First Measurement of Hard Exclusive π^{-}Δ^{++} Electroproduction Beam-Spin Asymmetries off the Proton.

The polarized cross-section ratio σ_{LT^{'}}/σ_{0} from hard exclusive π^{-}Δ^{++} electroproduction off an unpolarized hydrogen target has been extracted based on beam-spin asymmetry measurements using a 10.2  GeV/10.6  GeV incident electron beam and the CLAS12 spectrometer at Jefferson Lab.

First CLAS12 Measurement of Deeply Virtual Compton Scattering Beam-Spin Asymmetries in the Extended Valence Region.

Deeply virtual Compton scattering (DVCS) allows one to probe generalized parton distributions describing the 3D structure of the nucleon. We report the first measurement of the DVCS beam-spin asymmetry using the CLAS12 spectrometer with a 10.2 and 10.

First Measurement of Λ Electroproduction off Nuclei in the Current and Target Fragmentation Regions.

We report results of Λ hyperon production in semi-inclusive deep-inelastic scattering off deuterium, carbon, iron, and lead targets obtained with the CLAS detector and the Continuous Electron Beam Accelerator Facility 5.014 GeV electron beam. These results represent the first measurements of the Λ multiplicity ratio and transverse momentum broadening as a function of the energy fraction (z) in the current and target fragmentation regions.

Observation of Correlations between Spin and Transverse Momenta in Back-to-Back Dihadron Production at CLAS12.

We report the first measurements of deep inelastic scattering spin-dependent azimuthal asymmetries in back-to-back dihadron electroproduction in the deep inelastic scattering process. In this reaction, two hadrons are produced in opposite hemispheres along the z axis in the virtual photon-target nucleon center-of-mass frame, with the first hadron produced in the current-fragmentation region and the second in the target-fragmentation region. The data were taken with longitudinally polarized electron beams of 10.

Observation of Rayleigh-Lamb waves generated by the 2022 Hunga-Tonga volcanic eruption with the POLA detectors at Ny-Ålesund.

The eruption of the Hunga-Tonga volcano in the South Pacific Ocean on January 15, 2022, at about 4:15 UTC, generated a violent explosion, which created atmospheric pressure disturbances in the form of Rayleigh-Lamb waves detected all over the globe. Here we discuss the observation of the Hunga-Tonga shock-wave performed at the Ny-Ålesund Research Station on the Spitsbergen island, by the detectors of the PolarquEEEst experiment and their ancillary sensors. Online pressure data as well as the results of dedicated offline analysis are presented and discussed in details.

Observation of Azimuth-Dependent Suppression of Hadron Pairs in Electron Scattering off Nuclei.

We present the first measurement of dihadron angular correlations in electron-nucleus scattering. The data were taken with the CLAS detector and a 5.0 GeV electron beam incident on deuterium, carbon, iron, and lead targets.

Multidimensional, High Precision Measurements of Beam Single Spin Asymmetries in Semi-inclusive π^{+} Electroproduction off Protons in the Valence Region.

High precision measurements of the polarized electron beam-spin asymmetry in semi-inclusive deep inelastic scattering (SIDIS) from the proton have been performed using a 10.6 GeV incident electron beam and the CLAS12 spectrometer at Jefferson Lab. We report here a high precision multidimensional study of single π^{+} SIDIS data over a large kinematic range in Bjorken x, fractional energy, and transverse momentum of the hadron as well as photon virtualities Q^{2} ranging from 1-7  GeV^{2}.

Improved Λp Elastic Scattering Cross Sections between 0.9 and 2.0 GeV/c as a Main Ingredient of the Neutron Star Equation of State.

Strange matter is believed to exist in the cores of neutron stars based on simple kinematics. If this is true, then hyperon-nucleon interactions will play a significant part in the neutron star equation of state. Yet, compared to other elastic scattering processes, there is very little data on Λ-N scattering.

First Measurement of Timelike Compton Scattering.

We present the first measurement of the timelike Compton scattering process, γp→p^{'}γ^{*}(γ^{*}→e^{+}e^{-}), obtained with the CLAS12 detector at Jefferson Lab. The photon beam polarization and the decay lepton angular asymmetries are reported in the range of timelike photon virtualities 2.25 View Article and Find Full Text PDF

Observation of Beam Spin Asymmetries in the Process ep→e^{'}π^{+}π^{-}X with CLAS12.

The observation of beam spin asymmetries in two-pion production in semi-inclusive deep inelastic scattering off an unpolarized proton target is reported. The data presented here were taken in the fall of 2018 with the CLAS12 spectrometer using a 10.6 GeV longitudinally spin-polarized electron beam delivered by CEBAF at JLab.

Photoproduction of the f_{2}(1270) Meson Using the CLAS Detector.

The quark structure of the f_{2}(1270) meson has, for many years, been assumed to be a pure quark-antiquark (qq[over ¯]) resonance with quantum numbers J^{PC}=2^{++}. Recently, it was proposed that the f_{2}(1270) is a molecular state made from the attractive interaction of two ρ mesons. Such a state would be expected to decay strongly to final states with charged pions due to the dominant decay ρ→π^{+}π^{-}, whereas decay to two neutral pions would likely be suppressed.

Beam Spin Asymmetry in Semi-Inclusive Electroproduction of Hadron Pairs.

A first measurement of the longitudinal beam spin asymmetry A_{LU} in the semi-inclusive electroproduction of pairs of charged pions is reported. A_{LU} is a higher-twist observable and offers the cleanest access to the nucleon twist-3 parton distribution function e(x). Data have been collected in the Hall-B at Jefferson Lab by impinging a 5.

Extraction of Beam-Spin Asymmetries from the Hard Exclusive π^{+} Channel off Protons in a Wide Range of Kinematics.

We have measured beam-spin asymmetries to extract the sinϕ moment A_{LU}^{sinϕ} from the hard exclusive e[over →]p→e^{'}nπ^{+} reaction above the resonance region, for the first time with nearly full coverage from forward to backward angles in the center of mass. The A_{LU}^{sinϕ} moment has been measured up to 6.6  GeV^{2} in -t, covering the kinematic regimes of generalized parton distributions (GPD) and baryon-to-meson transition distribution amplitudes (TDA) at the same time.

Exploring the Structure of the Bound Proton with Deeply Virtual Compton Scattering.

In the past two decades, deeply virtual Compton scattering of electrons has been successfully used to advance our knowledge of the partonic structure of the free proton and investigate correlations between the transverse position and the longitudinal momentum of quarks inside the nucleon. Meanwhile, the structure of bound nucleons in nuclei has been studied in inclusive deep-inelastic lepton scattering experiments off nuclear targets, showing a significant difference in longitudinal momentum distribution of quarks inside the bound nucleon, known as the EMC effect. In this Letter, we report the first beam spin asymmetry (BSA) measurement of exclusive deeply virtual Compton scattering off a proton bound in ^{4}He.

Direct Observation of Proton-Neutron Short-Range Correlation Dominance in Heavy Nuclei.

We measured the triple coincidence A(e,e^{'}np) and A(e,e^{'}pp) reactions on carbon, aluminum, iron, and lead targets at Q^{2}>1.5  (GeV/c)^{2}, x_{B}>1.1 and missing momentum >400  MeV/c.

First Measurements of the Double-Polarization Observables F, P, and H in ω Photoproduction off Transversely Polarized Protons in the N^{*} Resonance Region.

First measurements of double-polarization observables in ω photoproduction off the proton are presented using transverse target polarization and data from the CEBAF Large Acceptance Spectrometer (CLAS) FROST experiment at Jefferson Lab. The beam-target asymmetry F has been measured using circularly polarized, tagged photons in the energy range 1200-2700 MeV, and the beam-target asymmetries H and P have been measured using linearly polarized, tagged photons in the energy range 1200-2000 MeV. These measurements significantly increase the database on polarization observables.

Center of Mass Motion of Short-Range Correlated Nucleon Pairs studied via the A(e,e^{'}pp) Reaction.

Short-range correlated (SRC) nucleon pairs are a vital part of the nucleus, accounting for almost all nucleons with momentum greater than the Fermi momentum (k_{F}). A fundamental characteristic of SRC pairs is having large relative momenta as compared to k_{F}, and smaller center of mass (c.m.

Novel Way to Search for Light Dark Matter in Lepton Beam-Dump Experiments.

A novel mechanism to produce and detect light dark matter in experiments making use of GeV electrons (and positrons) impinging on a thick target (beam dump) is proposed. The positron-rich environment produced by the electromagnetic shower allows us to produce an A^{'} via nonresonant (e^{+}+e^{-}→γ+A^{'}) and resonant (e^{+}+e^{-}→A^{'}) annihilation on atomic electrons. The latter mechanism, for some selected kinematics, results in a larger sensitivity with respect to limits derived by the commonly used A^{'}-strahlung.

Measurement of the Q^{2} Dependence of the Deuteron Spin Structure Function g_{1} and its Moments at Low Q^{2} with CLAS.

We measured the g_{1} spin structure function of the deuteron at low Q^{2}, where QCD can be approximated with chiral perturbation theory (χPT). The data cover the resonance region, up to an invariant mass of W≈1.9  GeV.

First Exclusive Measurement of Deeply Virtual Compton Scattering off ^{4}He: Toward the 3D Tomography of Nuclei.

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 uses the 6 GeV electron beam from the Continuous Electron Beam Accelerator Facility (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 is detected by CLAS and the photon by a dedicated electromagnetic calorimeter at forward angles.

Beam-Target Helicity Asymmetry for γ[over →]n[over →]→π^{-}p in the N^{*} Resonance Region.

We report the first beam-target double-polarization asymmetries in the γ+n(p)→π^{-}+p(p) reaction spanning the nucleon resonance region from invariant mass W=1500 to 2300 MeV. Circularly polarized photons and longitudinally polarized deuterons in solid hydrogen deuteride (HD) have been used with the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. The exclusive final state has been extracted using three very different analyses that show excellent agreement, and these have been used to deduce the E polarization observable for an effective neutron target.

Cross Sections for the Exclusive Photon Electroproduction on the Proton and Generalized Parton Distributions.

Unpolarized and beam-polarized fourfold cross sections (d^{4}σ/dQ^{2}dx_{B}dtdϕ) for the ep→e^{'}p^{'}γ reaction were measured using the CLAS detector and the 5.75-GeV polarized electron beam of the Jefferson Lab accelerator, for 110 (Q^{2},x_{B},t) bins over the widest phase space ever explored in the valence-quark region. Several models of generalized parton distributions (GPDs) describe the data well at most of our kinematics.

Towards a resolution of the proton form factor problem: new electron and positron scattering data.

There is a significant discrepancy between the values of the proton electric form factor, G(E)(p), extracted using unpolarized and polarized electron scattering. Calculations predict that small two-photon exchange (TPE) contributions can significantly affect the extraction of G(E)(p) from the unpolarized electron-proton cross sections. We determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections using a simultaneous, tertiary electron-positron beam incident on a liquid hydrogen target and detecting the scattered particles in the Jefferson Lab CLAS detector.

Longitudinal target-spin asymmetries for deeply virtual compton scattering.

A measurement of the electroproduction of photons off protons in the deeply inelastic regime was performed at Jefferson Lab using a nearly 6 GeV electron beam, a longitudinally polarized proton target, and the CEBAF Large Acceptance Spectrometer. Target-spin asymmetries for ep→e^{'}p^{'}γ events, which arise from the interference of the deeply virtual Compton scattering and the Bethe-Heitler processes, were extracted over the widest kinematics in Q^{2}, x_{B}, t, and ϕ, for 166 four-dimensional bins. In the framework of generalized parton distributions, at leading twist the t dependence of these asymmetries provides insight into the spatial distribution of the axial charge of the proton, which appears to be concentrated in its center.