Sven Schumann. DPG Spring Meeting

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2 Radiative 0 photoproduction with CB / TAPS Overview: Motivation Magnetic dipole moments of baryons (1232) resonance in p p 0 ' reactions Experimental set-up Energy-tagged photon beam Crystal Ball / TAPS detectors p p 0 ' measurement Data taking Analysis overview Results & interpretation Cross sections for p p 0 ' reactions Comparison with model predictions Status of + extraction Conclusion S.S., B. Boillat, E.J. Downie et al., Eur. Phys. J. A 43 (2010) 269

3 Magnetic moments Baryon octet: Stable or (sufficient) long lifetime p Baryon decuplet: (u,d,s), JP = 1/2+ n = 886 s ~ s 'Direct' experiments possible Different experimental approach required Test of baryon structure calculations Precise determination of magnetic moments p n N N 0.613± ± ± ± ±0.014, *, * unstable states e.g s e.g. spin precession in external magnetic fields (u,d,s), JP = 3/2+ ++ / N + / N Leinweber et al., PRD 46 (1992) N N N N N SU(6) z p 5.58 Latt. QCD Leinweber, PRD 46 (1992) 4.91±0.61 PT Butler, PRD 49 (1994) 4.00±0.40 QSM Yang, PRD 70 (2004) 5.40 BM Hong, nucl-th/ Experiment ± ± ±2.2exp±3.0theo p p 0 ' (TAPS@MAMI) M. Kotulla et al., PRL 89 (2002) Large statistical uncertainties only ~470 reconstructed events

4 Radiative 0 photoproduction Reaction mechanism: p (1232) resonance excitation ' p 0 ' initial reaction: p transition within resonance width 120 MeV intermediate transition ' Angular momentum & parity conservation (JP = 3/2+) J1 J2 l J1 + J2 E l: P = ( 1)l M l: P = ( 1)l+1 M1, (E2), (M3) multipoles for ' transition (1232) resonance decay final decay: p 0 Contributions to p 0 ' final state: Bremsstrahlung ' non-resonant Rescattering Theoretical reaction model for p p 0 ' required to extract +

5 MAMI-B/C electron accelerator facility Electron beam: 100% duty cycle cw accelerator Unpolarised beam: Imax = 100 µa Polarised beam: electron polarisation: Imax = 20 µa Pe ~ 85% MAMI-B: 3 accelerator stages Race Track Microtrons (RTM) E0 = MeV in steps of 15 MeV RTM1 Input energy Output energy Recirculations Energy gain / turn Magnetic flux Mass of magnets 3,97 MeV RTM2 RTM3 HDSM 14,86 MeV 180,0 MeV 883,1 MeV 14,86 MeV 180,0 MeV 883,1 MeV MAMI-C (since 2006): 1604 MeV ,6 MeV 3,2 MeV 7,5 MeV 15 MeV 0,10 T 0,56 T 1,28 T 1,53 T 2 1,3 t 2 43 t t t Energy resolution E = 13 kev Additional accelerator stage Harmonic Double-Sided Microtron (HDSM) E0 = 1604 MeV Energy resolution E = 110 kev

6 Photon tagging Photon beam (Glasgow-Mainz Tagger): Bremsstrahlung of e beam on radiator foil iron (~10 µm): diamond (~100 µm): unpolarised photons linear polarised photons Momentum determination of scattered e magnetic dipole spectrometer, B = 1.05 T e detection with plastic scintillators Energy-tagging of emitted Bremsstrahlung photons recoil energy of radiator nucleus negligible = E0 Ee Events / a.u. Photon energy Bremsstrahlung spectrum N ~ 1 Tagged photon energy range: % of E0 corresponding to = MeV at E0 = 883 MeV 352 (logical) detector channels Energy resolution: Ee = 2 MeV at E0 = 883 MeV Photon flux: s 1

7 Crystal Ball / TAPS Crystal Ball: 672 NaI(Tl) crystals 40,6 cm (15,7 radiation lengths) 93% solid angle coverage 20 < < < < 360 Photomultiplier readout Separate PMT for each crystal TAPS: 510 BaF2 crystals 25 cm (12 radiation lengths) Additional plastic scintillators 5 mm thickness, Charged Particle Veto (CPV) Coverage of forward angles 2 < < 20 0 < < 360 Inner detectors: Particle Identification Detector (PID) Cylindrical detector 24 plastic scintillator strips (2mm thickness) E/E identification of charged particles 2 cylindrical wire chambers (MWPC) Track reconstruction for charged particles Angular resolutions: = 1,3... 2,3 = 1,4

8 Beam times & analysis Data taking: July/August 2004 electron energy E0 = 883 MeV photon energies = MeV linear and circular polarisation 370 hours September 2004 January 2005 electron energy E0 = 883 MeV photonenergies = MeV linear and circular polarisation 120 hours electron energy E0 = 883 MeV photon energies = MeV linear and circular polarisation 190 hours January 2005 electron energy E0 = 570 MeV photon energies = MeV circular polarisation 120 hours Analysis: Exclusive measurement of p 0 ' p ' final state energy and direction information for all particles Inital p state determined by tagger & target Events/(320 MeV2) 800 hours data & additional 120 hours empty target Missing Mass mx2(p 0) photon energy from bremsstrahlung tagging, lh2-target in rest Reaction kinematically overdetermined Kinematical cuts reduction of p 0 and p 0 0 background reconstructed events Example: mx2(p 0) = (0 MeV)2 for p p 0 ' mx2(p 0) = (135 MeV)2 for p p 0 0

9 Energy-differential cross sections d /d ' = MeV, W = MeV = MeV, W = MeV = MeV, W = MeV d Diff. cross section d ' for ' CB-TAPS@MAMI, EPJ A 43 (2010) 269 M. Kotulla et al., PRL 89 (2002) Error bars: Histogram: stat. error syst. error

10 Angular-differential cross sections d /d ' = MeV, W = MeV = MeV, W = MeV = MeV, W = MeV d Diff. cross section d for ' ' CB-TAPS@MAMI, EPJ A 43 (2010) 269 M. Kotulla et al., PRL 89 (2002) Error bars: Histogram: stat. error syst. error

11 Soft-photon limit ' 0 Soft-photon limit: Gauge invariance model-independent relation between p p 0 ' and p p 0 For ' 0 p p 0 ' dominated by proton bremsstrahlung 3-fold cross section in soft-photon limit Angular weight function 1 e² d 0 d W(v) ' 2 ² d 'd d Integration over 0 angles 1 e² 1 d 0 d W(v) d ' 2 ² d ' d ' with angular weight function W(v) Cross section ratio 1 d 0 R ' 1 d '

12 Cross section ratio R = MeV, W = MeV = MeV, W = MeV = MeV, W = MeV d R = 1 ' d ' CB-TAPS@MAMI, EPJ A 43 (2010) 269 M. Kotulla et al., PRL 89 (2002) Error bars: Histogram: stat. error syst. error

13 Fits to cross section ratio = MeV, W = MeV Unitary isobar model calculation W.T. Chiang, M. Vanderhaeghen, S.N. Yang, D. Drechsel, Phys.Rev. C 71 (2005) ² fit to cross section ratio R = 400 ± 25 MeV ' = MeV 0.39 = ±0.39 at ²/ndf = 2.94 (ndf = 28) = MeV, W = MeV Chiral EFT calculation V. Pascalutsa, M. Vanderhaeghen, Phys. Rev. D 77 (2008) ² fit to cross section ratio R = 400 ± 25 MeV ' = MeV 0.18 = at ²/ndf = 2.72 (ndf = 16) Limitations of expansion scheme initial photon: final photon: around resonance excitation ' in soft photon range

14 Conclusion p p 0 ' reaction connected to anomalous magnetic moment large other contributions (e.g. proton Bremsstrahlung) model-dependent extraction of Radiative 0 photoproduction measurements with CB / TAPS [1] 800 hours of data in 2004/ reconstructed events Much higher statistics compared with pioneering + experiment [2] about 60 more reconstructed p p 0 ' events Difficulties in determination of + discrepancies between experimental data and model predictions problematic fit qualities limitations in energy ranges for and ' [1] S.S., B. Boillat, E.J. Downie et al., Eur. Phys. J. A 43 (2010) 269 [2] M. Kotulla et al., Phys. Rev. Lett. 89 (2002)

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