Hard exclusive processes: theoretical status

Generalized parton distributions: Status and perspectives

We summarize recent developments in understanding the concept of generalized parton distributions (GPDs), its relation to nucleon structure, and its application to high-Q2 electroproduction processes. Following a brief review of QCD factorization and transverse nucleon structure, we discuss (a) new theoretical methods for the analysis of deeply-virtual Compton scattering (t-channel-based GPD parametrizations, dispersion relations); (b) the phenomenology of hard exclusive meson production (experimental tests of dominance of small-size configurations, model-independent comparative studies); (c) …

杰斐逊实验室（Jlab）的硬遍举散射

Hard exclusive scattering at Jlab

该研究回顾了硬遍举过程的不同因子化方案以及这些方案在杰斐逊实验室（Jlab）相关硬遍举散射研究中的应用现状。

Spin effects in hard exclusive reactions

The present status of applications of perturbative QCD to large momentum transfer exclusive reactions is discussed. It is argued that in the region of momentum transfer accessible to present day experiments soft contributions dominate the exclusive processes.

Status of the Exclusive MSSM Higgs production at LHC after the Run I

We investigate the prospects for Central Exclusive Diffractive (CED) production of MSSM Higgs bosons at the LHC using forward proton detectors (FPD) proposed to be installed 220 m and 420 m from ATLAS and CMS detectors. We summarize the situation after the first and very successful data taking period of the LHC. The discovery of a Higgs boson and results from searches for additional MSSM Higgs bosons from the ATLAS and CMS, have recently led to a proposal of new low-energy MSSM benchmark scenarios. The CED signal cross section for the process H -> bb and its backgrounds are estimated in these …

Hard exclusive and semi-exclusive meson production

I review recent theory developments for hard exclusive and semi-exclusive production of mesons, emphasising the variety of physics issues that can be studied in these processes.

DVMP、DVCS 与 GPD 的研究现状

Status of DVMP, DVCS and GPDs

本文综述了手提袋方法下DVMP的分析及GPD参数化，重点讨论H和E，并展示其在DVCS等硬独家过程及Ji求和规则中的应用。

Factorization for Hard Exclusive Electroproduction

This talk summarized the proof of hard-scattering factorization for hard exclusive electroproduction processes: deeply virtual Compton scattering and exclusive meson production.

The Diquark Model for Exclusive Reactions

The present status of the diquark model for exclusive reactions at moderately large momentum transfer is reviewed. That model is a variant of the Brodsky-Lepage approach in which diquarks are considered as quasi-elementary constituents of baryons. Recent applications of the diquark model, relevant to high energy physics with electromagnetic probes, are discussed.

QCD et les processus exclusifs durs (QCD and hard exclusive processes)

We review the theory of hard exclusive reactions in QCD.

Hard exclusive wide-angle processes

In this talk the handbag approach to hard exclusive wide-angle processes is reviewed and applications, as for instance two-photon annihilations into pairs of mesons, are discussed.

Hard exclusive processes and higher-order QCD corrections

The short review of the higher order corrections to the hard exclusive processes is given. Different approaches are discussed and the importance of higher-order calculations is stressed.

HEPGEN - generator for hard exclusive leptoproduction

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.

Central exclusive production at RHIC

The present status and and future plans of the physics program of Central Exclusive Production at RHIC are described.

What is Measured in Hard Exclusive Electroproduction?

We examine the relation between amplitudes measured in exclusive lepto-production and the quark content of the nucleon. We show that in the limit of high energy and small t, the natural interpretation of amplitudes measured in these hard exclusive processes is in terms of the quark content of the meson cloud and not the target itself. In this regime, Regge amplitudes will make a significant contribution to these exclusive amplitudes. This leads to violation of QCD scaling.

Dispersion representations for hard exclusive processes

Several hard exclusive scattering processes admit a description in terms of generalized parton distributions and perturbative hard-scattering kernels. Both the physical amplitude and the hard-scattering kernels fulfill dispersion relations. We give a detailed investigation of their consistency at all orders in perturbation theory. The results shed light on the information about generalized parton distributions that can be extracted from the real and imaginary parts of exclusive amplitudes. They also provide a practical consistency check for models of these distributions in which Lorentz invari…

Soft Pion Emission in Hard Exclusive Pion Production

We investigate hard exclusive reactions on the nucleon with soft pion emission. A parametrization of corresponding hadronic matrix elements in terms of parton distributions for final pion-nucleon state is provided. These distributions are calculated in terms of nucleon and pion GPDs and the pion distribution amplitude via soft-pion theorems. Some observables for the process of hard charged pion production on the proton with soft pion emission are computed.

One Loop Chiral Corrections to Hard Exclusive Processes:I. Pion Case

We computed the leading non-analytic chiral corrections to the generalized parton distributions (GPDs) of the pion and to the two-pion distribution amplitudes. This allows us to obtain the corresponding corrections for the hard exclusive processes, such as gamma^* gamma -> pi pi, gamma^* N -> 2pi N' and deeply virtual Compton scattering on the pion target.

Dispersion representations for hard exclusive reactions

A number of hard exclusive scattering processes can be described in terms of generalized parton distributions (GPDs) and perturbative hard-scattering kernels. Both the physical amplitude and the hard-scattering kernels fulfill dispersion relations. We show that their consistency at all orders in perturbation theory is guaranteed if the GPDs satisfy certain integral relations. These relations are fulfilled thanks to Lorentz invariance.

Phenomenology of the exclusive rare semileptonic decay B -> K pi l^+ l^-

I review the phenomenology of exclusive Bbar -> Kbar pi l^+ l^- decays, with focus on the intermediate resonant Kbar^*(892) mode. The status of angular observables at both ends of the dilepton mass spectrum is revisited. Constraints on the b -> s Wilson coefficients C_7,C_9,C_10 based on 2011 data from the B factories, CDF and LHCb are presented.

Exclusive Hard Diffraction at HERA (DVCS and Vector Mesons)

Recent results obtained at HERA on deeply virtual Compton scattering and exclusive vector meson production are reviewed, with the emphasis on the transition from soft to hard diffraction and on spin dynamics.

Hard Exclusive and Diffractive Processes in QCD

Exclusive and semi-exclusive processes, the diffractive dissociation of hadrons into jets, and hard diffractive processes such as vector meson leptoproduction provide new testing grounds for QCD and essential information on the structure of light-cone wavefunctions of hadrons, particularly the pion distribution amplitude. I review the basic features of the leading-twist QCD predictions and the problems and challenges of studying QCD at the amplitude level. The application of the light-cone formalism to the exclusive semi-leptonic decay of heavy hadrons is also discussed.

硬遍举介子电产生中的自旋效应

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。

On the role of hard rescattering in exclusive diffractive Higgs production

We discuss the contribution of so-called semi-enhanced hard rescattering corrections to central exclusive diffractive Higgs production, pp -> p+H+p, at the LHC. We present arguments to show that these corrections are small. We confirm these expectations by considering HERA data for leading neutron production.

Lepton-pair production in hard exclusive hadron-hadron collisions

We investigate lepton-pair production in hard exclusive hadron-hadron collisions. We consider a double handbag (DH) mechanism in which the process amplitude factorizes in hard subprocesses, qq -> qq gamma* and qg -> qg gamma*, and in soft hadron matrix elements parameterized as generalized parton distributions (GPDs). Employing GPDs extracted from exclusive meson electroproduction, we present predictions for the lepton-pair cross section at kinematics typical for the LHC, NICA and FAIR. It turns out from our numerical studies that the quark-gluon subprocess dominates by far, the quark-quark (a…

Status of Hard Interactions (Jets and Heavy Flavor)

We review the status of hard interactions, in particular of jet and heavy flavor production, at HERA and LEP. Emphasis is given to recent theoretical developments. Instantons, event shapes, and prompt photons are also briefly discussed.

Hard exclusive photoproduction of charmed mesons

{We investigate the photoproduction process $ p γ\rightarrow Λ_{c}^{+} \overline{D^{0}}$ within the handbag approach, which we assume to be the dominant mechanism at energies well above the production threshold and in the forward scattering hemisphere.

Dispersion relations and subtractions in hard exclusive processes

We study analytical properties of the hard exclusive processes amplitudes. We found that QCD factorization for deeply virtual Compton scattering and hard exclusive vector meson production results in the subtracted dispersion relation with the subtraction constant determined by the Polyakov-Weiss $D$-term. The relation of this constant to the fixed pole contribution found by Brodsky, Close and Gunion and defined by parton distributions is proved, while its manifestation is spoiled by the small $x$ divergence. The continuation to the real photons limit is considered and the numerical corresponde…

Hard Exclusive Meson Production and Nonforward Parton Distributions

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 Status and Future of Color Transparency and Nuclear Filtering

40 years after its introduction, the phenomenon of color transparency remains a domain of controversial interpretations of experimental data. We review present evidence for or against its manifestation in various exclusive hard scattering reactions. The nuclear transparency experiments reveal whether short distance processes dominate a scattering amplitude at some given kinematical point. We plead for a new round of nuclear transparency measurements in a variety of experimental set-ups, including near-forward exclusive reactions related to generalized parton distribution (GPD) physics and near…

Hard Exclusive Production of Tensor Mesons

We point out that hard exclusive production of tensor mesons $f_2(1270)$ with helicity $λ=\pm 2$ is dominated by the gluon component in the meson wave function and can be used to determine gluon admixture in tensor mesons in a theoretically clean manner. We present a detailed analysis of the tensor meson distribution amplitudes and calculate the transition form factor $γ+γ^*\to f_2(1270)$ for one real and one virtual photon.

Neutrino induced hard exclusive D_s production

Motivated by the possibility to use high intensity neutrino beams for neutrino-nucleon scattering experiments we analyze charged current induced exclusive meson production within the framework of generalized parton distributions. The cross section for hard exclusive D_s production is estimated in this formalism to leading order in QCD. The integrated cross section proves to be sizable. It is shown that the considered process is well suited to provide novel information on the gluon structure of nucleons, which is contained in the generalized gluon parton distribution.

Hard Exclusive QCD Processes at the Linear Collider

The next generation of $e^+e^-$ colliders will offer a possibility of clean testing of QCD dynamics. Recent progress in the theoretical description of exclusive processes permits for many of them a consistent use of the perturbative QCD methods. We find that already on the basis of Born approximation, the exclusive diffractive production of two $ρ$ mesons from virtual photons at very high energies should be measurable at the linear collider (LC).

Diquarks as effective particles in hard exclusive scattering

In the context of hard hadronic reactions diquarks are a useful phenomenological device to model non-perturbative effects still observable in the kinematic range accessible by present-day experiments. In the following we present diquark-model predictions for $γγ\to p \bar{p}$ and $Λ\barΛ$. We also sketch how the (pure quark) hard-scattering formalism for exclusive reactions involving baryons can be reformulated in terms of quarks and diquarks. As an application of these considerations we analyze the magnetic proton form factor with regard to its quark-diquark content.

The Use of Dispersion Relations in Hard Exclusive Processes and the Partonic Interpretation of Deeply Virtual Compton Scattering

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.

Exclusive photoproduction of hard dijets and magnetic susceptibility of QCD vacuum

We argue that coherent production of hard dijets by linearly polarized real photons can provide direct evidence for chirality violation in hard processes, the first measurement of the magnetic susceptibility of the quark condensate and the photon distribution amplitude. It can also serve as a sensitive probe of the generalized gluon parton distribution. Numerical calculations are presented for HERA kinematics.

Hard exclusive production of a vector meson

The processes of a light neutral vector meson, V=ρ^0, ω, ϕ, electroproduction and a heavy quarkonium, V=J/Ψ, Υ, photoproduction are studied in the framework of QCD factorization. We derive a complete set of hard-scattering amplitudes which describe these processes at next-to-leading order (NLO).

Transversity Distribution Does Not Contribute to Hard Exclusive Electroproduction of Mesons

We show that in hard exclusive electroproduction, ep-->eVp, the leading-twist hard-scattering coefficient for the production of a transversely polarized vector meson V vanishes to all orders of perturbation theory. This implies that this process cannot be used to measure the skewed transversity distribution of quarks in a hadron. In contrast, a recent calculation obtained a non-zero value at NLO. We show that this calculation is incorrect because it failed to include the necessary collinear subtractions. Our method of proof also applies to other processes whose hard-scattering coefficients are…

Angular distributions in hard exclusive production of pion pairs

Using the leading order amplitudes of hard exclusive electroproduction of pion pairs we have analyzed the angular distribution of the two produced particles. At leading twist a pion pair can be produced only in an isovector or an isoscalar state. We show that certain components of the angular distribution only get contributions from the interference of the I=1 and the (much smaller) I=0 amplitude. Therefore our predictions prove to be a good probe of isospin zero pion pair production. We predict effects of a measurable size that could be observed at experiments like HERMES. We also discuss how…

Hard Pomeron in exclusive meson production at ILC

We calculate the exclusive process gamma* gamma* ->rho rho at high energy. The Born order estimate and the leading (LLA) and next to leading order (NLLA) BFKL resummation effects show the feasibility of experimental detection in a quite large range of Q2 values at future high energy e+e- linear colliders.

Hard exclusive $J/ψ$ leptoproduction on polarized targets

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.

Hard exclusive photoproduction of $Φ$ and $J/Ψ$ mesons

We present predictions for differential cross sections for the reaction $γp \to Φp$ and give an outlook to which extent our calculations may be generalized to the photoproduction of $J/Ψ$ mesons. Our results are obtained within perturbative QCD treating the proton as a quark-diquark system.

Exclusive Hadronic Processes and Color Transparency

We review the current status of high energy exclusive processes and color transparency.

Hard exclusive electroproduction of decuplet baryons in the large N_c limit

The cross sections and transverse spin asymmetries in the hard exclusive electroproduction of decuplet baryons are calculated in the large N_c limit and found to be comparable to that of octet baryons. Large N_c selection rules for the production amplitudes are derived, leading to new sensitive tests of the spin aspects of the QCD chiral dynamics both in the nonstrange and strange sectors. Importance of such studies for the reliable extraction of the pion form factor from pion electroproduction is explained.

Hard exclusive reactions and the two-gluon components of eta and eta' mesons

The formalism for treating the leading-twist contributions of the two-gluon Fock components occurring in hard exclusive processes that involve eta and eta' mesons is reviewed. The calculation of the eta, eta'--photon transition form factor in next-to-leading order in alpha_s, as well as, the analysis of the g* g* eta' vertex and the electro- and photoproduction of eta, eta' mesons are presented. Applications of this formalism to other relevant quantities such as glueballs are also discussed.

Hard Exclusive Photoproduction of Phi and J/Psi Mesons

We calculate the leading-order perturbative contribution to $γp \to M_{V} p$, with $M_V$ being a $Φ$ or $J/Ψ$ meson, in the kinematic region of large energy and scattering angle.

Analytic Properties of Hard Exclusive Amplitudes

Analytic properties of hard exclusive processes described by Generalized Parton Distributions (GPD's) are considered. The analytic continuation of GPD is provided by Generalized Distribution Amplitudes (GDA). The GDA's for the production of two $ρ-$mesons may give an access to four-quark exotic states. The crucial role in the proof of analyticity is played by the Cavalieri conditions (polynomiality), resulting in the "holographic" property of GPD, when the full information about various hard processes is contained in the one dimensional sections ($x=\pm ξ$)of GPD. The applicability of analytic…

Diquark model predictions for photon induced exclusive reactions

The present status of the diquark model for exclusive reactions at moderately large momentum transfer is reviewed. That model is a variant of the Brodsky-Lepage approach in which diquarks are considered as quasi-elementary constituents of baryons. Recent applications of the diquark model, relevant to high energy physics with electromagnetic probes, are discussed: electromagnetic form factors of baryons in both the space-like and the time-like region, photoproduction of mesons, two-photon annihilations into proton-antiproton pairs as well as real and virtual Compton scattering on which the main…

Hard exclusive production of two pions: Dipion mass distributions in gamma* N -->pi pi N and gamma* gamma --> pi pi

The leading twist parametrization of dipion mass distribution in hard exclusive reactions is proposed. Its parameters are related to quark distributions (usual and skewed) in the pion and to distributions amplitudes of mesons ($π$, $ρ$, etc.). We show that measurements of the shape of dipion mass distribution in hard exclusie reactions can give important information about partonic structure of the pion. The expression for the amplitude of the reaction $γ^*γ\to ππ$ near the threshold in terms of singlet quark distribution in the pion is presented.

Hard Exclusive Electroproduction of Pseudoscalar Mesons and QCD axial anomaly

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.

On hard exclusive reactions in the time-like region

The proton form factor, two-photon annihilations into $p\bar{p}$ as well as exclusive charmonium decays are critically examined. It will be argued that the standard perturbative QCD analysis of these reactions fails, i.e. the need for additional contributions can convincingly be demonstrated. Possible dynamical mechanisms such as colour-octet admixtures to the charmonium states or diquarks inside baryons, will be discussed and compared to the data.

Subprocess Size in Hard Exclusive Scattering

The interaction region of hard exclusive hadron scattering can have a large transverse size due to endpoint contributions, where one parton carries most of the hadron momentum. The endpoint region is enhanced and can dominate in processes involving multiple scattering and quark helicity flip. The endpoint Fock states have perturbatively short lifetimes and scatter softly in the target. We give plausible arguments that endpoint contributions can explain the apparent absence of color transparency in fixed angle exclusive scattering and the dimensional scaling of transverse rho photoproduction at…

Current status of $\varepsilon_K$ in lattice QCD

We present the current status of $\varepsilon_K$ evaluated directly from the standard model using lattice QCD inputs. The lattice QCD inputs include $\hat{B}_K$, $ξ_0$, $ξ_2$, $|V_{us}|$, $m_c(m_c)$, and $|V_{cb}|$. Recently, FLAG has updated $\hat{B}_K$, exclusive $|V_{cb}|$ has been updated with new lattice data in the $\bar{B}\to D\ell\barν$ decay mode, and RBC-UKQCD has updated $ξ_0$ and $ξ_2$. We find that the standard model evaluation of $\varepsilon_K$ with exclusive $|V_{cb}|$ (lattice QCD inputs) is $3.2σ$ lower than the experimental value, while that with inclusive $|V_{cb}|$ (heavy …

Hard Exclusive Electroproduction of Pions

We investigate the exclusive electroproduction of pi^+ mesons from nucleons. To leading twist, leading order alpha_s accuracy the corresponding production amplitude can be decomposed into pseudovector and pseudoscalar parts. Both can be expressed in terms of quark double distribution functions of the nucleon. While the pseudovector contribution is connected to ordinary polarized quark distributions, the pseudoscalar part can be related to one pion t-channel exchange. We observe that the pseudovector part of the production amplitude is important at x < 0.1. On the other hand, for 0.1 < x < 0.4 …

Exclusive electroproduction and off-diagonal parton distributions

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.