Recent results from experiments at MAMI

Progress in Particle and Nuclear Physics 67 (2012) 607–611

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Progress in Particle and Nuclear Physics journal homepage: www.elsevier.com/locate/ppnp

Review

Recent results from experiments at MAMI U. Müller Institut für Kernphysik, Johannes-Gutenberg-Universität Mainz, Germany

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Keywords: Electron scattering Nucleon form factors Photoproduction of mesons Parity violation

abstract The Mainz Microtron MAMI is an ideal tool for studying the structure of strongly interacting systems with an electromagnetic probe. With the new HDSM accelerator stage of MAMI C, a continuous-wave electron beam with an energy of up to 1604 MeV and excellent beam quality is available for precision experiments. In addition, polarisation degrees of freedom can be exploited with polarised beams and either polarised targets or recoil polarimetry. This paper presents selected results of the A1, A2, and A4 collaborations. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The Mainz Microtron MAMI [1] is a continuous-wave electron accelerator operated at the Institute for Nuclear Physics at Mainz University. The main components of the accelerator are three racetrack microtron (RTM) stages and a harmonic double-sided microtron (HDSM) [2] that went into routine operation in 2007 and has increased the maximum energy from 855 to 1604 MeV. Electron beams with currents of up to 100 µA unpolarised or 40 µA with 85% polarisation and an excellent beam quality are available for precision experiments. A floor plan of the MAMI C facility is shown in Fig. 1. 2. A1 collaboration The A1 collaboration does electron scattering experiments, using a setup of three magnetic spectrometers (A, B, and C) [3] with a momentum resolution of 1p/p ≈ 10−4 , a solid angle acceptance of up to 28 msr, and momentum acceptance of up to 25%. For out-of-plane measurements, spectrometer B can be inclined to angles of up to 10°. Spectrometer A is equipped with a proton recoil polarimeter, allowing for double polarisation measurements if combined with beam or target polarisation. In addition, a short-orbit spectrometer (SOS) with 1.5 m path length for detection of low-momentum pions and a kaon spectrometer (KAOS) capable of measuring high-momentum particles at small scattering angles are available. The setup can be further completed by neutron detectors or a neutron recoil polarimeter. Precise results of the elastic electron–proton scattering cross section were obtained in a measurement in about 1400 different kinematical settings, with negative four-momentum transfers squared from Q 2 = 0.004 to 1 (GeV/c)2 . Statistical errors are below 0.2%, and great effort was spent to reduce systematic errors down to the 1% level. The electric and magnetic form factors of the proton were extracted by fits of a large variety of form factor models directly to the cross sections, and they are depicted in Fig. 2. The form factors show some features at the scale of the pion cloud. The charge and magnetic radii 2 1/2 ⟩ = 0.777(13)stat (9)syst (5)model (2)group fm. are determined to be ⟨rE2 ⟩1/2 = 0.879(5)stat (4)syst (2)model (4)group fm and ⟨rM The electric radius is in complete agreement with the CODATA 2010 value of 0.8775(51) fm [5] based mostly on atomic measurements. However, the results from very recent Lamb shift measurements on muonic hydrogen [6] are smaller by 5 standard deviations. This difference is unexplained yet. ⃗ (⃗e, e′ n)pp double-polarisation The electric form factor of the neutron was measured at Q 2 = 1.5 (GeV/c)2 in a 3 He experiment [7] using a polarised helium gas target. Preliminary results indicate a value of GnE that is compatible with the Galster parameterisation. E-mail address: [email protected]. 0146-6410/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ppnp.2012.01.036

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U. Müller / Progress in Particle and Nuclear Physics 67 (2012) 607–611

Fig. 1. Floor plan of MAMI C with the RTM and HDSM accelerator stages. Also shown are the experimental areas of the A1, A2, A4, and X1 collaborations.

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Fig. 2. The form factors GE and GM normalised to the standard dipole and GE /GM as a function of Q 2 . Black line: best fit to the data. The error bands, ordered from innermost to outermost, indicate the statistical 68% pointwise confidence band, the experimental systematic error, and the variation of the Coulomb correction by ±50%, respectively. The different data points depict previous measurements and dashed lines are previous fits to the old data; see Ref. [4] and references therein.

3. A2 collaboration The A2 collaboration studies reactions induced by circular or linear polarised real photons with energies of up to 1.5 GeV. The individual energies of the beam photons are determined by a tagger system [8]. The central part of the experimental

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Fig. 3. Preliminary data of the photon asymmetry Σ versus pion angle (left) and photon energy (right) for π 0 photoproduction near threshold. Circles: data from [10], squares: new preliminary data [11]. Curves: DMT from [12], ChPT from [13].

setup used for detection of multi-photon final states is the Crystal Ball (CB) electromagnetic calorimeter [9] consisting of 672 NaI(Tl) scintillating detectors covering 93% of the full solid angle and with an energy resolution of 1E /E = 1.7% at 1 GeV. Particles emitted under small forward angles are detected by the TAPS calorimeter consisting of 384 BaF2 detectors. For tracking and identification of charged particles, organic scintillation counters and two layers of multi-wire proportional chambers in a cylindrical geometry are surrounding the experimental target. In order to measure spin degrees of freedom, the experimental setup is being completed by polarised targets and a recoil polarimeter. Pion photoproduction near threshold is a testing ground for low-energy theorems (LETs). In a previous experiment [10], total and differential cross sections and the photon asymmetry Σ had been measured. With this data set, the S wave multipole E0+ and three P wave combinations P1 , P2 , P3 could be separated and compared with results from chiral perturbation theory (ChPT). In order to measure over a range of energies and to improve accuracy, a new measurement has been performed [11]. Fig. 3 shows the new preliminary data for the photon asymmetry together with the previous data. The new asymmetries turned out significantly smaller than the previous ones. The reason for this discrepancy could be traced down to contributions from the target walls in the old data. The new data are in good agreement with the Dubna–Mainz–Taipeh (DMT) model [12]. They disagree with the ChPT curve [13] that had been fitted to the old data, however. For the γ⃗ p → π 0 ηp reaction, a very clean and high-statistics data set for photon energies between 0.95 and 1.4 GeV could be obtained in an experiment using the CB/TAPS setup [14]. In this region, the cross section seems to be dominated by one single resonance, D33 (1700). This dominance of the D33 partial wave amplitude is confirmed by the angular distributions of the pions in the π p rest frame. A single-resonance ansatz [15] using the ratio of the partial decay widths and the squared ratio of the helicity amplitudes as adjustable parameters gives a good description of the angular distributions. First precise measurements of the beam helicity asymmetry have also been carried out [16]. The results are shown in Fig. 4. Systematic deviations from the isobar model [17] at higher photon energies are observed. For the future, it is planned to measure further spin observables in this reaction channel with a polarised beam and a polarised target. The new Mainz/Dubna polarised frozen-spin target with longitudinal and transverse polarisation provides a proton polarisation of 90% with relaxation times of about 1000 h. A measurement of π 0 photoproduction with circular polarised photons and transverse polarised proton target has been performed and preliminary results for the double-polarisation variables F and T have been extracted. 4. A4 collaboration The A4 collaboration measures parity-violating cross-sectional asymmetries in elastic electron scattering, using an electromagnetic calorimeter consisting of 1022 PbF2 Cherenkov detectors covering a solid angle of 0.6 sr. The calorimeter can be positioned to cover either the forward or the backward angular region. A measurement of the parity-violating asymmetry in electron–proton scattering has been performed at Q 2 = 0.22 (GeV/c)2 under backward angles [18]. For the linear combination of the strange electric and magnetic form factors, a value of GsM + 0.26 GsE = −0.12(11)exp (11)FF is obtained, where the first error comes from the measurement and the

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Fig. 4. Angular distributions of the cross-sectional asymmetry for the γ⃗ p → π 0 ηp reaction [16]. Dotted lines: Fourier fit with three terms; solid lines: full isobar model with six resonances [17]; dashed lines: similar model with only the D33 (1700) resonance.

Fig. 5. The linear combination of GsE + ζ GsM , as extracted from backward angle [18] (steep band) and forward angle [19] (gradual band) measurements at Q 2 = 0.22 (GeV/c)2 . The bands represent the possible values of GsE + ζ GsM within one standard deviation, with statistical and systematic error added in quadrature. The ellipses show the 68% and 95% confidence level constraints in the GsE − ζ GsM plane.

second from the uncertainty in the axial and electromagnetic form factors of the nucleon. Combined with the previous results from the measurement at forward angles [19], the strange electric and magnetic form factors at this momentum transfer could be disentangled for the first time: GsE = 0.050(38)exp (19)FF and GsM = −0.14(11)exp (11)FF , as shown in Fig. 5. Recently, a measurement in quasi-elastic electron–deuteron scattering under backward angle has been carried out, being sensitive to the isoscalar axial form factor GA and the strange magnetic form factor. For the linear combination of the form factors, a preliminary result of GA + 0.61 GsM = −0.55(35) is obtained [20]. Acknowledgements The physics programme at MAMI is supported by the state of Rhineland-Palatinate, by the German Research Foundation via the Collaborative Research Centre 443, and by the European Union under the FP6 (Hadron Physics) and FP7 (Hadron Physics 2) framework programmes.

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