The f1(1285) meson with mass 1281.0 ± 0.8 MeV/c 2 and width 18.4 ± 1.4 MeV (FWHM) was measured for the first time in photoproduction from a proton target using CLAS at Jefferson Lab. Differential cross sections were obtained via the ηπ + π − , K +K 0 π − , and K − K 0 π + decay channels from threshold up to a center-of-mass energy of 2.8 GeV. The mass, width, and an amplitude analysis of the ηπ + π − final-state Dalitz distribution are consistent with the axial-vector J P = 1 + f1(1285) identity, rather than the pseudoscalar 0 − η(1295). The production mechanism is more consistent with s-channel decay of a high-mass N * state, and not with t-channel meson exchange. Decays to ηππ go dominantly via the intermediate a ± 0 (980)π ∓ states, with the branching ratio Γ(a0π (noKK))/Γ(ηππ (all)) = 0.74±0.09. The branching ratios Γ(KKπ)/Γ(ηππ) = 0.216±0.033 and Γ(γρ 0 )/Γ(ηππ) = 0.047 ± 0.018 were also obtained. The first is in agreement with previous data for the f1(1285), while the latter is lower than the world average.

We report on the first measurement of the beam-spin asymmetry in the exclusive process of coherent deeply virtual Compton scattering off a nucleus. The experiment used the 6 GeV electron beam from the CEBAF accelerator at Jefferson Lab incident on a pressurized 4 He gaseous target placed in front of the CEBAF Large Acceptance Spectrometer (CLAS). The scattered electron was detected by CLAS and the photon by a dedicated electromagnetic calorimeter at forward angles. To ensure the exclusivity of the process, a specially designed radial time projection chamber was used to detect the recoiling 4 He nuclei. We measured beam-spin asymmetries larger than those observed on the free proton in the same kinematic domain. From these, we were able to extract, in a model-independent way, the real and imaginary parts of the only 4 He Compton form factor, HA. This first measurement of coherent deeply virtual Compton scattering on the 4 He nucleus, with a fully exclusive final state via nuclear recoil tagging, leads the way toward 3D imaging of the partonic structure of nuclei.

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