Baryon Spectroscopy with Polarised Photon Beams at ELSA

Transcription

1 Baryon Spectroscopy with Polarised Photon Beams at ELSA Daniel Elsner Physikalisches Institut University of Bonn on behalf of the CBELSA/TAPS collaboration DFG - SFB/TR16 PANIC11 25 July 2011 Daniel Elsner 1

2 Outline Introduction Experimental setup polarised photon beams at ELSA long. and transv. polarised target Crystal Barrel detector setup Results meson photoproduction + single & double polarisation observables Summary PANIC11 25 July 2011 Daniel Elsner 2

3 Motivation baryon spectroscopy energy ev MeV atom nucleon PANIC11 25 July 2011 Daniel Elsner 3

4 Motivation W / GeV p X p p + + +p p + +p p + + +p K + + +p p + E γ / GeV PANIC11 25 July 2011 Daniel Elsner 4

5 Motivation photo production,, K,,,, avoid biased selection (e.g. πn coupling) photo production, measure many decay channels N,,,,? sensitivity to low intensity resonances polarisation observables PANIC11 25 July 2011 Daniel Elsner 5

6 Observables in meson photo production single pseudoscalar meson photoproduction photon target recoil (target + recoil) x y z x x z z - x y z x z x z unpolarised σ 0 0 T 0 0 P 0 T x -L x T z L z linearly pol. -Σ H (-P) -G O x (-T) O z (-L z ) (T z ) (-L x ) (-T x circularly pol. 0 F 0 -E -C x 0 -C z Knöchlein, Drechsel, Tiator Z.Phys. A 352 (1995) Example: Longitudinally polaris. protons: P T,z Linearly polarised photons: P γ lin Circularly polaris. photons: P γ circ dσ = dσ 0 [ 1- P lin γ ( Σ cos 2Φ - G PT,z sin 2Φ) - E Pγ circ PT,z ] PANIC11 25 July 2011 Daniel Elsner 6

7 Electron Stretcher Accelerator BGO - OD BGO e - polarisation 1.9 GeV 2.4 GeV 0.5 GeV to 3.5 GeV (polarised) electron beam I ext of 1 na with 83% duty factor PANIC11 25 July 2011 Daniel Elsner 7

8 Linearly polarised photons via coherent bremsstrahlung d de incoherent coherent coh./incoh. structure (diamond crystal) Laue-Bragg condition q=ng kinematical constraint (pancake) E /E 0 z x z h x v Polarisationsgrad relative Intensität Pol max 61% Pol max 58% Pol max 54% PANIC11 25 July 2011 Daniel Elsner 8

9 Circularly polarised photons & Møller polarimetry longitudinally polarised electrons helicity transfer to photon measurement of electron polarisation using Møller polarimeter count rate asymmetry A = N - N zz N + N beam polarisation P = A B zz P T a zz Polarisation / % Date PANIC11 25 July 2011 Daniel Elsner 9

10 Experimental hall (2008) Crystal Barrel set-up BGO - Open Dipole set-up PANIC11 25 July 2011 Daniel Elsner 10

11 Crystal Barrel / Taps setup electron beam PANIC11 25 July 2011 Daniel Elsner 11

12 Crystal Barrel setup Crystal Barrel detector 1230 CsI crystals forward detector (FWPlug) 90 CsI crystals with PM s, 12 deg. 30 deg. inner-detector cylinder of 513 scintillating fibers forward detector (MiniTAPS) 216 BaF 2, 1 deg. -12 deg. (not shown) Close to 4π coverage PANIC11 25 July 2011 Daniel Elsner 12

13 Frozen spin target polarisation: DNP at high B-field (2.5 T) chem. doped Butanol (C 4 H 9 OH) freeze up the spin (0.4 T) at 50 mk horizontal cryostat ( 4π accept.) with integrated solenoid ( B/B ~10-4 ) long. polarisation ~75% (max. 83%) Longitudinally polarised Solenoid with low mass layer G, E data taken Transversely polarised T, P, H data taken B H = A Bochum: W. Meyer, S. Reichertz Bonn: H. Dutz, S. Goertz sc. race track coil 2 dipoles transverse field 515 windings / 7 layers Wall: 2 mm PANIC11 25 July 2011 Daniel Elsner 13

14 Polarisation observables at ELSA Linearly polarised photons: P γ lin Circularly polaris. photons: P γ circ Longitudinally polaris. protons: P T,z photo production q 2 = 0 real photons dσ = dσ 0 [ 1- P lin γ ( Σ cos 2Φ - G PT,z sin 2Φ) - E Pγ circ PT,z ] Transversely polaris. protons: P T,x & P T,y dσ = dσ 0 [ 1- P lin γ ( H P T,x sin 2Φ + P P T,y cos 2Φ) + F Pγ circ P T,x + T P T,y ] PANIC11 25 July 2011 Daniel Elsner 14

15 Meson reconstruction γ + p p + γ γ PANIC11 25 July 2011 Daniel Elsner 15

16 Photon beam asymmetry Σ γ + p p + π 0 A. Thiel E γ = 733 MeV E γ = 767 MeV E γ = 800 MeV E γ = 833 MeV cosθ π E γ = 867 MeV E γ = 900 MeV E γ = 933 MeV E γ = 967 MeV E γ = 1033 MeV E γ = 1067 MeV E γ = 1100 MeV E γ = 1000 MeV Preliminary results CB GRAAL data BoGa SAID MAID PANIC11 25 July 2011 Daniel Elsner 16

17 Double pol. observable G in γ + p p + π PANIC11 25 July 2011 Daniel Elsner 17

18 (Effective) dilution factor target material: butanol unpolarised C and O γp pγγ γp c pγγ 10 H-Atoms and 32 protons in C and O dilution factor f = 10/(10+32) = 0.24 reconstruction efficiency different for both reactions fermi motion, final state interaction J. Hartmann need effective dilution factor f = NHydrogen / N Butanol PANIC11 25 July 2011 Daniel Elsner 18

19 Photon beam asymmetry Σ γ + p p + η A. Thiel E γ = 750 MeV E γ = 850 MeV cosθ η E γ = 950 MeV E γ = 1050 MeV Preliminary results CB GRAAL data CB data 2003 BoGa SAID MAID PANIC11 25 July 2011 Daniel Elsner 19

20 η-photoproduction (γp pη) photon beam asymmetry η-maid BnGa sensitive not sens. D 13 (1520) MAID/BnGa D 15 (1675) MAID BnGa P 11 (1710) MAID BnGa P 13 (1720) BnGa MAID Elsner D. et al., EPJ A 33, 147 (2007) CBELSA/TAPS dataset 2003 PANIC11 25 July 2011 Daniel Elsner 20

21 Double polarisation observable G γ + p p + η A. Thiel G cosθ η cosθ η G Preliminary results CB BoGa SAID MAID PANIC11 25 July 2011 Daniel Elsner 21

22 Polarisation observables Σ ( π) & G (π) γ + p p + ω 8.9% π 0 γ γγγ H. Eberhardt Σ& G E θ ω Φ p' reaction plane Define 2nd decay plane Σ π & G π γ E p' θ ω π 0 Φ π decay plane PANIC11 25 July 2011 Daniel Elsner 22

23 Σ Polarisation observables Σ & G γ + p p + ω G E γ = MeV H. Eberhardt Σ π G π Klein et al., Phys. Rev. D 78 (1108MeV 1200MeV) (1200MeV 1300MeV) A.V. Sarantsev et al., Eur. Phys. J. A 39 (t-kanal) Ajaka et al., Phys. Rev Lett. 96 PANIC11 25 July 2011 A.V. Daniel Sarantsev Elsner et al., Eur. Phys. J. A 39 (incl. s-kanal) 23

24 Helicity dependent cross section γ + p p + π 0 M. Gottschall circularly polarised photon beam longitudinally polarised proton target P 33 (1232) N 1/2 -N 3/2 with acceptance correction no normalisation PANIC11 25 July 2011 Daniel Elsner 24

25 Helicity dependent cross section γ + p p + π 0 M. Gottschall E PANIC11 25 July 2011 Daniel Elsner 25 BoGa SAID MAID

26 Helicity dependent total cross section γ + p p + η circularly polarised photon beam longitudinally polarised proton target Preliminary results (J. Müller) S 11 (1535) N 1/2 -N 3/2 P 11 (1710) P 13 (1720) no acceptance correction PANIC11 25 July 2011 Daniel Elsner 26

27 E 1 Double pol. observable E in γ + p p + ω 1109 MeV < E γ < 1200 MeV 1200 MeV < E γ < 1300 MeV 1300 MeV < E γ < 1400 MeV H. Eberhardt MeV < E γ < 1500 MeV 1500 MeV < E γ < 1600 MeV 1600 MeV < E γ < 1700 MeV MeV < E γ < 1800 MeV 1800 MeV < E γ < 1900 MeV 1900 MeV < E γ < 2000 MeV A.V. Sarantsev et al., Eur. Phys. J. A 39 (t-kanal) A.V. Sarantsev et al., Eur. Phys. J. A 39 (incl. s-kanal) cos θ ω PANIC11 25 July 2011 Daniel Elsner 27 CMS

28 Polarisation observables at ELSA Linearly polarised photons: P γ lin Circularly polaris. photons: P γ circ Longitudinally polaris. protons: P T,z photo production q 2 = 0 real photons dσ = dσ 0 [ 1- P lin γ ( Σ cos 2Φ - G PT,z sin 2Φ) - E Pγ circ PT,z ] Transversely polaris. protons: P T,x & P T,y dσ = dσ 0 [ 1- P lin γ ( H P T,x sin 2Φ + P P T,y cos 2Φ) + F Pγ circ P T,x + T P T,y ] PANIC11 25 July 2011 Daniel Elsner 28

29 Target asymmetry T Jan Hartmann unpolarised beam transversely polarised target (direction of pol.: β = 99 ) f = eff. dilution factor PANIC11 25 July 2011 Daniel Elsner 29

30 Recoil polarisation P and observable H linearly polarised photon beam (angle of pol. plane: α = 45 ) transversely polarised target (direction of pol.: β = 99 ) Jan Hartmann f = eff. dilution factor PANIC11 25 July 2011 Daniel Elsner 34

31 Cross sections γ p K 0 Σ + Cross section measurements suggest strong t-channel dependency Average K*Σ + / K*Λ threshold Black points: Ewald [1] Red points: Casteljins [2] [1] R. Ewald. PhD Thesis, Universitat Bonn (2010) Taken from [1] [2] R. Casteljins. Eur. Phys. J. A. 35:39 (2008) PANIC11 25 July 2011 Daniel Elsner 37

32 Cross sections γ p K 0 Σ + No existing partial wave parameterisation reproduces total cross section. Summing cross sections from K 0 Σ + and K* Σ + [3] yields red points The strength of the dip in the cross section for K 0 Σ + contributes to the K*Σ + channel Kaon-MAID [1] SAID [2] Dotted lines: Ghent PWA variants Speculate that just below threshold, K* couples strongly to a K 0 and π. K 0 is observed in the final state, π reabsorbed by the hyperon (*) therefore contributes to K 0 Σ + contribution below K* threshold. Above threshold it does not contribute and K* is produced as a free particle (*) Average K*Σ + / K*Λ threshold [1] (version ) [2] gwdac.phys.gwu.edu/ [3] M.Nanova et al. (CBELSA/TAPS Collab.) Eur.Phys. J. A35 PANIC11 25 July 2011 Daniel Elsner 38

33 Quasi-free photoproduction of η-mesons off the deuteron no cut on spectator momentum spectator momenta p < 100 MeV I. Jaegle et al., EPJA, in print position: W 1.67 GeV width: Γ 25 MeV (FWHM) narrow structure in excitation fct. of γ n n η: B. Krusche Parallel 2C Recent Baryon Results Mon 25/07, 17:00 Photoproduction of Mesons off the Deuteron GRAAL: W 1680 MeV, Γ <30 MeV Tohoku-LNS: W 1666 MeV, Γ <40 MeV ELSA: W 1685 MeV, Γ <40 MeV MAMI-C: W 1675 MeV, Γ <40 MeV PANIC11 25 July 2011 Daniel Elsner 39

34 Summary linearly and circularly pol. photon beams available at ELSA degree of polarisation: P lin max = %, Pcirc max = 65% longitudinally and transversely polarised target in operation the Crystal Barrel/TAPS experiment at ELSA ideally suited for neutral mesons in the final state almost complete coverage of solid angle high quality data for double polarisation observables preliminary results for the double polarisation observables: Σ,G, E and T,P,H essential for model independent PWA (complete experiment) PANIC11 25 July 2011 Daniel Elsner 40

35 New BGO OD setup ToF Walls BGO - OD set-up Drift Chambers Open Dipole forward spectrometer Dipole Magnet Scifi2 & MOMO Beam dump Photon tagging BGO calorimeter MWPCs inner tracking Si strips fw tracking Target system electron beam Bi 4 Ge 3 O 12 = Bismuth germanate PANIC11 25 July 2011 Daniel Elsner 41

36 BGO OD experiment PANIC11 25 July 2011 Daniel Elsner 42