We present the achievements of the last years of the experimental and theoretical groups working on hadronic cross section measurements at the low-energy
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colliders in Beijing, Frascati, ...Ithaca, Novosibirsk, Stanford and Tsukuba and on
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decays. We sketch the prospects in these fields for the years to come. We emphasise the status and the precision of the Monte Carlo generators used to analyse the hadronic cross section measurements obtained as well with energy scans as with radiative return, to determine luminosities and
τ
decays. The radiative corrections fully or approximately implemented in the various codes and the contribution of the vacuum polarisation are discussed.
We report a measurement of the spin polarization of the recoiling neutron in deuterium photodisintegration, utilizing a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured ...photon energy range of 300-700 MeV provides the first measurement of recoil neutron polarization at photon energies where the quark substructure of the deuteron plays a role, thereby providing important new constraints on photodisintegration mechanisms. A very high neutron polarization in a narrow structure centered around E_{γ}∼570 MeV is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition suggests this behavior could be related to the excitation of the d^{*}(2380) hexaquark.
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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.
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.
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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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