The cross section of the p(e,e′π+)n reaction has been measured for five kinematic settings at an invariant mass of W=1094 MeV and for a four-momentum transfer of Q2=0.078 (GeV/c)2. The measurement has been performed at MAMI using a new short-orbit spectrometer (SOS) of the A1 collaboration, intended for detection of low-energy pions. The transverse and longitudinal cross section terms were separated using the Rosenbluth method and the transverse-longitudinal interference term has been determined from the left-right asymmetry. The experimental cross section terms are compared with the calculations of three models: DMT2001, MAID2007 and χMAID. The results show that we do not yet understand the dynamics of the fundamental pion.

A new Short-Orbit Spectrometer (SOS) has been constructed and installed within the experimental facility of the A1 collaboration at Mainz Microtron (MAMI), with the goal to detect low-energy pions. It is equipped with a Browne–Buechner magnet and a detector system consisting of two helium–ethane based drift chambers and a scintillator telescope made of five layers. The detector system allows detection of pions in the momentum range of 50–147 MeV/c, which corresponds to 8.7–63 MeV kinetic energy. The spectrometer can be placed at a distance range of 54–66 cm from the target center. Two collimators are available for the measurements, one having 1.8 msr aperture and the other having 7 msr aperture. The Short-Orbit Spectrometer has been successfully calibrated and used in coincidence measurements together with the standard magnetic spectrometers of the A1 collaboration.

The differential cross section was measured for the 12C(e,e'pp)10Be g.s. reaction at energy and momentum transfers of 163MeV and 198MeV/c, respectively. The measurement was performed at the Mainz Microtron by using two high-resolution magnetic spectrometers of the A1 Collaboration and a newly developed silicon detector telescope. The overall resolution of the detector system was sufficient to distinguish the ground state from the first excited state in 10 Be. We chose a super-parallel geometry that minimizes the effect of two-body currents and emphasizes the effect of nucleon-nucleon correlations. The obtained differential cross section is compared to the theoretical results of the Pavia reaction code in which different processes leading to two-nucleon knockout are accounted for microscopically. The comparison shows a strong sensitivity to nuclear-structure input and the measured cross section is seen to be dominated by the interplay between long- and short-range nucleon-nucleon correlations. Microscopic calculations based on the ab initio self-consistent Green's function method give a reasonable description of the experimental cross section.

An experiment on the radiative π-meson photoproduction from the proton (γp → γπn) was carried out at the Mainz Microtron MAMI in the kinematic region 537 MeV < Eγ < 817 MeV, 140 ≤ θ γγ′ ≤ 180. The π-meson polarizabilities have been determined from a comparison of the data with the predictions of two different theoretical models, the first one being based on an effective pole model with pseudoscalar coupling while the second one is based on diagrams describing both resonant and nonresonant contributions. The validity of the models has been verified by comparing the predictions with the present experimental data in the kinematic region where the pion polarizability contribution is negligible (s1 < 5m 2 π) and where the difference between the predictions of the two models does not exceed 3%. In the region, where the pion polarizability contribution is substantial (5 < s1/m 2 π < 15, −12 < t/mπ < −2), the difference (α − β)π+ of the electric (α) and the magnetic (β) polarizabilities has been determined. As a result we find: (α− β)π+ = (11.6± 1.5stat ± 3.0syst ± 0.5mod)× 10−4fm. This result is at variance with recent calculations in the framework of chiral perturbation theory. PACS: 12.38.Qk Experimental tests – 13.40.-f Electromagnetic processes and properties – 13.60.Le Meson production E-mail: filkov@sci.lebedev.ru

Double-polarization observables in the reaction (e) over right arrow ep -> e'(p) over right arrow'gamma have been measured at Q(2) = 0.33 (GeV/c)(2). The experiment was performed at the spectrometer setup of the A1 Collaboration using the 855 MeV polarized electron beam provided by the Mainz Microtron (MAMI) and a recoil proton polarimeter. From the double-polarization observables the structure function P-LT(perpendicular to) is extracted for the first time, with the value (-15.4 +/- 3.3((stat.)-2.4) (+1.5) (syst.)) GeV-2, using the low-energy theorem for virtual Compton scattering. This structure function provides a hitherto unmeasured linear combination of the generalized polarizabilities of the proton.

Information on the size and shape of the neutron skin on (208)Pb is extracted from coherent pion photoproduction cross sections measured using the Crystal Ball detector together with the Glasgow tagger at the MAMI electron beam facility. On exploitation of an interpolated fit of a theoretical model to the measured cross sections, the half-height radius and diffuseness of the neutron distribution are found to be c(n)=6.70±0.03(stat.)  fm and a(n)=0.55±0.01(stat.)(-0.03)(+0.02)(sys.)  fm, respectively, corresponding to a neutron skin thickness Δr(np)=0.15±0.03(stat.)(-0.03)(+0.01)(sys.)  fm. The results give the first successful extraction of a neutron skin thickness with an electromagnetic probe and indicate that the skin of (208)Pb has a halo character. The measurement provides valuable new constraints on both the structure of nuclei and the equation of state for neutron-rich matter.

A new precise determination of the \( \eta\) meson mass is presented. It is based on a measurement of the threshold for the \( \gamma p\rightarrow p\eta\) reaction using the tagger focal-plane microscope detector at the MAMI-B facility in Mainz. The tagger microscope 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. Special emphasis was put on a very careful energy calibration of the electron beam and the tagging device, which reduced considerably the systematic uncertainty compared to the previous MAMI experiment. The result \( m_{\eta}=(547.865 \pm 0.031_{stat.}\pm 0.062_{syst.})\) MeV agrees very well with the precise values of the NA48, KLOE, CLEO and COSY-ANKE Collaborations and deviates by \( 6\sigma\) from the smaller value obtained by the GEM Collaboration at COSY.

We report new p(e, e′p) π ° measurements in the Δ + (1232) resonance at the low momentum transfer region utilizing the magnetic spectrometers of the A1 Collaboration at MAMI. The mesonic cloud dynamics are predicted to be dominant and appreciably changing in this region while the momentum transfer is sufficiently low to be able to test chiral effective field theory calculations. The results disagree with predictions of constituent quark models and are in reasonable agreement with dynamical calculations with pion cloud effects, chiral effective field theory and lattice calculations. The reported measurements suggest that improvement is required to the theoretical calculations and provide valuable input that will allow their refinements.

We report new p$(\vec{e},e^\prime p)\pi^\circ$ measurements in the $\Delta^{+}(1232)$ resonance at the low momentum transfer region utilizing the magnetic spectrometers of the A1 Collaboration at MAMI. The mesonic cloud dynamics are predicted to be dominant and appreciably changing in this region while the momentum transfer is sufficiently low to be able to test chiral effective calculations. The results disagree with predictions of constituent quark models and are in reasonable agreement with dynamical calculations with pion cloud effects, chiral effective field theory and lattice calculations. The reported measurements suggest that improvement is required to the theoretical calculations and provide valuable input that will allow their refinements.

Precise total cross-sections and invariant-mass distributions have been measured for photoproduction of pion pairs off the proton producing \(\ensuremath p\pi^0\pi^0\) and \(\ensuremath n\pi^+\pi^0\) final states from the threshold region up to 800MeV incident photon energy. Additionally, beam helicity asymmetries have been measured in the second resonance region (550MeV-820MeV). 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.

A new Silicon Detector Telescope has been constructed and installed within the experimental facility of the A1 collaboration at Mainz Microtron, with the goal to detect low-energy protons. It consists of seven silicon layers for energy and angle measurement and a plastic scintillator for triggering purposes. The detector subtends a solid angle up to 88 msr, depending on the distance from the target and covers the proton kinetic energy range of 25-41 MeV with the mean energy resolution sigma(E) = 0.47 MeV, operating at 500 kHz. Digital signal processing methods applied for energy reconstruction have been important for keeping the acceptable energy resolution at high counting rates. The Silicon Detector Telescope has been successfully used in double and triple coincidence measurements along with the magnetic spectrometers of the A1 collaboration. (C) 2012 Elsevier B.V. All rights reserved.

In arXiv:1108.3058v1 [nucl-ex], Arrington criticizes the Coulomb corrections we applied in the analysis of high precision form factor data (see Phys.Rev.Lett.105:242001, 2010, arXiv:1007.5076v3 [nucl-ex]). We show, by comparing different calculations cited in the Comment, that the criticism of the Comment neglects the large uncertainty of "more modern" TPE corrections. This uncertainty has also been seen in recent polarized measurements. We rerun our analysis using one of these calculations. The results show that the Comment exaggerates the quantitative effect at small Q^2.

New, accurate data are presented on the near threshold p(e,e'p)pi^0 reaction in the range of four-momentum transfers between Q^2=0.05 and 0.15GeV^2/c^2. The data were taken with the spectrometer setup of the A1 Collaboration at the Mainz Microtron MAMI. The complete center of mass solid angle was covered up to a center of mass energy of 4MeV above threshold. These results supersede the previous analysis based on three separate experiments, and are compared with calculations in Heavy Baryon Chiral Perturbation Theory and with phenomenological models.

A Reply to the Comment by J. Arrington.Received 22 June 2011DOI:https://doi.org/10.1103/PhysRevLett.107.119102© 2011 American Physical Society

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.

Hadron physics addresses a variety of fundamental phenomena arising from the many-body structure of strongly interacting systems. This special issue of the European Physical Journal contains 21 selected papers and review articles. They address key questions in low energy hadron physics and summarise the experimental and theoretical results which have been obtained within the Collaborative Research Center CRC443 - "Many-body structure of strongly interacting systems".

The reaction gamma p -> p pi(0)gamma' has been measured with the Crystal Ball/TAPS detectors using the energy-tagged photon beam at the electron accelerator facility MAMI-B. Energy and angular differential cross-sections for the emitted photon gamma' and angular differential cross-sections for the pi(0) have been determined with high statistics in the energy range of the Delta(+)(1232)-resonance. Cross-sections and the ratio of the cross-section to the nonradiative process gamma p -> p pi(0) are compared to theoretical reaction models, having the anomalous magnetic moment kappa(Delta+) as free parameter. As the shape of the experimental distributions is not reproduced in detail by the model calculations, currently no extraction of kappa(Delta+) is feasible.