Measurement of the double polarisation observable G with the Crystal Barrel ELSA

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1 Measurement of the double polarisation observable G with the Crystal Barrel ELSA for the CBELSA/TAPS Collaboration Helmholtz-Institut für Strahlen- und Kernphysik der Uni Bonn Supported by SFB/TR 16

2 Content baryon spectroscopy the Crystal Barrel/TAPS ELSA polarisation observables data analysis results summary 2

3 baryon spectrosopy 3

4 Crystal ELSA radiator target tagging system polarised target flux monitor Crystal Barrel calorimeter gas Čerenkov detector MiniTAPS calorimeter 4

5 Crystal ELSA radiator target tagging system polarised target flux monitor Crystal Barrel calorimeter gas Čerenkov detector 5

6 Crystal ELSA radiator target tagging system polarised target flux monitor Crystal Barrel calorimeter gas Čerenkov detector MiniTAPS calorimeter 6

7 Crystal ELSA radiator target tagging system polarised target flux monitor Crystal Barrel calorimeter gas Čerenkov detector MiniTAPS calorimeter 7

8 Crystal ELSA radiator target tagging system polarised target flux monitor Crystal Barrel calorimeter gas Čerenkov detector MiniTAPS calorimeter 8

9 Crystal ELSA radiator target forward detector: 90 CsI(Tl) crystals coverage in θ photomultiplier readout delivers trigger signal charge identification by 180 plastic scintillators tagging system flux monitor Crystal Barrel calorimeter: 1230 CsI(Tl) crystals angular coverage in θ high detection efficiency for photons scintillating fibre detector for charge identification of reaction products 9

10 Crystal ELSA radiator target Mini Taps calorimeter: 216 BaF crystals coverage 1 12 in θ trigger information for small θ tagging system flux monitor 10

11 The complete experiment A complete model independent partial wave analysis demands: and 3 single polarisation observables 4 properly chosen double polarisation observables [Chiang, Tabakin, Phys. Rev C55 (1997)] Crystal ELSA: Photon Polarisation Target Polarisation X Y Z unpolarised T linear - H ( P) G circular F E 11

12 Event selection for cuts on raw data: multiplicity: 3 hits in the calorimeters charge: 1x charged 2x neutral time coplanarity: colinearity: proton missing mass meson mass meson mass[mev] missing mass[mev] 12

13 The observables and G longitudinally polarised protons Target material butanol contribution of reactions on protons bound in (C,O) 13

14 Meson Asymmetries p 0 p E = 1066 MeV 14

15 Beam asymmetry 0 p p E = 1050 MeV A.Thiel ELSA D.Elsner CBELSA/TAPS GRAAL (O. Bartalini, Eur. Phys. J A26, 399 (2005)) Bonn-Gatchina PWA SAID-Analysis MAID-Analysis 15

16 Beam asymmetry 0 p p in = 110 MAID without without without A.Thiel ELSA (1400) (1520) (1680) 16

17 0 p p Observable G in = 110 G MAID Without without without A.Thiel ELSA (1440) (1520) (1680) 17

18 Observable G in G E = 1050 MeV G E = 950 MeV Bonn-Gatchina PWA SAID-Analysis MAID-Analysis A.Thiel ELSA 18

19 summary Polarisation observables are necessary to describe the nucleon excitation spectrum by a partial wave analysis without ambiguities. With the Crystal Barrel/TAPS ELSA it is possible to measure single and double polarisation observables in different reaction channels. These measured datasets provide new constraints for the partial wave analysis. supported by SFB/TR 16 19

20 The Dilution Factor 20

21 and photoproduction N N, N N N N The nucleon excitation spectrum has overlapping resonances. For a complete description of the spectrum with a partial wave analysis, it is necessary to measure polarisation observables. 21

22 Production of linearily polarised photons with a goniometer different bremsstrahl radiators can be positioned in the beam with high accuracy coherent scattering of the degrees of up to 65% photons are tagged in energy by a tagging spectrometer system beam on a diamond cristal creates polarisation 22

23 main detector components Crystal Barrel calorimeter: 1230 CsI(Tl) crystals angular coverage in θ high detection efficiency for photons scintillating fibre detector for charge identification of reaction products forward detektor: 90 CsI(Tl) crystals angular coverage in θ photomultiplier readout delivers trigger signal charge identification by 180 plastic scintillators Mini Taps calorimeter: 216 BaFl crystals coverage 1 12 in θ trigger information for small θ 23