Neutron skin of (208)pb from coherent pion photoproduction

C M Tarbert; D P Watts; D I Glazier; P Aguar; J Ahrens; J R M Annand; H J Arends; R Beck; V Bekrenev; B Boillat; A Braghieri; D Branford; W J Briscoe; J Brudvik; S Cherepnya; R Codling; E J Downie; K Foehl; P Grabmayr; R Gregor; E Heid; D Hornidge; O Jahn; V L Kashevarov; A Knezevic; R Kondratiev; M Korolija; M Kotulla; D Krambrich; B Krusche; M Lang; V Lisin; K Livingston; S Lugert; I J D MacGregor; D M Manley; M Martinez; J C McGeorge; D Mekterovic; V Metag; B M K Nefkens; A Nikolaev; R Novotny; R O Owens; P Pedroni; A Polonski; S N Prakhov; J W Price; G Rosner; M Rost; T Rostomyan; S Schadmand; S Schumann; D Sober; A Starostin; I Supek; A Thomas; M Unverzagt; Th Walcher; L Zana; F Zehr; (Crystal Ball at MAMI and A2 Collaboration)

doi:10.1103/PhysRevLett.112.242502

Neutron skin of (208)pb from coherent pion photoproduction

Phys Rev Lett. 2014 Jun 20;112(24):242502. doi: 10.1103/PhysRevLett.112.242502. Epub 2014 Jun 18.

Authors

C M Tarbert¹, D P Watts¹, D I Glazier¹, P Aguar², J Ahrens², J R M Annand³, H J Arends², R Beck⁴, V Bekrenev⁵, B Boillat⁶, A Braghieri⁷, D Branford¹, W J Briscoe⁸, J Brudvik⁹, S Cherepnya¹⁰, R Codling³, E J Downie³, K Foehl¹, P Grabmayr¹¹, R Gregor¹², E Heid², D Hornidge¹³, O Jahn², V L Kashevarov¹⁰, A Knezevic¹⁴, R Kondratiev¹⁵, M Korolija¹⁴, M Kotulla⁶, D Krambrich⁴, B Krusche⁶, M Lang⁴, V Lisin¹⁵, K Livingston³, S Lugert¹², I J D MacGregor³, D M Manley¹⁶, M Martinez², J C McGeorge³, D Mekterovic¹⁴, V Metag¹², B M K Nefkens⁹, A Nikolaev⁴, R Novotny¹², R O Owens³, P Pedroni⁷, A Polonski¹⁵, S N Prakhov⁹, J W Price⁹, G Rosner³, M Rost², T Rostomyan⁷, S Schadmand¹², S Schumann⁴, D Sober¹⁷, A Starostin⁹, I Supek¹⁴, A Thomas², M Unverzagt⁴, Th Walcher², L Zana¹, F Zehr⁶; (Crystal Ball at MAMI and A2 Collaboration)

Affiliations

¹ SUPA, School of Physics, University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom.
² Institut für Kernphysik, University of Mainz, Germany.
³ SUPA, Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, United Kingdom.
⁴ Institut für Kernphysik, University of Mainz, Germany and Helmholtz-Institut für Strahlen- und Kernphysik, University Bonn, Germany.
⁵ Petersburg Nuclear Physics Institute, Gatchina, Russia.
⁶ Institut für Physik, University of Basel, Basel, Switzerland.
⁷ INFN Sezione di Pavia, Pavia, Italy.
⁸ Center for Nuclear Studies, The George Washington University, Washington, D.C. 20052, USA.
⁹ University of California at Los Angeles, Los Angeles, California 90095, USA.
¹⁰ Lebedev Physical Institute, Moscow, Russia.
¹¹ Physikalisches Institut Universität Tübingen, Tübingen, Germany.
¹² II. Physikalisches Institut, University of Giessen, Germany.
¹³ Mount Allison University, Sackville, New Brunswick E4L 1E6, Canada.
¹⁴ Rudjer Boskovic Institute, Zagreb, Croatia.
¹⁵ Institute for Nuclear Research, Moscow, Russia.
¹⁶ Kent State University, Kent, Ohio 44240, USA.
¹⁷ The Catholic University of America, Washington, D.C. 20064, USA.

PMID: 24996085
DOI: 10.1103/PhysRevLett.112.242502

Abstract

Information on the size and shape of the neutron skin on (208)Pb is extracted from coherent pion photoproduction cross sections measured using the Crystal Ball detector together with the Glasgow tagger at the MAMI electron beam facility. On exploitation of an interpolated fit of a theoretical model to the measured cross sections, the half-height radius and diffuseness of the neutron distribution are found to be c(n)=6.70±0.03(stat.) fm and a(n)=0.55±0.01(stat.)(-0.03)(+0.02)(sys.) fm, respectively, corresponding to a neutron skin thickness Δr(np)=0.15±0.03(stat.)(-0.03)(+0.01)(sys.) fm. The results give the first successful extraction of a neutron skin thickness with an electromagnetic probe and indicate that the skin of (208)Pb has a halo character. The measurement provides valuable new constraints on both the structure of nuclei and the equation of state for neutron-rich matter.