Exclusive meson leptoproduction from nucleons in the deeply virtual exchanged boson limit can be described by generalized parton distributions (GPDs). Including spin dependence in the description requires 8 independent quark-parton and gluon-parton functions. The chiral even subset of 4 quark-nucleon GPDs are related to nucleon form factors and to parton distribution functions. The chiral odd set of 4 quark-nucleon GPDs are related to transversity, the tensor charge, and other quantities related to transversity. Different meson or photon production processes access different combinations of GPDs. This is analyzed in terms of t-channel exchange quantum numbers, $J^{PC}$ and it is shown that pseudoscalar production can isolate chiral odd GPDs. There is a sensitive dependence in various cross sections and asymmetries on the tensor charge of the nucleon and other transversity parameters. We also consider, briefly, the analyticity of the GPDs and the problems with the partonic interpretation in the ERBL region.
Deep virtual vector meson leptoproduction at small $\xbj$ is analyzed in the framework of generalized parton distributions. It is shown that the inclusion of transverse degrees of freedom leads to reasonable agreement with the cross section data for light mesons.
Hard exclusive leptoproductions of real photons, lepton pairs and mesons are the most promising tools to unravel the three-dimensional picture of the nucleon, which cannot be deduced from conventional inclusive processes like deeply inelastic scattering.
HEPGEN is a generator of Monte Carlo events, which is dedicated to studies of hard exclusive single photon or meson production processes at the COMPASS experiment kinematic domain. In addition HEPGEN allows to generate also single photon or meson production accompanied by the diffractive dissociation of the nucleon, which is one of the main sources of background in analyses of exclusive processes.
The semi-analytical approach to the model independent leptonic QED corrections to exclusive $ρ^0$ meson leptoproduction (i.e. electron and muon scattering experiments) is presented. The corrections to $ρ^0$ production at large $Q^2$ as well as to $ρ^0$ photoproduction are studied in details. The numerical results are calculated for two different experimental analyses: NMC (muoproduction at large $Q^2$) and ZEUS at HERA (photoproduction). It is shown that the corrections are 2-5 \% for NMC and below 2 \% for the ZEUS measurement. The application of the presented approach to other vector mesons …
We consider the exclusive production of $J/ψ$ mesons in polarized virtual-photon--proton collisions. We derive helicity amplitudes for the dominant subprocess $γ^* g \to J/ψg$ and show that polarization asymmetries vanish in the case of collinear scattering. Thus, contrary to what has been suggested earlier, this process is not a good probe of the polarized gluon distribution $Δg(x,Q^2)$ of the proton.
The physics of diffractive vector meson production in virtual photon nucleon scattering at NMC energies is reviewed. A particular attention is paid to the physical aspects of the reaction and how they influence the observables. The reaction is a good probe to investigate both soft exchange mechanisms and hadronic wave functions. Extension to either HERA or ELFE kinematics is sketched out.
The role of transversity or helicity-flip generalized parton distributions (GPDs) in leptoproduction of vector mesons is investigated within the framework of the handbag approach. The transversity GPDs in combination with twist-3 meson wave functions, occur in the amplitudes for transitions from a transversely polarized virtual photon to a longitudinal polarized vector meson. The importance of the transversity GPDs can be examined in some of the spin density matrix elements and in transverse target spin asymmetries. Using suitable parametrizations of both helicity-flip and non-flip GPDs, which…
We investigate exclusive electroproduction of vector and pseudoscalar mesons at large photon virtuality $Q^2$. These reactions were analyzed within the handbag approach where amplitudes factorize into hard subprocesses and generalized parton distributions (GPDs). The essential role of transversity effects were found in pseudoscalar and light vector meson leptoproduction. These contributions are important at low $Q^2$ and determined by twist-3 effects accompanied by transversity GPDs. The transversity contributions lead to large transverse cross sections for most reactions of pseudoscalar meson…
Generalized parton distributions (GPDs) extracted from exclusive meson leptoproduction within the handbag approach are briefly reviewed. Only the GPD E is discussed in some detail. Applications of these GPDs to virtual Compton scattering (DVCS) and to Ji's sum rule are also presented.
Sufficiently inclusive processes, like the deep inelastic scattering (DIS), are described in terms of scale-dependent parton distributions, which correspond to the density of partons with a given longitudinal momentum fraction, integrated over the parton transverse momentum. For less inclusive processes, one needs to consider densities unintegrated over the transverse momentum. This work focuses on the unintegrated gluon distribution (UGD), describing the probability that a gluon can be emitted by a colliding proton, with definite longitudinal fraction and transverse momentum. Through the lept…
The contribution of exclusive radiative tail to the cross section of semi-inclusive hadron leptoproduction has been calculated exactly for the first time. Although the experience of inclusive data analyses suggests us that the contribution of radiative tail from the elastic peak is of particular importance, similar effects in the semi-inclusive process were only recently estimated in the peaking approximation. The explicit expressions for the lepton part of the lowest order QED contribution of exclusive radiative tail to the five-fold differential cross section are obtained and discussed. Nume…
We analyze the light meson leptoproduction within the handbag approach. We show that effects determined by the transversity Generalized Parton Distributions (GPDs), $H_T$ and $\bar E_T$ are essential in the description of pseudoscalar and vector meson leptoproduction.
We apply a previously developed scheme to consistently include the twist-3 distribution amplitudes for transversely polarized rho meson in order to evaluate, in the framework of kT factorization, the helicity amplitudes for exclusive leptoproduction of a light vector meson, at leading order in alphaS. We compare our results with high energy experimental data for the ratios of helicity amplitudes T11/T00 and T01/T00 and get a good description of the data.
The light meson electroproduction is analyzed within the handbag approach. We consider the leading-twist contribution together with the transversity effects. We show that the transversity Generalized Parton Distributions (GPDs), $H_T$ and $\bar E_T$ are essential in the description of spin effects in the pseudoscalar and vector meson leptoproduction.
Light vector meson leptoproduction is analyzed on the basis of the generalized parton distributions. Our results on the cross section and spin effects are in good agrement with experiment at HERA, COMPASS and HERMES energies. Predictions for $A_{UT}$ asymmetry for various reactions are presented.
On the basis of the handbag approach we study cross sections and spin asymmetries for leptoproduction of various vector and pseudoscalar mesons. Our results are in good agrement with high energy experiments. We analyse what information about Generalized Parton Distributions (GPDs) can be obtained from these reactions.
We present an analysis of twist-2, leading order QCD amplitudes for hard exclusive leptoproduction of charged vector mesons. These processes are determined by nonforward parton distribution functions which are nondiagonal in quark flavor. We derive relations between flavor diagonal and nondiagonal distribution functions based on isospin symmetry. Furthermore, we discuss general features of rho^+ and rho^- production cross sections, and present estimates based on a simple model for nonforward distribution functions.
The amplitude of diffractive $ρ$ (and open quark) leptoproduction on a polarized target is calculated in the leading log approximation of pQCD using the hadron-parton duality hypothesis. The spin-spin asymmetry is expressed in terms of the spin dependent gluon and quark structure functions in the small $x$ region. Therefore the $γ^*+p\to ρ+p$ reaction provides a promising tool to study the spin dependent gluon distribution $ΔG(x,q^2)$.
We investigate exclusive leptoproduction of $ρ^0$ meson. These reactions were analyzed within the factorizing handbag approach. In our model good agreement of observables for light meson production with experimental data in a wide energy range was found. Using the model results we calculate the ratio of different helicity amplitudes for a transversely polarized proton target to the leading twist longitudinal amplitude. Our results are close to the amplitude ratios measured by HERMES.
We calculate the energy--momentum distribution of the charmed quarks produced in neutrino reactions on protons, quantifying the importance of mass and current non--conservation effects. We study the strange and charm distributions probed in neutrino interactions in the presently accessible kinematical region. Some ambiguities inherent to the extraction of the parton densities from dimuon data are pointed out.
We point out the role of intrinsic transverse momentum of partons in the study of azimuthal asymmetries in deep-inelastic 1-particle inclusive leptoproduction. Leading asymmetries often appear in combination with spin asymmetries. This leads not only to transverse momentum dependence in the parton distribution functions, but also to functions beyond the ones known from inclusive deep-inelastic scattering (DIS). We use Lorentz invariance and the QCD equations of motion to study the evolution of functions that appear at leading (zeroth) order in a 1/Q expansion in azimuthal asymmetries. This inc…
We discuss a few examples of structure functions for polarized, semi-inclusive scattering processes to show the richness of structure. Then we indicate how polarization and particle production can be used to study the quark and gluon structure of hadrons going further than the well-known parton densities and fragmentation functions.
We describe the hard leptoproduction of transversally polarized rho-meson, up to twist 3 accuracy, including 2- and 3- particles Fock-states, in the HERA kinematics of high center-of-mass energy. We first build a model based on a simple approach to the unintegrated gluon density (the parton impact factor) that we compare with H1 and ZEUS data for the ratios of helicity amplitudes T(gamma*T -> rhoT)/T(gamma*L -> rhoL) and T(gamma*T -> rhoL)/T(gamma*L -> rhoL) and get a good description of the data. We also show how saturation effects can be included in this model by extending the dipole represe…
We analyse spin effects in diffractive vector meson leptoproduction at small $x$ on the basis of the generalized parton distribution (GPD) approach. We take into account quark transverse degrees of freedom in the hard subprocess. We calculate amplitudes for the longitudinally and transversely polarized photons and vector mesons. Our results on the cross section and spin density matrix elements (SDME) are in fair agreement with the DESY experiments. Predictions for HERMES and COMPASS energy range are made. The predicted double spin longitudinal $A_{LL}$ asymmetry is not small at HERMES energies
We consider the pion pole contribution and transversity effects determined by the $H_T$ and $\bar E_T$ Generalized Parton Distributions (GPDs) which are essential in hard pseudoscalar and vector meson leptoproduction. We investigate spin effects in the $ω$ and $ρ^0$ reactions. It is shown that the pion pole contribution is very important in the $ω$ production. Such effects in the $ρ^0$ channel are much smaller. Our results on spin asymmetries and spin density matrix elements in these reactions were found to be in good agreement with HERMES data.
In this paper we analyze deep inelastic one-particle inclusive processes for the case of spin-one targets or for the case of spin-one produced hadrons, such as rho mesons. This allows the measurement of new distribution and fragmentation functions not available in the spin-half case, and provides new ways to probe functions otherwise difficult to measure. We will analyze only contributions leading order in 1/Q, but we will include effects of the transverse momentum of partons. We also include time-reversal odd functions.
We present formulae for the momentum (z) distributions of D-mesons produced in neutrino deep-inelastic scattering off strange partons. The expressions are derived within the variable flavor scheme of Aivazis et al. (ACOT-scheme), which is extended from its fully inclusive formulation to one-hadron inclusive leptoproduction. The dependence of the results on the assumed strange quark mass m_s is investigated and the m_ \to 0 limit is compared to the corresponding MSbar results. The importance of O(alpha_s) quark-initiated corrections is demonstrated for the m_s=0 case.
The information about generalized parton distributions (GPDs) extracted from exclusive meson leptoproduction within the handbag approach is summarized. Details are only discussed for the GPD E and the transversity ones. It is also commented on results for deep virtual Compton scattering evaluated from these GPDs
The Regge inspired k_T-factorization scheme expresses the scattering amplitude for exclusive leptoproduction of a light vector meson rho in terms of two impact factors, one for the gamma* - rho transition and the other one for the nucleon to nucleon transition, with, at leading order, a two "reggeized" gluon exchange in the t-channel. We report here on a recent phenomenological study where we apply a previously developped scheme which consistently takes into account the twist 3 distribution amplitudes for transversely polarized rho meson. The computation of the gamma* to rho impact factor are …
We review here two approaches to describe hard leptoproduction of transversally polarized rho-meson, based on recent calculation of the gamma*T -> rhoT impact factor up to twist 3 accuracy in the collinear factorization frame, including 2- and 3- particles Fock-states. The first approach uses a model of the unintegrated gluon density (the proton impact factor) which allows a comparison of our predictions with H1 and ZEUS data for the ratios of helicity amplitudes T(gamma*T -> rhoT)/T(gamma*L -> rhoL) and T(gamma*T -> rhoL)/T(gamma*L -> rhoL). In the second approach, we transform the gamma*T ->…
I review several topics in electroproduction which test fundamental aspects of QCD. These include the role of final-state interactions in producing diffractive leptoproduction processes, the shadowing of nuclear structure functions, and target-spin asymmetries. The antishadowing of nuclear structure functions is shown to be quark-flavor specific, suggesting that some part of the anomalous NuTeV result for $\sin^2θ_W$ could be due to the non-universality of nuclear antishadowing for charged and neutral currents. I also discuss the physics of the heavy-quark sea, hidden color in nuclear wavefunc…
We discuss specific observables that can be measured in deep inelastic leptoproduction in the case of 1-particle inclusive measurements, namely azimuthal asymmetries. These asymmetries contain information on the intrinsic transverse momentum of partons, with close connection to the gluon dynamics in hadrons.
Spin and transverse momentum dependent parton distributions - Generalized Parton Distributions (GPDs) - are at the interface between the QCD structure of the hadrons and observable quantities. The GPDs are linear superpositions within helicity amplitudes. The amplitudes are probed in high energy leptoproduction processes through angular dependent cross sections and polarization asymmetries. Phenomenological extraction of the amplitudes and the distributions is a challenging task. We present observables that connect particularly with the chiral odd quark-nucleon helicity amplitudes for Deeply V…
Generalized parton distributions of the nucleon are accessed via exclusive leptoproduction of the real photon. While earlier analytical considerations of phenomenological observables were restricted to twist-three accuracy, i.e., taking into account only terms suppressed by a single power of the hard scale, in the present study we revisit this differential cross section within the helicity formalism and restore power-suppressed effects stemming from the process kinematics exactly. We restrict ourselves to the phenomenologically important case of lepton scattering off a longitudinally polarized…
We present an analysis of twist-2, leading order QCD amplitudes for hard exclusive leptoproduction of mesons in terms of double/nonforward parton distribution functions. After reviewing some general features of nonforward nucleon matrix elements of twist-2 QCD string operators, we propose a phenomenological model for quark and gluon nonforward distribution functions. The corresponding QCD evolution equations are solved in the leading logarithmic approximation for flavor nonsinglet distributions. We derive explicit expressions for hard exclusive pi^0, eta, and neutral vector meson production am…
The spatial distribution of partons in the proton can be probed in suitable exclusive scattering processes. I report on recent performance estimates for parton imaging at a proposed Electron-Ion Collider.
In addition to the inclusive cross sections discussed within the QCD-parton model, in the regime of multiple parton interactions, different and more exclusive cross sections become experimentally viable and may be suitably measured. Indeed, in its study of double parton collisions, the quantity measured by CDF was an "exclusive" rather than an inclusive cross section. The non perturbative input to the "exclusive" cross sections is different with respect to the non perturbative input of the inclusive cross sections and involves correlation terms of the hadron structure already at the level of s…
Recent accurate data on F_2(x,Q) and on F_2^c(x,Q) from HERA at small-x require a more precise treatment of charm production in the global QCD analysis of parton distributions. We improve on existing global analyses by implementing the leptoproduction formalism of Aivazis et al. which represents a natural generalization of the conventional zero mass QCD parton framework to include heavy quark mass effects. We also perform analyses based on the fixed-flavor-number scheme, which is widely used in the literature, and demonstrate their uses and limitations. We discuss the implications of the impro…
We calculate cross sections for the exclusive diffractive leptoproduction of $ρ$-mesons, $γ^*~p~\to~ρ~p$, within the framework of high-energy factorization. Cross sections for longitudinally and transversally polarized mesons are shown. We employ a wide variety of unintegrated gluon distributions available in the literature and compare to HERA data. The resulting cross sections strongly depend on the choice of unintegrated gluon distribution. We also present predictions for the proton target in the kinematics of the Brookhaven EIC.
The overlap representation of skewed parton distributions is discussed and applications to wide-angle Compton scattering and electroproduction of mesons are presented. The amplitudes for these processes factorise into parton-level subprocess amplitudes and form factors representing 1/x-moments of skewed parton distributions.
The exclusive photon-photon production of the $K$-meson pairs has been analyzed within the partonic model of QCD for the kinematical conditions of BELLE experiment. The cross section dependences on partonic masses, charges, and a shape of $K$-meson wave function have been studied for the process under discussion.
The handbag mechanism for wide-angle exlusive scattering reactions is discussed and compared with other theoretical approaches. Its application to Compton scattering, meson photoproduction and two-photon annihilations into pairs of hadrons is reviewed.
We define and study hard ``semi-exclusive'' processes of the form $A+B \to C + Y$ which are characterized by a large momentum transfer between the particles $A$ and $C$ and a large rapidity gap between the final state particle $C$ and the inclusive system $Y$. Such reactions are in effect generalizations of deep inelastic lepton scattering, providing novel currents which probe specific quark distributions of the target $B$ at fixed momentum fraction. We give explicit expressions for photo- and leptoproduction cross sections such as $γp \to πY$ in terms of parton distributions in the proton and…
In a simplified model of Multiple Parton Interactions the inclusive cross sections, of processes with large momentum transfer exchange, acquire the statistical meaning of factorial moments of the distribution in multiplicity of interactions, while more exclusive cross sections, which can provide complementary information on the interaction dynamics, become experimentally viable. Inclusive and exclusive cross sections are linked by sum rules, which can be tested experimentally.
Recently dispersion relations have been applied to hard exclusive processes such as deeply virtual Compton scattering, and a holographic principle was proposed that maps out the generalized parton distributions entering the soft matrix elements for the processes from their values on a given kinematical ridge. We examine possible pitfalls associated with the implicit, direct identification in this approach of the physical hadronic states with colored partons, and suggest an improved treatment of this assumption.
We study the low-virtuality inclusive leptoproduction of open charm, $p^\uparrow l\rightarrow D^0+X$ as a probe of the gluon Sivers function. We perform the analysis in a generalised parton model framework. At leading order, this process is sensitive only to the gluon content of the proton. Hence any detection of a transverse single-spin asymmetry in this process would be clear indication of a non-zero gluon Sivers function (GSF). Considering COMPASS and a future Electron-Ion Collider (EIC), we present predictions for asymmetry using fits for the GSF available in literature. Predictions for pe…
Off-diagonal parton distributions occur in several hard exclusive reactions. They extend the study of hadron structure beyond what can be learned from ordinary distributions and have a particularly rich spin structure. The hard scattering subprocesses in electroproduction of mesons and of real photons satisfy helicity selection rules, which provide powerful tools to test leading-twist dominance at a given value of the hard scale.
The large luminosity and the increased kinematical domain accessible at the LHC will allow to isolate large numbers of events with multiple parton collisions. Interestingly, the hadron is probed in different points contemporarily in the process, which allows to obtain information on the correlations between partons in the hadron structure. The whole experimentally accessible information on multiple parton collisions consists in the probability distribution of the elementary interactions, while the inclusive cross sections, usually considered in large p_t processes, acquire a precise statistica…
Exclusive $π^o$ electroproduction is suggested for extracting both the tensor charge and the transverse anomalous magnetic moment from experimental data. A connection between partonic degrees of freedom, given in terms of Generalized Parton Distributions, and Regge phenomenology is discussed. Calculations are performed using a physically motivated parametrization that is valid at values of the skewness, $ζ\neq 0$. Our method makes use of information from the nucleon form factor data, from deep inelastuc scattering parton distribution functions, and from lattice results on the Mellin moments of…
There are presently two approaches to calculating heavy quark production for the deeply inelastic scattering process in current literature. The conventional fixed-flavor scheme focuses on the flavor creation mechanism and includes the heavy quark only as a final state particle in the hard scattering cross section; this has been computed to next-to-leading order--\alphas^2. The more recently formulated variable-flavor scheme includes, in addition, the flavor excitation process where the initial state partons of all flavors contribute above their respective threshold, as commonly accepted for ca…
We present a scheme for the generation of central exclusive final states in the Pythia 8 program. The implementation allows for the investigation of higher order corrections to such exclusive processes as approximated by the initial-state parton shower in Pythia 8. To achieve this, the spin and colour decomposition of the initial-state shower has been worked out, in order to determine the probability that a partonic state generated from an inclusive sub-process followed by a series of initial-state parton splittings can be considered as an approximation of an exclusive colour- and spin-singlet…
We evaluate perturbative next-to-leading order corrections to the hard exclusive leptoproduction of pi+ mesons on a transversely polarized proton target. A model dependent study shows that these corrections can be large. We analyze the scale dependence and explore the Brodsky-Lepage-Mackenzie scale setting procedure. Although the predictions for the cross section suffer from theoretical uncertainties, the transverse nucleon single spin asymmetry turns out to be a rather stable observable since higher order effects approximately cancel there.
Exclusive emissions of vector mesons in forward directions of rapidity offer us a faultless chance to probe the proton structure at small-$x$. A high-energy factorization formula is established within BFKL, given as the convolution of an impact factor depicting the forward-meson emission and of an unintegrated gluon distribution (UGD) driving the gluon evolution at small-$x$. As a nonperturbative quantity, the UGD is not well known and several models for it exist. We present recent progresses on the study of the exclusive forward $ρ$-meson leptoproduction at HERA and EIC energies, showing how …
We present a phenomenological model for the helicity amplitudes T11 and T00 of the rho meson exclusive diffractive leptoproduction in the forward limit. This model leads to a very good description of the polarized cross-sections sigmaT and sigmaL when compared to HERA data. This model is based on the impact factor representation of the helicity amplitudes. The gamma* -> rho impact factor is computed within the light-cone collinear factorization scheme, the impact parameter space representation allowing to factorize out the dipole-target amplitude. Finally our description combines a model for t…
