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 …

(γ,2N) reaction inC12

The $^{12}$C(\ensuremath{\gamma},pn) and $^{12}$C(\ensuremath{\gamma},pp) reactions have been measured for photon energies between 80 and 157 MeV using a photon tagging spectrometer and plastic scintillator detectors. The overall energy resolution was \ensuremath{\sim}7 MeV, sufficient to determine the initial shells of the emitted nucleons. Corrections were made for solid angle and threshold effects by means of Monte Carlo simulations. For the (\ensuremath{\gamma},pn) reaction both the missing energy and recoil momentum distributions are largely consistent with a two-nucleon absorption process on p-shell an sp nucleon pairs. For the much smaller $^{12}$C(\ensuremath{\gamma},pp) cross secti…

Double pi(0) photoproduction off the proton at threshold

The reaction $\gamma p \to \pi^0 \pi^0 p$ has been measured using the TAPS BaF$_2$ calorimeter at the tagged photon facility of the Mainz Microtron accelerator. Chiral perturbation theory (ChPT) predicts that close to threshold this channel is significantly enhanced compared to double pion final states with charged pions. In contrast to other reaction channels, the lower order tree terms are strongly suppressed in 2$\pi^0$ photoproduction. The consequence is the dominance of pion loops in the 2$\pi^0$ channel close to threshold - a result that opens new prospects for the test of ChPT and in particular its inherent loop terms. The present measurement is the first which is sensitive enough fo…

Determination of theE2/M1ratio in theγN→Δ(1232)transition from a simultaneous measurement ofp(γ→,p)π0andp(γ→,π+)n

Tagged linearly polarized photons have been used at the Mainz Microtron (MAMI) for simultaneous measurements of the $p(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},p){\ensuremath{\pi}}^{0}$ and $p(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},{\ensuremath{\pi}}^{+})n$ reaction channels to study the $\ensuremath{\gamma}\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\Delta}(1232)$ transition. The energy dependence of the magnetic dipole ${M}_{1+}^{3/2}$ and electric quadrupole ${E}_{1+}^{3/2}$ amplitudes have been extracted from these data in the photon energy range from $270$ to 420 MeV. The $E2/M1$ ratio for the $\ensuremath{\gamma}\stackrel{\ensuremath{\rightarrow…

C(,pn) photon asymmetry for Eγ=180–340 MeV

The photon asymmetry (Σ) of the 12C(,pn) reaction has been measured using linearly polarised tagged photons at the Mainz microtron MAMI for Eγ=180–340 MeV. The data have been analysed in separate missing energy (Em) regions corresponding to the ejection of nucleons from (1p)2 and (1p)(1s) shells. The measured Σ values for both Em regions are smaller in magnitude than corresponding 2H data, but the (1p)2 results have a similar Eγ dependence. Calculations of direct two-nucleon emission overestimate the magnitude of Σ at all photon energies.

Dependence of the C12(γ→,pd) reaction on photon linear polarisation

Abstract The sensitivity of the C 12 ( γ → , pd ) reaction to photon linear polarisation has been determined at MAMI, giving the first measurement of the reaction for a nucleus heavier than 3He. Photon asymmetries and cross sections were measured for E γ = 170 to 350 MeV. For E γ below the Δ resonance, reactions leaving the residual 9Be near its ground state show a positive asymmetry of up to 0.3, similar to that observed for 3He suggesting a similar reaction mechanism for the two nuclei.

Near threshold photoproduction of eta mesons off the proton.

We have measured precise total and differential cross sections for the reaction $\ensuremath{\gamma}p\ensuremath{\rightarrow}p\ensuremath{\eta}$ from threshold to 790 MeV at the MAMI accelerator in Mainz with the neutral meson spectrometer TAPS. Resonance parameters of the ${S}_{11}$(1535) resonance and the electromagnetic coupling $\ensuremath{\gamma}p\ensuremath{\rightarrow}{S}_{11}$ have been extracted from the data. Contributions from the ${D}_{13}$(1520) resonance to $\ensuremath{\eta}$ photoproduction in the threshold region have been identified for the first time via interference terms in the angular distributions.

Coherent π0-photoproduction from atomic nuclei

Differential and total cross sections for coherent pi0-photoproduction from 12C, 40Ca, 93Nb and Pb targets have been measured throughout the region of the Delta(1232)-resonance. The experiments were performed with the TAPS-detector at the Mainz accelerator MAMI. The characteristic proportionality of the cross section to the square of the atomic mass number and to the nuclear mass form factor is clearly demonstrated. The data allow for the first time detailed tests of model predictions for this reaction. The comparison of the data to model predictions shows that the Delta-nucleus interaction saturates: it is described for heavy nuclei with the same potential parameters as for 4He.

Quasifreeπ+production studied using theC12(γ, π+n)B11reaction in theΔ(1232)resonance region

Results are presented from a coincidence study of the {sup 12}C({gamma},{pi}{sup +}{ital n}){sup 11}B quasifree pion production reaction made in the {Delta}-resonance region using tagged photons. Cross sections for reactions originating on 1{ital p}-shell protons are found to be significantly larger than predicted by calculations based on quasifree pion production. It is suggested that more sophisticated calculations, perhaps including medium effects, may be required to reproduce the data. {copyright} {ital 1996 The American Physical Society.}

Photon Asymmetry of Deuteron Photodisintegration Between 160 and 410 MeV

The photon asymmetry Σ of the reaction has been measured across the photon energy range 160–410 MeV at the Mainz Microtron MAMI. Linearly polarized photons were obtained from coherent bremsstrahlung and their energies were determined with the Glasgow tagging spectrometer. The large acceptance detector DAPHNE enabled the outgoing protons to be measured over the complete azimuthal range and for polar angles from 35° to 155° in the c.m. system. The data are compared with existing results and recent theoretical calculations.

Investigation of multiparticle final states in12C photoreactions

The ${}^{12}\mathrm{C}(\ensuremath{\gamma},pn)$, ${}^{12}\mathrm{C}(\ensuremath{\gamma},pp)$, ${}^{12}\mathrm{C}(\ensuremath{\gamma},ppp)$, ${}^{12}\mathrm{C}(\ensuremath{\gamma},pp{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}})$, ${}^{12}\mathrm{C}(\ensuremath{\gamma},p{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}})$ reactions have been studied using tagged photons of energy ${E}_{\ensuremath{\gamma}}=250$--600 MeV. A model which includes all major absorption mechanisms and final state interactions gives results that agree well with the shapes of missing energy spectra, but for some channels the predicted cross sections are too large.

The 16O(γ, 2N) reaction measured with tagged photons

The results of tagged-photon measurements of the 4He(γ, pn)d, 4He(γ, pn)pn and 4He(γ, pp)nn reactions, carried out with photons of energies 80–131 MeV, are presented. The (γ, pn)d three-body-breakup cross section dominates the four-body-breakup cross section. The three-body-breakup cross section and the corresponding deuteron recoil-momentum spectrum are reasonably accounted for by a mechanism of photon absorption on correlated proton-neutron pairs. In contrast the four-body-breakup mechanism appears to have two components: absorption on correlated proton-neutron pairs and a more complex additional component which has a distribution similar to phase space. The data rule out substantial phot…

The reaction at Eγ = 60 and 80 MeV

Abstract The tagged photon technique has been used to measure the 12C photoproton cross section over the angular range θp = 30°–135° for Eγ = 60 and 80 MeV. Comparison of the results with quasifree knockout and quasideuteron calculations shows that photon interaction with two-nucleon pairs is dominant at least for high excitation energies in the residual nucleus.

Krusche et al. reply.

Neutron Skin ofPb208from Coherent Pion Photoproduction

Information on the size and shape of the neutron skin on (208)Pb is extracted from coherent pion photoproduction cross sections measured using the Crystal Ball detector together with the Glasgow tagger at the MAMI electron beam facility. On exploitation of an interpolated fit of a theoretical model to the measured cross sections, the half-height radius and diffuseness of the neutron distribution are found to be c(n)=6.70±0.03(stat.)  fm and a(n)=0.55±0.01(stat.)(-0.03)(+0.02)(sys.)  fm, respectively, corresponding to a neutron skin thickness Δr(np)=0.15±0.03(stat.)(-0.03)(+0.01)(sys.)  fm. The results give the first successful extraction of a neutron skin thickness with an electromagnetic p…

The angular distribution of the 12C(γ,pn) reaction for Eγ=120−150 MeV

The angular distribution of nucleons emitted in the 12C(γ,pn) reaction has been measured using tagged photons at the Mainz microtron MAMI. The variation of the reaction strength with the polar angles of the two emitted nucleons is reported for Eγ=120−150 MeV. The proton angular distribution peaks at more backward angles than the 2H(γ,p) differential cross section indicating a departure from the simple quasi-deuteron model of 2N photo-emission. The distribution shape is in reasonable agreement with microscopic theoretical models which include both π- and ρ-exchange.

First measurement of the polarisation transfer on the proton in the reactions $$H(\vec e,e'\vec p)$$ and $$D(\vec e,e'\vec p)$$

The measurement of the polarisation transfer to the proton in the reactions\(H(\vec e,e'\vec p)\) and\(D(\vec e,e'\vec p)\) performed with longitudinally polarised electrons in quasi-free kinematics is presented. The coincidence measurement was executed atQ2≈8fm−2 using the 855 MeV, c.w. beam of the Mainz Microtron MAMI. The recoil polarisation was determined by means of a carbon analyser. The experiment shows that the binding of the nucleon does not modify the polarisationPx of the recoil proton within an error ofΔPx/Px≈10%. The measured polarisation agrees with recent theoretical predictions. Implications for the measurement of the electric form factor of the neutron using the\(D(\vec e,e…

Determination of the η-transition form factor in the γp→pη→pγe+e− reaction

The Dalitz decay η→γe+e− has been measured using the combined Crystal Ball and TAPS photon detector setup at the electron accelerator MAMI-C. Compared to the most recent transition form-factor measurement in the e+e− channel, statistics have been improved by one order of magnitude. The e+e− invariant mass distribution shows a deviation from the QED prediction for a point-like particle, which can be described by a form-factor. Using the usual monopole transition form-factor parameterization, F(m2)=(1−m2/Λ2)−1, a value of Λ−2=(1.92±0.35(stat)±0.13(syst)) GeV−2 has been determined. This value is in good agreement with a recent measurement of the η Dalitz decay in the μ+μ− channel and with rece…

Mechanisms in theC12(γ,pn)and(γ,pp)Reactions

${}^{12}\mathrm{C}(\ensuremath{\gamma},pn)$ and $(\ensuremath{\gamma},pp)$ cross sections have been measured in kinematics which emphasize direct $2N$ knockout for ${E}_{\ensuremath{\gamma}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}120--400\mathrm{MeV}$ and ${\ensuremath{\theta}}_{p}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}23\ifmmode^\circ\else\textdegree\fi{}--152\ifmmode^\circ\else\textdegree\fi{}$. The cross sections for both reactions peak strongly in the $\ensuremath{\Delta}$-resonance region. Theoretical calculations that account for the medium dependence of the $\ensuremath{\Delta}$ propagator predict a similar photon energy dependence to the data and describe the …

Proton photoproduction from 12C

Abstract The 12 C(γ,p) differential cross-section in the Δ-resonance region has been measured using tagged photons. Data are presented for 20 MeV wide photon energy bins at 200, 300, 400 and 500 MeV and for proton angles from 52° to 128°. The present 12 C(γ,p) measurements are compared with previous experiments carried out at Frascati, Tokyo and Bonn. Comparisons are also made with theoretical calculations developed at Gent and Valencia which provide an interpretation of the photon reaction mechanism in terms of the major contributing processes. Coincidence data with correlated neutrons, protons and pions are also presented.

The12C(γ,NN)reaction studied over a wide kinematic range

The ${}^{12}\mathrm{C}(\ensuremath{\gamma},np)$ and ${}^{12}\mathrm{C}(\ensuremath{\gamma},pp)$ reactions have been studied using the Glasgow photon tagging spectrometer at the Mainz MAMI electron microtron for ${E}_{\ensuremath{\gamma}}=150--700$ MeV over a kinematic range which extends well beyond the approximately back-to-back detector arrangements of previous work. For ${}^{12}\mathrm{C}(\ensuremath{\gamma},np)$ the general trends of the missing energy distributions are reproduced over a wide range of kinematics and photon energies by the theory developed by the Valencia group. The corresponding ${}^{12}\mathrm{C}(\ensuremath{\gamma},pp)$ channel is overestimated by a factor of \ensurem…

In-medium modifications of the pi pi interaction in photon-induced reactions

Differential cross sections of the reactions $(\gamma,\pi^\circ\pi^\circ)$ and $(\gamma,\pi^\circ\pi^++\pi^\circ\pi^-)$ have been measured for several nuclei ($^1$H,$^{12}$C, and $^{\rm nat}$Pb) at an incident-photon energy of $E_{\gamma}$=400-460 MeV at the tagged-photon facility at MAMI-B using the TAPS spectrometer. A significant nuclear-mass dependence of the $\pi\pi$ invariant-mass distribution is found in the $\pi^\circ\pi^\circ$ channel. This dependence is not observed in the $\pi^\circ\pi^{+/-}$ channel and is consistent with an in-medium modification of the $\pi\pi$ interaction in the $I$=$J$=0 channel. The data are compared to $\pi$-induced measurements and to calculations within …

Two-body photodisintegration of the deuteron from 100 to 800 MeV

Abstract The total and the differential cross sections for the D(γ,p)n reaction have been measured over the photon energy range 100-800 MeV at the 855 MeV MAMI Microtron in Mainz. Using the large acceptance detector DAPHNE in conjunction with the Glasgow tagging spectrometer, high precision results with small systematic errors were obtained. The data are presented in the form of thirty-five angular distributions at c.m. proton angles between 30°–160° in 10° intervals and at photon energies in steps of 20 MeV. Previous experimental work is reassessed in the light of the present results and comparison with some recent theoretical calculations.

Mass and Γ3π0/Γγγ decay branching ratio of theη-meson from the p(γ, η)p reaction

Near threshold photoproduction ofη-mesons from the proton has been measured at the MAMI accelerator with the TAPS spectrometer. The mass of theη-meson was deduced from the threshold energy forη-photoproduction. The result of m η =(547.12 ± 0.06 ± 0.25) MeV supports the low value of theη-mass reported from a dp →3Heη measurement at SATURNE in 1992. Theη-decay branching ratio Γ3π0/Γγγ was measured to be (0.832±0.005±0.012).

Incoherent neutral pion photoproduction on 12C.

We present the first detailed measurement of incoherent photoproduction of neutral pions to a discrete state of a residual nucleus. The C-12(gamma,pi(0))C-12(4.4 MeV)* reaction has been studied with the Glasgow photon tagger at MAMI employing a new technique which uses the large solid angle Crystal Ball detector both as a pi(0) spectrometer and to detect decay photons from the excited residual nucleus. The technique has potential applications to a broad range of future nuclear measurements with the Crystal Ball and similar detector systems elsewhere. Such data are sensitive to the propagation of the Delta in the nuclear medium and will give the first information on matter transition form fa…

Determination of the neutron electric form factor in the D(e,e′n)p reaction and the influence of nuclear binding

The electric form factor of the neutron GE,n has been determined at the Mainz Microtron MAMI at the low momentum transfer Q2= 0.15 (GeV/c)2 in a measurement of the recoil polarisation ratio Px/Pz in the quasifree reaction D(e,e′n)p. At this Q2 the influence of the nuclear binding is strong. A purely kinematical model is used to get some insight into the effect of the initial Fermi momentum distribution of the neutron. The influence of the final state interaction is determined quantitatively by a model of Arenhovel et al.. After the corresponding corrections a value of GE,n(0.15 (GeV/c)2) = 0.0481±0.0065stat±0.0053syst is obtained.

Angular distribution of coherent bremsstrahlung

The angular distribution of the linearly polarised photon beam produced by coherent bremsstrahlung from an aligned diamond radiator has been measured at the MAMI A2 tagged photon facility. The measurements were made with a prototype position sensitive photon detector which utilises the pair production process and a double sided silicon strip detector. This polarised photon beam is used for nuclear and hadronic experiments and in their analysis the polarisation is obtained from a calculation, which matches the experimental intensity spectrum. As the polarisation is related to the photon beam angular distribution, the present measurements can be used to test this calculation. The overall agre…

The Electric Formfactor of the Neutron Determined by Quasielastic Scattering of Longitudinally Polarized Electrons from 3He and 2D

The recent availability of polarized electron beams at electron accelerator laboratories adds another technique to probing the electromagnetic structure of nuclear matter [1]. The present paper discusses the application of polarized electron scattering to the determination of the electric formfactor of the neutron. Its value at medium energies is only poorly known hitherto. Electron scattering is dominated by interaction with the neutron magnetic moment. Therefore the contribution of the charge distribution to the scattering cross section is scarcely detectable in case of unpolarized collision partners. The elastic scattering of longitudinally polarized electrons from a polarized neutron ta…

12C(γ,π+n) and 12C(γ,π+p) reactions across the Δ resonance region

Abstract Differential cross-sections from (γ,π+n) and (γ,π+p) experiments on 12C are presented together with a microscopic model calculation. The comparison of experimental data with theory confirms that quasifree pion production is the dominant process for the (γ,π+n) reaction. The comparison of (γ,π+p) with (γ,π+n) data shows the importance of final state interactions. first time to experimentally study the strength and the possible mechanisms involved in the final state interaction.

Three-nucleon mechanisms in photoreactions

The $^{12}$C$(\gamma,ppn)$ reaction has been measured for E$_{\gamma}$=150-800 MeV in the first study of this reaction in a target heavier than $^3$He. The experimental data are compared to a microscopic many body calculation. The model, which predicts that the largest contribution to the reaction arises from final state interactions following an initial pion production process, overestimates the measured cross sections and there are strong indications that the overestimate arises in this two-step process. The selection of suitable kinematic conditions strongly suppresses this two-step contribution leaving cross sections in which up to half the yield is predicted to arise from the absorptio…

Nuclear pion photoproduction in theΔresonance region

A measurement of the /sup 12/C( gamma , pi /sup +/n)/sup 11/B reaction in quasifree pi -production kinematic regimes has been performed using tagged photons in conjunction with large solid angle pi and n detectors. The aim of the experiment was to investigate predicted modifications to the Delta excitation of nucleons and their subsequent propagation and decay, brought about by the nuclear medium. Differential cross sections are presented for photon energies spanning the Delta (1232) excitation region. The measurements are consistent with distorted wave impulse approximation calculations in which the amplitude for proton Delta excitation, followed by Delta propagation and decay to pi /sup +…

Differential cross section for the reaction 2H(γ, p)n from 133 to 158 MeV

The cross section for the reaction 2H(γ, p)n has been measured at laboratory photon energies Eγ = 133−158 MeV and c.m. angles between 30° and 150°. The reaction was induced by a tagged bremsstrahlung photon beam incident on a liquid deuterium target. The uncertainty in the absolute cross sections is ⩽ 5%. There is now reasonable agreement between recent measurements in this energy region and the overall data set now defines the cross section sufficiently well to provide a test of current models of the reaction.

First measurement of the electric formfactor of the neutron in the exclusive quasielastic scattering of polarized electrons from polarized 3He

Abstract A first measurement of the asymmetry in quasielastic scattering of longitudinally polarized electrons from a polarized 3 He gas target in coincidence with the knocked out neutron is reported. This measurement was made feasible by the cw beam of the 855 MeV Mainz Microtron MAMI. It allows a determination of the electric formfactor of the neutron G E n independent of binding effects to first order. At Q 2 =0.31 ( GeV /c) 2 two asymmetries A ∥ ( S He ∥ q ) and A ⊥ ( S He ⊥ q ) have been measured giving A ∥ =(−7.40±0.73)% and A ⊥ =(0.89±0.30)% . The ratio A ⊥ / A ∥ is independent of the absolute value of the electron and target polarization and yields G E n =0.035±0.012±0.005.

Scaling of the 4He(γ,p)t reaction in the Eγ = 80–160 MeV region

The 4He(γ,p) 3H reaction cross section has been measured with tagged photons of energy 81–158 MeV. Protons were detected over the angular range 55°–125°. The scaling of the data as a function of momentum mismatch has been examined for two reaction models, the modified quasideuteron mechanism and quasifree knockout. Both models show clear scaling behaviour but comparison of the predicted and measured scaling function shapes favours the modified quasideuteron mechanism at momenta up to ≈500 MeV/c.

Determination of the neutron electric form factor from the reaction 3 He(e,e'n) at medium momentum transfer

The electric form factor of the neutron GEn has been determined in double polarized exclusive 3He(e,e'n) scattering in quasi–elastic kinematics by measuring asymmetries A⊥, A∥ of the cross section with respect to helicity reversal of the electron, with the nuclear spin being oriented perpendicular to the momentum transfer q in case of A⊥ and parallel in case of A∥. The experiment was performed at the 855 MeV c. w. microtron MAMI at Mainz. The degree of polarization of the electron beam and of the gaseous 3He target were each about 50%. Scattered electrons and neutrons were detected in coincidence by detector arrays covering large solid angles. Quasi–elastic scattering events were reconstruc…

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 …

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).

Radiative Pion Photoproduction from the Proton and π + Meson Polarizabilities

We study the possibility of investigating radiative pion photoproduction from the proton at the microtron MAMI-B with the aim to obtain an experimental information about the π + meson polarizabilities. It is shown that an exposition time of about 30 days will allow to determine the π + meson polarizability with quite high accuracy.

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.

Polarized photon measurements of the12C(γ→,pp)and12C(γ→,pn)reactions forEγ=160−350MeV

The ${}^{12}\mathrm{C}(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},pp)$ and ${}^{12}\mathrm{C}(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},pn)$ reactions have been studied for ${E}_{\ensuremath{\gamma}}=160\ensuremath{-}350$ MeV using linearly polarized photons from the Glasgow tagged photon spectrometer at the Mainz microtron MAMI. Both reaction channels show a negative photon asymmetry $\ensuremath{\Sigma}$ for missing energies ${E}_{m}$ below 70 MeV, where direct emission nucleon pairs is expected. A strong peak at low ${E}_{m}$ is observed in ${\ensuremath{\Sigma}}_{(\ensuremath{\gamma},pp)},$ but not in ${\ensuremath{\Sigma}}_{(\ensuremath{\gamma},pn)}.$ Furth…

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…

C12(γ,p)11B cross section from 80 to 157 MeV

The $^{12}\mathrm{C}$(\ensuremath{\gamma},p${)}^{11}$B differential cross section has been measured over proton angles ranging from 58\ifmmode^\circ\else\textdegree\fi{} to 128\ifmmode^\circ\else\textdegree\fi{}, using tagged photons of energy 80--157 MeV, for low-lying regions of residual excitation energy in $^{11}\mathrm{B}$. The data have been compared with four different types of calculation. It is shown that scaling of the cross section with momentum mismatch occurs for both the ground-state and excited-state data.

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.

"Table 1" of "Differential cross section for the reaction $^2$H(��,p)n from 133 to 158 MeV"

No description provided.

"Table 6" of "In-medium modifications of the pi pi interaction in photon-induced reactions."

Differential cross section for PI0PI+/- production with a Lead target. Errors are statistical only.

"Table 2" of "In-medium modifications of the pi pi interaction in photon-induced reactions."

Differential cross section for PI0PI0 production with a Carbon target. Errors are statistical only.

"Table 2" of "C-12 (gamma,p) B-11 cross-section from 80 to 157 MeV"

EX = 7 (FROM 6.5 TO 8.0 MEV). ANGULAR BINS OF 5 DEG WIDTH.

"Table 3" of "Scaling of the $^{4}$He(��,p)t reaction in the $E_\gamma$ = 80���160 MeV region"

No description provided.

"Table 2" of "The 16O(��, 2N) reaction measured with tagged photons"

PROPOSED THAT THE REACTION IS DUE TO A QUASI DEUTERON MECHANISM AND P N PAIRS E]ECT FROM THE 1P SHELL.

"Table 1" of "In-medium modifications of the pi pi interaction in photon-induced reactions."

Differential cross section for PI0PI0 production with a proton target. Errors are statistical only. Note that the data given in this table are sightly different (newer) than the data points presented in the paper.

"Table 4" of "In-medium modifications of the pi pi interaction in photon-induced reactions."

Differential cross section for PI0PI+ production with a proton target. Errors are statistical only. Note that the data given in this table are sightly different (newer) than the data points presented in the paper.

"Table 6" of "Scaling of the $^{4}$He(��,p)t reaction in the $E_\gamma$ = 80���160 MeV region"

No description provided.

"Table 3" of "C-12 (gamma,p) B-11 cross-section from 80 to 157 MeV"

EX = 13 (FROM 12.0 TO 13.5 MEV). ANGULAR BINS OF 5 DEG WIDTH.

"Table 4" of "Scaling of the $^{4}$He(��,p)t reaction in the $E_\gamma$ = 80���160 MeV region"

No description provided.

"Table 3" of "In-medium modifications of the pi pi interaction in photon-induced reactions."

Differential cross section for PI0PI0 production with a Lead target. Errors are statistical only.

"Table 1" of "Scaling of the $^{4}$He(��,p)t reaction in the $E_\gamma$ = 80���160 MeV region"

No description provided.

"Table 1" of "Double pi0 photoproduction off the proton at threshold."

Total cross section measurement. Statistical errors only.

"Table 7" of "Scaling of the $^{4}$He(��,p)t reaction in the $E_\gamma$ = 80���160 MeV region"

No description provided.

"Table 5" of "In-medium modifications of the pi pi interaction in photon-induced reactions."

Differential cross section for PI0PI+/- production with a Carbon target. Errors are statistical only.

"Table 1" of "First measurement of the polarization transfer on the proton in the reactions H (e (polarized), e-prime p (polarized)) and D (e (polarized), e-prime p (polarized))"

No description provided.

"Table 1" of "C-12 (gamma,p) B-11 cross-section from 80 to 157 MeV"

EX IN 0 MEV REGION (FROM 0 TO 1.5 MEV). ANGULAR BINS OF 5 DEG WIDTH.

"Table 2" of "Differential cross section for the reaction $^2$H(��,p)n from 133 to 158 MeV"

No description provided.

"Table 2" of "Scaling of the $^{4}$He(��,p)t reaction in the $E_\gamma$ = 80���160 MeV region"

No description provided.

"Table 5" of "Scaling of the $^{4}$He(��,p)t reaction in the $E_\gamma$ = 80���160 MeV region"

No description provided.

"Table 1" of "The 16O(��, 2N) reaction measured with tagged photons"

PROPOSED THAT THE REACTION IS DUE TO A QUASI DEUTERON MECHANISM AND P N PAIRS E]ECT FROM THE 1P SHELL.