arXiv subjects
Publications and source records attributed to B. Pire.
The generalized parton distributions and the generalized distribution amplitudes give access to a deeper understanding of the quark and gluon content of hadrons. In this short review, we select some new developments of their interesting connections with the physics information that one can extract from exclusive reactions at medium and high energies.
We study Pomeron-Odderon interference effects giving rise to charge and single-spin asymmetries in diffractive electroproduction of a $π^+ π^-$ pair. We calculate these asymmetries in the Born approximation of QCD in a kinematical domain accessible to HERA experiments.
We review the results of our studies on the charge and the spin asymmetries in the electroproduction of $π^+ π^-$ which are the observables sensitive to the Pomeron-Odderon interference.
We study Pomeron-Odderon interference effects giving rise to charge and single-spin asymmetries in diffractive electroproduction of a pi+ pi- pair. We calculate these asymmetries originating from both longitudinal and transverse polarizations of the virtual photon in the framework of QCD and in the Born approximation, in a kinematical domain accessible to HERA experiments. We predict a sizable charge asymmetry with a characteristic dependence on the invariant mass of the pi+ pi- pair, which makes this observable very important for establishing the magnitude of the Odderon exchange in hard processes. The single spin asymmetry turns out to be rather small. We briefly discuss future improvements of our calculations and their possible effects on the results.
The interference of Pomeron and Odderon amplitudes gives rise to a charge asymmetry in the diffractive electroproduction of a pi+pi- -pair. We calculate this charge asymmetry in perturbative QCD in the Born approximation and on the leading Q^2-level. The numerical evaluation shows a sizeable asymmetry in an experimentally accessible kinematical region. We find a characteristic m_{pi+pi} -dependence mainly dictated by the relevant Breit-Wigner-amplitudes and the corresponding phase-shifts.
Charge asymmetries in diffractive electroproduction of two mesons are proportional to the interference of Pomeron and Odderon exchange amplitudes. We calculate in the framework of QCD and in the Born approximation a forward-backward charge asymmetry which turns out to be sizable in a kinematical domain accessible to HERA experiments. We predict a distinctive dependence of this asymmetry on the invariant mass of the two pions. Testing this prediction is a crucial step in the discovery of the QCD-Odderon.
We study deeply virtual Compton scattering (DVCS) on a deuteron target. We model the Generalized Quark Distributions in the deuteron by using the impulse approximation for the lowest Fock-space state. Numerical predictions are given for the unpolarized cross sections for the kinematical regimes relevant for JLab and HERMES at HERA. Differential cross sections show the same pattern as for the proton case and at low values of t they are of comparable size.
We study the exclusive reactions pi- p -> l+ l- n and pi+ n -> l+ l- p$ in view of possible future experiments with high-intensity pion beams. For large invariant mass of the lepton pair l+ l- and small squared momentum transfer to the nucleon these are hard-scattering processes providing access to generalized parton distributions. We estimate the cross section for these reactions, explore their connection with the pion form factor, and discuss the role they can play in improving our understanding of the relevant reaction mechanisms.
We investigate the exclusive photoproduction of a heavy timelike photon which decays into a lepton pair, gamma p -> l+ l- p. This can be seen as the analog of deeply virtual Compton scattering, and we argue that the two processes are complementary for studying generalized parton distributions in the nucleon. In an unpolarized experiment the angular distribution of the leptons readily provides access to the real part of the Compton amplitude. We estimate the possible size of this effect in kinematics where the Compton process should be dominated by quark exchange.
We introduce generalized quark and gluon distributions in the deuteron, which can be measured in exclusive processes like deeply virtual Compton scattering and meson electroproduction. We discuss the basic properties of these distributions, and point out how they probe the interplay of nucleon and parton degrees of freedom in the deuteron wave function.
We critically examine recently published results on the thermal production of massive vector bosons in a quark-gluon plasma. We claim the production rate is a collinear safe observable.
The $γ^* γ\to πππ$ reaction is analysed in QCD at large photon virtuality Q and small hadronic mass W. Its amplitude factorizes in a $3 π$ generalized distribution amplitude and a hard subprocess $γ^* γ\to q \bar q$ or $γ^* γ\to g g$. The interference of this process with a Brehmsstrahlung process in e e or $e γ$ collisions gives rise to interesting single spin asymmetries, particularly when W is near the $ω$ mass.
We analyze in detail the problem of gauge invariance of the deeply vitual Compton scattering (DVCS) amplitude. Using twist 3 one-gluon exchange diagram contributions and the QCD equations of motion, we derive the general gauge invariant expression of the DVCS amplitude on a (pseudo)scalar particle (pion, $He^4$). Similarly to the case of deep inelastic scattering, the amplitude does not depend on the twist-3 quark-gluon correlations at the Born level. The contribution of the derived amplitude to the single-spin asymmetry with longitudinally polarized lepton is calculated.
We discuss two aspects of polarisation in hard exclusive meson production: the leading-twist selection rule for the meson helicity, and the different partial waves of a (pi pi)-pair which may or may not be due to the decay of a rho.
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.
Using the similarity between twist 3 contributions to transverse spin for usual parton distributions and transverse momentum for non-diagonal parton distributions, we determine for the simple case of a scalar (pion, $He^4$) target the linear in transverse momentum twist-3 contribution to Deeply Virtual Compton amplitude. This allows to write a gauge invariant amplitude for this non-forward process.
Exclusive two-pion production near threshold in the collision of a highly virtual photon with a real one offers a new possibility to unravel the partonic content of hadrons. We explain the dynamics of this regime, i.e. the separation of the amplitude in terms of partonic diagrams computable in QCD perturbation theory and generalized distribution amplitudes, which are nonperturbative functions describing the exclusive transition of a parton pair into two hadrons. The same quantities also appear in gamma* N -> pi pi N in the kinematical regime where the momentum transfer of the nucleon and the invariant mass of the pion pair are both small; this allows to extend the QCD analysis of exclusive rho electroproduction outside the resonance region of the two produced pions.
Off-diagonal parton distributions occur in several hard exclusive reactions. They extend the study of hadron structure beyond what can be learned from ordinary distributions and have a particularly rich spin structure. The hard scattering subprocesses in electroproduction of mesons and of real photons satisfy helicity selection rules, which provide powerful tools to test leading-twist dominance at a given value of the hard scale.