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

Exclusive neutrino-production of a charmed meson

We calculate the leading order in $\alpha_s$ QCD amplitude for exclusive neutrino and antineutrino production of a $D$ pseudoscalar charmed meson on an unpolarized nucleon. We work in the framework of the collinear QCD approach where generalized parton distributions (GPDs) factorize from perturbatively calculable coefficient functions. We include both $O(m_c)$ terms in the coefficient functions and $O(M_D)$ mass term contributions in the heavy meson distribution amplitudes. We emphasize the sensitivity of specific observables on the transversity quark GPDs.Comment: 17 pages,16 figures. Version 2, to be published in Phys Rev D, misprint corrected in unnumbered equation after eq.43, plots corrected after numerical bug repaire

Hard exclusive neutrino production of a light meson

We update the leading order in $\alpha_s$ QCD amplitude for deep exclusive neutrino and antineutrino production of a light meson on an unpolarized nucleon. The factorization theorems of the collinear QCD approach allow us to write the amplitude as the convolution of generalized parton distributions (GPDs) and perturbatively calculable coefficient functions. We study both the pseudoscalar meson and longitudinally polarized vector meson cases. It turns out that, contrarily to the electroproduction case, the leading twist scattering amplitudes for $\pi$ and $\rho_L$ productions are proportional to one another, which may serve as an interesting new test of the leading twist dominance of exclusive processes at medium scale. The dominance of the gluonic contribution to most cross sections is stressed.Comment: 9 pages, 8 figures. second version (to be published in Phys Rev D): misprint in name of second author corrected; one added reference; minor misprints corrected. arXiv admin note: text overlap with arXiv:1702.0031

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

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

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

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