Recent results from MAMI

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1 Recent results from MAMI Ulrich Müller Institut für Kernphysik Johannes-Gutenberg-Universität Mainz The Mainz Microtron MAMI A: Electron scattering Proton form factors Electric form factor of the neutron A2: Real photon experiments π photoproduction near threshold π η photoproduction Transverse spin observable F in γ p π p A4: Strangeness in the nucleon Strangeness form factors G s E and Gs M Axial form factor (H 2 / D 2 ) Summary Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 / 43

2 The Mainz Microtron MAMI MAMI C Beam energy max. 6 MeV CW beam µa unpolarised 4 µa at 8 % polarisation Excellent beam quality Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 2 / 43

3 A: Electron scattering Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 3 / 43

4 Three-spectrometer setup of the A collaboration Spectrometer A: α > 2 p < 735 MeV/c Ω = 28 msr p/p = 2% Spectrometer B: α > 8 p < 87 MeV/c Ω = 5.6 msr p/p = 5% Spectrometer C: α > 55 p < 655 MeV/c Ω = 28 msr p/p = 25% Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 4 / 43

5 Nucleon form factors Elastic electron scattering: Cross section and form factors Cross section: with: ( ) dσ dσ dω = ɛg2 E (Q2 ) + τg 2 M (Q2 ) dω Mott ɛ ( + τ) ( τ = Q2, ɛ = + 2 ( + τ) tan 2 θ e 4m 2 p 2 ) Fourier transform of G E, G M spatial distribution (Breit frame) Electric and magnetic radius: r 2 E = 6 h 2 dg E dq 2 Q2 = r 2 M = 6 h 2 d(g M /µ) dq 2 Q2 = Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 5 / 43

6 Proton form factors: Motivation Form factors from elastic ep scattering Two classes of experimental methods: Unpolarised scattering: Rosenbluth separation Separated G E (Q 2 ) and G M (Q 2 ), but contribution from two photon exchange (TPE) Polarised scattering: polarised electrons scattered from polarised targets polarisation transfer from electron to nucleon Only ratio G E (Q 2 )/G M (Q 2 ), little contribution from two photon exchange (TPE)? As always in physics: What accuracy can be reached? Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 6 / 43

7 Proton form factor: Measured settings dσ dω = ( ) dσ ɛg2 E(Q 2 ) + τg 2 M(Q 2 ) dω ɛ ( + τ) Mott Q [GeV/c] Q 2 [(GeV/c) 2 ] 4 settings.4.6 > 9 events spectrometer A spectrometer B spectrometer C ε Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 7 / 43

8 Background subtraction Simulation: Background from elastic and quasi-elastic scattering at target walls Model for energy loss and small angle scattering Input: momentum-, angular-, vertex resolution data simulated background residue de [MeV] Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 8 / 43

9 Description of the radiative tail counts E [MeV] integrated tail exp./sim. [arb. units] E [MeV] Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 9 / 43

10 Cross sections / standard dipole.7 +.5, 855 MeV.6 σ exp /σ std. dipole , 72 MeV +.3, 585 MeV +.2, 45 MeV +., 35 MeV scattering angle 8 MeV Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 / 43

11 Extraction of form factors. Traditionally: Rosenbluth separation at constant Q 2 2. Super-Rosenbluth Separation : fit of form factor models directly to the cross sections Feasible due to fast computers All data at all Q 2 and ɛ values contribute to the fit no projection to constant Q 2 no limit of kinematics Easy fixing of normalisation Model dependence? For extraction of radii: Need a fit anyway! Classical Rosenbluth: Extracted G E and G M highly correlated Error propagation very involved Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 / 43

12 Cross sections: 8 MeV σ exp /σ std. dipole Polynomial Poly. + dip. Poly. dip. Inv. poly. Spline Spline dip. Friedrich-Walcher Double Dipole Extended G.K Q 2 [(GeV/c) 2 ] Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 2 / 43

13 Form factor results G E /G std. dipole Christy et al. Simon et al. Price et al. Berger et al. Hanson et al. Borkowski et al. Janssens et al. Murphy et al G M /(µ p G std. dipole ) Christy et al. Price et al. Berger et al. Hanson et al. Borkowski et al. Janssens et al. Bosted et al. Bartel et al G E /G std. dipole Arrington et al. Friedrich/Walcher Simon et al. Price et al. Borkowski et al. Janssens et al. Murphy et al µ p G E /G M Arrington w/o TPE Jones et al..85 Arrington w/ TPE Crawford et al. Pospischil et al. Dieterich et al..8 Gayou et al. Milbrath et al. Ron et al. Punjabi et al Q 2 / (GeV/c) 2.8 Jan C. Bernauer et al., High-precision determination of the electric and magnetic form factors of the proton, PRL 5 (2) 242, arxiv:7.576 Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 3 / 43

14 Radii of the proton from electron scattering MAMI result with Coulomb correction (McKinley and Feshbach): r 2 E /2 =.879 ±.5 stat. ±.4 syst. ±.2 mod. ±.4 grp fm r 2 M /2 =.777 ±.3 stat. ±.9 syst. ±.5 mod. ±.2 grp fm PRL 5 (2) 242 MAMI result with TPE correction (Borisyuk and Kobushkin): r 2 E /2 =.876 ±.5 stat. ±.4 syst. ±.2 mod. ±.4 grp fm r 2 M /2 =.83 ±.3 stat. ±.9 syst. ±.5 mod. ±.2 grp fm PRL 7 (2) 92 Magnetic radius from hyperfine splitting in hydrogen: r 2 M /2 =.778(29) fm A. V. Volotka, V. M. Shabaev, G. Plunien, G. Soff: EPJ D 33 (25) 23 Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 4 / 43

15 Overview of different proton charge radius results Pohl et al. (2) Bernauer et al. (2) CODATA 6 (28) CODATA 2 (25) Melnikov et al. (2) Udem et al. (997) Blunden et al. (25) Sick et al. (23) Rosenfelder et al. (2) Mergell et al. (996) Wong et al. (994) Eschrich et al. (2) McCord et al. (99) Simon et al. (98) Borkowski et al. (975) Akimov et al. (972) Frerejacque et al. (966) Hand et al. (963) Filled circles: results from new measurements Hollow circles: reanalysis of existing data Talks by: S. Dubnicka (thursday) I. Sick (friday) <r E > [fm] Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 5 / 43

16 Electric form factor of the neutron Problem: No free neutron target available small compared to Gn M Rosenbluth separation gives large errors: G n E dσ dω ag2 E(Q 2 ) + bg 2 M(Q 2 ) Solution: Double polarisation experiments on 2 H or 3 He 2 H( e, e n) 3 He( e, e n) Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 6 / 43

17 Double polarisation experiments on 3 He Beam target asymmetry: A = N( ) N( ) N( ) + N( ) ag n E = P e P Gn M sin θ + bgn 2 M cos θ n cg n2 E + dg n M2 Ratio of asymmetries: A(θ = 9 ) A(θ = ) = A = a G n E A b G n M Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 7 / 43

18 28 measurement: GnE at Q2.5 (GeV/c)2 Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 8 / 43

19 Polarised 3 He target Relaxation due to surfaces pressure (5 bar) field gradients electron beam T 45 h Change of the target cell every 2 hours J. Krimmer et al., NIM 6 (29) 8 Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 9 / 43

20 G n E from double polarisation experiments..8.6 Published H(e, e n) 2 H(e, e n) 3 He(e, e n) Galster fit MAMI G E n.4 Preliminary 28 data Q 2 [(GeV/c) 2 ] S. Schlimme: PhD thesis, Mainz (2) Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 2 / 43

21 A2: Real photon experiments Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 2 / 43

22 A2: Crystal Ball / TAPS Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 22 / 43

23 π photoproduction near threshold Test of LETs Close to threshold only s and p waves contribute: l = E + s wave l = M +, M, E + p waves so that dσ dω (k, θ) = q k [A + B cos θ + C cos2 θ] where A, B, and C are functions of E +, M +, M, and E + Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 23 / 43

24 π photoproduction near threshold Previous data A. Schmidt et al., PRL 87 (2) 2325 S. Kamalov et al., PR C 64 (2) 322 V. Bernard et al., EPJ A (2) 29 Discrepancy with DMT prediction? Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 24 / 43

25 π photoproduction near threshold CB/TAPS 28 data CB/TAPS 28: Much better statistics Subtraction of target window contributions Energy dependence of Σ New fit of multipoles under way, publication in preparation Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 25 / 43

26 γp π ηp cross section Blue line: best fit with D 33 (7), P 33 (6), P 3 (75), F 35 (95), and Born terms Partial contributions: Red line: D 33 (7) Green line: P 33 (6) Black line: P 3 (75) Dashed red line: F 35 (95) Dashed blue line: Born terms Dominated by D 33 (7) V. Kashevarov et al., EPJ A 42 (29) 4 A. Fix et al., EPJ A 36 (28) 6 Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 26 / 43

27 γp π ηp beam helicity asymmetry Dotted red line: Fourier fit (3 terms) Dashed blue line: D 33 (7) only Black line: Isobar model, 6 resonances (A. Fix et al.) Phys. Lett. B 693 (2) 55 More spin observables will be measured (T and F, pol. beam and pol. target) Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 27 / 43

28 Polarized frozen spin target for Crystal Ball Frozen spin target in operation since beginning of 2 3 He- 4 He dilution refrigerator (Mainz / JINR Dubna) Material: Butanol, polarisation > 9 % hours relaxation time & low He consumption Running with transverse polarised target! Now: D-Butanol, P = 75 %, t R 3 h (Bochum) Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 28 / 43

29 First measurement of transverse spin observable F in γ p π p ( ) dσ dσ [ dω = Pγ lin Σ(θ) cos(2φ) dω unpol [ + P x P lin γ H(θ) sin(2φ) + Pγ circ F(θ) ] [ + P y T(θ) + P lin γ P(θ) cos(2φ) ] + P z [ P lin γ G(θ) sin(2φ) + P circ γ E(θ) ]] T asymmetry: transverse polarised target F asymmetry: circular polarised photons, transverse polarised target Need to separate contribution from 2 C, 6 O (Butanol), and 3/4 He Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 29 / 43

30 γ p π p preliminary results Transverse target asymmetry T Blue line MAID 27 Green line SAID Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 3 / 43

31 γ p π p preliminary results Double polarisation observable F 44 MeV 47 MeV 5 MeV 53 MeV 56 MeV MeV.5 62 MeV.5 65 MeV 68 MeV 7 MeV MeV 77 MeV 8 MeV 83 MeV 86 MeV MeV 92 MeV MeV 98 MeV MeV * Θ π [deg] 5 5 * Θ π [deg] 5 5 * Θ π [deg] 5 5 * Θ π [deg] * Θ π [deg] Blue line MAID 27 Green line SAID Black line BG 2-2 Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 3 / 43

32 A4: Strangeness in the nucleon Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 32 / 43

33 A4: Lead flouride calorimeter Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 33 / 43

34 Parity violation asymmetry e γ e e Z e σ j γ,µ J µ γ + j Z,µ J µ Z q 2 M 2 Z 2 p p p p A RL = A + A S with A S = aρ eq ɛg p E Gs E + τgp M Gs M ɛ(g p E )2 + τ(g p M )2 A RL = σ R σ L σ R + σ L 5 A = A V + A A : can be calculated in Standard Model A RL : can be measured Strangeness contribution A S can be determined in experiment Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 34 / 43

35 Measurement of strangeness form factors Three quantities to be measured: G s E, Gs M : strangeness contribution G A : (isoscalar) axial form factor, large electroweak corrections For one specific four-momentum transfer Q 2 : at least three measurements Scattering experiment e + p elastic, forward angle e + p elastic, backward angle e + d quasi-elastic, backward angle e + 4 He elastic, forward angle sensitive to G s E and Gs M G s M and G A G s M and G A G s E Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 35 / 43

36 Setup of the A4 experiment Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 36 / 43

37 A4: Lead flouride calorimeter Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 37 / 43

38 A4: Energy spectrum Comparison with Monte Carlo simulation Backward angle (35 MeV) Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 38 / 43

39 A4: Strangeness form factor at Q 2 =.23 (GeV/c) 2 G s E =.5 ±.42(±.38 exp ±.9 FF ) G s M =.4 ±.6 (±. exp ±. FF ) Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 39 / 43

40 A4: Measurement under backward angle (D 2 ) Asymmetry in quasi-elastic ed scattering: A = ( 2.76 ±.96 stat ±.76 syst ) ppm (preliminary, but including all corrections) Preliminary result: G A +.6 G s M =.55 ±.35 Experiment: G A =.47 ±.3 Zhu et al.: G A =.77 ±.35 Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 4 / 43

41 A4: Axial form factor (H 2 / D 2 ) Comparison with G results: Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 4 / 43

42 Strangeness form factors: World data (At least two measurements at same Q 2 ) A4 measurements: H 2, Q 2 =.23 (GeV/c) 2 forward H 2, Q 2 =. (GeV/c) 2 backward H 2, Q 2 =.23 (GeV/c) 2 backward D 2, Q 2 =.23 (GeV/c) 2 backward H 2, Q 2 =.62 (GeV/c) 2 forward Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 42 / 43

43 Summary A: Electron scattering High precision electron proton scattering data from MAMI Q 2 range from.3 to (GeV/c) 2 Data point for G n E with polarised 3 He at Q 2.5 (GeV/c) 2 A2: Real photon experiments New data for π threshold photoproduction Photon asymmetry Σ consistent with DMT model γp π ηp, dominated by D 33 (7) First measurement of transverse spin observable F in γ p π p A4: Strangeness in the nucleon Separated strangeness form factors G s E and Gs M at Q2 =.23, (GeV/c) 2 Axial form factor G A Ulrich Müller (Univ. Mainz) Recent results from MAMI Erice 2 43 / 43