NH Colloquium

C.M. Tarbert et al., PRL 112, 242502 (2014):

Neutron Skin of 208Pb from Coherent Pion

Photoproduction

Presentation by T. Gogami2015/6/15 (Mon)

Equation state of neutron matter

where,

Symmetry energy factor

Density dependence of the symmetry energy

HIC exp.

Heavy ion collision Nuclear binding energy Isobaric analog state Neutron skin thickness

M.B.Tsang, …, T.Murakami et al., PRC 86, 015803 (2012)

𝑆 (𝜌 )=𝑆0−𝐿𝜀+12𝐾 𝑠𝑦𝑚 𝜀

2+𝑂 [𝜀3]

𝜀=(𝜌0−𝜌)/(3𝜌 0)

Neutron skin thickness:

Δ 𝑟𝑛𝑝 (¿𝑆 )=√¿𝑟𝑛2>¿−√¿𝑟𝑝

2 >¿¿¿Difference of RMS radii between neutrons and protons

L vs

Neutron skin thickness of 208Pb Constraint for L

L.W. Chen et al., PRC 72, 064309 (2005)

Measurements up to nowProton scattering:

fm [20]Pion scattering:

fm [21]Annihilation of antiprotons:

fm [22,23]Iso-spin diffusion in heavy ion collisions:

fm [24]Pygmy dipole resonances and electric dipole

polarizabilities of nuclei: fm [15,17,25]

[22] B. Klos et al., PRC 76 (2007) 014311[23] B.A. Brown et al., PRC 76 (2007) 034305[24] L.W. Chen et al., PRC 72 (2005) 064309 [25] A. Krasznahorkay et al., PRL 66 (1991) 1287

[15] A. Carbone et al., PRC 81 (2010) 041301[17] A. Tamii et al., PRL 107 (2011) 062502[20] J. Zenihiro et al., PRC 82 (2010) 044611[21] E. Friedman, NPA 396 (2012) 46

Experiments with electromagnetic probes

Different systematic errors from strongly interacting probes No successful measurements of the neutron distribution

208Pb Radius Experiment (PREX)Elastic scattering of polarized electrons from 208Pb Parity-violating asymmetry:

Weak Form factor Neutron density

S. Abrahamyan et al., PRL 108, 112502 (2012)

X. Roca-Maza et al., PRL 106, 252501 (2011)𝜟𝒓𝒏𝒑=𝟎 .𝟑𝟑 ±𝟎 .𝟏𝟕

Coherent Photoproduction of π0 meson from 208Pb

dependenceTwo-parameter Fermi distribution (2PF):

MAMI at Johannes Gutenberg-Universität Mainz

Okazaki

Gutenberg

Exp. hall

Schematic drawing of Experimental setup

Beam: 180 – 240 MeV tagged photonTagged photon resolution ~ 2 MeV full width2×105γ s-1MeV-1

Enriched 208Pb(0.52 mm, 99.5%)

Crystal Ball: 672 NaI(94% of 4π)

C.M. Tarbert et al.,PRL 100, 132301 (2008)

Schematic drawing of Experimental setup

Beam: 180 – 240 MeV tagged photonTagged photon resolution ~ 2 MeV full width2×105γ s-1MeV-1

Enriched 208Pb(0.52 mm, 99.5%)

Crystal Ball: 672 NaI(94% of 4π)

C.M. Tarbert et al.,PRL 100, 132301 (2008)

Analysis

Energies and angles of 2γπ0 momentumq = (Pγ - Pπ)Cross section as a function of q

Analysis (Yield of Coherent Reaction)

Δ 𝐸𝜋diff .=𝐸𝜋

c .m .−𝐸𝜋det .

π0 energy assuming coherent reaction

Efficiency etc.

• π0 detection efficiency: ~40%• Photon tagging efficiency: ~40%• π0 not originating from target: ~1%

Fitting to the experimental results

𝜌 (𝑟 )=(𝑍 / 𝐴)𝜌𝑝 (𝑟 )+(𝑁 / 𝐴)𝜌𝑛(𝑟 )

Known: ap=0.447 fm, cp=6.680 fm

cn = 6.28 to 7.07 fm an = 0.35 to 0.65 fm σq = 0.02 – 0.03 fm-1 depending on Eγ

Results

fm

fm

fm

Neutron skin of 208Pb Halo character

Summary

Coherent photoproduction of π0 from 208Pb fmConsistent with previous experiments