Baryon Spectroscopy at ELSA

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1 Baryon Spectroscopy at ELSA R. Beck, University Bonn CBELSA/TAPS-collaboration EuNPC, March 15-20, 2009, Bochum Motivation ELSA accelerator Crystal Barrel experiment Recent Results Summary and Outlook supported by the DFG within the SFB/TR16

2 Introduction Baryons are extended objects with internal structure : ELSA: focus on excitation spectrum

3 Introduction 3.2 GeV photon beam at ELSA used to study meson photoproduction Breit Wigner Resonances Total Photon Absorption Cross Section Spectroscopic Notation X 2 I 2 J Isospin Spin X = π π S ( l = 0); P( l = 1);K

4 Introduction PDG 2008: Status on nucleon resonances only 7 N* and 5 Δ* established in the region 1400 MeV < W < 2000 MeV Energy pattern for the dominant states Constituent Quark Models Dynamical Models Lattice QCD Various nucleon models predict many more states weak coupling to πn final state insufficient data for PWA

5 Experimental program for N* Common effort at ELSA, JLab and MAMI, Precision data for different final states Polarization experiments (beam, target and recoil) γ + p X γ + p > p + π + π + γ +p > p + π 0 + π 0 γ +p > p + π 0 γ +p > K + + Λ γ +p > p + η

6 Observables in Meson Photoproduction Photon polarization Target polarization Recoil nucleon polarization Target and recoil polarizations X Y Z(beam) X Y Z X X Z Z X Z X Z unpolarized linear circular σ Σ - - T - H (-P) G F - E - P - O x (-T) O z C x - C z T x L x T z L z (-L z ) (T z ) (L x ) (-T x ) data only for: data needed for: Differential cross section: Beam asymmetry: Double polarization: σ Σ E Target asymmetry: Recoil polarization: Double polarization: T P G Sensitive to: P ) Re( 1 P2 Sensitive to: P ) Im( 1 P2 Crystal Barrel experiment at ELSA: polarized photons, polarized targets and 4π acceptance

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8 Crystal Barrel Set Up at ELSA

9 Crystal Barrel Set Up at ELSA Crystal Barrel detector 1230 CsI crystals π 0 γγ η γγ 0 η 3π Inner-detector cylinder of 513 scintillating fibers forward detector (FWPlug) 90 CsI crystals with PM s, forward detector (MiniTAPS) 216 BaF 2, Close to 4π coverage more details about performance and future upgrade: HK 52.4, HK 62.2, HK 62.3, HK 62.5 und HK 75.4

10 Polarized Photons Linearly polarized photons: - coherent bremsstrahlung - diamond radiator Circularly polarized photons: - longitudinally polarized electrons - helicity transfer to photon Linearly polarized photons Circularly polarized photons high polarization at low photon energies Cir Lin p γ = 70% high polarization at p = 65% γ high photon energies (H. Eberhard HK69.8) (S. Kammer)

11 Polarized Target Frozen Spin Target liquid helium from the still internal superconducting 'holding coil target horizontal cryostat with integrated solenoid to freeze up the spin beam 60 mm Target: Butanol (C 4 H 9 OH) Polarization: DNP at high B-field (2.5 T) freeze up the spin (0.4 T) relaxation time T~500h 100 mm Butanol (C 4 H 9 OH) Bonn: H. Dutz, S. Goertz Bochum: W. Meyer, S. Reichertz

12 Polarized Target Running time over 2500 hours in year 2008 High. polarization P + = 83.4 % P - = % fast build-up Pol.-time 05h04min (May/June) 05h39min (August) 06h10min horizontal cryostat in experimental area data taking

13 Polarization Observables Linearly polarized photons > beam asymmetry Σ Circularly polarized photons Longitud. polarized protons Linearly polarized photons Longitud. polarized protons Crystal Barrel experiment at ELSA: New results for different final states New preliminary results for G and E > double polarization asymmetry E > double polarization asymmetry G HK 14.7, HK 14.8, HK 25.4, HK 56.3, HK 57.9, HK 69.3, HK 69.4, HK 69.6, HK 69.8

14 Crystal Barrel/TAPS Results r γ + p p +η V. Crede, O. Bartolomy et al., PRL 94 (2005) , EPJ A33 (2007) 133 Meson photoproduction S 11 (1535) GRAAL S 11 (1650) MAMI CLAS CB-ELSA D 15 (2070) PWA Photon helicity couplings: A 1/2 und A 3/2 S 11 (1535) : A 1/2 (S 11 (1535)) only D 13 (1520) : A 1/2 (D 13 (1520)) and A 3/2 (D 13 (1520)) Total cross section: σ tot ~ A 1/2 (S 11 ) 2 + A 1/2 (D 13 ) 2 + A 3/2 (D 13 ) 2 +..

15 Crystal Barrel/TAPS Results Beam asymmetry: Σ r γ + p p +η Higher sensitivity because of interference between different resonance contributions Σ ~ A 1/2 (S 11 ) A 1/2 (D 13 ) +.. D. Elsner et al., EPJ A33 (2007) 147 Σ η-maid Σ D 15 (1675) D 13 (1520) = 1250 MeV D 13 (1520) P 13 (1720) BoGa PWA θ η cm Θ cm

16 reaction: Helicity dependent total cross section r γ + p r X circularly polarized photons longitudinally polarized proton P 33 (1232)

17 reaction: Helicity dependent cross section r 0 γ + p r p + π circularly polarized photons longitudinally polarized proton count rate difference preliminary acceptance correction Preliminary results (M. Gottschall HK14.7) P 33 (1232) S 11 (1535) N 1/2 -N 3/2 F 15 (1680) +

18 Helicity dependent cross section reaction: r 0 γ + p r p + π Angular distributions sensitive to interference between resonances Preliminary results (M. Gottschall HK14.7) N 1/2 -N 3/2 count rate difference with acceptance correction

19 reaction: Helicity dependent cross section r γ + p r p +η circularly polarized photons longitudinally polarized proton Preliminary results (M. Gottschall HK14.7) S 11 (1535) N 1/2 -N 3/2 P 11 (1710) P 13 (1720)

20 reaction: Helicity dependent cross section r γ + p r p +η Preliminary results (M. Gottschall HK14.7) N 1/2 -N 3/2 count rate difference

21 reaction: Helicity dependent cross section r 0 γ + p + π + p r 0 π circularly polarized photons longitudinally polarized proton Preliminary results (D. Piontek) N 1/2 -N 3/2

22 Polarization Observables

23 reaction: Double Polarization Experiment for G r 0 γ + p r p + π dσ dσ Lin Lin ( θ, φ) = ( θ ) ( 1 pγ Σ cos(2φ ) pγ pz G sin(2φ )) dω dω first online spectra with linearly polarized photons longitudinally polarized proton

24 reaction: Double Polarization Experiment for G r 0 γ + p r p + π dσ dσ Lin Lin ( θ, φ) = ( θ ) ( 1 pγ Σ cos(2φ ) pγ pz G sin(2φ )) dω dω linearly polarized photons longitudinally polarized proton Clear effect from G observed

25 G-Asymmetry for pπp 0 =1050 MeV reaction: r 0 γ + p r p + π missing mass cut fit to the φ-distribution dσ dσ Lin Lin ( θ, φ) = ( θ ) ( 1 pγ Σ cos(2φ ) pγ pz G sin(2φ )) dω dω Preliminary results CB (A. Thiel HK69.6) GRAAL data =1050 MeV =1050 MeV Σ G SAID MAID

26 reaction: r 0 γ + p r p + π Asymmetries for pπp 0 linearly polarized photons longitudinally polarized proton =966 MeV =1000 MeV Preliminary results (A. Thiel HK69.6) Σ Σ GRAAL data G G prediction partial wave analysis BoGa SAID MAID

27 reaction: r 0 γ + p r p + π Asymmetries for pπp 0 =1033 MeV =1066 MeV Preliminary results (A. Thiel HK69.6) Σ Σ GRAAL data =1033 MeV =1066 MeV prediction partial wave analysis G G BoGa SAID MAID

28 reaction: r 0 γ + p r p + π Asymmetries for pπp 0 =1100 MeV =1133 MeV Preliminary results (A. Thiel HK69.6) Σ Σ GRAAL data G G prediction partial wave analysis BoGa SAID MAID

29 Energy dependence Beam-Asymmetry Σ at θ = 90 without P 11 (1440) without D 13 (1520) without F 15 (1680) Double-Polarization-Asymmetry G at θ = 90 Preliminary results (A. Thiel HK69.6)

30 Asymmetries for pηp =1050 MeV reaction: r γ + p r p +η missing mass cut fit to the φ-distribution dσ dσ Lin Lin ( θ, φ) = ( θ ) ( 1 pγ Σ cos(2φ ) pγ pz G sin(2φ )) dω dω =1050 MeV =1050 MeV Σ G SAID MAID

31 Asymmetries for pηp reaction: r γ + p r p +η linearly polarized photons longitudinally polarized proton =850 MeV =950 MeV =1050 MeV Preliminary results (A. Thiel HK69.6) Σ CB (D. Elsner) =850 MeV =950 MeV =1050 MeV prediction partial wave analysis BoGa G G G SAID MAID

32 World Data Base reaction: r 0 γ + p r p + π Differential cross section Beam asymmetry Σ Asymmetry G Asymmetry E First round of double polarization experiments with CB at ELSA: Energy range for G: MeV Energy range for E: MeV Future plans: Extend energy range Transversally polarized target in preparation

33 Summary First round of double polarization experiments with Crystal Barrel at ELSA Preliminary results for the double polarization observable G and E Aim: reach complete experiment Model independent partial wave analysis Will shed new light on the nucleon excitation spectrum

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