with the spin S greater or equal then 1 at collisions of high energy particles.
An exact expression is derived for the meson production current off the nucleon for real and virtual photons which cleanly separates, in a model-independent manner, a tree-level expression that manifestly preserves local gauge invariance in terms of a generalized Ward-Takahashi identity from transverse final-state-interaction (FSI) terms. As discussed in some detail, this exact formulation is particularly well suited for implementing approximation schemes of exchange-current and FSI contributions (which in practice generally will be necessary) at any level of sophistication without violating l…
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…
The process of a light neutral vector meson electroproduction is studied in the framework of QCD factorization in which the amplitude factorizes in a convolution of the nonperturbative meson distribution amplitude and the generalized parton densities with the perturbatively calculable hard-scattering amplitudes. We derive a complete set of hard-scattering amplitudes at next-to-leading order for the production of vector mesons, $V=ρ^0, ω, ϕ$.
Data for the differential cross sections for $γ^* p\toρ^0 p$, $γ^* p \to ϕp$ and $γ^* p\to J/ψp$, at all available values of $Q^2$, fit very well to a combination of soft pomeron, hard pomeron and $f_2$ exchange.
An analysis of light vector meson photoproduction at small Bjorken $x \leq 0.2$ is done on the basis of the generalized parton distributions (GPDs). Our results on the cross section and spin density matrix elements (SDME) are in good agreement with experiments.
