The cross section for coherent omega-meson photoproduction off the deuteron has been measured for the first time as a function of the momentum transfer t=(P-gamma - P-omega)(2) and photon energy ...E-gamma using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cross sections are measured in the energy range 1.4 < E-gamma < 3.4GeV. A model based on omega- N rescattering is consistent with the data at low and intermediate momentum transfer, vertical bar t vertical bar. For 2.8 < E-gamma < 3.4GeV, the total cross-section of omega - N scattering, based on fits within the framework of the Vector Meson Dominance model, is in the range of 30-40 mb.
We present studies of single-spin asymmetries for neutral pion electroproduction in semi-inclusive deep-inelastic scattering of 5.776 GeV polarized electrons from an unpolarized hydrogen target, ...using the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. A substantial sinϕh amplitude has been measured in the distribution of the cross section asymmetry as a function of the azimuthal angle ϕh of the produced neutral pion. The dependence of this amplitude on Bjorken x and on the pion transverse momentum is extracted with significantly higher precision than previous data and is compared to model calculations.
.
Coherent photoproduction of π
0
-mesons from threshold (
E
th
≈ 136 MeV) throughout the Δ-resonance region and of η-mesons close to the production threshold (
E
th
≈ 570 MeV for η has been measured ...for
7
Li nuclei. The experiment was performed using the tagged-photon beam of the Mainz MAMI accelerator with the Crystal Ball and TAPS detectors combined to give an almost 4π solid-angle electromagnetic calorimeter. The reactions were identified by a combined invariant-mass and missing-energy analysis. A comparison of the pion data to plane-wave impulse modelling tests the nuclear mass form factor. So far coherent η production had been only identified for the lightest nuclear systems (
2
H and
3
He . For
3
He a large enhancement of the cross section above plane-wave approximations had been reported, indicating the formation of a quasi-bound state. The present Li data for η production agree with a plane-wave approximation. Contrary to
3
He , neither a threshold enhancement of the total cross section nor a deviation of the angular distributions from the expected form factor dependence were observed.
We present a search for axion-like particles, \(a\), produced in photon-proton collisions at a center-of-mass energy of approximately 4 GeV, focusing on the scenario where the \(a\)-gluon coupling is ...dominant. The search uses \(a\to\gamma\gamma\) and \(a\to\pi^+\pi^-\pi^0\) decays, and a data sample corresponding to an integrated luminosity of 168 pb\(^{-1}\) collected with the GlueX detector. The search for \(a\to\gamma\gamma\) decays is performed in the mass range of \(180 < m_a < 480\) MeV, while the search for \(a\to\pi^+\pi^-\pi^0\) decays explores the \(600 < m_a < 720\) MeV region. No evidence for a signal is found, and 90% confidence-level exclusion limits are placed on the \(a\)-gluon coupling strength. These constraints are the most stringent to date over much of the mass ranges considered.
Photon beam asymmetry $\Sigma$ measurements for $\omega$ photoproduction in the reaction $\vec{\gamma} p \to \omega p$ are reported for photon energies from 1.152 to 1.876 GeV. Data were taken using ...a linearly-polarized tagged photon beam, a cryogenic hydrogen target, and the CLAS spectrometer in Hall B at Jefferson Lab. The measurements we obtained markedly increase the size of the database for this observable, extend coverage to higher energies, and resolve discrepancies in previously published data. Comparisons of these new results with predictions from a chiral-quark-based model and from a dynamical coupled-channels model indicate the importance of interferences between $t$-channel meson exchange and $s$- and $u$-channel contributions, underscoring sensitivity to the nucleon resonances included in those descriptions. Comparisons with the Bonn-Gatchina partial-wave analysis indicate the $\Sigma$ data reported here help to fix the magnitudes of the interference terms between the leading amplitudes in that calculation (Pomeron exchange and the resonant portion of the $J^P=3/2^+$ partial wave), as well as the resonant portions of the smaller partial waves with $J^P$= $1/2^-$, $3/2^-$, and $5/2^+$.
We report the first large-acceptance measurement of polarization transfer from a polarized photon beam to a recoiling nucleon. The measurement pioneers a novel polarimetry technique, which can be ...applied to many other nuclear and hadron physics experiments. The commissioning reaction of 1H(γ, p)π0 in the range 0.4<Eγ<1.4 GeV validates the technique and provides essential new data to constrain the excitation spectrum of the nucleon.
Precise total cross-sections and invariant-mass distributions have been measured for photoproduction of pion pairs off the proton producing
and
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
solid-angle electromagnetic calorimeter. The results are much more precise than any previous measurements and confirm the chiral perturbation theory predictions for the threshold behavior of these reactions. In the second resonance region, the invariant-mass distributions of meson-meson and meson-nucleon pairs are in reasonable agreement with model predictions, but none of the models reproduce the asymmetries for the mixed-charge channel.
The spin-exotic hybrid meson $\pi_{1}(1600)$ is predicted to have a large
decay rate to the $\omega\pi\pi$ final state. Using 76.6~pb$^{-1}$ of data
collected with the GlueX detector, we measure the ...cross sections for the
reactions $\gamma p \to \omega \pi^+ \pi^- p$, $\gamma p \to \omega \pi^0 \pi^0
p$, and $\gamma p\to\omega\pi^-\pi^0\Delta^{++}$ in the range $E_\gamma =$ 8-10
GeV. Using isospin conservation, we set the first upper limits on the
photoproduction cross sections of the $\pi^{0}_{1}(1600)$ and
$\pi^{-}_{1}(1600)$. We combine these limits with lattice calculations of decay
widths and find that photoproduction of $\eta'\pi$ is the most sensitive
two-body system to search for the $\pi_1(1600)$.
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