Photoproduction of Neutral Pseudoscalar Mesons on the Neutron

1. Photoproduction of Neutral Pseudoscalar Mesonson the Neutron Spokespersons: William Briscoe and D. Mark Manley

2. Photoproduction of p0, p0p0, p0 h0 on polarized deuterium • In order to extract reliable N* resonance parameters from meson photoproduction data, PWAs must be extended to include additional reactions. • Single-channel pN analyses alone do not provide the necessary constraints for a full and unambiguous determination of resonance properties. • This is particularly true for resonances that have only a weak coupling to the pN state.

3. Principal Measurements of Interest

4. Overview • We will use a polarized deuterium target to make double polarization measurements. • We propose to begin with a transversely polarized target to take advantage of the fact that JLab will not be ready to run transversely polarized deuterons until last 2010. • These experiments are complementary to charged final-state measurements and will further the investigation of low-lying resonances.

5. Pion Photoproduction • Pion photoproduction is the fundamental process used to determine baryon-resonance photo-decay amplitudes. • This process is defined by three independent isospin amplitudes (the isospin-3/2 amplitude and independent isospin-1/2 amplitudes for proton and neutron targets). • The isospin-3/2 and isospin-1/2 (proton) amplitudes are most reliably determined from experiments measuring gp→ p0p and gp→ p+n.

6. The Isospin-1/2 (Neutron) Amplitude • Assuming a good determination of the isospin-3/2 amplitude, the isospin-1/2 (neutron) amplitude can in principle be determined by measuring gn → p-p. • However, fits much above the D(1232) region are generally model-dependent and this model-dependence is greatest for the extraction of gn photo-decay amplitudes. • In general, the isospin-1/2 (neutron) amplitudes are more poorly determined than the isospin-1/2 (proton) amplitudes.

7. Existing gn→p0n Data

8. Polarization Measurements Eight observables can be measured w/o a polarimeter for the recoil nucleon:

9. Spin Observables

18. Neutral Pion PhotoproductionSummary • At present, only ds/dW and S have been measured with the Crystal Ball and TAPS. • With longitudinally and transversely polarized targets, most observables. • At MAMI-C energies, SAID and MAID give different predictions and no data are available for gn →p0n above 905 MeV. • Awaiting other speakers to see what of the π0π0 and π0η we should include here.

19. Polarization Observables in Coherent p0Photoproduction off Deuterium • We propose to measure T20, iT11, T21, and T22 at backward pion CM angles and photon energies between 0.5 and 1.4 GeV. • Our goal is to study the underlying reaction dynamics at moderate momentum transfer: 1 - 4 (GeV/c)2.

20. Physics Motivation: intermediate η rescattering • dπ0d shows strong sensitivity to intermediate η–rescattering through excitation of S11(1535) at photon energies around 800 MeV and cos(CM) < -0.6. • Unpolarized cross sections from Imanishi et al. (Phys. Rev. Lett. 54, 2497(1985)), as well as data from CLAS show an enhancement in the energy dependence around E = 0.7 GeV at pion CM angles above 130. The structure seems to get more pronounced as the pion CM angle increases.

21. CLAS DataY. Ilieva et al. (arXiv:nucl-ex/0703006)

22. Physics Motivation: intermediate η rescattering • Unpolarized cross sections from Asai et al. (Phys. Rev. B187, 249(1987)) do not show any structure around E = 0:7 GeV, but a smooth fall off. Currently this result disagrees with the CLAS data. • Similar enhancement around beam momentum of 0.7 GeV/c is observed in the excitation functions of backward πd elastic scattering.

23. Physics Motivation: intermediate η rescattering • The observed structure can be explained in terms of a contribution from formation and rescattering via S11 excitation in the intermediate state. • Interference with the background enhances the signal, and higher order rescattering might contribute to the magnitude of the structure

24. Physics Motivation: intermediate η rescattering • High statistics measurement of d/d at photon energies around 0.7 GeV and backward pion CM angles needed in order to solve the disagreement between the CLAS and the Asai et al. data and to study the underlying -rescattering amplitudes. • Previous measurement of at 130 shows potential sensitivity to the reaction dynamics.

25. Kondratyuk and Lev showed that this effect can be explained with the opening of the η-production threshold and indermediaterescattering.

26. Higher order rescattering terms might contribute to the magnitude of the structure A. Fix, arXiv:nucl-th/0509029 (2005).

27. Fits to the CLAS data were made in order to determine the relative magnitude of the sub-process.(A.E. Kudryavtsev et al. Phys.Rev.C71:035202,2005. e-Print: nucl-th/0408027)

28. Fits to the CLAS data were made in order to determine the relative magnitude of the sub-process.

29. Physics Motivation: intermediate η rescattering • The proper way to study the underlying reaction dynamics is through helicity amplitudes. • There are 9 independent helicity amplitudes depending on s and t. 17 independent measurements are necessary in order to determine those. • Key importance of high precision single- and double-polarization experiments. • Measuremnts will be made as paracite to neutron measurements.