Publications (45)73.63 Total impact
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ABSTRACT: 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.07/2013;  [Show abstract] [Hide abstract]
ABSTRACT: New, accurate data are presented on the near threshold p(e,e'p)pi^0 reaction in the range of fourmomentum 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.09/2011;  [Show abstract] [Hide abstract]
ABSTRACT: 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 BonnB nucleonnucleon 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.Physical Review Letters 10/2009; 103(15):152501. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Cross sections for the ^{3}He(e,e'pn)p 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 BonnB nucleonnucleon potentials overestimate the measured cross sections by a factor 5 at low recoil proton momentum with the discrepancy becoming much smaller at higher recoil momentum. Comment: 5, pages, 3 figures03/2009;  [Show abstract] [Hide abstract]
ABSTRACT: The differential crosssections σ0 = σT + εσL , σLT , and σTT of p0 \pi^{0}_{} electroproduction from the proton were measured from threshold up to an additional centerofmass energy of 40MeV, at a value of the photon fourmomentum transfer of Q 2 = 0.05 GeV2/c2 and a centerofmass angle of θ = 90° . By an additional outofplane 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 p+ \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.European Physical Journal A 09/2008; 38(1):2733. · 2.42 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The determination of nonspherical angular momentum amplitudes in nucleons at long ranges (low Q2) was accomplished through the p(e⃗,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 centerofmass 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.Phys. Rev. C. 08/2008; 78(2).  [Show abstract] [Hide abstract]
ABSTRACT: We report on new H(e,e'p)γ measurements in the Δ(1232) resonance at Q2=0.06 (GeV/c)2 carried out simultaneously with H(e,e'p)π0. It is the lowest Q2 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 dispersionrelation calculations in which the multipole amplitudes derived from H(e,e'p)π0 data are used as input, thus confirming the compatibility of the results. The derived resonant magnetic dipole amplitude M1+3/2=(40.60±0.70stat+sys)(103/mπ+) at W=1232 MeV is in excellent agreement with the value extracted from H(e,e'p)π0 measurements.Phys. Rev. C. 07/2008; 78(1).  [Show abstract] [Hide abstract]
ABSTRACT: The crosssection 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 crosssection is analyzed using the lowenergy 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 nontrivial Q 2evolution of the generalized polarizabilities and call for more measurements in the lowQ 2 region (≤ 1 (GeV/c)2).European Physical Journal A 06/2008; 37(1):18. · 2.42 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report on new H$(e,e^\prime p)\gamma$ measurements in the $\Delta(1232)$ resonance at $Q^2=0.06$ (GeV/c) carried out simultaneously with H$(e,e^\prime p)\pi^0$. 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 dispersionrelation calculations in which the multipole amplitudes derived from H$(e,e^\prime p)\pi^0$ data are used as input, thus confirming the compatibility of the results. The derived resonant magnetic dipole amplitude $M^{3/2}_{1+} = (40.60 \pm 0.70_{stat+sys})(10^{3}/m_{\pi^+})$ at $W=$ 1232 MeV is in excellent agreement with the value extracted from H$(e,e^\prime p)\pi^0$ measurements.05/2008;  [Show abstract] [Hide abstract]
ABSTRACT: We report new precise measurements at the peak of the Δ+(1232) resonance at performed at the Mainz Microtron (MAMI). The new data are sensitive to both the electric (E2) and the Coulomb (C2) quadrupole amplitudes of the γ∗N→Δ transition. They yield precise quadrupole to dipole amplitude ratios: CMR=(−5.09±0.28stat+sys±model0.30)% and EMR=(−1.96±0.68stat+sys±model0.41)% for . The new results are in disagreement with Constituent Quark Model predictions and in qualitative agreement with models that account for mesonic contributions, including recent Lattice calculations. They thus give further credence to the conjecture of deformation in hadronic systems favoring the attribution of the origin of deformation to the dominance of mesonic effects.Physics Letters B. 07/2007; 
Article: The COMPASS Experiment at CERN
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ABSTRACT: The COMPASS experiment makes use of the CERN SPS highintensitymuon and hadron beams for the investigation of the nucleon spin structure and the spectroscopy of hadrons. One or more outgoing particles are detected in coincidence with the incoming muon or hadron. A large polarized target inside a superconducting solenoid is used for the measurements with the muon beam. Outgoing particles are detected by a twostage, large angle and large momentum range spectrometer. The setup is built using several types of tracking detectors, according to the expected incident rate, required space resolution and the solid angle to be covered. Particle identification is achieved using a RICH counter and both hadron and electromagnetic calorimeters. The setup has been successfully operated from 2002 onwards using a muon beam. Data with a hadron beam were also collected in 2004. This article describes the main features and performances of the spectrometer in 2004; a short summary of the 2006 upgrade is also given.Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 07/2007; · 1.32 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The differential cross sections \sigma_0=\sigma_T+\epsilon \sigma_L, \sigma_{LT}, and \sigma_{TT} of \pi^0 electroproduction from the proton were measured from threshold up to an additional center of mass energy of 40 MeV, at a value of the photon fourmomentum transfer of Q^2= 0.05 GeV^2/c^2 and a center of mass angle of \theta=90^\circ. By an additional outofplane measurement with polarized electrons \sigma_{LT'} was determined. This showed for the first time the cusp effect above the \pi^+ threshold in the imaginary part of the swave. The predictions of Heavy Baryon Chiral Perturbation Theory 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.06/2007;  [Show abstract] [Hide abstract]
ABSTRACT: The beamhelicity asymmetry has been measured simultaneously for the reactions e® \;\stackrel{{\rightarrow}}{{e}}\; p→epγ and e® \;\stackrel{{\rightarrow}}{{e}}\; p→epπ 0 in the Δ(1232)resonance region at Q 2 = 0.35(GeV/c)2. The experiment was performed at MAMI with a longitudinally polarized beam and an outofplane detection of the proton. The results are compared with calculations based on dispersion relations for virtual Compton scattering and with the MAID model for pion electroproduction. There is an overall good agreement between experiment and theoretical calculations. The remaining discrepancies may be ascribed to an imperfect parametrization of some γ (*) N→πN multipoles, mainly contributing to the nonresonant background. The beamhelicity asymmetry in both channels (γ and π 0) shows a good sensitivity to these multipoles and should allow future improvement in their parametrization.European Physical Journal A 03/2007; 32(1):6975. · 2.42 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present a measurement of the deuteron spindependent structure function based on the data collected by the COMPASS experiment at CERN during the years 2002–2004. The data provide an accurate evaluation for , the first moment of , and for the matrix element of the singlet axial current, a0. The results of QCD fits in the next to leading order (NLO) on all g1 deep inelastic scattering data are also presented. They provide two solutions with the gluon spin distribution function ΔG positive or negative, which describe the data equally well. In both cases, at the first moment of ΔG(x) is found to be of the order of 0.2–0.3 in absolute value.Physics Letters B 03/2007; 647(1):817. · 6.02 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The beamhelicity asymmetry has been measured simultaneously for the reactions (e p \to e p \gamma) and (e p \to e p \pi^0) in the $\Delta (1232)$ resonance region at $Q^2=$ 0.35 (GeV/c)$^2$. The experiment was performed at MAMI with a longitudinally polarized beam and an outofplane detection of the proton. The results are compared with calculations based on Dispersion Relations for virtual Compton scattering and with the MAID model for pion electroproduction. There is an overall good agreement between experiment and theoretical calculations. The remaining discrepancies may be ascribed to an imperfect parametrization of some $\gamma^{(*)} N \to \pi N$ multipoles, mainly contributing to the nonresonant background. The beamhelicity asymmetry in both channels ($\gamma$ and $\pi^0$) shows a good sensitivity to these multipoles and should allow future improvement in their parametrization.01/2007;  [Show abstract] [Hide abstract]
ABSTRACT: We present a measurement of the deuteron spindependent structure function g^d_1 based on the data collected by the COMPASS experiment at CERN during the years 20022004. The data provide an accurate evaluation for \Gamma^d_1, the first moment of g^d_1(x), and for the matrix element of the singlet axial current, a_0. The results of QCD fits in the next to leading order (NLO) on all g1 deep inelastic scattering data are also presented. They provide two solutions with the gluon spin distribution function \Delta_G positive or negative, which describe the data equally well. In both cases, at Q^2 = 3(GeV/c)^2 the first moment of \Delta G is found to be of the order of 0:2  0:3 in absolute value.01/2007;  [Show abstract] [Hide abstract]
ABSTRACT: To determine nonspherical angularmomentum amplitudes in hadrons at long ranges (low Q2), data were taken for the p(¯e, e'p)π0 reaction in the Δ region at Q 2 = 0.060 (GeV/c)2 utilizing the magnetic spectrometers of the A1 Collaboration at MAMI. The results for the dominant transition magnetic dipole amplitude and the quadrupole to dipole ratios at W = 1232 MeV are M1+3/2=(40.33 ±0.63stat+syst ±0.61model)(103/mp+)\ensuremath M_{1+}^{3/2}=(40.33 \pm 0.63_{\rm stat+syst} \pm 0.61_{\rm model})(10^{3}/m_{\pi^+}) , Re( E1+3/2/M1+3/2)=(2.28 ±0.29stat+syst ±0.20model\ensuremath E_{1+}^{3/2}/M_{1+}^{3/2})=(2.28 \pm 0.29_{\rm stat+syst} \pm 0.20_{\rm model} )%, and Re( S1+3/2/M1+3/2) = (4.81 ±0.27stat+syst ±0.26modelS_{1+}^{3/2}/M_{1+}^{3/2})\ensuremath =(4.81 \pm 0.27_{\rm stat+syst} \pm 0.26_{\rm model} )%. These disagree with predictions of constituent quark models but are in reasonable agreement with lattice calculations with nonlinear (chiral) pion mass extrapolations, with chiral effective field theory, and with dynamical models with pion cloud effects. These results confirm the dominance, and general Q2 variation, of the pionic contribution at large distances.European Physical Journal A 11/2006; 30(3):471476. · 2.42 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: This paper reports on the first measurement of the 16O(e, e'pn)14N reaction. Data were measured in kinematics centred on a superparallel geometry at energy and momentum transfers of 215MeV and 316MeV/c. The experimental resolution was sufficient to distinguish groups of states in the residual nucleus but not good enough to separate individual states. The data show a strong dependence on missing momentum and this dependence appears to be different for two groups of states in the residual nucleus. Theoretical calculations of the reaction using the Pavia code do not reproduce the shape or the magnitude of the data.European Physical Journal A 08/2006; 29(3):261270. · 2.42 Impact Factor 
Article: Measurement of the LTasymmetry in πelectroproduction at the energy of the Δ(1232)resonance
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ABSTRACT: The reaction p(e, e'p)π0 has been studied at Q2 = 0.2 (GeV/c)2 in the region of W = 1232MeV. From measurements left and right of [(q)\vec] \vec{q}\, , crosssection asymmetries ρLT have been obtained in forward kinematics ρLT( qp0cm \theta_{{\pi^0}}^{{cm}} = 20°) = ( 11.68±2.36stat±2.36sys) and backward kinematics ρLT( qp0cm \theta_{{\pi^0}}^{{cm}} = 160°) = (12.18±0.27stat±0.82sys) π0. Multipole ratios Â \Re {S1+ * M1+}/ M1+2 and Â \Re {S0+ * M1+}/ M1+2 were determined in the framework of the MAID2003 model. The results are in agreement with older data. The unusally strong negative Â \Re {S0+ * M1+}/ M1+2 required to bring also the result of Kalleicher et al. in accordance with the rest of the data is almost excluded.European Physical Journal A 12/2005; 27(1):9197. · 2.42 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The electron target asymmetries A  and A ⊥ with target spin parallel and perpendicular to the momentum transfer $\ensuremath{\boldsymbol{q}}$ were measured for both the two and threebody breakup of 3He in the 3 $ \overrightarrow{\rm He}$ (¯e, e’p)reaction. Polarized electrons were scattered off polarized 3He in the quasielastic regime in parallel kinematics with the scattered electron and the knockedout proton detected using the ThreeSpectrometer Facility at MAMI. The results are compared to Faddeev calculations which take into account FinalState Interactions as well as Meson Exchange Currents. The experiment confirms the prediction of a large effect of FinalState Interactions in the asymmetry of the threebody breakup and of an almost negligible one for the twobody breakup.European Physical Journal A 05/2005; 25(2). · 2.42 Impact Factor
Publication Stats
413  Citations  
73.63  Total Impact Points  
Top Journals
Institutions

2009

University of Tuebingen
 Institute of Physical and Theoretical Chemistry
Tübingen, BadenWuerttemberg, Germany


2000–2008

Johannes GutenbergUniversität Mainz
 Institute of Nuclear Physics
Mainz, RhinelandPalatinate, Germany


2003

University of Saskatchewan
Saskatoon, Saskatchewan, Canada
