Exploring backward pion electroproduction in the scaling regime

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.Comment: Presented at the 7th International Conference on Quark Confinement and Hadron Spectrum (QCHS7), September 2-7 2006, Ponta Delgada, Portugal and at Soft-Pions in hard processes, August 3-5 2006, Regensburg, Germany, 4 pages, LaTeX, uses aip-6s.clo, aipproc.cls and aipxfm.sty (included

Lepton-pair production in ultraperipheral collisions at AFTER@LHC

We investigate the potentialities offered by the study of lepton-pair production in ultraperipheral collisions at a fixed-target experiment using the proton and ion LHC beams. In these collisions, exclusive or semi-exclusive lepton-pair production can be used as luminosity monitor as well as a check of the equivalent-photon approximation, via the measurement of the Bethe-Heitler cross section. It can also serve as a probe of the inner hadron structure via the measurement of the lepton-pair azimuthal asymmetry which is sensitive to the timelike virtual Compton scattering. We also briefly discuss the possibility offered by the study of eta(c) production. Finally, we outline the possibilities for lepton-pair production by Pomeron-Odderon fusion in exclusive pp and pA collisions.Comment: 21 pages, 19 figures, uses jheppub.sty (included

Hard exclusive processes in the backward region

We review the potentialities offered by the study of backward exclusive processes in a new scaling regime, i.e. involving a large -timelike or spacelike- Q2 photon and a baryonic exchange in the t-channel. We recall the concept of Transition Distribution Amplitudes (TDAs) containing unique information on the hadron structure, then discuss how they enter the description of processes such as backward electroproduction of a pion, antiproton-proton annihilations into a dilepton + meson as well as into J/psi + meson. We then discuss first phenomenological studies for processes that are being analysed at JLAB and HERMES or that will be measured by Panda at GSI-FAIR. Finally we present outlooks for their theoretical studies based on approaches such as the pion-cloud model.Comment: Contributed to the 35th International Conference on High Energy Physics -- ICHEP 2010, Paris, France, July 22-28, 2010. 5 pages, 3 figures, uses PoS.cls (included

Hard exclusive electroproduction of a pion in the backward region

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.Comment: 8 pages, 17 figures, RevTeX; v3: update according to an erratum to be published by Phys Rev D : equations (19), (28) and (29) are modified and Fig. 3 corrected accordingly; Conclusions unchange

Single Transverse-Spin Asymmetry in Hard-Exclusive Meson Electroproduction in the Backward Region

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.Comment: Invited talk at the 4th Workshop on Exclusive Reactions at High Momentum Transfer, May 18-21 2010, Thomas Jefferson National Accelerator Facility, Newport News, VA, USA. 8 pages, 2 figures, LaTeX, uses ws-procs9x6-notrimmark.cls (included

On the physical Relevance of the Study of gamma* gamma -> pi0 pi0 at small t and large Q2

We discuss the relevance of a dedicated measurement of exclusive production of a pair of neutral pions in a hard gamma* gamma scattering at small momentum transfer. In this case, the virtuality of one photon provides us with a hard scale in the process, enabling us to perform a QCD calculation of this reaction rate using the concept of Transition Distribution Amplitudes (TDA). Those are related by sum rules to the pion axial form factor F^pi_A and, as a direct consequence, a cross-section measurement of this process at intense beam electron-positron colliders such as CLEO, KEK-B and PEP-II, or Super-B would provide us with a unique measurement of the neutral pion axial form factor F^pi0_A at small scale.Comment: Invited talk at Photon 2009: International Conference On The Structure And Interactions Of The Photon: 18th International Workshop On Photon-Photon Collisions: International Workshop On High Energy Photon Linear Colliders, 11-15 May 2009, Hamburg, Germany. 7 pages, 3 figures, LaTeX, uses desyproc.cls (provided

Spin Observables in Transition-Distribution-Amplitude Studies

Exclusive hadronic reactions with a massive lepton pair (l^+l^-) in the final state will be measured with PANDA at GSI-FAIR and with Compass at CERN, both in p+p-bar -> l^+l^-+pi and pi+N -> N'+l^+l^-. Similarly, electroproduction of a meson in the backward region will be studied at JLAB. We discuss here how the spin structure of the amplitude for such processes will enable us to disentangle various mechanisms. For instance, target-transverse-spin asymmetries are specific of a partonic description, where the amplitude is factorised in terms of baryon to meson or meson to baryon Transition Distribution Amplitudes (TDAs) as opposed to what is expected from baryon-exchange contributions.Comment: Contributed to the 19th International Spin Physics Symposium (SPIN 2010), September 27 - October 2, 2010, Juelich, Germany. 8 pages, 7 figures, uses jpconf.cls, jpconf11.clo, iopams.sty (included

Hard exclusive reactions and hadron structure

The generalized Bjorken regime of exclusive reactions opens new ways to explore the hadron structure. We shortly review the present status of this domain where generalized parton distributions, generalized distribution amplitudes and transition distribution amplitudes describe various aspects of confinement physics.Comment: 7 pages LATEX, 5 figures, invited talk at the Vth Conference on Hadronic Physics held at ICTP, Trieste, Italy, May 200