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 present the results of our final analysis of the full data set of g p 1 (Q 2 ), the spin structure function of the proton, collected using CLAS at Jefferson Laboratory in [2000][2001] We compare our final results with various theoretical models and expectations, as well as with parametrizations of the world data. Our data, with their precision and dense kinematic coverage, are able to constrain fits of polarized parton distributions, test pQCD predictions for quark polarizations at large x, offer a better understanding of quark-hadron duality, and provide more precise values of higher twist matrix elements in the framework of the operator product expansion.

The quasi-free γd → π − p(p) differential cross section has been measured with CLAS at photon beam energies Eγ from 0.445 GeV to 2.510 GeV (corresponding to W from 1.311 GeV to 2.366 GeV) for pion center-of-mass angles cos θ c.m. π from −0.72 to 0.92. A correction for final state interactions has been applied to this data to extract the γn → π − p differential cross sections. These cross sections are quoted in 8428 (Eγ, cos θ c.m. π) bins, a factor of nearly three increase in the world statistics for this channel in this kinematic range. These new data help to constrain coupled-channel analysis fits used to disentangle the spectrum of N * resonances and extract their properties. Selected photon decay amplitudes N * → γn at the resonance poles are determined for the first time and are reported here.

This paper reports new exclusive cross sections for ep → e π + π − p using the CLAS detector at Jefferson Laboratory. These results are presented for the first time at photon virtualities 2.0 GeV 2 < Q 2 < 5.0 GeV 2 in the center-of-mass energy range 1.4 GeV < W < 2.0 GeV, which covers a large part of the nucleon resonance region. Using a model developed for the phenomenological analysis of electroproduction data, we see strong indications that the relative contributions from the resonant cross sections at W < 1.74 GeV increase with Q 2 . These data considerably extend the kinematic reach of previous measurements. Exclusive ep → e π + π − p cross section measurements are of particular importance for the extraction of resonance electrocouplings in the mass range above 1.6 GeV.

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