M. Martinez

The neutron skin of 208Pb from coherent pion photoproduction

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.)u2009u2009fm and a(n)=0.55±0.01(stat.)(-0.03)(+0.02)(sys.)u2009u2009fm, respectively, corresponding to a neutron skin thickness Δr(np)=0.15±0.03(stat.)(-0.03)(+0.01)(sys.)u2009u2009fm. 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.

Coherent photoproduction of η-mesons off 3He – search for η-mesic nuclei

Coherent photoproduction of eta-mesons off He-3, i.e. the reaction gamma He-3 -< eta He-3, has been investigated in the near-threshold region. The experiment was performed at the Glasgow tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball - TAPS detector. Angular distributions and the total cross section were measured using the eta -< gamma gamma and eta -< 3 pi(0) -< 6 gamma decay channels. The observed extremely sharp rise of the cross section at threshold and the behavior of the angular distributions are evidence for a strong eta He-3 final state interaction, pointing to the existence of a resonant state. The search for further evidence of this state in the excitation function of pi(0)-proton back-to-back emission in the gamma He-3 -< pi(0)pX reaction revealed a very complicated structure of the background and could not support previous conclusions

Narrow structure in the excitation function of η photoproduction off the neutron

The photoproduction of η mesons off nucleons bound in 2H and 3He has been measured in coincidence with recoil protons and recoil neutrons for incident photon energies from threshold up to 1.4 GeV. The experiments were performed at the Mainz MAMI accelerator, using the Glasgow tagged photon facility. Decay photons from the η→2γ and η→3π0 decays and the recoil nucleons were detected with an almost 4π electromagnetic calorimeter combining the Crystal Ball and TAPS detectors. The data from both targets are of excellent statistical quality and show a narrow structure in the excitation function of γn→nη. The results from the two measurements are consistent, taking into account the expected effects from nuclear Fermi motion. The best estimates for position and intrinsic width of the structure are W=(1670±5)u2009u2009MeV and Γ=(30±15)u2009u2009MeV. For the first time precise results for the angular dependence of this structure have been extracted.

K+Λ and K+Σ0 photoproduction with fine center-of-mass energy resolution

Measurements of γp→K+Λγp→K+Λ and γp→K+Σ0γp→K+Σ0 cross-sections have been obtained with the photon tagging facility and the Crystal Ball calorimeter at MAMI-C. The measurement uses a novel K+K+ meson identification technique in which the weak decay products are characterized using the energy and timing characteristics of the energy deposit in the calorimeter, a method that has the potential to be applied at many other facilities. The fine center-of-mass energy (W) resolution and statistical accuracy of the new data results in a significant impact on partial wave analyses aiming to better establish the excitation spectrum of the nucleon. The new analyses disfavor a strong role for quark–diquark dynamics in the nucleon.

Photoproduction of π0π0 - and π0π+ -pairs off the proton from threshold to the second resonance region

Precise total cross-sections and invariant-mass distributions have been measured for photoproduction of pion pairs off the proton producing

Measurement of the beam-helicity asymmetry I-circle dot in the photoproduction of pi(0)-pairs off the proton and off the neutron

ensuremath ppi^0pi^0

Incoherent neutral pion photoproduction on C-12

and

Measurement of the slope parameter {alpha} for the {eta}{yields}3{pi}{sup 0} decay with the Crystal Ball detector at the Mainz Microtron (MAMI-C)

ensuremath npi^+pi^0

Quasi-free photoproduction of η-mesons off 3He nuclei

final states from the threshold region up to 800MeV incident photon energy. Additionally, beam helicity asymmetries have been measured in the second resonance region (550MeV-820MeV). The experiment was performed at the tagged photon beam of the Mainz MAMI accelerator with the Crystal Ball and TAPS detectors combined to give an almost 4

Coherent photoproduction of π0- and η-mesons off 7Li

pi