Spin physics with CLAS and CLAS12

An extensive experimental program to measure the spin structure of the nucleon has been conducted in Hall B at Jefferson Lab with the CEBAF Large Acceptance Spectrometer (CLAS) in the last decade. Using a longitudinally polarized beam scattering off longitudinally polarized NH3 and ND3 targets, inclusive Deep Inelastic Scattering (DIS), Semi‐Inclusive DIS (SIDIS) and DVCS experiments were carried out that make a significant contributions to the existing data. The inclusive double spin asymmetry A∥ was measured over a large range in Q2 and W, providing data of impressively high precision that give a better understanding of the structure of the nucleon in the DIS and the valence quarks regions. Using parameterizations A2 and F1 from world data, the virtual photon asymmetry A1 and the structure function g1 were extracted in a Q2 range from 0.05 to 5 GeV2 and a W range from 1.08 to 3.0 GeV. As a result of the extended kinematical range, first moments of structure functions were measured over a large range in Q2 and duality was tested. Furthermore, newly proposed experiments, using an upgraded accelerator at Jefferson Laboratory and an improved CLAS detector (CLAS12), are expected to increase the statistical precision of the current measurements and extend them to kinematic regions presently not accessible, such as high x. This will improve significantly our knowledge of the structure of the nucleon, including parton distribution functions, duality and higher twists contributions

Nucleon spin structure at Jefferson Lab

In the past decade an extensive experimental program to measure the spin structure of the nucleon has been carried out in the three halls at Jefferson Lab. Using a longitudinally polarized beam scattering off longitudinally or transversely polarized 3He, NH3 and ND3 targets, the double spin asymmetries A∥ and A⊥ were measured, providing data of impressively high precision that gives a better understanding of the structure of the nucleon in the deep inelastic scattering and the valence quarks regions. The virtual photon asymmetries A1,2 and polarized structure functions g1,2 were also extracted for the proton, neutron and deuteron over large kinematic ranges, allowing the extraction of first moments and the testing of sum rules and duality

DVCS with longitudinally polarized target using CLAS at 6 GeV

Deeply Virtual Compton Scattering (DVCS) is one of the simplest processes that can be described in terms of Generalized Parton Distributions (GPDs). The target single‐spin asymmetry (target SSA) in the reaction ep⃗→epγ is directly proportional to the imaginary part of the DVCS amplitude, and gives access to a combination of GPDs namely H̃, H, and E. This asymmetry will be measured in a dedicated experiment at Jefferson Lab using the CEBAF 6‐GeV polarized electron beam, a polarized solid‐state 14NH3 target, and the CEBAF Large Acceptance Spectrometer (CLAS) together with the Inner Calorimeter (IC). The expected asymmetry from leading‐order calculations is in the range of 20% to 40%, depending on the kinematics and on the GPD model used. The DVCS amplitude will be mapped out in the Q2 region from 1 to 4 GeV2, xB from 0.15 to 0.55 and −t from 0.1 to 2 GeV2 providing new constraints on the GPDs

First measurement of target and double spin asymmetries for e [over→] p [over→]→ epπ^{0} in the nucleon resonance region above the Δ (1232)

The exclusive channel p→(e→,e′p)π0 was studied in the first and second nucleon resonance regions in the Q2 range from 0.187 to 0.770 GeV2 at Jefferson Lab using the CEBAF Large Acceptance Spectrometer. Longitudinal target and beam-target asymmetries were extracted over a large range of center-of-mass angles of the π0 and compared to the unitary isobar model MAID, the dynamic model by Sato and Lee, and the dynamic model DMT. A strong sensitivity to individual models was observed, in particular for the target asymmetry and in the higher invariant mass region. This data set, once included in the global fits of the above models, is expected to place strong constraints on the electrocoupling amplitudes A1/2 and S1/2 for the Roper resonance N(1400)P11 and the N(1535)S11 and N(1520)D13 states

Photodisintegration of He4 into p+t

The two-body photodisintegration of 4He into a proton and a triton has been studied using the CEBAF Large-Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. Real photons produced with the Hall-B bremsstrahlung-tagging system in the energy range from 0.35 to 1.55 GeV were incident on a liquid 4He target. This is the first measurement of the photodisintegration of 4He above 0.4 GeV. The differential cross sections for the γ4He→pt reaction were measured as a function of photon-beam energy and proton-scattering angle and are compared with the latest model calculations by J.-M. Laget. At 0.6-1.2 GeV, our data are in good agreement only with the calculations that include three-body mechanisms, thus confirming their importance. These results reinforce the conclusion of our previous study of the three-body breakup of 3He that demonstrated the great importance of three-body mechanisms in the energy region 0.5-0.8 GeV

Cascade production in the reactions γp→K+K+(X) and γp→K+K+π−(X)

Photoproduction of the cascade resonances has been investigated in the reactions γp→K+K+(X) and γp→K+K+π−(X). The mass splitting of the ground state (Ξ−,Ξ0) doublet is measured to be 5.4±1.8 MeV/c2, consistent with existing measurements. The differential (total) cross sections for the Ξ− have been determined for photon beam energies from 2.75 to 3.85 (4.75) GeV and are consistent with a production mechanism of Y+→K+Ξ− through a t-channel process. The reaction γp→K+K+π−[Ξ0] has also been investigated to search of excited cascade resonances. No significant signal of excited cascade states other than the Ξ−(1530) is observed. The cross-section results of the Ξ−(1530) have also been obtained for photon beam energies from 3.35 to 4.75 GeV

π0 photoproduction on the proton for photon energies from 0.675 to 2.875 GeV

Differential cross sections for the reaction γp→pπ0 have been measured with the CEBAF Large Acceptance Spectrometer (CLAS) and a tagged photon beam with energies from 0.675 to 2.875 GeV. The results reported here possess greater accuracy in the absolute normalization than previous measurements. They disagree with recent CB-ELSA measurements for the process at forward scattering angles. Agreement with the SAID and MAID fits is found below 1 GeV. The present set of cross sections has been incorporated into the SAID database, and exploratory fits have been extended to 3 GeV. Resonance couplings have been extracted and compared to previous determination

G0 electronics and data acquisition (forward-angle measurements)

The G0 parity-violation experiment at Jefferson Lab (Newport News, VA) is designed to determine the contribution of strange/anti-strange quark pairs to the intrinsic properties of the proton. In the forward-angle part of the experiment, the asymmetry in the cross-section was measured for elastic scattering by counting the recoil protons corresponding to the two beam-helicity states. Due to the high accuracy required to measure the few-part-per-million asymmetry, the G0 experiment was based on a custom experimental setup with its own associated electronics and data acquisition (DAQ) system. Highly specialized time-encoding electronics provided time-of-flight spectra for each detector for each helicity state. More conventional electronics, processing only a small fraction of the events, was used for monitoring (mainly FastBus). The time-encoding electronics and the DAQ system have been designed to handle events from the 128 detector pairs at a mean rate of 2 MHz per detector pair with low deadtime and with minimal helicity-correlated systematic errors. In this paper, we outline the general architecture and the main features of the electronics and the DAQ system dedicated to G0 forward-angle measurements