[en] The large momentum acceptance of the Kaos spectrometer at the Mainz Microtron MAMI requires the discrimination between kaons and pions for momenta ≥ 1 GeV/c. Silica aerogel with refractive index n∝1.07 has been chosen as a Cerenkov radiator for pion suppression. The area to be covered by the aerogel (H x W ∝ 40 x 200 cm²) is governed by the focal plane of the magnetic spectrometer. A prototype detector with two aerogel tiles of d=2 cm thickness and 20 x 10 cm² total area was designed, built and tested. Modifications to the basic geometry of the prototype were tried, e.g. different kinds of polished aluminum faces were used in the interior in order to improve the reflection of photons towards the PMTs. The behaviour of the prototype has been simulated by using Litrani, a C++/ROOT-based program. Different parameters of the aerogel (absorption and scattering) have been measured and used as input data to the simulation code. The characteristic lengths were of the order of 10 cm in the wavelength range of the produced Cerenkov photons. During two beam-times in Oct. and Nov. 2008 at MAMI the performance of the prototype was measured with electrons, pions, and protons. The absorption and scattering losses in the aerogel tiles lead to a very low number of detected photons. Detection efficiencies were determined at different threshold settings and particle momenta.

[en] A new exclusion limit for the electromagnetic production of a light U(1) gauge boson γ^' decaying to e⁺e^- was determined by the A1 Collaboration at the Mainz Microtron. Such light gauge bosons appear in several extensions of the standard model and are also discussed as candidates for the interaction of dark matter with standard model matter. In electron scattering from a heavy nucleus, the existing limits for a narrow state coupling to e⁺e^- were reduced by nearly an order of magnitude in the range of the lepton pair mass of 210 MeV/c²e⁺e^-<300 MeV/c². This experiment demonstrates the potential of high current and high resolution fixed target experiments for the search for physics beyond the standard model.

[en] The performance of scintillating fibre detectors was studied with electrons at the spectrometer facility of the Mainz microtron MAMI, as well as in a ¹²C beam of 2 AGeV energy and in a beam of different particle species at GSI. Multi-anode photomultipliers were used to read out one or more bundles of 128 fibres each in different geometries. For electrons a time resolution of FWHM ∼ 1 ns was measured in a single detector plane with a detection efficiency ε>99%. A time resolution of 310 ps (FWHM) between two planes of fibres was achieved for carbon ions, leading to a FWHM ∼220ps for a single detector. The hit position residual was measured with a width of FWHM=0.27mm. The variation in the measured energy deposition was ΔE/E=15-20% (FWHM) for carbon ions. In addition, the energy response to p/π⁺/d particles was studied. Based on the good detector performance fibre hodoscopes will be constructed for the KAOS/A1 spectrometer at MAMI and for the HypHI experiment at GSI

[en] In 2007 the Mainz Microtron MAMI has been upgraded to 1.5 GeV electron beam energy, crossing the energy threshold for open strangeness production. The strangeness quantum number, as carried by the strange quark, provides valuable information on the contribution of individual quark flavours to hadronic processes. Theoretically, the strange quark with its rest energy of order 150 MeV is particularly interesting because it can neither be treated as a massless nor as a heavy quark. Experimentally, an instrument of central importance for the charged kaon electro-production off the proton or light nuclei at MAMI is the magnetic spectrometer Kaos that was installed recently and is now routinely operated by the A1 collaboration. (authors)

[en] The investigation of light hypernuclei is quite important for understanding the basic YN interaction and the mechanism of hypernuclear structure. We started the commissioning of the decay pion spectroscopy of light hypernuclei at MAMI-C in 2011. In order to realize the K⁺ tagging efficiently, some detectors on KAOS spectrometer were upgraded or newly installed. The existing and well-studied spectrometers, SpekA, SpekC were used as pion spectrometers. The analysis is ongoing to estimate the detectors performance and develop the spectrometers for future experiments with higher beam intensity. The preliminary results of the particle identification are presented in this article. (author)

[en] Cross-section measurements of the exclusive p(e,e'K⁺)Λ, Σ⁰electroproduction reactions have been performed at the Mainz Microtron MAMI in the A1 spectrometer facility using for the first time the KAOS spectrometer for kaon detection. These processes were studied in a kinematical region not covered by any previous experiment. The nucleon was probed in its third-resonance region with virtual photons of low four-momenta, Q² = 0.030-0.055 (GeV/c)². The MAMI data indicate a smooth transition in Q² from photoproduction to electroproduction cross-sections. Comparison with predictions of effective Lagrangian models based on the isobar approach reveal that strong longitudinal couplings of the virtual photon to the N^* resonances can be excluded from these models. Modern isobar and Regge-plus-resonance models are in agreement with the data. (orig.)

[en] Wire chambers are routinely operated as tracking detectors in magnetic spectrometers at high-intensity continuous electron beams. Especially in experiments studying reactions with small cross-sections the reaction yield is limited by the background rate in the chambers. One way to determine the track of a charged particle through a multi-wire proportional chamber (MWPC) is the measurement of the charge distribution induced on its cathodes. In practical applications of this read-out method, the algorithm to relate the measured charge distribution to the avalanche position is an important factor for the achievable position resolution and for the track reconstruction efficiency. An algorithm was developed for operating two large-sized MWPCs in a strong background environment with multiple-particle tracks. Resulting efficiencies were determined as a function of the electron beam current and on the signal amplitudes. Because of the different energy-losses of pions, kaons, and protons in the momentum range of the spectrometer the efficiencies depend also on the particle species.

[en] The Mainz Microtron MAMI provides a continuous-wave unpolarized or spin-polarized electron beam with energies up to 1.6 GeV and high degrees of polarization. Electro-production of strange mesons is performed in the multi-spectrometer facility with the KAOS spectrometer for kaon detection and a high-resolution spectrometer for electron detection in plane or out of plane. Differential cross section measurements of exclusive p(e,e^′K⁺)Λ,Σ⁰ reactions at low four-momentum transfers in the nucleon's third resonance region have been done, followed by a measurement of the beam helicity asymmetry for p(e^→,e^′K⁺)Λ. These studies are important for the understanding of the effective coupling of photons to the hadrons and their resonances and electromagnetic form factors entering different effective Lagrangian models for photo- and electro-production of strangeness. The polarized structure function, extracted from the beam helicity asymmetry, shows an especially high sensitivity to model parameters

[en] At the Mainz Microtron hypernuclei are produced by (e,e^′K) reactions. A dedicated kaon spectrometer located at 0° with respect to the electron beam is used to detect kaons emitted in forward direction thus tagging events involving strangeness production. By measuring the momenta of pions from two body weak decays using high resolution magnetic spectrometers one gains direct access to the ground state masses of the produced hyperfragments. At FAIR the PANDA Collaboration intends to produce double-hypernuclei by numbers with an antiproton beam and study their high resolution γ-spectroscopy thus providing for the first time precise information on the level structure of these nuclei. The status of both experiments is reviewed

[en] Since 2008 the magnetic spectrometer KAOS, dedicated to the detection of charged kaons, is operating at the 1.5 GeV electron beam of MAMI at the Institut fuer Kernphysik in Mainz, Germany. The strangeness programme performed in 2008-9 is addressing some important issues in the field of elementary kaon photoelectro-production reactions. The identification of Λ and Σ⁰ hyperons in the missing mass spectra from kaon production off a liquid hydrogen target demonstrates the capability of the extended facility to perform strangeness electro-production spectroscopy at low momentum transfers Q²<0.5(GeV/c)². Systematic uncertainties in the cross-section extraction from the data are discussed.