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Coherent p 0 Photoproduction on Nuclei

Coherent p 0 Photoproduction on Nuclei. Claire Tarbert, University of Edinburgh. Spokesperson: Dan Watts. p 0 Photoproduction. Coherent A( g,p 0 )A Incoherent A( g,p 0 )A*. Coherent p 0 Photoproduction Takes place with ~same probability on n and p.

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Coherent p 0 Photoproduction on Nuclei

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  1. Coherent p0 Photoproduction on Nuclei Claire Tarbert, University of Edinburgh Spokesperson: Dan Watts Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

  2. p0 Photoproduction Coherent A(g,p0)A Incoherent A(g,p0)A* • Coherent p0 Photoproduction • Takes place with ~same probability on n and p. • Reaction amplitudes from all nucleons add coherently. • Cross section contains information on matter distribution • F2m(q) is Fourier Transform of Matter Density as a function of radius. • =>r.m.s matter radius Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

  3. Nuclear Matter Radii Elastic Electron Scattering • Charge radii (distribution of protons) well known from electron scattering etc. • Matter radii (protons and neutrons) less well known. • Theory predicts a neutron skin for n-rich nuclei (208Pb ~0.1 – 0.3 fm) • Traditionally use strong probes to probe matter radius e.g. p, a scattering • Encounter problems with model dependency – initial and final state interactions. Nuclear Charge Radii • Matter radii are important as: • A test of Nuclear Theories • A constraint for Atomic Parity Non-Conservation • A constraint on the properties of Neutron Stars Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

  4. 208Pb and Neutron Stars • Skin thickness on 208Pb gives info about compressibility of matter. • Calibrates symmetry energy as a function of density at low densities. • Large neutron skin => large crust on neutron star. Crab Pulsar 208Pb Neutron Skin Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh Possible Equations of State for Neutron Stars

  5. Analysis Framework Pion Missing Energy Calibration of Crystal Ball using low energy p0s. Particle identification. Select p0s. Separation of Coherent/Incoherent events. - - DEp = Ep(g1,g2) – Ep(Eg) Ep(g1,g2) = detected pion energy (cm) Ep(Eg) = calculated pion energy (cm) Incoherent p0s always less energetic than coherent equivalent. Nuclear decay gs Nuclear decay gs. Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

  6. Analysis Framework Angular distribution of photons • Sharp drop off in no of detected photons in region of phase space covered by TAPS. • See similar distribution for protons. • Now only use TAPS to veto charged particles. Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

  7. Fits to DEp Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

  8. Fits to DEp qp = (30-32)o Completed first iteration of fits to pion missing energy. s (MeV) Eg (MeV) Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh Crystal Ball TAPS 2001 data

  9. “Cross Sections” 208Pb • Still to include: • Better fits. • Simulated detection efficency (flat detection efficiency assumed at the moment). • Correction for cut on p0 invariant mass. Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

  10. “Cross Sections” 208Pb Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

  11. Comparison to Theory • Compare one energy bin to DREN • calculation – • rm ~ 5.78fm (cf rc = 5.45fm) + Data -- DREN • rm – rc ~ 0.33fm DREN calculation by Kamalov Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh Diffraction from a circular disc: => rm ~ 5.85fm

  12. Conclusion To do • Finalise coherent cross sections • improve fits to DEp • finalise calibrations • finish p0 detection efficiency simulations • Extract matter form factor via comparison to theory • Continue analysis of Incoherent p0 photoproduction using detection of nuclear decay gs Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

  13. Comparison to Theory 208Pb Theoretical Calculations by S.Kamalov --- PWIA --- DWIA --- DREN Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

  14. Just in case 40Ca References: APNC - Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

  15. Preliminary Analysis 208Pb • Coherent p0 Photoproduction • Theoretical Calculations: • PWIA (Plane Wave Impulse Approx ) • DWIA (Distorted Wave Impulse Approx) • DREN (Delta Resonance Energy Model) • DWIA, DREN take into account FSI. • Good agreement with theory. • Same quality of data for all targets. • F2m(q) is same for all Eg bins, but Ep increases => can fit to at least 40 spectra • for each target to extract form factor and • pion distortion parameters. Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

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