Electroproduction Of Pseudoscalar Mesons Research Articles

Moravcsik's theorem on complete sets of polarization observables reexamined

We revisit Moravcsik's theorem on the unique extraction of amplitudes from polarization observables, which has been originally published in 1985. The proof is (re-) written in a more formal and detailed way and the theorem is corrected for the special case of an odd number of amplitudes (this case was treated incorrectly in the original publication). Moravcsik's theorem, in the modified form, can be applied in principle to the extraction of an arbitrary number of $N$ helicity amplitudes. The uniqueness theorem is then applied to hadronic reactions involving particles with spin. The most basic example is Pion-Nucleon scattering ($N=2$), the first non-trivial example is pseudoscalar meson photoproduction ($N=4$) and the most technically involved case treated here is given by pseudoscalar meson electroproduction ($N=6$). The application of Moravcsik's theorem to electroproduction yields new results, which for the first time provide insights into the structure and content of complete sets for this particular process. The uniqueness-statements for the various reactions are compared and an attempt is made to recognize general patterns, which emerge under the application of the theorem.

Transversity in hard exclusive electroproduction of pseudoscalar mesons

Estimates for electroproduction of pseudoscalar mesons at small values of skewness are presented. Cross sections and asymmetries for these processes are calculated within the handbag approach which is based on factorization in hard parton subprocesses and soft generalized parton distributions (GPDs). The latter are constructed from double distributions. Transversity GPDs are taken into account; they are accompanied by twist-3 meson wave functions. For most pseudoscalar-meson channels a combination of H-tilde_T$ and E-bar_T$ plays a particularly prominent role. This combination of GPDs which we constrain by moments obtained from lattice QCD, leads with the exception of the pi+ and eta' channels, to large transverse cross sections.

A polarization relation and the measurement of the longitudinal response in pseudoscalar meson electroproduction off the nucleon

For pseudoscalar meson electroproduction off the nucleon in parallel kinematics a relation between three polarization observables is derived. It is shown that, without Rosenbluth separation, a measurement of the longitudinal strength can be achieved through three different ways. They are discussed with preliminary MAMI data for the \(p(\vec e, e' \vec p)\pi ^0 \) reaction in the energy range of the Δ(1232) resonance.

Measuring longitudinal amplitudes for electroproduction of pseudoscalar mesons using recoil polarization in parallel kinematics

We propose a new method for measuring longitudinal amplitudes for electroproduction of pseudoscalar mesons that exploits a symmetry relation for polarization observables in parallel kinematics. This polarization technique does not require variation of electron scattering kinematics and avoids the major sources of systematic errors in Rosenbluth separation.

Odderon and photon exchange in electroproduction of pseudoscalar mesons

We investigate the reaction \({\rm e}\;{\rm p} \rightarrow{\rm e}\;{\rm PS}\;{\rm X}\) where PS denotes a pseudoscalar meson \(\pi^0\), \(\eta\), \(\eta'\), or \(\eta_c\) and X either a proton or resonance or continuum state into which the proton can go by diffractive excitation. At high energies photon and odderon exchange contribute to the reaction. The photon exchange contribution is evaluated exactly using data for the total virtual photon-proton absorption cross section. The odderon exchange contribution is calculated in nonperturbative QCD, using functional integral techniques and the model of the stochastic vacuum. For the proton we assume a quark-diquark structure as suggested by the small odderon amplitude in pp and \( {\rm p} \bar{{\rm p}}\) forward scattering. We show that odderon exchange leads to a much larger inelastic than elastic PS production cross section. Observation of our reaction at HERA would establish the soft odderon as an exchange object on an equal footing with the soft pomeron and would give us valuable insight into both the nucleon structure and the mechanism of high energy diffractive scattering.

Hard exclusive pseudoscalar meson electroproduction and spin structure of the nucleon

The amplitude for hard exclusive pseudoscalar meson electroproduction off nucleon (nuclear) targets is computed in QCD within the leading ${\ensuremath{\alpha}}_{s}\mathrm{ln}{Q}^{2}/{\ensuremath{\Lambda}}_{\mathrm{QCD}}^{2}$ approximation. We show that the distribution of recoil nucleons depends strongly on the angle between the momentum of the recoil nucleon and the polarization vector of the target (or outgoing nucleon). This dependence is especially sensitive to the spin flip skewed parton distribution (SPD) $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{E}.$ We argue also that the scaling for this spin asymmetry sets in at lower ${Q}^{2}$ than that for the absolute cross section. Based on the chiral quark-soliton model of the nucleon we estimate quantitatively the spin asymmetry. In the case of ${\ensuremath{\pi}}^{+}$ production this asymmetry is dominated at small t by the contribution of the pion pole in the isovector SPD $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{E}$ as required by PCAC. In the case of ${K}^{0}$ production off a proton we find a large enhancement of the cross section as compared to the case of ${\ensuremath{\pi}}^{0}$ production. For the forward production of neutral pseudoscalar mesons off a deuteron target we find the cross section should be zero for the zero deuteron helicity (along the ${\ensuremath{\gamma}}^{*}D$ direction). We consider also cross sections of quasielastic processes off nuclei including the feasibility to implant ${K}^{+},\ensuremath{\rho}$ mesons into nuclear volume.

OddCPcontributions to diffractive processes

We investigate contributions to diffractive scattering, which are odd under C and P parity. Comparison of $p\ensuremath{-}\overline{p}$ and $p\ensuremath{-}p$ scattering indicates that these odderon contributions are very small and we show how a diquark clustering in the proton can explain this effect. A good probe for the odderon exchange is the photo- and electroproduction of pseudoscalar mesons. We concentrate on the ${\ensuremath{\pi}}^{0}$ and show that the quasielastic ${\ensuremath{\pi}}^{0}$ production is again strongly suppressed for a diquark structure of the proton whereas the cross sections for diffractive proton dissociation are larger by orders of magnitude and rather independent of the proton structure.

Regge description of charged pseudoscalar meson electroproduction above the resonance region

A gauge invariant model based on $\ensuremath{\pi}$ and $\ensuremath{\rho}$ Regge trajectory exchanges provides a successful and economical description of the forward charged pion electroproduction reactions above the resonance region. Its sensitivity to the meson form factors is investigated in kinematics where the pion electromagnetic form factor will be extracted at high ${Q}^{2}$ at TJNAF. An extension to ${K}^{+}$ electroproduction is presented.

Photo and electroproduction of mesons

The lecture provides an elementary introduction to photo and electroproduction of pseudoscalar mesons in the resonance region. After discussing the general structure of the elementary production amplitude and its symmetry properties the formalism is applied to some interesting physical problems.

SU(6)Wanalysis of the electroproduction of pseudoscalar mesons in the second resonance region

Recent data on the photoproduction and electroproduction of $\ensuremath{\pi}$ and $\ensuremath{\eta}$ mesons in the second resonance region are analyzed assuming that the electromagnetic current transforms on the $W$ spin as a mixture of $W=0$ and $W=1$, ${W}_{z}=0, +1$. The data favor the presence of a term transforming as $W=1$, ${W}_{z}=0$, with the $W=0$ and $W=1$, ${W}_{z}=0$ matrix elements showing similar ${q}^{2}$ dependence ${q}^{2}$ being the square of the mass of the virtual photon. The ratio between the two helicity cross sections $\frac{{\ensuremath{\sigma}}_{\frac{1}{2}}}{{\ensuremath{\sigma}}_{\frac{3}{2}}}$ is found to be small for $0<\ensuremath{-}{q}^{2}<1.5$ Ge${\mathrm{V}}^{2}$.