P. Kroll
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.
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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.
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 evaluate the leading order amplitude for the deep exclusive electroproduction of an exotic hybrid meson in the Bjorken regime. We show that, contrary to naive expectation, this amplitude factorizes at the twist 2 level and thus scales like usual meson electroproduction when the virtual photon and the hybrid meson are longitidinally polarized. Exotic hybrid mesons may thus be studied in electroproduction experiments at JLAB, HERA (HERMES) or CERN (COMPASS).
Phi meson electroproduction at small Bjorken-x
在手袋方法中分析小Bjorken-x下Phi介子电产生，振幅因子化为广义部分子分布和胶子子过程，截面和自旋密度矩阵与HERA数据一致。
Vector meson electroproduction is analyzed within the two-gluon (2G) model and the generalized parton distribution (GPD) approach at small $x$-Bjorken. We demonstrate that 2G and GPD models are not completely equivalent. At the same time, both models are in reasonable agreement with available experimental data on light vector meson electroproduction.
We outline $QCD$ predictions for the diffractive electroproduction of heavy flavor vector mesons and compare them with available data.
By means of an explicit one-loop calculation, it is shown that the leading twist contribution to the exclusive electroproduction of transversely polarized vector mesons from the nucleon vanishes. This confirms the all-orders proof by Collins and Diehl.
A systematic multiple scattering formalism for vector meson electroproduction from nuclei is developed, with emphasis on the formation, propagation and hadronization scales for quark-gluon fluctuations of the virtual high-energy photon. The theory is compared to HERMES measurements of rho electroproduction on 14N. The nuclear transparency as a function of the vector meson propagation length is well reproduced.
A nonperturbative calculation of elastic electroproduction of the J/psi meson is presented and compared to the experimental data. Our model describes well the observed dependences of the cross sections on the photon virtuality Q2 and on the energy, and the measured ratio R of longitudinal to transverse cross sections.
We present the first evaluation of the color dipole diffraction slope from the data on diffractive photo- and electroproduction of vector mesons. The energy and dipole size dependence of the found dipole diffraction slope are consistent with the color dipole gBFKL dynamics.
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.
We present the first evaluation of color dipole cross section from experimental data on diffractive photo- and electroproduction of vector mesons. The dipole-size and energy dependence of the found dipole cross section is consistent with expectations from the BFKL dynamics.
Deeply virtual vector meson electroproduction at small Bjorken-x
本文在手袋框架内结合广义部分子分布模型和修正微扰方法，分析了小Bjorken-x下矢量介子电产生，计算结果与HERA数据吻合良好。
We discuss the possibility of measuring generalized parton distributions in exclusive electroproduction of mesons off the nucleon and estimate the uncertainty from perturbatively induced higher-twist corrections. We find that, while the magnitude of the cross section changes significantly taking into account twist-four contributions modeled via renormalons, the transverse spin asymmetry is weakly sensitive to them and displays the precocious scaling.
The electromagnetic radiative correction to the cross section of the vector meson electroproduction is calculated. Explicit covariant formulae for the observed cross section are obtained. The dependence of the radiative correction on the experimental resolution and on the inelasticity cut is discussed. FORTRAN code DIFFRAD based on both exact (ultrarelativistic) and approximate sets of the formulae for the radiative correction to the cross section is presented. The detailed numerical analysis for kinematical conditions of the recent experiments on the diffractive electroproduction of vector me…
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 calculate the amplitude for the forward electroproduction of two light vector mesons in next-to-leading order BFKL. This amplitude is written as a convolution of two impact factors for the virtual photon to light vector meson transition with the BFKL Green's function. It represents the first next-to-leading order amplitude ever calculated for a collision process between strongly interacting colorless particles.
We show that hard electroproduction is a promising way to study exotic hybrid mesons, in particular through the hybrid decay channel $H \to πη$. We discuss the $πη$ generalized distribution amplitude, calculate the production amplitude and propose a forward-backward asymmetry as a signal for the hybrid meson production.
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.
By explicit calculation we demonstrate that, in variance with the classical prediction, the leading twist contribution to the exclusive electroproduction of transversely polarized vector mesons from the nucleon does not vanish beyond the leading order in perturbation theory. This appears to be due to a quantum-mechanical anomaly, in the sense that a classical symmetry of the field theory is broken by quantum corrections.
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 show that the leading twist contribution to exclusive electroproduction of transversely polarized vector mesons vanishes at all orders in perturbation theory. Therefore one cannot extract information on the quark transversity distribution of the nucleon from this reaction. In turn one has that the produced vector meson is purely longitudinal in the large-Q^2 limit, which provides a new test of the dominance of leading twist at finite Q^2.
Spin effects in hard exclusive electroproduction of mesons
该研究围绕硬遍举介子电产生过程的自旋效应展开，结合手袋方法的最新理论计算对相关实验数据进行讨论，为相关强子物理研究提供参考。
We calculate the amplitude for the forward electroproduction of two light vector mesons in next-to-leading order BFKL. This amplitude represents the first next-to-leading order amplitude ever calculated for a collision process between strongly interacting colorless particles.
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.
In the following, we will present a generalization of the proof of factorization for electroproduction of vector mesons to the production of several mesons such as $π^+π^-$ and a proof of factorization for exclusive $γ^* γ$ production of several hadrons for example $p \bar p$ or $π^+π^-$ to all orders in perturbation theory up to power suppressed terms.
The end-point contributions in the quark longitudinal momentum fraction of the virtual photon ($γ^*$) to vector meson ($V$) impact factor to the diffractive electroproduction amplitude can be factorized in terms of a generalized parton evolution of the target parton distribution. The result is used to model the helicity amplitudes $γ^* p \to V p$ in terms of small $x$ generalized parton distributions.
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.
We estimate the sizeable cross section for deep exclusive electroproduction of an exotic $J^{PC}=1^{-+}$ hybrid meson in the Bjorken regime. The production amplitude scales like the one for usual meson electroproduction, i.e. as $1/Q^2$. This is due to the non-vanishing leading twist distribution amplitude for the hybrid meson, which may be normalized thanks to its relation to the energy momentum tensor and to the QCD sum rules technique. The hard amplitude is considered up to next-to-leading order in $α_{S}$ and we explore the consequences of fixing the renormalization scale ambiguity through…
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 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 (NLO) for the production of vector mesons, $V=ρ^0, ω, ϕ$.
We use perturbative QCD and parton-hadron duality to estimate the cross sections for the diffractive electroproduction of rhoprime(1-) and rho(3-) resonances which occur in the 1.3-1.8 GeV mass interval. We present the cross sections and the ratios sigma_L/sigma_T as a function of Q^2. We compare the predictions with those for the diffractive electroproduction of rho mesons. We show how such diffractive electroproduction measurements at HERA can probe features of the perturbative QCD 'Pomeron'.
Electroproduction of two vector mesons with a large rapidity gap between them provides the first feasible selective access to chiral-odd GPDs. The scattering amplitude of the process $ γ^* N \to ρ_L^0 ρ_T^+ N'$ may be represented as a convolution of an impact factor describing the $γ^* \to ρ_L^0$ transition and an amplitude describing the $N\to ρ_T^+ N'$ transition, analogously to deeply exclusive electroproduction of a meson, the virtual photon being replaced by a Pomeron. The production of a transversely polarized vector meson $ρ_T^+$ selects a chiral-odd GPD in the proton.
We predict the cross-section for diffractive $ϕ$ electroproduction within the dipole model, using a holographic meson wavefunction for the $ϕ$. For the dipole cross-section, we use the Color Glass Condensate dipole model whose parameters are fitted to the latest 2015 combined HERA data on Deep Inelastic Scattering. Choosing a strange quark mass of $0.14$ GeV, we find good agreement with the available data.
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…
Transversity in exclusive meson electroproduction
本研究结合手袋框架的理论计算，综述硬遍举介子电产生的自旋效应，发现正电π介子电产生中光子-π介子跃迁的强贡献可由带twist-3 π波函数的横度GPD H_T建模。
We demonstrate that the explicit mass dependence of the exponent in the power--like energy behavior of the vector--meson production cross-section in the processes of virtual photon interactions with a proton gamma^*p --> Vp obtained in the off--shell extension of the approach based on unitarity is in a quantitative agreement with the high--energy HERA experimental data.
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…
Assuming the proton's light-cone wave function to be dominated by small parton virtualities and small intrinsic transverse momenta, we show that the electroproduction amplitudes at large momentum transfer factorize into parton-level subprocess amplitudes and form factors representing 1/x-moments of skewed parton distributions. On the basis of a wave function overlap model for the form factors we present detailed predictions for the electroproduction cross sections. We also comment on large momentum transfer photoproduction.
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 estimate the sizeable cross section for deep exclusive electroproduction of an exotic $J^{PC}=1^{-+}$ hybrid meson in the Bjorken regime. The production amplitude scales like the one for usual meson electroproduction, {\it i.e.} as $1/Q^2$. This is due to the non-vanishing leading twist distribution amplitude for the hybrid meson, which may be normalized thanks to its relation to the energy momentum tensor and to the QCD sum rules technique. The hard amplitude is considered up to next--to--leading order in $α_{S}$ and we explore the consequences of fixing the renormalization scale ambiguity…
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…
The hard exclusive electroproduction of $π^0,η,η'$ mesons by the longitudinal photons is considered. We calculate the ratios of the $π^0 : η: η'$ yields for the electroproduction processes off nucleons and deuterons based on the QCD factorization theorem, the low energy QCD chiral dynamics, and the QCD anomaly. Thus investigation of these ratios gives a new way to probe the physics of the spontaneous chiral symmetry breaking.
A consistent description of exclusive photoproduction of mesons at high momentum transfer t relies on a few effective degrees of freedom which can be checked against Lattice calculation or more effective models of QCD.
We discuss exclusive vector meson electroproduction within the QCD collinear factorization framework. In Bjorken kinematics the amplitude factorizes in a convolution of the nonperturbative meson distribution amplitude and the generalized parton densities with the perturbatively calculable hard-scattering amplitudes, which are presently known to next-to-leading order (NLO). At small $x_{\rm B}$ NLO corrections are very large. It is related to appearance of BFKL type logarithms in the hard-scattering amplitudes, that calls for a resummation of these effects at higher orders. Here we report the f…
Deep-inelastic scattering at low Bjorken x and elastic vector meson electroproduction are analyzed on the basis of the off-shell s-channel unitarity. We discuss behavior of the total cross-section of virtual photon-proton scattering and obtain, in particular, that the exponent in the power-like dependence of $σ^{tot}_{γ^* p}$ is related to the interaction radius of a constituent quark. The explicit mass dependence of the exponent in the power energy behavior of the vector meson electroproduction has been obtained. Angular distributions at large momentum transfers are considered. The energy dep…
We give a general formula for the cross section for diffractive vector meson electroproduction, gamma^* p -> Vp. We first calculate diffractive qqbar production, and then use parton-hadron duality by projecting out the J^P = 1^- state in the appropriate mass interval. We compare the Q^2 dependence of the cross section for the diffractive production of rho and J/psi mesons with recent HERA data. We include the characteristic Q^2 dependence associated with the use of the skewed gluon distribution. We give predictions for sigma_L/sigma_T for both rho and J/psi production.
We discuss the electroproduction of light vector mesons from transversely polarized photons. Here QCD factorization cannot be applied as shown explicitly in a leading order calculation of corresponding Feynman diagrams. It is emphasized that present infrared singular contributions cannot be regularized through phenomenological meson distribution amplitudes with suppressed endpoint configurations. We point out that infrared divergencies arise also from integrals over skewed parton distributions of the nucleons. In a phenomenological analysis of transverse vector meson production model dependent…
We compute the exclusive electroproduction, $γ^* p \rightarrow V p$, of heavy quarkonia $V$ to NLO in the collinear factorisation scheme, which has been formally proven for this process. The inclusion of an off-shell virtuality $Q^2$ carried by the photon extends the photoproduction phase space of the exclusive heavy quarkonia observable to electroproduction kinematics. This process is relevant for diffractive scattering at HERA and the upcoming EIC, as well as at the proposed LHeC and FCC.
We formulate and prove a QCD factorization theorem for hard exclusive electroproduction of mesons in QCD. The proof is valid for the leading power in Q and all logarithms. This generalizes previous work on vector meson production in the diffractive region of small x. The amplitude is expressed in terms of off-diagonal generalizations of the usual parton densities. The full theorem applies to all kinds of meson and not just to vector mesons. The parton densities used include not only the ordinary parton density, but also the helicity density (g_1 or $Δq$) and the transversity density ($h_1$ or …
We consider hard electroproduction of mesons at twist-3 level and focus our attention on helicity flip amplitudes corresponding to the production of transversely polarized mesons by transverse photons. We demonstrate that helicity-flip amplitude for the transversely polarized particles are closely related to the contributions of the matrix elements of the tensor gluon operator which can serve as pure gluonic probe of the hadrons. Using standard factorisation approach and Wanzura-Wilczek approximation for the twist-3 matrix elements we find that such amplitudes do not have divergencies from the…
Virtual radiative corrections due to the long range Coulomb forces of heavy nuclei with charge Z may lead to sizeable corrections to the Born cross section usually used for lepton-nucleus scattering processes. An introduction and presentation of the most important issues of the eikonal approximation is given. We present calculations for forward electroproduction production of rho mesons in a framework suggested by the VDM (vector dominance model), using the eikonal approximation. It turns out that Coulomb corrections may become relatively large. Some minor errors in the literature are correcte…
We consider the process gamma^* N -> M_1 M_2 N' with a large rapidity gap between the two mesons M_1 and M_2. Within the QCD collinear approximation, the scattering amplitude may be written as a convolution of an impact factor describing the gamma^* -> M_1 transition and an amplitude describing the N -> M_2 N' collinear process.
We present the detailed analysis of genuine twist-3 contributions to the exclusive electroproduction amplitude of transversely polarized vector mesons. Using the formalism based on the QCD factorization in the momentum representation we calculated all the genuine twist-3 terms and found the total expression of this amplitude at 1/Q level. Generally speaking, these terms violate standard factorization owing to the existence of the infrared divergencies in the amplitude of hard sub-processes, although the strongest divergencies cancel due to the QCD equations of motion. We discuss the possible t…
We give predictions for the leading order amplitudes for deeply virtual Compton scattering and hard meson electroproduction reactions at large Q^2 in the valence region in terms of skewed quark distributions. We give first estimates for the power corrections to these leading order amplitudes. In particular, we outline examples of experimental opportunities to access the skewed parton distributions at the current high-energy lepton facilities : JLab, HERMES and COMPASS.
We consider the electroproduction of two vector mesons with a large rapidity gap between them on a nucleon target in the process $ γ^* N \to ρ_1 ρ_2 N'$. We calculate the Born term for this process within the collinear factorization framework. The resulting scattering amplitude may be represented as a convolution of an impact factor describing the $γ^* \to ρ_1$ transition and an amplitude describing the $N\to ρ_2 N'$ transition. The latter amplitude is analogous to deeply virtual electroproduction of a meson, the virtual photon being replaced by two gluon (Pomeron) exchange . The long distance…
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