Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

Coherent 0 Photoproduction on Nuclei

Claire Tarbert, University of Edinburgh

Spokesperson: Dan Watts

Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

Coherent 0 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

Coherent A(0)A

Incoherent A(0)A*

(q)FdΩdσ 2

m

0 Photoproduction

Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

Nuclear Matter Radii

• 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.

Elastic Electron Scattering

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

Crab Pulsar

• 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.

208Pb and Neutron Stars

208Pb

Neutron Skin

Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

1. Calibration of Crystal Ball using low energy 0s.

2. Particle identification.

3. Select 0s.

4. Separation of Coherent/Incoherent events. -

-

Pion Missing Energy

Analysis Framework

E = E(1,2) – E(E)

E(1,2) = detected pion energy (cm)E(E) = calculated pion energy (cm)

Incoherent 0s always less energetic than coherent equivalent.

Nuclear decay s

Nuclear decay s.

Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

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

Fits to E

Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

Fits to E

(M

eV)

(MeV)

Completed first iteration of fits to pion missing energy.

= (30-32)o

Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

“Cross Sections” 208Pb

Still to include:• Better fits.• Simulated detection efficency (flat detection efficiency assumed at the moment).• Correction for cut on 0 invariant mass.

Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

“Cross Sections” 208Pb

Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

Comparison to Theory

+ Data-- DREN

Compare one energy bin to DRENcalculation –

rm ~ 5.78fm

(cf rc = 5.45fm)

DREN calculation by Kamalov

rm – rc ~ 0.33fm

Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

• Finalise coherent cross sections• improve fits to E

• finalise calibrations• finish 0 detection efficiency simulations

• Extract matter form factor via comparison to theory

• Continue analysis of Incoherent p0 photoproduction using detection of nuclear decay s

Conclusion

To do

Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

Comparison to Theory 208Pb

Theoretical Calculations by S.Kamalov

--- PWIA

--- DWIA

--- DREN

Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

Just in case

40Ca

References: APNC -

Crystal Ball Collaboration Meeting, Basel, October 2006 Claire Tarbert, Univeristy of Edinburgh

Preliminary Analysis

Coherent 0 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 E bins, but E increases

can fit to at least 40 spectra for each target to extract form factor and pion distortion parameters.

208Pb