We report the first, kinematically-complete measurement of the differential cross section of ϕ-meson photoproduction from deuterium near the production threshold for a proton using the CLAS detector ...and a tagged-photon beam in Hall B at Jefferson Lab. The measurement was carried out by a triple coincidence detection of a proton, K+ and K− near the theoretical production threshold of 1.57 GeV. The extracted differential cross sections dσdt for the initial photon energy range of 1.65–1.75 GeV are consistent with predictions based on a quasifree mechanism. Our finding is different from recent LEPS results on ϕ-meson photoproduction from deuterium in a similar incident photon energy range, but in a different momentum transfer region.
We report on the results of the first measurement of exclusive f{sub 0}(980) meson photoproduction on protons for E{sub {gamma}}=3.0-3.8 GeV and -t=0.4-1.0 GeV{sup 2}. Data were collected with the ...CLAS detector at the Thomas Jefferson National Accelerator Facility. The resonance was detected via its decay in the {pi}{sup +}{pi}{sup -} channel by performing a partial wave analysis of the reaction {gamma}p{yields}p{pi}{sup +}{pi}{sup -}. Clear evidence of the f{sub 0}(980) meson was found in the interference between P and S waves at M{sub {pi}{sup +}}{sub {pi}{sup -}}{approx}1 GeV. The S-wave differential cross section integrated in the mass range of the f{sub 0}(980) was found to be a factor of about 50 smaller than the cross section for the {rho} meson. This is the first time the f{sub 0}(980) meson has been measured in a photoproduction experiment.
Exclusive $\pi^0$ electroproduction at a beam energy of 5.75 GeV has been measured with the Jefferson Lab CLAS spectrometer. Differential cross sections were measured at more than 1800 kinematic ...values in $Q^2$, $x_B$, $t$, and $\phi_\pi$, in the $Q^2$ range from 1.0 to 4.6 GeV$^2$,\ $-t$ up to 2 GeV$^2$, and $x_B$ from 0.1 to 0.58. Structure functions $\sigma_T +\epsilon \sigma_L, \sigma_{TT}$ and $\sigma_{LT}$ were extracted as functions of $t$ for each of 17 combinations of $Q^2$ and $x_B$. The data were compared directly with two handbag-based calculations including both longitudinal and transversity GPDs. Inclusion of only longitudinal GPDs very strongly underestimates $\sigma_T +\epsilon \sigma_L$ and fails to account for $\sigma_{TT}$ and $\sigma_{LT}$, while inclusion of transversity GPDs brings the calculations into substantially better agreement with the data. There is very strong sensitivity to the relative contributions of nucleon helicity flip and helicity non-flip processes. The results confirm that exclusive $\pi^0$ electroproduction offers direct experimental access to the transversity GPDs.
The transition helicity amplitudes from the proton ground state to the P₁₁(1440) and D₁₃(1520) excited states (γvpN* electrocouplings) were determined from the analysis of nine independent one-fold ...differential π⁺π⁻p electroproduction cross sections off a proton target, taken with CLAS at photon virtualities of 0.25<Q2<0.60 GeV². The phenomenological reaction model was employed for separation of the resonant and nonresonant contributions to the final state. The P₁₁(1440) and D₁₃(1520) electrocouplings were obtained from the resonant amplitudes parametrized within the framework of a unitarized Breit-Wigner ansatz. They are consistent with results obtained in the previous CLAS analyses of the π⁺n and π⁰p channels. The successful description of a large body of data in dominant meson-electroproduction channels off protons with the same γvpN* electrocouplings offers clear evidence for the reliable extraction of these fundamental quantities from meson-electroproduction data. This analysis also led to the determination of the long-awaited hadronic branching ratios for the D₁₃(1520) decay into Δπ (24%–32%) and Nρ (8%–17%).
We have measured the nuclear transparency of the incoherent diffractive A(e,e′ρ0) process in 12C and 56Fe targets relative to 2H using a 5 GeV electron beam. The nuclear transparency, the ratio of ...the produced ρ0ʼs on a nucleus relative to deuterium, which is sensitive to ρA interaction, was studied as function of the coherence length (lc), a lifetime of the hadronic fluctuation of the virtual photon, and the four-momentum transfer squared (Q2). While the transparency for both 12C and 56Fe showed no lc dependence, a significant Q2 dependence was measured, which is consistent with calculations that included the color transparency effects.
We report on the first measurement of the F(2) structure function of the neutron from the semi-inclusive scattering of electrons from deuterium, with low-momentum protons detected in the backward ...hemisphere. Restricting the momentum of the spectator protons to ≲100 MeV/c and their angles to ≳100° relative to the momentum transfer allows an interpretation of the process in terms of scattering from nearly on-shell neutrons. The F(2)(n) data collected cover the nucleon-resonance and deep-inelastic regions over a wide range of Bjorken x for 0.65<Q(2)<4.52 GeV(2), with uncertainties from nuclear corrections estimated to be less than a few percent. These measurements provide the first determination of the neutron to proton structure function ratio F(2)(n)/F(2)(p) at 0.2≲x≲0.8 with little uncertainty due to nuclear effects.
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