We study the process e+e−→π+π−ηγ, where the photon is radiated from the initial state. About 8000 fully reconstructed events of this process are selected from the BABAR data sample with an integrated ...luminosity of 469 fb−1. Using the π+π−η invariant mass spectrum, we measure the e+e−→π+π−η cross section in the e+e− center-of-mass energy range from 1.15 to 3.5 GeV. The cross section is well described by the Vector-Meson dominance model with four ρ-like states. We observe 49±9 events of the J/ψ decay to π+π−η and measure the product ΓJ/Ψ→e+e−BJ/Ψ→π+π−η=2.34±0.43stat±0.16syst eV.
A cryogenic supersonic gas jet target was developed for the MAGIX experiment at the high-intensity electron accelerator MESA. It will be operated as an internal, windowless target in the ...energy-recovering recirculation arc of the accelerator with different target gases, e.g., hydrogen, deuterium, helium, oxygen, argon, or xenon. Detailed studies have been carried out at the existing A1 multi-spectrometer facility at the electron accelerator MAMI. This paper focuses on the developed handling procedures and diagnostic tools, and on the performance of the gas jet target under beam conditions. Considering the special features of this type of target, it proves to be well suited for a new generation of high-precision electron scattering experiments at high-intensity electron accelerators.
A first measurement of the polarisation transfer from a circularly-polarised photon to the final state neutron (Cx'n) in deuterium photodisintegration has been carried out. This quantity is ...determined over the photon energy range 370 – 700 MeV and for neutron centre-of-mass breakup angles ~ 45 - 120°. The polarisation of the final state neutrons was determined by an ancillary large-acceptance nucleon polarimeter, surrounding a cryogenic liquid deuterium target within the Crystal Ball detector at MAMI. The polarimeter characterised (n, p) charge exchange of the ejected neutrons to determine their polarisation. The new Cx'n data are also compared to a theoretical model based on nucleonic and nucleon resonance degrees of freedom constrained by the current world-database of deuterium photodisintegration measurements. Structures in Cx'n observed in the region of the d*(2380) could not be explained by conventional models of deuteron photodisintegration.
The light collection of several fiber configurations embedded in a box-shaped plastic scintillating counter was studied by scanning with minimum ionizing electrons. The light was read out by silicon ...photomultipliers at both ends. The light yield produced by the 855-MeV beam of the Mainz Microtron showed a strong dependence on the transverse distance from the beam position to the fibers. The observations were modeled by attributing the collection of indirect light inside of the counter and of direct light reaching a fiber to the total light yield. The light collection with fibers was compared to that of a scintillating counter without fibers. These studies were carried out within the development of plastic scintillating detectors as an active veto system for the DarkMESA electron beam-dump experiment that will search for light dark matter particles in the MeV mass range.
Virtual Compton scattering on the proton has been investigated at three yet unexplored values of the four-momentum transfer Q2: 0.10, 0.20, and 0.45 GeV2, at the Mainz Microtron. Fits performed ...using either the low-energy theorem or dispersion relations allowed the extraction of the structure functions PLL−PTT/ε and PLT, as well as the electric and magnetic generalized polarizabilities αE1(Q2) and βM1(Q2). These new results show a smooth and rapid falloff of αE1(Q2), in contrast to previous measurements at Q2=0.33 GeV2, and provide for the first time a precise mapping of βM1(Q2) in the low-Q2 region.
Abstract We present the findings of a study based on a new inelastic electron-scattering experiment on the $^{12}$$ 12 C nucleus focusing on the kinematic region of $$Q^2=0.8\,\textrm{GeV}^2/{c}^2$$ ...Q 2 = 0.8 GeV 2 / c 2 . The measured cross section is sensitive to the transverse response function and provides a stringent test of theoretical models, as well as of the theoretical assumptions made in Monte-Carlo event-generator codes developed for the interpretation of neutrino-nucleus experiments, such as DUNE and HyperK. We find that modern generators such as GENIE and GiBUU reproduce our new experimental data within 10 $$\%$$ % .
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