arXiv subjects
Publications and source records attributed to B. Pire.
Off-Forward Parton Distributions (OFPD's) are new hadronic objects which may be measured in various exclusive reactions. We derive non-trivial positivity constraints for them that should allow to get extra restrictions for model inputs.
Final state interactions in quark fragmentation may give origin to non zero values of the off-diagonal element rho_(1,-1) of the helicity density matrix of vector mesons V produced in current jets, with a large energy fraction x_E; the value of rho_(1,-1)(V) is related to the hard constituent dynamics and tests unusual properties of it. Some recent data on phi, K^* and D^* produced in e^+ e^- annihilations at LEP show such effects. Predictions are given here for rho_(1,-1) of heavy mesons produced in nucleon-nucleon, gamma-nucleon and lepton-nucleon interactions.
The spin structure of nucleon at twist 2 and 3 levels is analyzed. The contribution of quark and gluon spins to nucleon spin are Lorentz invariant, while it is not sure for orbital angular momentum. The conserved fractional moments of transversity distribution are considered. The scenario of decoupled total angular momentum, determined predominantly by the unpolarized scattering, is discussed.
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.
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.
We discuss the handbag approximation to exclusive deep virtual Compton scattering. After defining the kinematical region where this approximation can be valid, we propose tests for its relevance in planned electroproduction experiments, e + p -> e + p + gamma. We focus on scaling laws in the cross section, and the distribution in the angle between the lepton and hadron planes, which contains valuable information on the angular momentum structure of the Compton process. We advocate to measure weighted cross sections, which make use of the data in the full range of this angle and do not require very high event statistics.
We review the theory of hard exclusive reactions in QCD.
This is a pedagogical review on polarized structure functions .
Determining how much spin is carried by gluon in a polarized proton is a fundamental problem which cannot be resolved by completely inclusive deep inelastic measurements. Hadroproduction of heavy flavors is very sensitive to the gluon content of hadrons. We show that $χ_2 (3555)$ production in polarized proton-proton collisions is a good candidate reaction to address this challenging question.
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 of the helicity violating process as the energy or momentum transfer $Q^2$ is increased over the region 1 GeV^2 < Q^2 < 100 GeV^2. In the asymptotic region Q^2> 1000 GeV^2, a saddle point approximation with doubly logarithmic accuracy yields suppression by a fraction of power of Q^2. ``Chirally--odd" exclusive wave functions which carry non--zero orbital angular momentum and yet are leading order in the high energy limit, play an important role.
The difference between the timelike and spacelike meson form factors is analysed 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 measurable large \(Q^{2}\). This is in agreement with the trend shown by experimental data.