T. Gousset

PROBING PARTONIC STRUCTURE IN GAMMA GAMMA PI PI NEAR THRESHOLD

Hadron pair production gamma* gamma -> h hbar in the region where the c.m. energy is much smaller than the photon virtuality can be described in a factorized form, as the convolution of a partonic handbag diagram and generalized distribution amplitudes which are new non-perturbative functions describing the exclusive fragmentation of a quark-antiquark pair into two hadrons. Scaling behavior and a selection rule on photon helicity are signatures of this mechanism. The case where h is a pion is emphasized.

Timelike form factors at high energy.

The difference between the timelike and spacelike meson form factors is analyzed in the framework of perturbative QCD with Sudakov effects included. It is found that integrable singularities appear but that the asymptotic behavior is the same in the timelike and spacelike regions. The approach to asymptotia is quite slow and a rather constant enhancement of the timelike value is expected at a measurable large

Exclusive electroproduction of vector mesons and transversity distributions

{\mathit{Q}}^{2}

Hadron helicity violation in exclusive processes: Quantitative calculations in leading order QCD

. This is in agreement with the trend shown by experimental data.

Probing Partonic Structure in γ * γ → π π near Threshold

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.

Exclusive hadronization in gamma* gamma -> pi pi

We study a new mechanism for hadronic helicity flip in high energy hard exclusive reactions. The mechanism proceeds in the limit of perfect chiral symmetry, namely, without any need to flip a quark helicity. The fundamental feature of the new mechanism is the breaking of rotational symmetry of the hard collision by a scattering plane in processes involving independent quark scattering. We show that in the impulse approximation there is no evidence for strong suppression of the helicity-violating process as the energy or momentum transfer {ital Q}{sup 2} is increased over the region 1 GeV{sup 2}{lt}{ital Q}{sup 2}{lt}100 GeV{sup 2}. In the asymptotic region {ital Q}{sup 2}{gt}1000 GeV{sup 2}, a saddle-point approximation yields suppression by a fraction of a power of {ital Q}{sup 2}. {open_quote}{open_quote}Chirally odd{close_quote}{close_quote} exclusive wave functions which carry a nonzero orbital angular momentum, and yet are leading order in the high energy limit, play an important role. {copyright} {ital 1996 The American Physical Society.}

Deeply virtual Compton scattering: How to test handbag dominance?

Hadron pair production gamma* gamma -> h hbar in the region where the c.m. energy is much smaller than the photon virtuality can be described in a factorized form, as the convolution of a partonic handbag diagram and generalized distribution amplitudes which are new non-perturbative functions describing the exclusive fragmentation of a quark-antiquark pair into two hadrons. Scaling behavior and a selection rule on photon helicity are signatures of this mechanism. The case where h is a pion is emphasized.

Probing Partonic Structure inγ*γ→ππnear Threshold

We perform a QCD analysis of exclusive two-meson production in gamma* gamma collisions in the kinematical domain of large photon virtuality Q^2 and small hadronic invariant mass W^2. In these kinematics the amplitude factorizes into a perturbative subprocess gamma* gamma -> q qbar and a two-meson distribution amplitude. This opens the way to study new hadronic matrix elements related with the physics of hadronization and confinement.

Hard exclusive scattering in QCD

We propose detailed tests of the handbag approximation in exclusive deeply virtual Compton scattering. Those tests make no use of any prejudice about parton correlations in the proton which are basically unknown objects and beyond the scope of perturbative QCD. Since important information on the proton substructure can be gained in the regime of light cone dominance we consider that such a class of tests is of special relevance.

Polarisation in deeply virtual meson production

Hadron pair production gamma* gamma -> h hbar in the region where the c.m. energy is much smaller than the photon virtuality can be described in a factorized form, as the convolution of a partonic handbag diagram and generalized distribution amplitudes which are new non-perturbative functions describing the exclusive fragmentation of a quark-antiquark pair into two hadrons. Scaling behavior and a selection rule on photon helicity are signatures of this mechanism. The case where h is a pion is emphasized.