Neutron Skin ofPb208from Coherent Pion Photoproduction

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.

Equation of State for Nucleonic and Hyperonic Neutron Stars With Mass and Radius Constraints

“…Unfortunately, neutron skins are difficult to extract from experiments in a model-independent fashion. The new experiments to measure neutron skins are being designed with electroweak and electromagnetic probes where, unlike hadronic experiments, the interactions with the nucleus (Abrahamyan et al 2012), or at least the initial state interactions (Tarbert et al 2014), are not complicated by the strong force. The challenging, purely electroweak (nearly model-independent) measurement of the neutron skin of 208 Pb by parity violating electron scattering at JLab (Abrahamyan et al 2012;Horowitz et al 2012) has been able to provide Δr np =0.302±0.177 fm for this isotope (Horowitz et al 2012), although the data are not conclusive due to the large error bars (a follow-up measurement at JLab with better statistics has been proposed).…”

Neutron and weak-charge distributions of the 48Ca nucleus

“…The distribution of neutrons in nuclei determines the limits of the nuclear landscape 2 , gives rise to exotic structures and novel phenomena in rare isotopes [3][4][5] , and impacts basic properties of neutron stars [6][7][8] . Because of its fundamental importance, experimental efforts worldwide have embarked on an ambitious program of measurements of neutron distributions in nuclei using different probes, including hadronic scattering 9 , pion photoproduction 10 , and parityviolating electron scattering 11 . Electrons interact with nucleons by exchanging photons and Z 0 bosons.…”

Neutron skin studies in heavy nuclei with coherent p0 photo-production

“…The coherent π 0 photoproduction, has proven to be capable to access this information in a fast and efficient way [18], allowing a short measuring time per target in the order of 140h. This could permit a systematic scan over the periodic table.…”

“…Therefore, a Geant4 simulation is set up to estimate the detection efficiency of the system. In addition, an interpolated fit with the available theory [19] will then give the radius and diffuseness of the matter distribution [18]. Since the charge distribution is known [1,2], the neutron skin thickness can be extracted.…”

“…One powerful method to measure the neutron distribution using electromagnetic probes is coherent π 0 photoproduction [15][16][17][18]. Here, a photon beam impinges on a nucleus with a mass number A, probing the entire volume.…”