0000000000002818
AUTHOR
F. Wissmann
Photoproduction of neutral pion pairs from the proton
Double neutral pion photoproduction from the proton has been measured at MAMI for photon energies between threshold and 820 MeV. The reaction was identified by an invariant mass and missing mass analysis. From threshold up to 370 MeV the total cross-section does not exceed 30 nb. For higher energies it shows a smooth rise until it reaches a maximum of about 10 μb at Eγ = 740 MeV. Dalitz plots of m2(π0π0) versusm2(p,π0) for seven bins of incident photon energy have been analysed. For Eγ > 610 MeV, a strong contribution of a sequential decay is observed with the Δ(1232)-resonance as intermediate state. A comparison to model calculations shows that these sequential decays presumably originate …
Compton scattering from the free and bound proton at backward angles above π-threshold
Differential cross sections for Compton scattering from the free proton at Theta(gamma)(lab) = 130.7 degrees in the energy region from 200 MeV to 410 MeV and far quasi-free Compton scattering from the proton bound in the deuteron at Theta(gamma)(lab) = 148.8 degrees in the energy region from 200 MeV to 290 MeV have been measured. The free proton data are in agreement with dispersion-theory predictions based on standard parameters. The difference of the proton polarizabilities has been extracted from the quasi-free data. Our result, - = [9.1 +/- 1.7(stat + syst) +/- 1.2(mod)] x 10(-4) fm(3), is in reasonable agreement with the world average of the free proton data if the backward spin polari…
Test of Low-Energy Theorems forH1(γ→,π0)H1in the Threshold Region
The photon asymmetry in the reaction H-1(, pi (0))H-1 close to threshold has been measured for the first time with the photon spectrometer TAPS using linearly polarized photons from the tagged-photon facility at the Mainz Microtron MAMI. The total and differential cross sections were also measured simultaneously with the photon asymmetry. This allowed determination of the S-wave and all three P-wave amplitudes. The values obtained at threshold are E0+ = [-1.33 +/- 0.08(stat) +/- 0.03(syst)] x 10(-3)/m(pi+), P-1 = [9.47 +/- 0.08(stat) +/- 0.29(syst)] x 10(-3)q /m(pi+)(2) P-2 [-9.46 +/- 0.1(stat) +/- 0.29(syst)] x 10(-3)q/m(pi+)(2), and P-3 = [11.48 +/- 0.06(stat) +/- 0.35(syst)] x 10(-3)q/m(…
Compton Scattering by the Proton
Compton scattering by the proton has been measured over a wide range covering photon energies 250 MeV < E_\gamma < 800 MeV and photon scattering angles 30^0 < \theta^{lab}_\gamma < 150^0, using the tagged-photon facility at MAMI (Mainz) and the large-acceptance arrangement LARA. The data are in good agreement with the dispersion theory based on the SAID-SM99K parameterization of photo-meson amplitudes. From the subset of data between 280 and 360 MeV the resonance pion-photoproduction amplitudes were evaluated leading to the multipole E2/M1 ratio EMR(340 MeV) =(-1.6 \pm 0.4(stat+syst) \pm 0.2(model)%. From all data below 455 MeV the proton's backward spin polarizability was determined to be …
Quasi-free Compton Scattering and the Polarizabilities of the Neutron
Differential cross sections for quasi-free Compton scattering from the proton and neutron bound in the deuteron have been measured using the Glasgow/Mainz tagging spectrometer at the Mainz MAMI accelerator together with the Mainz 48 cm $\oslash$ $\times$ 64 cm NaI(Tl) photon detector and the G\"ottingen SENECA recoil detector. The data cover photon energies ranging from 200 MeV to 400 MeV at $\theta^{LAB}_\gamma=136.2^\circ$. Liquid deuterium and hydrogen targets allowed direct comparison of free and quasi-free scattering from the proton. The neutron detection efficiency of the SENECA detector was measured via the reaction $p(\gamma,\pi^+ n)$. The "free" proton Compton scattering cross sect…
Compton scattering by the proton through θcms = 75° and 90° in the Δ-resonance region
Abstract Differential cross sections for Compton scattering by the proton have been measured in the energy interval between 200 and 500 MeV at scattering angles of θcms = 75° and θcms = 90° using the CATS, the CATS/TRAJAN, and the COPP setups with the Glasgow Tagger at MAMI (Mainz). The data are compared with predictions from dispersion theory using photo-meson amplitudes from the recent VPI solution SM95. The experiment and the theoretical procedure are described in detail. It is found that the experiment and predictions are in agreement as far as the energy dependence of the differential cross sections in the Δ-range is concerned. However, there is evidence that a scaling down of the reso…
Intermediate resonance excitation in the γp→pπ0π0 reaction
The helicity dependence of the total cross section for the (gamma) over right arrow(p) over right arrow -> p pi(0)pi(0) reaction has been measured for the first time at incident photon energies from 400 to 800 MeV. The measurement, performed at the tagged photon beam facility of the MAMI accelerator in Mainz, used the large acceptance detector DAPHNE and a longitudinally polarized frozen-spin target. This channel is found to be excited predominantly when the photon and proton have a parallel spin orientation, most likely due to the intermediate production of the D-13(1520) resonance. However, the contribution of the antiparallel spin configuration, arising from other reaction mechanisms, is…
Enhancement of the linear polarization of coherent bremsstrahlung by collimation of the photon beam
A method is described to precisely predict the relative intensities and degrees of linear polarization of coherent bremsstrahlung from diamond crystals, taking into account the collimation of the photon beam and the lateral distribution and angular divergence of the electron beam in addition to the properties of the crystal. It is confirmed that the increase of the degree of linear polarization through collimation of the photon beam is a sizable effect. Compared to previous approaches considerable progress has been made in reproducing the experimentally observed relative intensities of collimated coherent bremsstrahlung, by taking into account the angular distribution of coherent bremsstrah…
A high angular-resolution and time-of-flight detector system for recoil protons from the reactions p(γ, γ)p and p(γ, π0)p
Using the Mainz NaI(Tl)BaF2 photon spectrometer CATS (Compton And Two photon Spectrometer) together with the prototype of a universal spectrometer for recoil protons TRAJAN (TRAjectory ANalyser) we have studied different methods to disentangle Compton scattering by the proton from the large π0 background through the energy range of the Δ resonance. CATS is a modular 48 cm ⊘ × 64 cm NaI(Tl) detector with an energy resolution of 1.5% combined with a 2π array of 61 BaF2 detectors. TRAJAN consists of a hodoscope with two planes, a wire chamber with horizontal and vertical wires and a grid of horizontal and vertical plastic scintillator strips, followed by a wall of nine NaI(Tl) detectors. By t…
Search for narrow dibaryon resonances in neutral pion photoproduction from the deuteron
The reaction γd↦π0 X has been measured with TAPS at MAMI in the energy range E γ = 140-300 MeV. Using the Glasgow tagging spectrometer a photon energy resolution of 0.8 MeV was achieved. The energy excitation functions of integral and differential total cross-sections show no structures of statistical significance > 2σ. Upper limits for the production of narrow isoscalar or isovector dibaryons with masses m? 2100 MeV/c2 were deduced. They are in the range 2-5 μb averaged over the 0.8 MeV energy resolution.
Coherent and incoherent π0 photoproduction from the deuteron
Differential cross-sections for the reactions d(gamma,pi (o))d and d(gamma,pi (o))pn have been measured at MAMI with the TAPS detector setup in the energy range 140 MeV > E- > 306 MeV. By use of the Glasgow tagging spectrometer an 0.8 MeV energy resolution for photons incident on the target was achieved. The (o) missing energy resolution was sufficient for a reliable separation of coherent and incoherent channels. The data for the break-up channel exhibit very strong final state interaction effects, whereas the observed angular dependence of the inclusive process d(gamma, pi (o))X is in quantitative agreement with predictions for a quasi-free process. The observed absolute d(gamma, pi (o))X…
Neutron polarizabilities investigated by quasi-free Compton scattering from the deuteron
Measuring Compton scattered photons and recoil neutrons in coincidence, quasifree Compton scattering by the neutron has been investigated at MAMI (Mainz) at ${\ensuremath{\theta}}_{\ensuremath{\gamma}}^{\mathrm{lab}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}136\ifmmode^\circ\else\textdegree\fi{}$ in an energy range from 200 to 400 MeV. From the data a polarizability difference of ${\ensuremath{\alpha}}_{n}\ensuremath{-}{\ensuremath{\beta}}_{n}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}9.8\ifmmode\pm\else\textpm\fi{}3.6(\mathrm{stat}{)}_{\ensuremath{-}1.1}^{+2.1}(\mathrm{syst})\ifmmode\pm\else\textpm\fi{}2.2(\mathrm{model})$ in units of ${10}^{\ensuremath{-}4}{\mathrm{fm}}^{…
Measurement of coherent and incoherent π0 photoproduction off the deuteron with tagged photons up to the Δ region
Coherent and incoherent pi(0) photoproduction off the deuteron has been measured for incident quasi-monoenergetic photons in the energy range from pi(0) threshold up to 300 MeV using the TAPS setup together with the Glasgow tagger at MAMI. Integral and differential cross sections over the full angular range have been obtained. Coherent and incoherent production are found to exhibit very different angle and energy dependences. The sum of both processes, the total pi(0) production off the deuteron is quantitatively extremely well described by a pure quasifree model, whereas the data for coherent and incoherent channels necessitate strong FSI effects for their description. Finally upper limits…
Beam Asymmetry of Coherentπ° Photoproduction on4Hein theΔ(1232) Range Measured using Coherent Bremsstrahlung
Using coherent bremsstrahlung at MAMI (Mainz), the beam asymmetry of the differential cross sections for coherent $\ensuremath{\pi}\ifmmode^\circ\else\textdegree\fi{}$ photoproduction on ${}^{4}\mathrm{He}$ has been measured in the energy interval from 200 to 500 MeV at an angle of ${\ensuremath{\theta}}_{\mathrm{lab}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}37\ifmmode^\circ\else\textdegree\fi{}$. The coherent ( $\ensuremath{\gamma},\ensuremath{\pi}\ifmmode^\circ\else\textdegree\fi{}$) reaction is used as a precise linear polarimeter for the first time. Agreement with predictions has been obtained on a $\ensuremath{\gtrsim}3%$ level of precision.
Low-energy Compton scattering and the polarizabilities of the proton
Differential cross-sections for Compton scattering from the proton have been measured at the MAMI tagged photon facility using the TAPS setup. The data cover an angular range of θlab γ = 59°-155° and photon energies ranging from 55 MeV to 165 MeV. Our results are in good agreement with those from previous experiments, but yield higher precision. Using dispersion relations the proton polarizabilities have been determined to be = [11.9±0.5stat.±1.3syst.±0.3mod.] . 10-4 fm 3 and = [1.2±0.7stat.±0.3syst.±0.4mod.)] . 10-4 fm 3. These results confirm the Baldin sum rule which was re-evaluated to be + = [13.8±0.4] . 10-4 fm 3. We can also conclude that there is no significant additional asymptotic…
First measurement of the helicity-dependent γ p↦pη differential cross-section
The helicity dependence of the (γ) over right arrow(p) over right arrow -+ peta reaction has been measured for the first time at a center-of-mass angle theta(eta)*= 70degrees in the photon energy range from 780 MeV to 790 MeV. The experiment, performed at the Mainz microtron MAMI, used a 4pi-detector system, a circularly polarized, tagged photon beam. and a longitudinally polarized frozen-spin target. The helicity 3/2 cross-section is found to be small and the results for helicity 1/2 agree with predictions from the MAID analysis.
Photoemission of protons from4He in theΔ-resonance region
High resolution proton energy spectra from the4He(γ,p) reaction have been measured with tagged photons in the range Eγ = 130−525 MeV using the large Mainz NaI(T1) spectrometer atΘplab=37.1°. Three separate reaction channels were identified, viz.4He(γ,p)t two-body breakup,4He photodisintegration via two-nucleon photon absorption processes and the quasifree pion production channel. Differential cross sections are presented for each of these channels as a function of photon energy. The sum of the two-nucleon photon absorption and the quasifree pion production differential cross sections, in the CM system, resembles that of the corresponding free-nucleon differential cross section when Fermi mo…
Backward spin polarizabilityγπof the proton
Using the Mainz $48\mathrm{cm}\ensuremath{\emptyset}\ifmmode\times\else\texttimes\fi{}64\mathrm{cm}$ NaI(Tl) detector and the segmented G\"ottingen recoil detector SENECA in coincidence, Compton scattering by the proton at ${\ensuremath{\theta}}_{\ensuremath{\gamma}}^{\mathrm{lab}}=136\ifmmode^\circ\else\textdegree\fi{}$ has been measured at MAMI (Mainz) in the energy range from 200 to 470 MeV. The new data confirm the previous observation that there is a systematic discrepancy between MAMI and LEGS (Brookhaven) data leading to different spin polarizabilities ${\ensuremath{\gamma}}_{\ensuremath{\pi}}=\ensuremath{-}38.7\ifmmode\pm\else\textpm\fi{}1.8$ and $\ensuremath{-}27.2\ifmmode\pm\else\…
Helicity dependence of the γ→p→→nπ+π0 reaction in the second resonance region
The helicity dependence of the total cross section for the (γ) over right arrow(p) over right arrownpi(+) pi(0) reaction has been measured for the first time at incident photon energies from 400 to 800 MeV The measurement was performed with the large acceptance detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. This channel is found to be excited predominantly when the photon and proton have a parallel spin orientation, due to the intermediate production of the D-13 resonance. (C) 2002 Elsevier Science B.V. All rights reserved.
Exclusive measurement of quasi-free η-photoproduction from deuterium
Quasi-free photoproduction of eta-mesons from the deuteron has been measured at the tagged photon facility of the Mainz microtron MAMI with the photon spectrometer TAPS for incident photon energies from the production threshold at 630 MeV up to 820 MeV. In a fully exclusive measurement eta-mesons and recoil nucleons were detected in coincidence. At incident photon energies above the production threshold on the free nucleon, where final state interaction effects are negligible, an almost constant ratio of sigma(n)/sigma(p)=0.66+/-0.10 was found. At lower incident photon energies the ratio rises due to re-scattering effects. The average ratio agrees with the value extracted from a comparison …
Angular and polarization dependence of Compton scattering from in the Δ-resonance region
Abstract Using linearly polarized tagged photons from coherent bremsstrahlung, differential cross sections and beam asymmetries for Compton scattering by 4 He have been measured at MAMI in the energy interval between 150 MeV and 500 MeV for scattering angles of θ γ lab =37°, 93° and 137°, thus largely increasing the available data base. Improved calculations in terms of the Δ -hole model completely fail to describe the data at large scattering angles. The same proved to be true for a schematic model, even after taking into account properties of nuclear photo-absorption in very detail.
Helicity Dependence ofγp→Nπbelow 450 MeV and Contribution to the Gerasimov-Drell-Hearn Sum Rule
The helicity dependence of the single pion photoproduction on the proton has been measured in the energy range from 200 to 450 MeV for the first time. The experiment, performed at the Mainz microtron MAMI, used a 4pi-detector system, a circularly polarized, tagged photon beam, and a frozen-spin target. The data obtained provide new information for multipole analyses of pion photoproduction and determine the main contributions to the Gerasimov-Drell-Hearn sum rule and the forward spin polarizability gamma(0).
Double- π0 photoproduction from the deuteron
The photoproduction of two neutral pions from the deuteron has been studied for incident photon energies from 200 MeV to 820 MeV with the TAPS spectrometer at MAMI (Mainz Microtron). The total cross-section was determined and used to deduce the cross-section from the neutron. Due to the good statistical quality of the data Dalitz plots for the three particles in the exit channel (pi (0)pi N-0) could be constructed. The invariant mass distributions derived from them are presented in this paper. They indicate that the important reaction mechanism in the second resonance region is a sequential decay pattern involving the population of the Delta (1232)-resonance as an intermediate state.
Elastic and inelastic photon scattering from 12C
Abstract The elastic and inelastic photon scattering from 12 C was investigated in the energy range of the Δ-resonance ( E γ = 200–500 MeV) at a scattering angle of 40°. The experiment was performed with the tagged photon facility at the cw electron accelerator MAMI ( E e − = 855 MeV). By using a large NaI(Tl) detector having an energy resolution of 1.5%, the elastic scattering could be separated from the inelastic process to the first excited state ( E x = 4.44 MeV of 12 C. The differential cross section for the coherent scattering is compared with a simple static ansatz, that uses the known charge form factor, and with a recent Δ-hole calculation.
Compton scattering from the free and bound proton above π-threshold
The differential cross sections for Compton scattering from the proton have been measured at MAMI with three different detector setups. There is a good agreement among all the results. The theoretical calculation based on dispersion relations shows that there is no drastic change necessary in the parameters of this approach.
Compton Scattering by the Proton using a Large-Acceptance Arrangement
Compton scattering by the proton has been measured over a wide range covering photon energies 250 MeV lt or approximately=E/sub gamma / lt or approximately=800 MeV and photon scattering angles 30 degrees lt or approximately= theta /sub gamma //sup lab/ lt or approximately=150 degrees , using the tagged-photon facility at MAMI (Mainz) and the large-acceptance arrangement LARA. The previously existing data base on proton Compton scattering is greatly enlarged by more than 700 new data points. The new data are interpreted in terms of dispersion theory based on the SAID-SM99K parametrization of photo-meson amplitudes. It is found that two-pion exchange in the t-channel is needed for a descripti…
Direct Observation of aρDecay of theD13(1520)Baryon Resonance
The reaction {gamma}p{yields}{pi}{sup +}{pi}{sup 0}n has been measured at MAMI for photon energies up to 820MeV. Invariant mass spectra of the particles in the final state ({pi}{sup +}n) , ({pi}{sup 0}n) , ({pi}{sup +}{pi}{sup 0}) have been determined for several bins of incident photon energy. Differences in {pi}{sup +}{pi}{sup 0} and simultaneously measured {pi}{sup 0}{pi}{sup 0} invariant mass distributions are assigned to a {rho} branch of the D{sub 13}(1520) nucleon resonance.
Near threshold photoproduction of η mesons from 4 He
Photoproduction of η mesons from 4He has been measured for the first time. The experiment was performed at the Mainz Microtron (MAMI) using the TAPS photon spectrometer and the Glasgow/Mainz tagged photon facility. η mesons were identified in coincidence with recoil nucleons. Total and differential cross sections are presented in the photon energy range between threshold and 818 MeV. The exclusive data are used to determine the ratio of the elementary production cross sections on bound neutrons and protons, σn/σp= 0.68 ± 0.02 ± 0.09. In addition, upper limits for the total coherent cross section have been derived.
Threshold enhancement in η photoproduction from 2H and 4He
The photoproduction of η-mesons from 2H and 4He has been studied for energies close to the production thresholds. The experiments were carried out with the tagged photon beam of the Mainz MAMI accelerator. The η-mesons were detected via their two photon decays with the electromagnetic calorimeter TAPS. Total cross-sections, angular and momentum distributions of the η-mesons have been determined for both reactions. The total cross-sections in the threshold region show a large enhancement over the predictions of a participant-spectator model, indicating significant final-state interaction effects. The results are compared to recent model calculations taking into account nucleon-nucleon and nu…
"Table 3" of "Quasi-free Compton Scattering and the Polarizabilities of the Neutron"
Energy dependence of the triple differential cross section w.r.t. the scattered proton.
"Table 1" of "Compton scattering from the free and bound proton at backward angles above pi threshold."
No description provided.
"Table 2" of "Quasi-free Compton Scattering and the Polarizabilities of the Neutron"
Energy dependence of the quasi-free proton differential cross section.
"Table 5" of "Quasi-free Compton Scattering and the Polarizabilities of the Neutron"
Energy dependence of the free neutron differential cross section.
"Table 4" of "Quasi-free Compton Scattering and the Polarizabilities of the Neutron"
Energy dependence of the triple differential cross section w.r.t. the scattered neutron.
"Table 1" of "Test of low-energy theorems for p(gamma(pol.),pi0)p in the threshold region."
Polarized photon beam.
"Table 1" of "Angular and polarization dependence of Compton scattering from He-4 in the Delta resonance region"
Axis error includes +- 0.0/0.0 contribution.
"Table 1" of "Quasi-free Compton Scattering and the Polarizabilities of the Neutron"
Energy dependence of the free-proton differential cross section.