Status of the recoil nucleon polarimeter

1. Status of the recoil nucleon polarimeter Dan Watts, Derek Glazier, Mark Sikora (SUPA PhD student) (University of Edinburgh, UK)

2. Outline • Physics motivation • Polarimeter operation • Beam test - proof of polarimeter concept • First results - beam helicity transfer observabes (Cx) • Outlook

3. Physics motivation: Nucleon excitation spectrum Excitation spectrum is fundamental to nucleon structure - but not firmly established Particularly disappointing given the potential advances from theory QCD models Constituent quark models Holographic dual of QCD Lattice QCD

4. Polarisation observables • s just one of 16 observables in pseudo scalar meson photoproduction • Complete measurement requires 8 well chosen observables • Only possible with double polarisation measurements Longitudinally polarised proton target  Transversely polarised  g+ N → N + m Recoil polarimeter - enable the first complete measurement of observables Fully constrain the reaction amplitudes Linear Polarisation  Circular polarisation 

5. Double-polarisation in pseudo-scalar meson photoproduction Polarisation of g target recoil Observable

6. Nucleon scattering and polarisation q n(q,f) =no(q){1+A(q)[Pycos(f)–Pxsin(f)] Number of nucleons scattered In the direction q, f x and y (transverse) components of nucleon polarisation Polar angle distribution for unpolarised nucleons Analysing power of scatterer

7. The polarimeter setup

8. Test data results - p(g,p0) yield Ee=1.5 GeV

9. Test data analysis – p(g,p0)p Cx’ • 2 x 3 day beam times (Ee=0.85 and 1.5 GeV) - First data for Cx!! Cx’ Cx’ Photon energy (MeV) Photon energy (MeV)

10. 2p0 – test of helicity bit Single spin beam helicity asymmetry

11. Test data analysis - p(g,h) Eg <0.9 geV All qh First measurement of beam helicity transfer in h photoproduction!! Eg =0.9 -1.1 GeV All qh Azimuthal scatter angle in polarimeter

12. Summary and outlook • Succesful nucleon polarimeter test - now ready for production beamtime • Formalism for extraction of Ox, T we developed for CB proposal used succesfully to extract observables in JLAB kaon photoproduction measurement • New Edinburgh PhD student to work on project • CB@MAMI poised to provide unique measurements of double-polarisation observables for for p and h meson photoproduction

13. MAID predictions and expected data accuracy - p(g,p)N 300 hrs MAMI B 500 hrs MAMI C Qcm=120o±10

14. Tagged nucleon events For events with nuclear scatter in polarimeter

15. Sensitivity to Roper P11(1440) MAID PWA No Roper • Present PWA solutions indicate sensitivity of observables to specific resonances Linear Polarisation + RECOIL Linear Polarisation Asymmetry Cross section Eg = 500 MeV • Recoil observables give large sensitivities to poorly established resonances e.g. Roper P11(1440)

16. Summary • High quality meson photoproduction data with polarisation observables can be expected from MAMI • Determination of beam, target and recoil polarisation will give a “complete” measurement of observables • Commissioning data for nucleon polarimeter expected in 2007

17. Double polarisation in meson photoproduction • Many overlapping resonances • are a problem • Double polarisation • observables give new • constraints on resonance • properties and reaction • mechanisms g + p → N + meson g recoil target Polarised targets Polarised g beams

18. For events Scattered in polarimeter

20. Present knowledge of the spectrum “Roper” Resonance DMass ~ ±20 MeV DWidth ~ ±100 MeV!! Large discrepancies between analyses of sameexperimental data with different amplitude analysis methods D(1232) P11(1440) D13(1520) F15(1680)

21. Intense tagged photon beam, circularly or linearly polarised Longitudinally polarised proton and neutron targets Approved programme of measurements

22. 4 complex amplitudes - 16 observables in meson photoproduction To fix the 4 amplitudes unambiguously need to measure 8 real quantities ds + 3 single polarisation + 5 double polarisation Cannot choose from same set Need recoil polarisation measurements Why measure double polarisation observables? gtarget recoil

23. Photon Tagger upgrade

24. Predicted sensitivity to poorly established resonances Resonance parameters from quark model (Capstick and Roberts) Cx’ (g + recoil) – theoretical predictions Solid – SAID Dashed – background + **** Dotdash- background + **** +N-3/2(1960) Dutta, Gao and Lee, PRC 65, 044619 (2002)

25. Previous experimental data – SAID database P T Data for all CM breakup angles Ox’ Cx’ Recent JLAB data not in database

26. Recent Cx’ measurement at JLab • First determination • p(g,p)p0 in 2002 • Hall A JLab • MAID & SAID • poor description • of new data Polarisation transfer Cx’ Photon energy (MeV)

27. The proposed experimental setup Useful scattered event Select events with scattering angles larger than ~10 degrees : arising from nuclear interaction Initial path of proton Polarimeter acceptance : ±20o polar angle (target at centre) Most events suffer only coulomb scattering Hydrogen target cell g beam TAPS Graphite sheet Crystal Ball n(q,f) =no(q){1+A(q)[Pycos(f)–Pxsin(f)]

28. GEANT simulation of polarimeter • Simulation includes realistic • smearing of energy deposits due to experimental energy resolution • and proper cluster finding algorithms • Finite target size and Eg resolution included No Graphite With Graphite scatterer Angle between qN(Eg,qp) and TAPS hit

29. qp(CM) >~130o Kinematic acceptance of polarimeter p(g,p)N Polarimeter acceptance Pion angle in CM (deg) Eg=150 MeV Eg=200 Eg=300 Eg=500 Eg=750 Eg=1000 Eg=1500 Nucleon angle in lab (deg)

30. More forward recoils than for pion production. Almost all recoils are incident on polarimeter up to ~0.8 GeV Kinematic acceptance of polarimeter p(g,h)N Polarimeter acceptance CM h angle (degrees) Eg=720 Eg=820 Eg=920 Eg=1520 Lab nucleon angle (degrees)

31. Expected data accuracy Common parameters: Photon beam: 2.5x105g sec-1 MeV-1 Bin ±12.5 MeV Target: 2.11023 nuclei / cm2 Meson: e(p0) = 80% e(h) = 35% Bin ±10o Polarimeter: 3% probability for a (detected) nuclear scatter Average analysing power ~0.4

32. MAID predictions and expected data accuracy - p(g,p)N 300 hrs MAMI B 500 hrs MAMI C

33. MAID predictions and expected data accuracy - p(g,p)N 300 hrs MAMI B Full MAID No P11(1440)

34. Summary • The different sensitivities offered by recoil polarisation observables will give new constraints on the excitation spectrum of the nucleon. • Data will be complimentary to the beam-target measurement programmes in place at MAMI and other facilities • UK EPSRC grant already awarded to help setup the facility (including 2 year postdoc and graphite)

35. Cx’ – Extraction and expected accuracy Cx’ 0 180 360 Photon energy (MeV) • Pg=0.7, Eg=±25MeV, qp=130±10 • s ~ 1 mb/sr →DCx ~ 0.015 • s ~ 0.1 mb/sr →DCx ~0.05 • Greatly improved data quality Plot difference in f distributions for two helicity states (cut on region of q with reasonable A(q)) Left with simple sin(f) Dependence. Extract Px

37. Ox’ – linearly polarised g and recoil One measurement : p(g,p+)n Yerevan 80’s DP~2/√(A2N) Pg=0.4, Eg=±25 MeV, qm=130±10 s ~ 1 mb/sr →DOx ~ 0.04 s ~ 0.1 mb/sr →DOx ~0.12 Polarimeter - full f acceptance - determine T as the y component. Periodically change polarisation direction by ±45o - eliminate detector effects.

38. Lx (Longitudinally polarised Target + recoil) No previous measurements Mainz target: ~80% polarisation PT=0.7, Eg=±25MeV, qm=130±10 s ~ 1 mb/sr →DLx ~ 0.015 s ~ 0.1 mb/sr →DLx ~0.05 BUT: Limitations in beam intensity and dilution from polarised target Must measure background contribution from non-proton events. Prompt to background 1:1 worsens error by √2 • Transversely polarised (Tx)?

39. Cross sections Eg bin +-25 MeV Pion bin +-10 degrees, 500 hrbeamtime p(g,N)p Cross sections as low as 1mb/sr (>2*106 n bin1-) p(g,N)h, p(g,N)h/ Cross sections as low as 0.1mb/sr (0.2*106 nucleons per bin) Assume 1% of nucleons undergo nuclear interaction in proposed graphite sheet (select high analysing power with theta cut)

40. Estimate of polarimetry accuracy Take ds/dW~1mb/sr, qp=130±10 eDA ~eCB-TAPS~0.7, Ng=2.5x105 g sec-1 MeV-1 NNucleons = NT x Ng x eDA x eCB-TAPS x s = 2222 day-1 MeV-1 500 hour beamtime have 2.3x106 nucleons in Eg=±25MeV bin Polarimeter efficiency 2% gives 4.6x104 useful nucleons Absolute error in polarisation DP~√(2/A2N) ~ 0.02 (A~0.4 for 12C) For 0.1 mb/sr absolute uncertainty in polarisation DP~0.06 For double polarisation must divide error by beam(target) polarisation p(g,p0)p qp=130 p(g,h)p

41. ds/dW~1mb/sr, qp=130±10, eDA ~0.7; eCB-TAPS~0.5, Ng=2.5x105 g s-1MeV-1 NNucleons = NT x Ng x eDA x eCB-TAPS x s = ~5556 day-1 MeV-1 20 days beam, Eg=±25MeV → 5.5x106 nucleons e(polarimeter)~2% → 11.1x104 useful nucleons Analysing power A~0.4 for 12C s~1 mb/sr →DP~√(2/A2N) ~ 0.010 (abs. error) s~0.1 mb/sr →DP~0.026 For double polarisation must include further effects of degree of beam(target) polarisation Estimate of polarimetry accuracy p(g,p0)p qp=130 p(g,h)p qh=130

43. Principles of nucleon polarimetry • Well established technique – relies on spin-orbit interaction in Nucleon-Nucleon interaction • Polarimeters - exploited nucleon or nuclear targets (2H, 4He, 12C, 28Si) – tended to use materials with well known analysing powers A1 FPP Kent state GEn Polarimeter pomme

44. qscat Polarimetry basics • Measure direction of nucleon before and after the scatterer with sufficient accuracy to determine an analysing reaction has taken place. For incident protons also have multiple (coulomb) scattering Qscat=5-20o

45. Scattered nucleon detection in TAPS • 1 TAPS block ~ position resolution for hit • TAPS~0.9m from scatterer N Straight through 10o scatter 20o scatter p

46. Detrimental side-effects of scatterer material • To hit polarimeter TN>100 MeV in g(p,p)N above the D • Proton energy loss <10 MeV for Tp>100 MeV. • Multiple scattering <1o FWHM for Tp>100 MeV • 0.37 radiation lengths g conversion ~ 30% Tp after graphite Energy loss Tp exit proton (MeV) Tp incident proton (MeV) Coulomb scattering FWHM scattering angle (deg) Proton energy (MeV)