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 measure the nucleon spin polarizabilities is through polarized Compton scattering. Double-polari
zed Compton scattering asymmetries on the proton were measured in the $Delta(1232)$ 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 $gamma_{E1E1} = -3.5 pm 1.2$, $gamma_{M1M1}= 3.16 pm 0.85$, $gamma_{E1M2} = -0.7 pm 1.2$, and $gamma_{M1E2} = 1.99 pm 0.29$, in units of $10^{-4}$ fm$^4$.
A new measurement of the rare, doubly radiative decay eta->pi^0 gamma gamma was conducted with the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. New data on the dependence of th
e partial decay width, Gamma(eta->pi^0 gamma gamma), on the two-photon invariant mass squared, m^2(gamma gamma), as well as a new, more precise value for the decay width, Gamma(eta->pi^0 gamma gamma) = (0.33+/-0.03_tot) eV, are based on analysis of 1.2 x 10^3 eta->pi^0 gamma gamma decays from a total of 6 x 10^7 eta mesons produced in the gamma p -> eta p reaction. The present results for dGamma(eta->pi^0 gamma gamma)/dm^2(gamma gamma) are in good agreement with previous measurements and recent theoretical calculations for this dependence.
The Dalitz decay eta -> e^+ e^- gamma has been measured in the gamma p -> eta p reaction with the Crystal Ball and TAPS multiphoton spectrometers, together with the photon tagging facility at the Mainz Microtron MAMI. The experimental statistic used
in this work is one order of magnitude greater than in any previous measurement of eta -> e^+ e^- gamma. The value obtained for the slope parameter 1/Lambda^2 of the eta transition form factor, 1/Lambda^2 = (1.95 +/- 0.15_stat +/- 0.10_syst) [1/GeV^2], is in good agreement with recent measurements conducted in eta -> e^+ e^- gamma and eta -> mu^+ mu^- gamma decays, as well as with recent form-factor calculations. The uncertainty obtained in the value of 1/Lambda^2 is lower compared to results from previous measurements of the eta -> e^+ e^- gamma decay.
A precision measurement of the differential cross sections $dsigma/dOmega$ and the linearly polarized photon asymmetry $Sigma equiv (dsigma_perp - dsigma_parallel) slash (dsigma_perp + dsigma_parallel)$ for the $vec{gamma} p rightarrow pi^0p$ reactio
n in the near-threshold region has been performed with a tagged photon beam and almost $4pi$ detector at the Mainz Microtron. The Glasgow-Mainz photon tagging facility along with the Crystal Ball/TAPS multi-photon detector system and a cryogenic liquid hydrogen target were used. These data allowed for a precise determination of the energy dependence of the real parts of the $S$- and all three $P$-wave amplitudes for the first time and provide the most stringent test to date of the predictions of Chiral Perturbation Theory and its energy region of agreement with experiment.
A new precise determination of the {eta} meson mass is presented. It is based on a measurement of the threshold for the {gamma}p -> p{eta} reaction using the tagger focal-plane microscope detector at the MAMI-B facility in Mainz. The tagger microscop
e has a higher energy resolution than the standard tagging spectrometer and, hence, allowed an improvement in the accuracy compared to the previous {eta} mass measurement at MAMI-B. The result m{eta} = (547.851 +- 0.031 stat. +- 0.062 syst.) MeV agrees very well with the precise values of the NA48, KLOE and CLEO collaborations and deviates by about 5{sigma} from the smaller, but also very precise value obtained by the GEM collaboration at COSY.
Precise total cross-sections and invariant-mass distributions have been measured for photoproduction of pion pairs off the proton producing $ppi^0pi^0$ and $npi^+pi^0$ final states from the threshold region up to 800 MeV incident photon energy. Addit
ionally, beam helicity asymmetries have been measured in the second resonance region (550 MeV - 820 MeV). The experiment was performed at the tagged photon beam of the Mainz MAMI accelerator with the Crystal Ball and TAPS detectors combined to give an almost 4$pi$ solid-angle electromagnetic calorimeter. The results are much more precise than any previous measurements and confirm the chiral perturbation theory predictions for the threshold behavior of these reactions. In the second resonance region, the invariant-mass distributions of meson-meson and meson-nucleon pairs are in reasonable agreement with model predictions, but none of the models reproduce the asymmetries for the mixed-charge channel.
The dynamics of the $etato 3pi^0$ decay have been studied with the Crystal Ball multiphoton spectrometer and the TAPS calorimeter. Bremsstrahlung photons produced by the 1.5-GeV electron beam of the Mainz microtron MAMI-C and tagged by the Glasgow ph
oton spectrometer were used for $eta$-meson production. The analysis of $3 times 10^6$ $gamma p to eta p to 3pi^0 p to 6gamma p$ events yields the value $alpha=-0.032pm0.003$ for the $etato 3pi^0$ slope parameter, which agrees with the majority of recent experimental results and has the smallest uncertainty. The $pi^0pi^0$ invariant-mass spectrum was investigated for the occurrence of a cusplike structure in the vicinity of the $pi^+pi^-$ threshold. The observed effect is small and does not affect our measured value for the slope parameter.
A precise measurement of the Dalitz plot parameter, alpha, for the eta->3pi^0 decay is presented. The experiment was performed with the Crystal Ball and TAPS large acceptance photon detectors at the tagged photon beam facility of the MAMI-B electron
accelerator in Mainz. High statistics of 1.8*10^6 eta->3pi^0 events were obtained, giving the result alpha = -0.032 +/- 0.002(stat) +/- 0.002(syst).
Total and differential cross sections for the reaction gamma p -> pi^o eta p have been measured with the Crystal Ball/TAPS detector using the tagged photon facility at the MAMI C accelerator in Mainz. In the energy range E_gamma=0.95-1.4 GeV the reac
tion is dominated by the excitation and sequential decay of the Delta(1700)D33 resonance. Angular distributions measured with high statistics allow us to determine the ratio of hadronic decay widths Gamma_{eta Delta}/Gamma_{pi S11} and the ratio of the helicity amplitudes A_{3/2}/A_{1/2} for this resonance.
We present the first detailed measurement of incoherent photoproduction of neutral pions to a discrete state of a residual nucleus. The $^{12}$C$(gamma,pi^{0})^{12}$C$^{*}_{4.4MeV}$ reaction has been studied with the Glasgow photon tagger at MAMI emp
loying 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. The data are sensitive to the propagation of the $Delta$ in the nuclear medium and will give the first information on matter transition form factors from measurements with an electromagnetic probe. The incoherent cross sections are compared to two theoretical predictions including a $Delta$-hole model.
