Measurement of cross sections and polarisation observables in η photoproduction from neutrons and protons bound in light nuclei

The structure of matter, i.e. the binding of nucleons to nuclei and the formation of quarks to nucleons or other hadrons, is governed by the strong interaction. The underlying Gauge theory, Quantum Chromodynamics (QCD), is well established and has a characteristic property: the coupling constant is decreasing as a function of the momentum transfer (energy). In high-energy reactions, quarks and gluons behave as free particles and the coupling constant is small. This regime of QCD, where quarks and gluons interact only weakly, is called asymptotic freedom and pertubative calculations can be used to predict interactions. However, at small energies, the quarks interact strongly and virtual gluons can produce gluon-gluon pairs and confine quarks in colorless hadrons. Due to the large coupling constant, pertubative calculations of QCD are unreliable at low energies and cannot explain the confinement. In this low-energy range, only phenomenological models such as quark models or numerical calculations (lattice QCD) can be used to solve QCD. To verify QCD models at low energies, the excitation spectrum of the nucleon is of particular interest. Comparison of the model predictions and the experimentally observed states have shown a large discrepancy in number and ordering of the levels. Many more states are predicted than have been experimentally observed, which is known as the problem of missing resonances. This mismatch may either originate from the effective degrees of freedom of the models or from experimental bias. In the beginning of hadron spectroscopy, most results have been obtained from pion-nucleon scattering experiments. However, since the intermediate nucleon resonance depends on the production mechanism, only resonances that couple to π N have been observed. In the last decades, these results have been supplemented with data on unpolarised cross sections obtained from meson photoproduction at various acceleration facilities. These results could clarify the situation to some extent. Nevertheless, the problem of missing resonances persists, which is mainly caused by the fact that many resonances are broad and overlapping. Thus, current experiments focus on the measurement of single and double polarisation observables, which may improve the situation since observables are sensitive to interference terms and thus can enhance weak contributions from resonances. In this work, η photoproduction from quasi-free protons and neutrons has been studied. Photoproduction of η mesons is of particular interest since former results of different collaborations have shown an unusual narrow structure in the cross section on the neutron, which is not visible on the proton. Various theoretical models exist that try to explain this effect, but no conclusive solution has been found yet. Thus, to get a final interpretation of this effect, unpolarised cross sections, the double polarisation observable E and the helicity dependent cross sections σ1/2 and σ3/2 have been extracted in this work. Unpolarised total and differential cross sections have been determined for protons and neutrons bound in light nuclei, i.e. deuterium and 3He. Data have been measured with the CBELSA/TAPS experiment at the Electron Stretcher Accelerator (ELSA) in Bonn (deuterium, December 2008) and with the A2 experiment at the Mainzer Microtron (MAMI) in Mainz (3He, November 2008). Both setups used energy-tagged photon beams to produce η mesons from cryogenic liquid targets. The target was surrounded by an almost 4π covering detector setup. At CBELSA/TAPS the combined setup of Crystal Barrel (CBB) and MiniTAPS was used, at A2 the main detectors were Crystal Ball (CB), TAPS. Furthermore, experiments aiming at the extraction of the double polarisation observable E, have been run at both acceleration facilities. A circularly polarised photon beam and a longitudinally polarised deuterated butanol (dButanol) target have been used. The results obtained in this work give input to new partial wave analysis and help to straighten out the situation of η photoproduction from the neutron

Measurement of the transverse target and beam-target asymmetries in η\eta meson photoproduction at MAMI

We present new data for the transverse target asymmetry T and the very first data for the beam-target asymmetry F in the $\vec \gamma \vec p\to\eta p$ reaction up to a center-of-mass energy of W=1.9 GeV. The data were obtained with the Crystal-Ball/TAPS detector setup at the Glasgow tagged photon facility of the Mainz Microtron MAMI. All existing model predictions fail to reproduce the new data indicating a significant impact on our understanding of the underlying dynamics of $\eta$ meson photoproduction. The peculiar nodal structure observed in existing T data close to threshold is not confirmed.Comment: 5 pages, 3 figures, accepted for publication in PR

T and F asymmetries in π0 photoproduction on the proton

The γp→π0p reaction was studied at laboratory photon energies from 425 to 1445 MeV with a transversely polarized target and a longitudinally polarized beam. The beam-target asymmetry F was measured for the first time and new high precision data for the target asymmetry T were obtained. The experiment was performed at the photon tagging facility of the Mainz Microtron (MAMI) using the Crystal Ball and TAPS photon spectrometers. The polarized cross sections were expanded in terms of associated Legendre functions and compared to recent predictions from several partial-wave analyses. The impact of the new data on our understanding of the underlying partial-wave amplitudes and baryon resonance contributions is discussed

A new measurement of the neutron detection efficiency for the NaI Crystal Ball detector

We report on a measurement of the neutron detection efficiency in NaI crystals in the Crystal Ball detector obtained from a study of single p0 photoproduction on deuterium using the tagged photon beam at the Mainz Microtron. The results were obtained up to a neutron energy of 400 MeV. They are compared to previous measurements made more than 15 years ago at the pion beam at the BNL AGS

K+ΛK^+\Lambda and K+Σ0K^+\Sigma^0 photoproduction with fine center-of-mass energy resolution

Measurements of $\gamma p \rightarrow K^{+} \Lambda$ and $\gamma p \rightarrow K^{+} \Sigma^0$ cross-sections have been obtained with the photon tagging facility and the Crystal Ball calorimeter at MAMI-C. The measurement uses a novel $K^+$ meson identification technique in which the weak decay products are characterized using the energy and timing characteristics of the energy deposit in the calorimeter, a method that has the potential to be applied at many other facilities. The fine center-of-mass energy ($W$) resolution and statistical accuracy of the new data results in a significant impact on partial wave analyses aiming to better establish the excitation spectrum of the nucleon. The new analyses disfavor a strong role for quark-diquark dynamics in the nucleon.Comment: 7 pages, 8 figure

Measurement of pi^0 photoproduction on the proton at MAMI C

Differential cross sections for the gamma p -> pi^0 p reaction have been measured with the A2 tagged-photon facilities at the Mainz Microtron, MAMI C, up to the center-of-mass energy W=1.9 GeV. The new results, obtained with a fine energy and angular binning, increase the existing quantity of pi^0 photoproduction data by ~47%. Owing to the unprecedented statistical accuracy and the full angular coverage, the results are sensitive to high partial-wave amplitudes. This is demonstrated by the decomposition of the differential cross sections in terms of Legendre polynomials and by further comparison to model predictions. A new solution of the SAID partial-wave analysis obtained after adding the new data into the fit is presented.Comment: 13 pages, 12 figures, 1 tabl

Photoproduction of π0\pi^0-pairs off protons and off neutrons

Total cross sections, angular distributions, and invariant-mass distributions have been measured for the photoproduction of $\pi^0\pi^0$ pairs off free protons and off nucleons bound in the deuteron. The experiments were performed at the MAMI accelerator facility in Mainz using the Glasgow photon tagging spectrometer and the Crystal Ball/TAPS detector. The accelerator delivered electron beams of 1508 and 1557~MeV, which produced bremsstrahlung in thin radiator foils. The tagged photon beam covered energies up to 1400~MeV. The data from the free proton target are in good agreement with previous measurements and were only used to test the analysis procedures. The results for differential cross sections (angular distributions and invariant-mass distributions) for free and quasi-free protons are almost identical in shape, but differ in absolute magnitude up to 15\%. Thus, moderate final-state interaction effects are present. The data for quasi-free neutrons are similar to the proton data in the second resonance region (final state invariant masses up to $\approx$1550~MeV), where both reactions are dominated by the $N(1520)3/2^-\rightarrow \Delta(1232)3/2^+\pi$ decay. At higher energies, angular and invariant-mass distributions are different. A simple analysis of the shapes of the invariant-mass distributions in the third resonance region is consistent with strong contributions of an $N^{\star}\rightarrow N\sigma$ decay for the proton, while the reaction is dominated by a sequential decay via a $\Delta\pi$ intermediate state for the neutron. The data are compared to predictions from the Two-Pion-MAID model and the Bonn-Gatchina coupled channel analysis.Comment: accepted for publication in Eur. Phys. J.

The {\eta}'-carbon potential at low meson momenta

The production of $\eta^\prime$ mesons in coincidence with forward-going protons has been studied in photon-induced reactions on $^{12}$C and on a liquid hydrogen (LH$_2$) target for incoming photon energies of 1.3-2.6 GeV at the electron accelerator ELSA. The $\eta^\prime$ mesons have been identified via the $\eta^\prime\rightarrow \pi^0 \pi^0\eta \rightarrow 6 \gamma$ decay registered with the CBELSA/TAPS detector system. Coincident protons have been identified in the MiniTAPS BaF$_2$ array at polar angles of $2^{\circ} \le \theta _{p} \le 11^{\circ}$. Under these kinematic constraints the $\eta^\prime$ mesons are produced with relatively low kinetic energy ($\approx$ 150 MeV) since the coincident protons take over most of the momentum of the incident-photon beam. For the C-target this allows the determination of the real part of the $\eta^\prime$-carbon potential at low meson momenta by comparing with collision model calculations of the $\eta^\prime$ kinetic energy distribution and excitation function. Fitting the latter data for $\eta^\prime$ mesons going backwards in the center-of-mass system yields a potential depth of V = $-$(44 $\pm$ 16(stat)$\pm$15(syst)) MeV, consistent with earlier determinations of the potential depth in inclusive measurements for average $\eta^\prime$ momenta of $\approx$ 1.1 GeV/$c$. Within the experimental uncertainties, there is no indication of a momentum dependence of the $\eta^\prime$-carbon potential. The LH$_2$ data, taken as a reference to check the data analysis and the model calculations, provide differential and integral cross sections in good agreement with previous results for $\eta^\prime$ photoproduction off the free proton.Comment: 9 pages, 13 figures. arXiv admin note: text overlap with arXiv:1608.0607

Measurements of double-polarized compton scattering asymmetries and extraction of the proton spin polarizabilities

The spin polarizabilities of the nucleon describe how the spin of the nucleon responds to an incident polarized photon. The most model-independent way to extract the nucleon spin polarizabilities is through polarized Compton scattering. Double-polarized Compton scattering asymmetries on the proton were measured in the Δ(1232) region using circularly polarized incident photons and a transversely polarized proton target at the Mainz Microtron. Fits to asymmetry data were performed using a dispersion model calculation and a baryon chiral perturbation theory calculation, and a separation of all four proton spin polarizabilities in the multipole basis was achieved. The analysis based on a dispersion model calculation yields γE1E1=−3.5±1.2, γM1M1=3.16±0.85, γE1M2=−0.7±1.2, and γM1E2=1.99±0.29, in units of 10−4  fm4