crystal ball collaboration meeting, mainz, october 2007 claire tarbert, univeristy of edinburgh...

Crystal Ball Collaboration Meeting, Mainz, October 2007 Claire Tarbert, Univeristy of Edinburgh

Coherent 0 Photoproduction on Nuclei

Claire Tarbert, University of Edinburgh

Spokesperson: Dan Watts


Overview

1. Very briefly… background, motivation

2. Analysis Outline

3. Cross Sections

4. Status and Future work


• 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

• Similarities with elastic electron scattering.

• Complicated by presence of pion – nucleus final state interactions (FSI) extraction of form factor is model dependent.

Coherent A(0)A

(q)FdΩdσ 2

m2A

CoherentNuclear0 Photoproduction


Nuclear Matter Radii

• Neutron Skin: Difference between rms proton and neutron radii.

• Matter radii (protons and neutrons) poorly 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


Crab Pulsar

208Pb and the Equation of State

Energy per particle in infinite nuclear matteras a function of density and isospin asymmetry: E/A(n,I)Where n is the number density (sometimes written as r) and I = (N – Z)/A = (nn – np)/n

From this all Infinite Nuclear Medium parameters follow.

Example: Semi-empirical mass formulaGives a good description of symmetric

nuclear matter at nuclear denisities.

Extending the equation of state to higher densities and more asymmetric matter – e.g. neutron stars.


Crab Pulsar

• Calibrates symmetry energy as a function of density at low densities.

• Large neutron skin large crust on neutron star.

• Large neutron skin + small neutron star radius phase transition in EOS strange quark matter?

• PREX measurement @ JLAB cited 80 times according to SPIRES (Phys. Rev. C63, 025501 (2001))

208Pb and Neutron Stars

Derivative of Eos

Ne

utr

on

Sk

in T

hic

kn

es

s

Ne

utr

on

Sk

in T

hic

kn

es

s

Derivative of Symmetry Energy


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

2. Particle identification.

3. Select 0s, don’t detect recoil.

4. Separation of Coherent/Incoherent events. -

-

Analysis Framework

E = E(1,2) – E(E)

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

Not trivial – typical energy of first nuclear excited state a few MeV!

Nuclear decay s.

Experimental Details

208Pb

40Ca

16O

12C

2 weeks April 2005

E = 883MeV

M2 trigger


• Fitting to the pion missing energy obviously difficult.

• Don’t try to fit entire incoherent background.

• Exploit areas that are clearly dominated by coherent peak to get a good sample of coherent shape.

• Use this information on width, position to constrain fits in regions where coherent is not dominant.

Analysis Framework: Fits to E


Analysis Framework: Fits to E

• Widths of coherent peaks taken from fits to coherent dominated part of spectrum.

• Confirmed widths via comparison with simulation.


Analysis Framework: Empty Target


Analysis Framework: H20 Target

Hydrogen


Analysis Framework: Nuclear Decay s

12C 16O


Analysis Framework: Nuclear Decay s

Coherent minimum

Coherent maximum

16O

Try to sample pion missing energy from incoherent by cutting on decay gammas.

Will develop this technique with full simulation of decay gammas.


Analysis Framework: 0 Detection

Efficiency

• Throw0s isotropically – 100000/MeV

•See effect of Pb target – important to get radius (2.1cm) right in simulation

• See effect of missing TAPS at higher photon energies.

+ 208Pb

+ 40Ca

+ 12C

+ 16O


Analysis: 4x4 effect in TDCs

Pb M3 trigger

Pb M2 trigger

Ca M2 trigger

O M2 trigger

C M2 trigger

• See familiar 4x4 structure in Ca-40, Pb-208 (M2) and Pb-208 (M3) data.• Rate dependent: Ladder OR 3 times higher for Pb than C12, CB OR 4 times higher for Pb than C12.


Final 208Pb Cross Sections

+ CB data

+ Glasgow TAPS data

-- Delta Resonance Energy Model (DREN - Kamalov)

DREN includes effect of pion-nucleus final state scattering via a pion-nucleus optical potential. Also includes modification and propagation of in the medium.



+ CB data

+ Glasgow TAPS data



208Pb Total Cross Section

+ CB data

+ Glasgow TAPS data


40Ca Cross Sections

+ CB data

+ Glasgow TAPS data



16O Cross Sections

+ CB data

+ Glasgow TAPS data-- Delta Resonance Energy Model (DREN - Kamalov)


12C Cross Sections


Kinematic Fitting

• Have dabbled with Derek Glazier’s kinematic fitting classes.• Use only one constraint – mass of the 0.• Definetely see different shape to missing energy.• Possibly gain ~0.5MeV in widths of coherent peaks at low energys.• Need new resolutions for solid targets from simulation.

+ No kin fit

+ Kin fit

E = (190-200)MeV, = 40-42(o)

16O


What can we say about the 208Pb neutron skin?

Neutron radius = proton radius (no skin)

0.15fm neutron skin

208Pb208Pb


What can we say about the 208Pb neutron skin?

Compare with Krusche’s TAPS data – see a definite shift in minimum to larger momentum transfer i.e. larger neutron skin.

+ Krusche TAPS

+ Glasgow TAPS

+ CB


The Way Forward to the 208Pb Matter Distribution

• 208Pb cross sections in theta are now finalised.

• Need the equivalent as a function of momentum transfer.

• Use improved Kamalov calculations to correct for pion FSI.

• Extract form factor and transform to a matter distribution using the techniques pioneered for electron scattering expts.


Cross Sections:Pb Differential cross sections look good and are essentially

finalised. See good agreement in magnitude of cross section with previous data but with better statistics, angular resolution and a more physical shape.

Ca Differential cross sections look good.

O, C Should benefit from a kinematic fit – first attempts look promising.

Matter Distributions:• See hints of neutron skin on Pb.• Next step: full extraction done in terms of the momentum transfer to the

nucleus.

Conclusions & Future Work


Future Work:• 6 month post doc at Edinburgh• Extract matter radius of 208Pb and publish.

• Also start investigating decay gammas seriously:• Incoherent 0 photoproduction• Nuclear astrophysics applications.

Conclusions & Future Work


•END


Comparison to Theory

+ Data-- DREN

Compare one energy bin to DRENcalculation –

rm ~ 5.78fm

(cf rc = 5.45fm)

rm – rc ~ 0.33fm

DREN calculation by Kamalov


Crab Pulsar

208Pb and the Equation of State


Analysis: Tagging Efficiencies

+ 208Pb

+ 40Ca

+ 12C

+ 16O

• 4mm collimator

crystal ball collaboration meeting, mainz, october 2007 claire tarbert, univeristy of edinburgh...

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