Search

Measurements of the total and differential cross sections for η-meson photoproduction on 1H, 2D, and 14N liquid targets from threshold to 1.2 GeV have been taken using the tagged Bremsstrahlung photon beam produced by the electrons extracted from the ELSA storage ring at Bonn. The reaction was identified by detecting the η decay products in the neutral meson spectrometer SPESO-2π, while the recoil baryons (proton, neutron, or deuteron) were detected by a variety of large angle scintillator detectors. We present here our experimental results on the deuterium target, both the comparison of the quasi-free proton and neutron as well as results on the coherent production. Our preliminary results for the counting rates integrated on the S11 excitation show significantly smaller figures for the neutrons than for the protons. This is consistent with an isoscalar part of the amplitude roughly ten times smaller than the isovector one and also explains the very low rate of coherent deuterons observed.

A formalism, based on an isobaric approach, via Feynman diagrammatic technique, including the nucleonic (spin ≤ 5/2), hyperonic (spin 1/2) and kaonic resonances is developed. Using this formalism, a thorough investigation of all the electromagnetic strangeness processes, for which experimental results are available, is performed and satisfactory agreement with the data is obtained. The reaction mechanism is found to include a reasonable number of baryonic resonances, with the extracted main kaon-hyperon-nucleon coupling constants in agreement with the SU(3)-symmetry predictions. The sensitivity of this model to the form factors of the relevant hadrons is emphasized.

We have measured total and differential cross sections for η‐meson photo‐production on 7 cm thick 1H, 2D, and 14N liquid targets from threshold to 1.2 GeV, using the tagged Bremsstrahlung photon beam produced by the electrons extracted from the ELSA storage ring at Bonn. The photoreaction was identified by detecting the η decay products in the neutral meson spectrometer SPES0‐2π, while the recoil baryons (proton, neutron, or deuteron) were detected by a variety of large angle scintillator detectors. Some of our recent preliminary results, both theoretical and experimental, will be discussed.

Investigations in progress on the associated strangeness photo‐ and electroproduction of the proton in the few GeV region and the negative kaon radiative capture are reported, and results for the K+∑0 channel are presented.

Spin observables for the reaction $\gamma p \rightarrow K^+ \Lambda$ are examined using three recent dynamical models and are compared to the general features of such observables deduced earlier by Fasano, Tabakin and Saghai. These general features, such as the energy dependence of spin observables and the location of nodes in their angle dependence, are realized. Several instructive surprises, which occur in this comparison to the conjectures of Fasano et al., are then discussed. The sensitivity of spin observables to isobar and {\it t-}channel dynamics is analyzed and suggestions for selecting experiments which provide important dynamical information are presented., Comment: 22 pages, latex, 10 figures

International audience; The reaction pp→pp$\pi$$^0$ has been investigated at the Satume accelerator using a transversely polarized beam of 800 MeV incident kinetic energy and the DIOGENE detector. We have measured the total cross section, the differential cross section as a function of the pion energy and the anisotropy parameter b. We find a strongly dominant contribution, greater than 90%, of the Delta resonance to this process. We have determined the analyzing power A$_y$ of the reaction and get negative values.

The associated strangeness production in pp collisions was studied at bombarding energies of 2.3 and 2.7 GeV by detecting the outgoing K + particles with a high resolution magnetic spectrometer at forward angles up to 23.5° laboratory angle. The kaons were separated from an immense background of protons and pions by a highly resolving TOF electronics addition to vetoes from Cerenkov detectors utilizing either β differing Cerenkov light cones in lucite or different thresholds for light production in aerogel. The hyperon-nucleon missing mass spectra obtained show strong deviations from pure phase space which is ascribed to final state interactions of the pΛ and the Nσ systems. The structures found are discussed and compared with results from K − d → π − YN experiments, with theoretical calculations in the frame of OBE models, and with the prediction of sharp strange dibaryon resonances from several quark-bag models.

The cross section for exclusive ${\mathrm{\ensuremath{\pi}}}^{+}$ electroproduction on the proton has been measured near threshold for the first time at two different values of the virtual photon polarization (\ensuremath{\varepsilon}\ensuremath{\sim}0.2 and \ensuremath{\varepsilon}\ensuremath{\sim}0.7). Using the low energy theorem for this reaction we deduce the axial and pseudoscalar weak form factors ${\mathit{G}}_{\mathit{A}}$ and ${\mathit{G}}_{\mathit{P}}$ at \ensuremath{\Vert}t\ensuremath{\Vert}=0.073, 0.139, and 0.179 (GeV/c${)}^{2}$. The slope of ${\mathit{G}}_{\mathit{A}}$ agrees with the value obtained in neutrino experiments. ${\mathit{G}}_{\mathit{P}}$ satisfies the pion pole dominance hypothesis, which is thus verified for the first time in this range of transfer.

A direct experimental determination has been made of the ratio of the forward-angle positive-pion electroproduction cross section for a proton bound in the deuteron with that of a free proton for invariant masses of 1160 and 1232 MeV. A significant quenching of the reaction in the deuteron is observed.

A strong enhancement at the (Σ Open image in new window )+ threshold is observed in missing mass spectra of the reaction pp→K+X experiment performed at the French Saturn National Lab (LNS). This effect is related to the coupling of the (Σ Open image in new window )+ and Λp channels. It could be the signature of a strangeS=−1 resonance the H1* (2130),SU(3) analog partner of the deuteron.

We have measured inclusive electron scattering cross sections on 3 He at incident energies up to 667 MeV. The transverse and longitudinal response functions were extracted at four-momentum transfers between −0.1 (GeV/ c ) 2 and −0.3 (GeV/ c ) 2 and up to an energy transfer of 450 MeV. The data are compared to recent theoretical calculations which include many-body effects.

We have measured the deep-inelastic electron scattering from carbon up to and including the Δ-region at 36°, 60°, 90° and 145°. The systematic decomposition of the transverse and longitudinal response functions has been obtained by means of a Rosenbluth-type analysis of the data in the momentum transfer interval 200 MeV/c /s |q| /s 600 MeV/c. A comparison with theoretical calculations which extend over the quasielastic and Δ-peak regions is presented. A reduction of the differences between our data and theory seems obtainable through the introduction of meson-exchange currents, resonant and non-resonant meson production, and the use of the shell model. Our experimental Coulomb sum-rule estimates at higher | q | agree with independent particle model predictions. We compare our results in the vicinity of the Δ-peak with the total absorption cross section for real photons.

We have studied the π 0 Dalitz decay π 0 → γ e + e − using the pion beam and a magnetic pair spectrometer at the Saclay Linear Accelerator. A shape analysis of the (e + e − ) invariant mass squared x , carried out on 32 000 events, gives a direct measurement of the π 0 electromagnetic transition form factor squared | F ( x )| 2 , and of its slope parameter a in the expression F ( x ) = 1 + ax . We find a = −0.11±0.03(stat.)±0.08(syst.) m −2 π 0 .

There has been a long-standing problem in the coupled \ensuremath{\pi}NN-NN theory concerning its apparent failure in reproducing the tensor polarization ${T}_{20}$ in the elastic \ensuremath{\pi}d scattering at medium energies. A mechanism recently proposed by Jennings to solve this problem is examined. It is then found that it comes from the positive-energy pole in one of the nucleon propagators in the exchange impulse diagram within the covariant picture which is usually overlooked when devising the spectator on-mass-shell approximation, and that it does not lead to the claimed near cancellation of the Pauli exchange contribution, but is important to be included. The inclusion of this contribution is shown to considerably improve in reproducing the pion-deuteron spin observables including ${T}_{20}$, particularly above the delta resonance energy.

Deep-inelastic electron scattering from 12C was measured at 60°, 90°, 130° and 145° until 560 MeV incident electron energy. Longitudinal-transverse separation was achieved for ¦q2μ¦ < 6 fm−2 and energy-loss ω < 360 MeV. The longitudinal sum rule exhibits 80 % of the strength predicted by single particle model. The measured cross section between the quasi-elastic peak and the free Δ peak is compared to different theoretical models. Mesonic exchange current in proton-neutron pairs seems to play an important role.

We have studied the reaction: π − p→ne + e − at rest, i.e. inverse pion electroproduction at threshold, using the pion and a magnetic pair spectrometer at the Saclay Linear Accelerator. With 30 000 analyzed events, we proceed for the first time to the separation between the transverse and the longitudinal parts of the process, leading to model-independent measurements of the longitudinal multipole L π − 0+ for five values of the photon mass k 2 and the k 2 dependence of the transverse multipole E π − 0+ in the small transfer region.

Measurements of absolute elastic-scattering differential cross sections for π ± He at T lab π = 30.2 MeV, 45.1 MeV and 65.0 MeV and for π ± - 4 − 4 He at T lab π = 25.1 MeV and 51.1 MeV have been made in the angular range 40° to 145°. An overall accuracy of about ±5% was achieved. The results are compared with available theoretical calculations.

The effect of the coupling to the ..pi..d channel on the intermediate energy NN scattering is studied using the recent NN..-->..NN and NN..--> pi..d partial wave analyses as well as the coupled NN..--> pi..NN theory. Contrary to some recent claims, it is found that this coupling is important in the /sup 1/D/sub 2/ and /sup 3/F/sub 3/ partial waves in which the intermediate N..delta.. configuration dominates.

Measurements of absolute differential cross sections for π ± d elastic scattering at T π lab = 64.9 ± 0.7 MeV with an overall accuracy better than ±4% and an angular distribution ranging from 41° to 143° are reported and compared with available calculations. The need for more realistic calculations is emphasized; in particular the Coulomb interaction treatment should be improved before any conclusion is drawn about possible charge symmetry breaking in the strong interaction.

The sensitivity of π ± d differential cross section, vector analyzing power, tensor polarization and charge asymmetry parameters to different Coulomb contributions to the total scattering amplitudes is investigated at eleven energies in the energy range T π lab = 47 to 360 MeV. For that purpose we derive a simple on-shell approximation for the Coulomb-corrected hadronic T -matrix, which is superior to widely used semiclassical descriptions of the Coulomb corrections. In addition we use relativistic Coulomb amplitudes and phase shifts, which consider also the pion and deuteron charge extension. It is shown that the vector analyzing power it 11 is more sensitive to different Coulomb prescriptions than the differential cross section and tensor polarization t 20 . We have also succeeded in describing the charge asymmetry parameter of differential cross section A π satisfactorily and find that the charge asymmetry parameter of vector analyzing power A it 11 is even more sensitive than A π to different Coulomb treatments. For a search of charge-symmetry-breaking effects in terms of the π d charge asymmetry parameter, the necessity of including our careful treatment of Coulomb contributions to the total scattering amplitude is emphasized.

The absorption of π − at rest on 12 C leading to 11 Be is measured using the activation technique. The probability for single proton emission per stopped pion is (4.5 ± 0.8) × 10 −4 . Production of 8 Li is also observed with a probability of (8 ± 4) × 10 −3 per stopped pion.

Elastic scattering differential cross section data ofπ±d at 65 MeV andπ±4He at 51 MeV are presented and compared respectively to fully relativistic 3-body and optical potential calculations including true pion absorption.

The coupled πNN-NN theory is examined in its dominant πN P33 (or Δ) input by a direct calculation of the NN → NΔ observables which are then compared with the recent data on pp → Δ+ + n extracted from the pp → π+ pn reaction at medium energies.

A new interest to study the S = -1, B = 2 system has recently been raised due to theoretical predictions coming either from both potential [1] and quark bag model calculations [2] about the existence of narrow exotic states. In particular the NIJMEGEN GROUP [j] predicts in the MIT bag model a large number of 6 quark configurations with strangeness S = 0, -1 or -2. The stability of these states against quark dissociation or two baryon emission increases with the number of strange quarks involved, the optimum case beeing for the S = -2 system (uu dd ss) that could even present a stable di-hyperon state [4]. No experimental evidence for the observation of such S = -2 states has yet been found, but the cross section level reached in these different experimental researches [5] does not rule out their existence.

We present a quark-parton model to describe polarized and unpolarized nucleon structure functions. The twist-two matrix elements for the QCD evolution analysis of lepton-hadron scattering are calculated within a light-front covariant quark model. The relativistic effects in the three-body wave function are discussed for both the polarized and unpolarized cases. Predictions are given for the polarized gluon distributions as will be seen in future experiments.