We report a measurement of the spin polarization of the recoiling neutron in deuterium photodisintegration, utilizing a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured ...photon energy range of 300-700 MeV provides the first measurement of recoil neutron polarization at photon energies where the quark substructure of the deuteron plays a role, thereby providing important new constraints on photodisintegration mechanisms. A very high neutron polarization in a narrow structure centered around E_{γ}∼570 MeV is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition suggests this behavior could be related to the excitation of the d^{*}(2380) hexaquark.
A precise measurement of the differential cross sections dσ/dΩ and the linearly polarized photon beam asymmetry Σ_{3} for Compton scattering on the proton below pion threshold has been performed with ...a tagged photon beam and almost 4π detector at the Mainz Microtron. The incident photons were produced by the recently upgraded Glasgow-Mainz photon tagging facility and impinged on a cryogenic liquid hydrogen target, with the scattered photons detected in the Crystal Ball/TAPS setup. Using the highest statistics Compton scattering data ever measured on the proton along with two effective field theories (both covariant baryon and heavy-baryon) and one fixed-t dispersion relation model, constraining the fits with the Baldin sum rule, we have obtained the proton electric and magnetic polarizabilities with unprecedented precision: α_{E1}=10.99±0.16±0.47±0.17±0.34, β_{M1}=3.14±0.21±0.24±0.20±0.35; in units of 10^{-4} fm^{3} where the errors are statistical, systematic, spin polarizability dependent, and model dependent.
The spin polarizabilities of the nucleon describe how the spin of the nucleon responds to an incident polarized photon. The most model-independent way to extract the nucleon spin polarizabilities is ...through polarized Compton scattering. Double-polarized Compton scattering asymmetries on the proton were measured in the Δ(1232) region using circularly polarized incident photons and a transversely polarized proton target at the Mainz Microtron. Fits to asymmetry data were performed using a dispersion model calculation and a baryon chiral perturbation theory calculation, and a separation of all four proton spin polarizabilities in the multipole basis was achieved. The analysis based on a dispersion model calculation yields γ(E1E1)=-3.5±1.2, γ(M1M1)=3.16±0.85, γ(E1M2)=-0.7±1.2, and γ(M1E2)=1.99±0.29, in units of 10(-4) fm(4).
Rotational levels of molecular free radicals can be tuned to degeneracy by using laboratory-scale magnetic fields. Because of their intrinsically narrow width, these level crossings of ...opposite-parity states have been proposed for use in the study of parity-violating interactions and other applications. We experimentally study a typical manifestation of this system using BaF138. Using a Stark-mixing method for detection, we demonstrate level-crossing signals with spectral width as small as 6 kHz. We use our data to verify the predicted line shapes, transition dipole moments, and Stark shifts and to precisely determine molecular magnetic g factors. Our results constitute an initial proof of concept for use of this system to study nuclear spin-dependent parity-violating effects.
We present new data for the transverse target asymmetry T and the very first data for the beam-target asymmetry F in the γover →pover →→ηp reaction up to a center-of-mass energy of W=1.9 GeV. The ...data were obtained with the Crystal-Ball/TAPS detector setup at the Glasgow tagged photon facility of the Mainz Microtron MAMI. All existing model predictions fail to reproduce the new data indicating a significant impact on our understanding of the underlying dynamics of η meson photoproduction. The peculiar nodal structure observed in existing T data close to threshold is not confirmed.
Arrestins quench the signaling of a wide variety of G protein-coupled receptors by virtue of high-affinity binding to phosphorylated activated receptors. The high selectivity of arrestins for this ...particular functional form of receptor ensures their timely binding and dissociation. In a continuing effort to elucidate the molecular mechanisms responsible for arrestin's selectivity, we used the visual arrestin model to probe the functions of its N-terminal beta-strand I comprising the highly conserved hydrophobic element Val-Ile-Phe (residues 11-13) and the adjacent positively charged Lys(14) and Lys(15). Charge elimination and reversal in positions 14 and 15 dramatically reduce arrestin binding to phosphorylated light-activated rhodopsin (P-Rh*). The same mutations in the context of various constitutively active arrestin mutants (which bind to P-Rh*, dark phosphorylated rhodopsin (P-Rh), and unphosphorylated light-activated rhodopsin (Rh*)) have minimum impact on P-Rh* and Rh* binding and virtually eliminate P-Rh binding. These results suggest that the two lysines "guide" receptor-attached phosphates toward the phosphorylation-sensitive trigger Arg(175) and participate in phosphate binding in the active state of arrestin. The elimination of the hydrophobic side chains of residues 11-13 (triple mutation V11A, I12A, and F13A) moderately enhances arrestin binding to P-Rh and Rh*. The effects of triple mutation V11A, I12A, and F13A in the context of phosphorylation-independent mutants suggest that residues 11-13 play a dual role. They stabilize arrestin's basal conformation via interaction with hydrophobic elements in arrestin's C-tail and alpha-helix I as well as its active state by interactions with alternative partners. In the context of the recently solved crystal structure of arrestin's basal state, these findings allow us to propose a model of initial phosphate-driven structural rearrangements in arrestin that ultimately result in its transition into the active receptor-binding state.
The Dalitz decays η → e+e-γ and ω → π0e+e- have been measured in the γp → ηp and γp → ωp reactions, respectively, with the A2 tagged-photon facility at the Mainz Microtron. The value obtained for the ...slope parameter of the electromagnetic transition form factor of η, Λ$-2\atop{η}$ = (1.97 ± 0.11tot) GeV-2, is in good agreement with previous measurements of the η → e+e-γ and η → μ+μ-γ decays. The uncertainty obtained in the value of Λ$-2\atop{η}$ is lower than in previous results based on the η → e+e-γ decay. The value obtained for the ω slope parameter, Λ$-2\atop{ωπ^0}$ = (1.99 ± 0.21tot) GeV-2, is somewhat lower than previous measurements based on ω → π0μ+μ-, but the results for the ω transition form factor are in better agreement with theoretical calculations, compared to earlier experiments.
Precise helicity-dependent cross sections and the double-polarization observable E were measured for η photoproduction from quasifree protons and neutrons bound in the deuteron. The η → 2γ and η → ...3π0 → 6γ decay modes were used to optimize the statistical quality of the data and to estimate systematic uncertainties. The measurement used the A2 detector setup at the tagged photon beam of the electron accelerator MAMI in Mainz. A longitudinally polarized deuterated butanol target was used in combination with a circularly polarized photon beam from bremsstrahlung of a longitudinally polarized electron beam. The reaction products were detected with the electromagnetic calorimeters Crystal Ball and TAPS, which covered 98% of the full solid angle. The results show that the narrow structure observed earlier in the unpolarized excitation function of η photoproduction off the neutron appears only in reactions with antiparallel photon and nucleon spin (σ1/2). It is absent for reactions with parallel spin orientation (σ3/2) and thus very probably related to partial waves with total spin 1/2. The behavior of the angular distributions of the helicity-dependent cross sections was analyzed by fitting them with Legendre polynomials. The results are in good agreement with a model from the Bonn-Gatchina group, which uses an interference of P11 and S11 partial waves to explain the narrow structure.
The Compton double-polarization observable Σ 2z has been measured for the first time in the Δ (1232) resonance region using a circularly polarized photon beam incident on a longitudinally polarized ...proton target at the Mainz Microtron. This paper reports these results, together with the model-dependent extraction of four proton spin polarizabilities from fits to additional asymmetry data using either a dispersion relation calculation or a baryon chiral perturbation theory calculations with the weighted average of these two fits resulting in γE1E1=–2.87±0.52, γM1M1=2.70±0.43, γE1M2=–0.85±0.72 , and γM1E2=2.04±0.43 , in units of 10–4 fm4 .
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