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.

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.

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.

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.

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.

Cross sections for the 3 He ð e; e 0 pn Þ 1 H reaction were measured for the first time at energy transfers of 220 and 270 MeV for several momentum transfers ranging from 300 to 450 MeV =c . Cross sections are presented as a function of the momentum of the recoil proton and the momentum transfer. Continuum Faddeev calculations using the Argonne V 18 and Bonn-B nucleon-nucleon potentials overestimate the measured cross sections by a factor 5 at low recoil proton momentum with the discrepancy becoming smaller at higher recoil proton momentum.

Cross sections for the 3He(e,e' pn)1H reaction were measured for the first time at energy transfers of 220 and 270 MeV for several momentum transfers ranging from 300 to 450 MeV/c. Cross sections are presented as a function of the momentum of the recoil proton and the momentum transfer. Continuum Faddeev calculations using the Argonne V18 and Bonn-B nucleon-nucleon potentials overestimate the measured cross sections by a factor 5 at low recoil proton momentum with the discrepancy becoming smaller at higher recoil proton momentum.

D.G. Middleton,* J. R.M. Annand, M. Ases Antelo, C. Ayerbe, P. Barneo, D. Baumann, J. Bermuth, J. Bernauer, H. P. Blok, R. Böhm, D. Bosnar, M. Ding, M.O. Distler, J. Friedrich, J. Garcı́a Llongo, D. I. Glazier, J. Golak, W. Glöckle, P. Grabmayr, T. Hehl, J. Heim, W.H.A. Hesselink, E. Jans, H. Kamada, G. Jover Mañas, M. Kohl, L. Lapikás, I. J. D. MacGregor, I. Martin, J. C. McGeorge, H. Merkel, P. Merle, K. Monstad, F. Moschini, U. Müller, A. Nogga, R. Pérez-Benito, Th. Pospischil, M. Potokar, G. Rosner, M. Seimetz, R. Skibiński, H. de Vries, Th. Walcher, D. P. Watts, M. Weinriefer, M. Weiss, H. Witała, and B. Zihlmann Kepler Centre for Astro and Particle Physics, Physikalisches Institut, Universität Tübingen, D-72076 Tübingen, Germany Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, Scotland Institut für Kernphysik, Johannes Gutenberg-Universität Mainz, D-55099 Mainz, Germany Nikhef, P.O. Box 41882, 1009 DB Amsterdam, The Netherlands Department of Physics, VU-university, Amsterdam, The Netherlands Department of Physics, University of Zagreb, Zagreb, Croatia M. Smoluchowski Institute of Physics, Jagiellonian University, PL-30059 Kraków, Poland Institut für Theoretische Physik II, Ruhr-Universität Bochum, D-44780 Bochum, Germany Department of Physics, Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan Institute for Advanced Simulation, Institut für Kernphysik, and Jülich Center for Hadron Physics, Forschungszentrum Jülich, D-52425 Jülich, Germany Institute Jožef Stefan, University of Ljubljana, Ljubljana, Slovenia (Received 6 March 2009; published 7 October 2009)

The determination of nonspherical angular momentum amplitudes in nucleons at long ranges (low Q2) was accomplished through the p(→e,e′p)π0 reaction in the Δ region at Q2=0.060, 0.127, and 0.200 (GeV/c)2 at the Mainz Microtron with an accuracy for the cross sections of 4%. The results for the dominant transition magnetic dipole amplitude and the quadrupole to dipole ratios have been obtained with an estimated model uncertainty that is approximately the same as the experimental uncertainty. Lattice and effective field theory predictions agree with our data within the relatively large estimated theoretical uncertainties. Phenomenological models are in good agreement with experiment when the resonant amplitudes are adjusted to the data. To check reaction model calculations additional data were taken for center-of-mass energies below resonance and for the σLT′ structure function. These results confirm the dominance, and general Q2 variation, of the pionic contribution at large distances.9 MoreReceived 14 March 2008DOI:https://doi.org/10.1103/PhysRevC.78.025209©2008 American Physical Society

The differential cross-sections σ 0 = σ T + εσ L , σ LT , and σ TT of π^0_ electroproduction from the proton were measured from threshold up to an additional center-of-mass energy of 40MeV, at a value of the photon four-momentum transfer of Q 2 = 0.05 GeV 2 /c 2 and a center-of-mass angle of θ = 90 ° . By an additional out-of-plane measurement with polarized electrons σ LT' was determined. This showed for the first time the presence of an imaginary part of the s -wave above the π^+_ threshold, which is usually interpreted as a unitary cusp. The predictions of the Heavy Baryon Chiral Perturbation Theory (HBChPT) are in disagreement with these data. On the other hand, the data are somewhat better predicted by the MAID phenomenological model and are in good agreement with the dynamical model DMT.

The cross-section of the ep → e′p′γ reaction has been measured at Q 2 = 0.33 (GeV/c)2. The experiment was performed using the electron beam of the MAMI accelerator and the standard detector setup of the A1 Collaboration. The cross-section is analyzed using the low-energy theorem for virtual Compton scattering, yielding a new determination of the two structure functions PLL - PTT/ε and PLT which are linear combinations of the generalized polarizabilities of the proton. We find somewhat larger values than in the previous investigation at the same Q 2. This difference, however, is purely due to our more refined analysis of the data. The results tend to confirm the non-trivial Q 2-evolution of the generalized polarizabilities and call for more measurements in the low-Q 2 region (≤ 1 (GeV/c)2).

The determination of nonspherical angular momentum amplitudes in nucleons at long ranges (low Q(2)) was accomplished through the p((e) over bar e'p)pi(0) reaction in the Delta region at Q(2) = 0.060, 0.127, and 0.200 (GeV/c)(2) at the Mainz Microtron with an accuracy for the cross sections of 4%. The results for the dominant transition magnetic dipole amplitude and the quadrupole to dipole ratios have been obtained with an estimated model uncertainty that is approximately the same as the experimental uncertainty. Lattice and effective field theory predictions agree with our data within the relatively large estimated theoretical uncertainties. Phenomenological models are in good agreement with experiment when the resonant amplitudes are adjusted to the data. To check reaction model calculations additional data were taken for center-of-mass energies below resonance and for the sigma(LT)' structure function. These results confirm the dominance, and general Q(2) variation, of the pionic contribution at large distances.

The differential cross-sections σ0 = σT + εσL , σLT , and σTT of \( \pi^{0}_{}\) electroproduction from the proton were measured from threshold up to an additional center-of-mass energy of 40MeV, at a value of the photon four-momentum transfer of Q 2 = 0.05 GeV2/c2 and a center-of-mass angle of θ = 90° . By an additional out-of-plane measurement with polarized electrons σLT' was determined. This showed for the first time the presence of an imaginary part of the s -wave above the \( \pi^{+}_{}\) threshold, which is usually interpreted as a unitary cusp. The predictions of the Heavy Baryon Chiral Perturbation Theory (HBChPT) are in disagreement with these data. On the other hand, the data are somewhat better predicted by the MAID phenomenological model and are in good agreement with the dynamical model DMT.

The differential cross-sections sigma(0) = sigma(T) + epsilon sigma(L) , sigma(LT) , and sigma(TT) of pi 0selectroproduction from the proton were measured from threshold up to an additional center-of-mass energy of 40MeV, at a value of the photon four-momentum transfer ofQ 2 = 0.05 GeV2/c and a center-of-mass angle of theta = 90 . By an additional out-of-plane measurement with polarized electrons sigma(LT') was determined. This showed for the first time the presence of an imaginary part of thes -wave above the pi threshold, which is usually interpreted as a unitary cusp. The predictions of the Heavy Baryon Chiral Perturbation Theory (HBChPT) are in disagreement with these data. On the other hand, the data are somewhat better predicted by the MAID phenomenological model and are in good agreement with the dynamical model DMT.

We report on new H(e,e(')p)gamma measurements in the Delta(1232) resonance at Q(2)=0.06 (GeV/c)(2) carried out simultaneously with H(e,e(')p)pi(0). It is the lowest Q(2) for which the virtual Compton scattering (VCS) reaction has been studied in the first resonance region. The VCS measured cross sections are well described by dispersion-relation calculations in which the multipole amplitudes derived from H(e,e(')p)pi(0) data are used as input, thus confirming the compatibility of the results. The derived resonant magnetic dipole amplitude M-1+(3/2)=(40.60 +/- 0.70(stat+sys))(10(-3)/m(pi)(+)) at W=1232 MeV is in excellent agreement with the value extracted from H(e,e(')p)pi(0) measurements.