arXiv subjects
Publications and source records attributed to B. Pire.
The QCD analysis of the recent experimental data (L3@LEP) of the hard exclusive $ρρ$ production in two photon collisions shows that these data can be understood as a signal for the existence of an exotic isotensor resonance with a mass around $1.5 {\rm GeV}$. We also argue that hard exclusive reactions are a powerful tool for an experimental study of exotic hybrid mesons with $J^{PC}=1^{-+}$.
We estimate the sizeable cross section for deep exclusive electroproduction of an exotic $J^{PC}=1^{-+}$ hybrid meson in the Bjorken regime. The production amplitude scales like the one for usual meson electroproduction, {\it i.e.} as $1/Q^2$. This is due to the non-vanishing leading twist distribution amplitude for the hybrid meson, which may be normalized thanks to its relation to the energy momentum tensor and to the QCD sum rules technique. The hard amplitude is considered up to next--to--leading order in $α_{S}$ and we explore the consequences of fixing the renormalization scale ambiguity through the BLM procedure.
We analyse deeply-virtual Compton scattering on a proton target, gamma* P -> P' gamma in the backward region and in the scaling regime. We define the transition distribution amplitudes which describe the proton to photon transition. Model-independent predictions are given to test this description, for current or planned experiments at JLab or by Hermes.
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.
We calculate the lowest order QCD amplitude, i.e. the quark exchange contribution, to the forward production amplitude of a pair of longitudinally polarized $ρ$ mesons in the scattering of two virtual photons $γ^*(Q_1) γ^*(Q_2) \to ρ^0_L ρ^0_L$. We show that the scattering amplitude simultaneously factorizes in two quite different ways: the part with transverse photons is described by the QCD factorization formula involving the generalized distribution amplitude of two final $ρ$ mesons, whereas the part with longitudinally polarized photons takes the QCD factorized form with the $γ^*_L \to ρ^0_L$ transition distribution amplitude. Perturbative expressions for these, in general, non-perturbative functions are obtained in terms of the $ρ-$meson distribution amplitude.
The chiral-odd generalized parton distribution (GPD), or transversity GPD, of the nucleon can be accessed experimentally through the photo- or electroproduction of two vector mesons on a polarized nucleon target, γ^(*) N --> ρ_1 ρ_2 N', where ρ_1 is produced at large transverse momentum, ρ_2 is transversely polarized, and the mesons are separated by a large rapidity gap. We predict the cross section for this process for both transverse and longitudinal ρ_{2} production. To this end we propose a model for the transversity GPD H_T(x,ξ,t), and give an estimate of the relative sizes of the transverse and longitudinal ρ_{2} cross sections. We show that a dedicated experiment at high energy should be able to measure the transversity content of the proton.
We study the exclusive production of pi pi and rho pi in hard gamma* gamma scattering in the forward kinematical region where the virtuality of one photon provides us with a hard scale in the process. The newly introduced concept of Transition Distribution Amplitudes (TDA) is used to perform a QCD calculation of these reactions thanks to two simple models for TDAs. Cross sections for rho pi and pi pi production are evaluated and compared to the possible background from the Bremsstrahlung process. This picture may be tested at intense electron-positron colliders such as CLEO and B factories. The cross section e gamma -> e' pi0 pi0 is finally shown to provide a possible determination of the pi0 axial form factor, FA, at small t, which seems not to be measurable elsewhere.
We present a theoretical study of exotic hybrid meson (JPC=1-+) production in photon-photon collisions where one of the photons is deeply virtual, including twist 2 and twist 3 contributions. We calculate the cross section of this process which turns out to be large enough to imply sizeable counting rates in the present high luminosity electron-positron colliders. We emphasize theimportance of the pi eta decay channel for the detection of the hybrid meson candidate pi_1(1400) and calculate the cross section and the angular distribution for pi eta pair production in the unpolarized case. This angular distribution is a useful tool for disentangling the hybrid meson signal from the background. Finally, we calculate the single spin asymmetry associated with one initial longitudinally polarized lepton.
The QCD analysis of the hard exclusive production of $ρ^+ρ^-$ and $ρ^0ρ^0$ mesons in two photon collisions shows that the recent experimental data obtained by the L3 Collaboration at LEP can be understood as a signal for the existence of an exotic isotensor resonance with a mass around $1.5 {\rm GeV}$.
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.
A QCD analysis of the reactions A B -> gamma^* gamma and gamma^* gamma -> A B in the forward region shows that they can be factorized in a way which is quite similar to the frameword developped for deeply virtual Compton scattering. The generalized parton distributions (GPDs) related to this latter process being replaced by new non perturbative hadronic matrix elements, the transition distribution amplitudes (TDAs).
We argue that the electroproduction of exotic particles is a useful tool for study of their partonic structure. In the case of hybrid mesons, the magnitude of their cross sections shows that they are accessible for measurements in existing electroproduction experiments.
We study the scaling regime of nucleon-anti-nucleon annihilation into a deeply virtual photon and a photon or meson, \bar p N -> gamma ^* pi, \bar p N -> gamma ^* gamma, in the forward direction. The leading twist amplitude factorizes into an antiproton distribution amplitude, a short-distance matrix element and a long-distance dominated transition distribution amplitude (TDA) which describes the nucleon to meson or photon transition. The impact representation of this TDA maps out the transverse locations of the small size core and the meson or photon cloud inside the proton.
Exotic hybrid mesons H, with quantum numbers J^{PC}= 1^{- +} may be copiously produced in the hard exclusive processes gamma^*(Q^2) gamma -> H and gamma^*(Q^2) P(p) -> H P(p') because they have a leading twist distribution amplitude with a sizable coupling constant f_H, which may be estimated through QCD sum rules. The reaction rates scale in the same way as the corresponding rates for usual mesons.
We calculate the cross-section for the diffractive exclusive process gamma*_L (Q_1^2) gamma*_L(Q_2^2) -> rho^0_L rho^0_L, in view of its study in the future high energy e+e- linear collider. The Born order approximation of the amplitude is completely calculable in the hard region Q_1, Q_2 >> Lambda_QCD. The resulting cross-section is large enough for this process to be measurable with foreseen luminosity and energy, for Q_1 and Q_2 in the range of a few GeV.
We present a first estimate of the cross-section for the exclusive process gamma^*_L (Q_1^2) gamma^*_L(Q_2^2) -> rho^0_L rho^0_L, which will be studied in the future high energy e^+ e^- linear collider. As a first step, we calculate the Born order approximation of the amplitude for longitudinally polarized virtual photons and mesons, in the kinematical region s >> -t, Q_1^2, Q_2^2. This process is completely calculable in the hard region Q_1^2, Q_2^2 >> Lambda^2_{QCD}. We perform most of the steps in an analytical way. The resulting cross-section turns out to be large enough for this process to be measurable with foreseen luminosity and energy, for Q_1^2 and Q_2^2 in the range of a few GeV^2.
We present a theoretical estimate for the cross-section of exclusive $ρ^+ρ^-$ and $ρ^0ρ^0$-meson production in two photon collisions when one of the initial photons is highly virtual. We focus on the discussion of the twist 4 contributions which are related to the production of an exotic isospin 2 resonance of two $ρ$ mesons. Our analysis shows that the recent experimental data obtained by the L3 Collaboration at LEP can be understood as a signal for the existence of an exotic isotensor resonance with a mass around $1.5 {\rm GeV}$.
We study the scaling regime of hadron-(anti)-hadron annihilation into a deeply virtual photon and a real photon, H anti-H -> gamma^* gamma, and deep backward virtual Compton scattering, gamma^* H -> H gamma. We advocate that there is a kinematical region where the scattering amplitude factorizes into a short-distance matrix element and a long-distance dominated object: a transition distribution amplitude which describes the hadron to photon transition.