Session EH: Mini-Symposium: New Results from CLAS12 I

The CLAS12 Experiment at Jefferson Lab

The CLAS12 spectrometer in Hall B at Jefferson Lab has been designed to support an extensive program in hadronic and nuclear physics making use of the high intensity and high polarization electron beam provided by the CEBAF accelerator. The physics program of the experiment is built around three main pillars, the study of the structure and multi-dimensional imaging of the nucleon, hadron spectroscopy in the light quark sector with the search for exotic baryons and mesons, and the study of hadronization in the nuclear medium, color transparency and short range correlations. After a first commissioning run in 2017, CLAS12 completed the engineering run in 2018 and entered the production phase, with the first data taking periods on hydrogen and deuterium target at 10.2-10.6 GeV. This presentation will provide an overview of the status of the experiment and the first results from the ongoing data analysis, toward the realization of the CLAS12 program.   

Deeply virtual $\pi^0$ electroproduction measurements with CLAS12 at Jefferson Lab

The recently upgraded Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab combined with CEBAF Large Acceptance Spectrometer (CLAS12) provides rich opportunities to measure deeply virtual exclusive reactions such as hard electroproduction of photons and mesons. This presentation will focus on the measurements of deeply virtual $\pi^0$ production (DV$\pi^0$P) with CLAS12 and the planned analysis in terms of underlying Generalized Parton Distributions (GPD). The experimental measurements of $\pi^0$ electroproduction allow us to constrain largely unknown chiral-odd GPDs $\bar E_T$ and $H_T$ which contain information on quark transverse spin densities in unpolarized and polarized nucleons. The first data collected during 2018 and 2019 using 10.6 GeV polarized electron beam on liquid hydrogen target provide unique access to a large kinematic range with photon virtuality $Q^2$ up to 8 GeV$^2$. In this talk we will report the current status of the DV$\pi^0$P analysis and present preliminary results from CLAS12 data.   

SIDIS Single Pion Beam Spin Asymmetry measurements with CLAS12

The CLAS12 detector at Jefferson Laboratory (JLab) started data taking with a polarized 10.6 GeV electron beam, interacting with an unpolarized liquid hydrogen target in February 2018. The collected statistics enable a high precision study of the moment A$_{\mathrm{LU}}^{\mathrm{sin(\phi )\thinspace }}$corresponding to the polarized electron beam spin asymmetry in semi-inclusive deep inelastic scattering. A$_{\mathrm{LU}}^{\mathrm{sin(\phi )\thinspace }}$is a twist-3 quantity which provides information about the quark gluon correlations in the nucleon. Based on the available statistics, a multidimensional analysis becomes possible. This contribution will present a simultaneous study of all three pion channels ($\pi^{\mathrm{+}}$, $\pi^{\mathrm{0}}$ and $\pi ^{\mathrm{-}})$ over a large kinematic range of z, x$_{\mathrm{B}}$, P$_{\mathrm{T}}$ and Q\texttwosuperior with virtualities Q$^{\mathrm{2}}$ ranging from 1 GeV\texttwosuperior up to 8 GeV\texttwosuperior .   

Deeply Virtual Compton Scattering with CLAS12 at Jefferson Laboratory

Generalized Parton Distributions (GPDs) provide the opportunity to obtain a 3-dimensional, tomographic picture of the nucleon. GPDs are related to total angular momentum, mass, and pressure distributions inside the nucleon via QCD-based sum rules. These distribution functions are experimentally accessible via Deeply Virtual Compton Scattering (DVCS), the reaction in which a highly virtual photon interacts with the proton, emitting a high-energy photon in the final state. At Jefferson Lab, the new CLAS12 spectrometer has been commissioned and collected the first DVCS data with a 10.6 GeV electron beam in 2018. Data calibrations are well advanced and the DVCS final state has been cleanly identified from first analysis. In this contribution, the current status of the analysis will be reviewed and first results on the DVCS beam spin asymmetry will be presented.   

Deeply Virtual Compton Scattering at CLAS12 with Multi-Energy Polarized Electron Beams.

Deeply Virtual Compton Scattering (DVCS) provides the cleanest access to the 3D imaging of nucleon structure encoded in the Generalized Parton Distributions. In the DVCS process, the interaction of a quark inside the nucleon with the virtual photon from the scattered electron results in the nucleon’s emission of a high-energy real photon. DVCS naturally occurs with Bethe-Heitler (BH) process where a photon is instead emitted by the scattered electron, resulting in the same final-stated particles. The DVCS amplitude can be separated from DVCS-BH interference amplitude by performing experiments at different beam energies, allowing the extraction of the gravitational D(t) form factor, which may shed light on nucleon’s confinement mechanism. High luminosity and high polarization of Jefferson Lab’s electron beam together with the large-acceptance of the CLAS12 detector system in Hall B provide the ideal environment for multi-energy experiments requiring efficient particle detection in broad kinematic ranges. DVCS data were collected with CLAS12 in 2018 at 6.5 GeV, 7.5 GeV and at 10.6 GeV electron beam energies, on hydrogen target. We will present preliminary results of experiments at different energies, focusing on the Beam-Spin Asymmetry which is particularly sensitive to D(t).   

Hadronization of quarks and correlated di-hadron production in DIS

The production of two hadrons in Deep Inelastic Scattering (DIS) provides access to partonic dynamics and correlations between orbiting partons not accessible in the single-hadron inclusive DIS (SIDIS). The interpretation of di-hadron production, as well as interpretation of single-hadron production is intimately related to contributions to those samples from correlated di-hadrons in general, and vector mesons, in particular. Single hadron production, being part of correlated di-hadron production, could be described in the factorized approach, as a convolution of set of different spin-dependent and independent distribution and fragmentation functions. However, there are certain applications where the simple fragmentation functions, depending on the fraction of the virtual photon energy carried by the hadron and transverse momentum generated in the fragmentation, may not be enough for precision description of the hadronization process. The list of these applications, some of them critical for interpretation of the data, includes modeling of hadronization of polarized quarks, procedures for accessing transverse momentum of quarks, fraction of secondary lepton pairs produced in electroproduction, and even the procedure for radiative corrections in SIDIS. In this talk, we present latest results on di-hadron multiplicities from CLAS12 detector at Jefferson Lab, indicating that most of the pions in SIDIS at relatively small transverse momentum come from decays of vector mesons.   

Charged dihadron beam-spin asymmetries from CLAS12

Azimuthal correlations in the production of hadron pairs in semi-inclusive deep-inelastic scattering provide rich information on nucleon structure. The additional degree of freedom present in the two hadron final state allows for studies of correlations in the hadronization process, which can access novel fragmentation functions not accessible in single hadron production. Consequently, several PDFs such as the higher-twist collinear PDF $e(x)$ can be extracted. We present preliminary charged pion beam-spin asymmetries sensitive to $e(x)$ from a subset of the data taken in 2018 with the CLAS12 detector at Jefferson Lab. The data were taken with a 10.6 GeV polarized electron beam and an unpolarized liquid-hydrogen target. The large kinematic acceptance and sample size allows for a multidimensional analysis in Bjorken $x$, $Q^2$, $z$ and the invariant mass of the hadron pair.