neutral pion photoproduction and neutron radii dan watts, claire tarbert university of edinburgh...
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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