Recoil Polarisation Measurements in Meson Photoproduction

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1 Recoil Polarisation Measurements in Meson Photoproduction Polarisation Observables and Partial Wave Analysis Bad Honnef 2009 Derek Glazier, D.P. Watts University of Edinburgh

2 Helpful for PWA At least 8 observables From at least two of Beam Recoil, Beam Target or Target Recoil Proton and Neutron targets Producing Small enough E and bins to resolve resonance structure

3 Beam Recoil Observables

4 Measuring Nucleon Polarisation Use Spin Orbit Interaction Azimuthal distribution related to transverse polarisation

5 Polarimeter Reference Frame

6 JLab Hall A Results for photoproduction o Phys.Rev. C, (2002) Used Focal Plane Polarimeter on magnetic spectrometer ~30 Data points for P, Cx and Cz 0.8<E <4.1GeV, 60< <135 Also a reasonable number of previous P measurements below 1.5 GeV P cx

7 CrystalBall at MAMI E~2 MeV I~108 /s CB 672 NaI(Ti) Crystals TAPS Spherically arranged 384 BaF2 Crystals around target О Forward 21 93% phase space 1.5m upstream MAMI C provides 1.5GeV e Tag Brem. Photons to 1.4GeV Linear or circular pol.

8 Proton Polarimeter Experiment MC MC no had. inter.

9 Kinematics and Acceptance

10 Polarimeter Data Analysis Algorithm

11 Beam Helicity Asymmetries sc sc

12 Analysing Power Polarised scattering model Parameterisation based on world pc scattering data set Function of T and p sc Tp sc + GEANT4 tracking model Include all polarimeter components Track polarisation=+1 and 1

13 Results of Analysing Power Integration Perform full data analysis then, Fit to MC Asymmetries Analysing Power Same binning as for real data sc E Overall detection eff. ~2 3%, analysing power~0.2

14 Preliminary Cx Results Divide real data asymmetries by MC analysing pow. E E

15 Linearly Polarised Beam Beam linearly polarised through CohBrem P= between MeV T Ox SAID 0.84 MAID 0.4

16 Recap...we would like : CHARGED and neutral mesons Protons and NEUTRONS Large acceptance (will need central and forward components)

17 New Central Nucleon Polarimeter Design Not to scale! Scattered vector from wirechambers + Track back to graphite for incident vector Use np charge exchange as neutron analyser sc, sc POLARISATION

18 Example p n Implement new design into CrystalBall Monte Carlo model Track n events and analyse ouput ID from E E technique n/p charge exchange from wire chamber track with no scintillator hit Polarimetry information comes just from track

19 Simulation Spectra for n

20 Simulation Results for n For Polarisation and Analysing power=1 Asymmetry ~ 0.7 Detection Efficiency ~1% We can produce measurable asymmetries!

21 Analysing Power for n/p charge exchange D. Glazier, PhD Thesis, University of Glasgow, 2003 n/p Charge exchange on CH2 at 400 MeV well modelled by quaifree model see also NPOL3, NIM A547, (2005) 569 +C(n,p) scattering papers Analysing Power Tn>150 MeV Model Experiment

22 Comparison of old and new Relative figure of merit for proton and neutron polarimeter, Proton beamtime=2.5 weeks, Therefore 10 weeks required for similar results with neutrons i.e 6 8 bins, 50 MeV E bins, (100 MeV above 1.3GeV) With Deuteron target would simultaneously measure 4 times as many bins with proton in final state Also measure polarisation in deuteron photodisintegration

23 Forward Wall Design still to be developed More space should allow larger efficiency Possibilities, Traditional neutron polarimeter, 2 walls of scintillator +High efficiency, higher analysing power (from nn) Complicated acceptance, poor forward photon detection Another tracker design

24 Other Facilities Could extend measurements above 1.4GeV Linearly polarised measurements above 0.7GeV Method requires central tracking detectors For example Bonn B1, similar detector setup, forward dipole magnet/tracking CLAS...

25 Summary Recoil polarisation measurements are important for a complete measurement Beam Recoil measurements are possible at current experimental facilities Can be very cost effective Should be possible to measure many final states