Neutral pion photoproduction and neutron radii

Dan Watts, Claire TarbertUniversity of Edinburgh

Crystal Ball and A2 collaboration at MAMI

Eurotag Meeting September 2008Presented by Bob Owens

Glasgow University

• 0 photoproduction from nuclei

Coherent - Accurate Matter form factors Neutron skins of stable nuclei (neutron stars)

Incoherent - Transition matter form factors New observable for production amplitude

Talk Outline

Ex, q

Why measure the matter form factor?

e.g. 208Pb RMS charge radius accuracy < 0.001 fm RMS neutron radius accuracy ~0.2 fm !!

• Our knowledge of the shape of stable nuclei is presently incomplete

Horowitz et al. PRC63 025501 (2001)Piekarewicz et al. NPA 778 (2006)

Relativistic mean field

Skyrme HF

Mass Number (A)

Proton fraction as a function of density in neutron star

Thick neutron skin→ Low transition density in neutron star

New data fromX-Ray telescopes→ mass, radii, tempof neutron stars !

208Pb Neutron skin and Neutron stars

Liquid

Solid

Direct URCA Cooling

n → p + e- +

e- + p → n +

Matter form factor and neutron stars

Rutel et al, PRL 95 122501 (2005)Horowitz, PRL 86 5647 (2001) Horowitz, PRC 062802 (2001)Carriere, Astrophysical Journal 593 (2003)Tsuruta, Asttrophysical Journal Lett. 571 (2002)

• Angular distribution of 0 → PWIA contains the matter form factor

• 0 final state interactions - use latest complex optical potentials tuned to -A scattering data. Corrections modest at low pion momenta

Coherent pion photoproduction

Photon probe Interaction well understood 0 meson – produced with

~equal probability on protons AND neutrons.

Select reactions which leave nucleus in ground state

Reconstruct

from →decay

d/dPWIA) = (s/mN2) A2 (q*/2k) F2(E

,)2 |F

m(q)|2 sin2

TAPSCrystal Ball528 BaF2 crystals

672 NaI crystals

E~ 2 MeV108 sec-1

Coherent pion photoproduction

208Pb

Coherent maxima

208Pb

Ed

iff

Ed

iff theta (deg)

Non-coherent contributions

E=210±10 MeV

theta (deg)

Coherent pion photoproduction - analysis

E=175±5 MeV

Ediff = E

measured- Ecalc

208Pb data

208Pb : Momentum transfer distributions

▬ Unitary isobar model () with complex optical potential Dreschel et. al. NPA 660 (1999)

|q| = p-p0 (fm-1)

E=180-190 MeV E=180-190 MeVE=190-200 MeV E=200-220 MeV

E=220-240 MeV E=240-260 MeV

208Pb: Simple correction for distortion

• For first preliminary assessment 1) Carry out simple correction of q shift using the theory 2) Analyse corrected minima - fit to determine qmin

E=200-220 MeV

208Pb neutron skin – preliminary assessment

• See effects of a neutron skin of ~0.1 fm (preliminary!!)• More detailed analysis in progress ▬ implement various predicted FF ▬ “model independent” analysis

No Skin

0.1 fm skin

0.2 fm skin

Pro

ton

sca

tter

ing

NP

A 7

78 (

200

6) 1

0

An

tip

roto

nic

ato

mP

RC

76

034

305

(20

07)

0.3 fm skin

Incoherent nuclear 0 photoproduction

• Measurement of neutral pion production to a discrete excited nuclear state has proven elusive for many decades

• → Detect nuclear decay photon also in the Crystal Ball

• Important if want to extract matter form factorsfor light nuclei (Coherent/incoherent goes as A2/A = A)

Ex, q

In coincidence with 12C(0)

E=200-220 MeV

Takaki -hole model (NPA 443 p570 (1985)) ▬ Full calculation --- Without -N interaction

Tryasuchev model (Phys At.Nuc. 70 827 (2007))

--- Full calculation

CM Tarbert et. al., PRL 100 132301 (2008)

Incoherent cross section12C(0)12C(4.4MeV)E=220 10 MeV

0

• Strong sin2(2) distribution for 4.4 MeV photons

• 2+ to 0+ transition from aligned residual nucleus

• Degree of alignment - new experimental observable!!

Alignment of recoiling 12C nucleus

sin2(2) distributionPassed through Detector acceptance

Alpha (deg)

Coun

ts [a

rb. U

nits

]

q

• New high quality nuclear 0 photoproduction data will give timely constraints on nuclear structure and neutron stars

• Nuclear decay photon detection to tag incoherent processes -> accurate matter form factors for lighter nuclei

Summary

672 NaI crystals

342 BaF2 crystals

• Strong sin2(2) distribution for 4.4 MeV photons - Spin independent amplitude dominant () (Tryasuchev and Kolchin Phys. At. Nuc. 70 827 (2007))

• Spin dependent predicted to give cos2(2) – use to separate the in medium amplitude?

Alignment of recoiling 12C nucleus

Alpha (deg)

Co

un

ts [

arb

. Un

its]

q

sin2(2)

cos2(2) Incl..detector acceptance

12C(0)12C(4.4MeV)E=300 10 MeV

Edependence of spin dependent / independent in medium

12C(0)12C(4.4MeV)E=175 5 MeV

12C(0)12C(4.4MeV)E=310 10 MeV

12C(0)12C(4.4MeV)E=420 20 MeV

Decay angular distributions – other targets

Ca40(0)40Ca*E=230 10 MeV

O160)16O*E=230 10 MeV

16O40Ca

Symmetrised Fermi distributions

Coherent production on Oxygen

Coherent production on Calcium