Narrow Results

Studies of the structure of excited baryons are key factors to the N* program at Jefferson Lab (JLab). Within the first year of data taking with the Hall B CLAS12 detector following the 12 GeV upgrade, a dedicated experiment will aim to extract the N* electrocouplings at high photon virtualities Q². This experiment will allow exploration of the structure of N* resonances at the highest photon virtualities ever achieved, with a kinematic reach up to Q² = 12 GeV². This high-Q² reach will make it possible to probe the excited nucleon structures at distance scales ranging from where effective degrees of freedom, such as constituent quarks, are dominant through the transition to where nearly massless bare-quark degrees of freedom are relevant. In this document, we present a detailed description of the physics that can be addressed through N* structure studies in exclusive meson electroproduction. The discussion includes recent advances in reaction theory for extracting N* electrocouplings from meson electroproduction off protons, along with Quantum Chromodynamics (QCD)-based approaches to the theoretical interpretation of these fundamental quantities. This program will afford access to the dynamics of the nonperturbative strong interaction responsible for resonance formation, and will be crucial in understanding the nature of confinement and dynamical chiral symmetry breaking in baryons, and how excited nucleons emerge from QCD.

In an effort towards a "complete" experiment for the ω meson, we present studies from an experiment with an unpolarized target and a circularly polarized photon beam (g1c), carried out using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. The experiment was analyzed using an extended maximum likelihood fit with partial wave amplitudes. New likelihood functions were calculated to account for the polarization of the photon beam. Both circular and linear polarizations are explored. The results of these fits are then used to project out the spin density matrix for the ω. First measurements of the ρ³ spin density matrix elements will be presented using this method.

Measurement results of photoproduced excited hyperon states using the CLAS detector at Jefferson Lab are shown. The invariant mass distribution of the Λ(1405) has recently been shown to be different for each of the three Σπ channels that it decays to, showing that there is prominent interference between the isospin I = 0 and I = 1 isospin amplitudes. Measurements of the differential and total cross sections of the three hyperons Λ(1405), Σ⁰(1385), and Λ(1520) are presented and compared. Prospects of future studies using a 12 GeV beam with the GlueX detector are briefly given.

Preliminary measurements of the differential cross sections of the γn → K*(892)⁰Λ and γn → K⁺Σ*(1385)^- reactions are shown using data from the Jefferson Lab Hall B CLAS g13 experiment. No experimental cross section data have yet been published on the γn → K*(892)⁰Λ reaction, and the only published cross section data on the γn → K⁺Σ*(1385)^- reaction are at forward angles, where t-channel K⁺ and K*⁺ exchanges are predicted to dominate. These data can be used to contribute to the search for the "missing" N* resonances, some of which are predicted to have non-negligible couplings to the excited strangeness channels.

The exclusive electroproduction process ep → e′nπ⁺ was measured in the range of the invariant mass for nπ⁺ system 1.6 GeV ≤ W ≤ 2.0 GeV, and the photon virtuality 1.8 GeV² ≤ Q² ≤ 4.0 GeV² using CLAS. For the first time, these kinematics are probed in exclusive π⁺ production from the protons with nearly full coverage in the azimuthal and polar angles of the nπ⁺ center-of-mass system. In this experiment, approximately 39,000 differential cross-section data points were measured. In this conference proceeding, preliminary results of our latest analysis work are presented on differential cross sections and structure functions as well as Legendre Moments.

Baryon spectroscopy and structure studies are the key to a deeper understanding of the processes which happened during the first few microseconds of the universe. The understanding of the fundamental degrees of freedom inside a proton or a neutron and how they change with varying quark masses can help us to obtain a deeper understanding on the origin of confinement and its connection to chiral symmetry breaking. While photoproduction experiments are have established as a very effective tool to search and identify new resonances, a hard scale, provided by electroproduction is needed to probe the internal structure of the N* states and to reveal their true nature. The paper will give an overview on the spectroscopy of light baryon states, with a special focus on recent progress in hadron structure studies with CLAS at JLAB. The CLAS spectrometer offers a unique dataset to determine the electrocouplings of N* and Δ resonances using both single and double pion electroproduction. As an outlook, an overview of the planed baryon resonance studies with CLAS12 at the recently upgraded 12 GeV CEBAF electron accelerator will be presented.

Besides a GPD based description, the collinear factorisation theorem of QCD also allows a TDA based description of the hard exclusive pion electroproduction. Both descriptions provide access to different kinematic regions and therefore give us insights into different aspects of the 3 dimensional nucleon structure. To understand the complete reaction mechanism and the validity of the different models, experimental studies over the full kinematic range have to be performed. In this work measurements of the beam-spin asymmetry have been performed over a wide range of kinematics in the deep inelastic regime for the hard exclusive e p → e n π⁺ reaction using the CEBAF Large Acceptance Spectrometer (CLAS) and a 5.5 GeV polarized electron beam at Jefferson Lab (JLAB). The ϕ dependence of the BSA as well as the Q², x_B and t dependence of the extracted moments will be presented. A clear sign change of the BSA can be observed between pions emitted in forward and backward direction with a smooth transition around 90° in CM.

The beam spin asymmetries of the reaction in the Bjorken regime were measured over a wide kinematical domain using the CLAS detector and a new lead-tungstate calorimeter. Through the interference of the Bethe-Heitler process with Deeply Virtual Compton Scattering, those asymmetries provide constraints for the nucleon Generalized Parton Distributions models. The observed shapes are in agreement with twist-2 dominance predictions.

We extracted the multipoles near pion threshold for the first time at high momentum transfers (Q²) in the nπ⁺ final state channel. The dominance of the S-wave transverse multipole (E₀₊) is expected in this region, which allows us to access the new generalized form factors G₁, G₂ within the Light-Cone-Sum-Rule (LCSR) framework. In this analysis, we used one of the recent CLAS experimental data sets which utilized the high-energy polarized electron beam on unpolarized proton target using the 4π detector coverage. We measured the differential cross sections and extracted the multipoles using two methods, LCSR and multipole fit. Both methods show consistent results, which are almost Q² independent.

We present an overview of selected CLAS nucleon excitations results: electro-production of single pion from the Δ(1232), Roper, D₁₃(1520) resonances and electro-production of double pion Nπ⁺π^- . These and other CLAS measurements, coupled with recent theoretical developments, will allow non-perturbative approaches to shed lights on the role of quarks and gluons in nuclei and address issues such as confinement and the non-zero quark mass in the chiral limit.