The double-polarization observable \(E\) was studied for the reaction \(\gamma p\to p\omega\) using the CEBAF Large Acceptance Spectrometer (CLAS) in Hall B at the Thomas Jefferson National ...Accelerator Facility and the longitudinally-polarized frozen-spin target (FROST). The observable was measured from the charged decay mode of the meson, \(\omega\to\pi^+\pi^-\pi^0\), using a circularly-polarized tagged-photon beam with energies ranging from the \(\omega\) threshold at 1.1 to 2.3 GeV. A partial-wave analysis within the Bonn-Gatchina framework found dominant contributions from the \(3/2^+\) partial wave near threshold, which is identified with the sub-threshold \(N(1720)\,3/2^+\) nucleon resonance. To describe the entire data set, which consisted of \(\omega\) differential cross sections and a large variety of polarization observables, further contributions from other nucleon resonances were found to be necessary. With respect to non-resonant mechanisms, \(\pi\) exchange in the \(t\)-channel was found to remain small across the analyzed energy range, while pomeron \(t\)-channel exchange gradually grew from the reaction threshold to dominate all other contributions above \(W \approx 2\) GeV.
The \(f_0\)(1500) meson resonance is one of several contenders to have significant mixing with the lightest glueball. This resonance is well established from several previous experiments. Here we ...present the first photoproduction data for the \(f_0\)(1500) via decay into the \(K_S^0 K_S^0\) channel using the CLAS detector. The reaction \(\gamma p\) -> \(f_0 p\) -> \(K_S^0 K_S^0 p\), where J = 0, 2, was measured with photon energies from 2.7 to 5.1 GeV. A clear peak is seen at 1500 MeV in the background subtracted invariant mass spectra of the two kaons. This is enhanced if the measured 4-momentum transfer to the proton target is restricted to be less than 1.0 GeV2. By comparing data with simulations, it can be concluded that the peak at 1500 MeV is produced primarily at low t, which is consistent with a t-channel production mechanism.
Exclusive photoproduction cross sections have been measured for the process \(\gamma p \rightarrow p\pi^0(e^+e^-(\gamma))\) with the Dalitz decay final state using tagged photon energies in the range ...of \(E_{\gamma} = 1.275-5.425\) GeV. The complete angular distribution of the final state \(\pi^0\), for the entire photon energy range up to large values of \(t\) and \(u\), has been measured for the first time. The data obtained show that the cross section \(d\sigma/dt\), at mid to large angles, decreases with energy as \(s^{-6.89\pm 0.26} \). This is in agreement with the perturbative QCD quark counting rule prediction of \(s^{-7} \). Paradoxically, the size of angular distribution of measured cross sections is greatly underestimated by the QCD based Generalized Parton Distribution mechanism at highest available invariant energy \(s=11\) GeV\(^2\). At the same time, the Regge exchange based models for \(\pi^0\) photoproduction are more consistent with experimental data.
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 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 \(5/2^+\).
The three-body photodisintegration of {sup 3}He has been measured with the CLAS detector at Jefferson Lab, using tagged photons of energies between 0.35 GeV and 1.55 GeV. The large acceptance of the ...spectrometer allowed us for the first time to cover a wide momentum and angular range for the two outgoing protons. Three kinematic regions dominated by either two- or three-body contributions have been distinguished and analyzed. The measured cross sections have been compared with results of a theoretical model, which, in certain kinematic ranges, have been found to be in reasonable agreement with the data.
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A cost-effective computational methodology designed to study astatine (At) chemistry in aqueous solution has been established. It is based on two-component spin-orbit density functional theory ...calculations and solvation calculations using the conductor-like polarizable continuum model in conjunction with specific astatine cavities. Theoretical calculations are confronted with experimental data measured for complexation reactions between metallic forms of astatine (At(+) and AtO(+)) and inorganic ligands (Cl(-), Br(-) and SCN(-)). For each reaction, both 1:1 and 1:2 complexes are evidenced. The experimental trends regarding the thermodynamic constants (K) can be reproduced qualitatively and quantitatively. The mean signed error on computed Log K values is -0.4, which corresponds to a mean signed error smaller than 1 kcal mol(-1) on free energies of reaction. Theoretical investigations show that the reactivity of cationic species of astatine is highly sensitive to spin-orbit coupling and solvent effects. At the moment, the presented computational methodology appears to be the only tool to gain an insight into astatine chemistry at a molecular level.
Spin transfer from circularly polarized real photons to recoiling hyperons has been measured for the reactions $\vec\gamma + p \to K^+ + \vec\Lambda$ and $\vec\gamma + p \to K^+ + \vec\Sigma^0$. The ...data were obtained using the CLAS detector at Jefferson Lab for center-of-mass energies $W$ between 1.6 and 2.53 GeV, and for $-0.85<\cos\theta_{K^+}^{c.m.}< +0.95$. For the $\Lambda$, the polarization transfer coefficient along the photon momentum axis, $C_z$, was found to be near unity for a wide range of energy and kaon production angles. The associated transverse polarization coefficient, $C_x$, is smaller than $C_z$ by a roughly constant difference of unity. Most significantly, the {\it total} $\Lambda$ polarization vector, including the induced polarization $P$, has magnitude consistent with unity at all measured energies and production angles when the beam is fully polarized. For the $\Sigma^0$ this simple phenomenology does not hold. All existing hadrodynamic models are in poor agreement with these results.
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Eur. Phys. J. A (2013) 49:1-16 The exclusive electroproduction of $\pi^+$ above the resonance region was
studied using the $\rm{CEBAF}$ Large Acceptance Spectrometer ($\rm{CLAS}$) at
Jefferson ...Laboratory by scattering a 6 GeV continuous electron beam off a
hydrogen target. The large acceptance and good resolution of $\rm{CLAS}$,
together with the high luminosity, allowed us to measure the cross section for
the $\gamma^* p \to n \pi^+$ process in 140 ($Q^2$, $x_B$, $t$) bins:
$0.16<x_B<0.58$, 1.6 GeV$^2<$$Q^2$$<4.5$ GeV$^2$ and 0.1 GeV$^2<$$-t$$<5.3$
GeV$^2$. For most bins, the statistical accuracy is on the order of a few
percent. Differential cross sections are compared to two theoretical models,
based either on hadronic (Regge phenomenology) or on partonic (handbag diagram)
degrees of freedom. Both can describe the gross features of the data reasonably
well, but differ strongly in their ingredients. If the handbag approach can be
validated in this kinematical region, our data contain the interesting
potential to experimentally access transversity Generalized Parton
Distributions.
We have measured the nuclear transparency of the incoherent diffractive
$A(e,e'\rho^0)$ process in $^{12}$C and $^{56}$Fe targets relative to $^2$H
using a 5 GeV electron beam. The nuclear ...transparency, the ratio of the
produced $\rho^0$'s on a nucleus relative to deuterium, which is sensitive to
$\rho A$ interaction, was studied as function of the coherence length ($l_c$),
a lifetime of the hadronic fluctuation of the virtual photon, and the
four-momentum transfer squared ($Q^2$). While the transparency for both
$^{12}$C and $^{56}$Fe showed no $l_c$ dependence, a significant $Q^2$
dependence was measured, which is consistent with calculations that included
the color transparency effects.
Results are presented for the first measurement of the double-polarization helicity asymmetry E for the \(\eta\) photoproduction reaction \(\gamma p \rightarrow \eta p\). Data were obtained using the ...FROzen Spin Target (FROST) with the CLAS spectrometer in Hall B at Jefferson Lab, covering a range of center-of-mass energy W from threshold to 2.15 GeV and a large range in center-of-mass polar angle. As an initial application of these data, the results have been incorporated into the J\"ulich model to examine the case for the existence of a narrow \(N^*\) resonance between 1.66 and 1.70 GeV. The addition of these data to the world database results in marked changes in the predictions for the E observable using that model. Further comparison with several theoretical approaches indicates these data will significantly enhance our understanding of nucleon resonances.
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