Session SL: Hadrons: DVCS and DVMP II

Measuring Deeply Virtual Compton Scattering on the Neutron with CLAS12 at Jefferson Lab

A key step towards understanding nucleon structure in terms of generalized parton distributions (GPDs) is the measurement of deeply virtual Compton scattering on the neutron (nDVCS). This talk will report on a recently concluded experiment at Jefferson Lab, utilizing the upgraded 11 GeV CEBAF polarized electron beam, the CLAS12 detector, and a liquid deuterium target. Preparation of the raw data for analysis is currently ongoing. Preliminary beam-spin asymmetries for nDVCS ($ed\to e'n\gamma(p)$), extracted from approximately half of the total dataset, and a summary of detector performance and data quality will be presented. This beam-spin asymmetry measurement, when taken with complementary pDVCS measurements, gives us access to the flavor separation of relevant quark GPDs, only accessible in linear combinations within proton and neutron GPDs. In particular, the measurement of GPD E via nDVCS can be used to gather information on the quark total angular momentum and a more complete picture of the nucleon structure.   

Exclusive Pion Photoproduction from a Nucleon in the Wide Angle Regime

The measured differential cross sections for the single neutral/charge pion photoproduction from a nucleon in the wide angle regime (s,-t,-u much larger hadronic scale) disagree with theoretical calculations by {\bf almost two orders of magnitude} in spite of many years of investigation. A solution was proposed by P.~Kroll and K. Passek-Kumericki, whose GPD-based theory includes both twist-2 and twist-3 amplitudes. The signatures of the twist-3 amplitude (compared to twist-2) is a increase in the cross section and has opposite signs of the double polarization observables K$_{LL}$ and A$_{LL}$ at backward production angles. An experimental check of the prediction would provide valuable information on the validity of the handbag mechanism in the GPD framework in the accessible energy range. The proposed experiment will measure K$_{LL}$. It will be performed in Hall A at Jefferson Lab. The experiment will use the 6.6 GeV CEBAF electron beam to impinge photons in the energy range E$_\gamma \ge 4.0$ GeV on a deuterium target. Overview and projected results will be presented in this talk.   

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

DVCS provides access to the 3D imaging of the nucleon structure encoded in the Generalized Parton Distributions, which correlate the 1D longitudinal momentum fraction of the nucleon’s constituent to its 2D transverse position. In the DVCS reaction, the virtual photon from the scattered electron interacts with a quark inside the nucleon, resulting in the nucleon’s emission of a high-energy real photon. DVCS naturally comes with Bethe-Heitler (BH) process, which has the same final-state particles but with the photon emitted instead by the scattered electron. By conducting DVCS experiments at different beam energies, separation of the DVCS amplitude from DVCS-BH interference amplitude can be performed and this allows for the extraction of the D(t) form factor, which may shed light on nucleon’s confinement mechanism. Data were collected with the CLAS12 detector at high luminosity and at different electron beam energies, focusing on the Beam-Spin Asymmetry, which is particularly sensitive to the D(t) term. CLAS12 provides the ideal setup for multi-energy DVCS experiments with efficient particle detection in broad kinematic ranges. First multi-energy DVCS results will be presented, and plans for the extraction of the D(t) form factors will be discussed.   

Deeply virtual compton scattering on Proton with CLAS12

While it has been known since the 60s that nucleons are composed of quarks and gluons, very little is understood about the mechanisms responsible for the emergence of nucleons from these partons. Generalized Parton Distributions (GPDs) provide the opportunity to obtain a 3-dimensional, tomographic picture of a nucleon. Moreover, GPDs are related to total angular momentum, mass and pressure distributions inside the nucleon. GPDs are experimentally accessible via the deeply virtual Compton scattering (DVCS), i.e. the absorption of a highly virtual photon by the proton and the subsequent emission of a high-energy photon. At Jefferson Lab, the CLAS12 spectrometer has collected DVCS data on unpolarized proton with a longitudinally polarized 10.6-GeV electron beam in 2018. Central silicon and micromegas trackers within a 5T-solenoidal field surrounding the liquid hydrogen target are ideal to detect the recoil proton of a DVCS event. The forward detectors, placed in a toroidal magnetic field, detect the associated scattered electron and high energy photon. We will present preliminary results associated to the entire fall 2018 run period. After a careful subtraction of the background and a refined binning, a more detailed picture of the nucleon can be revealed by these new data.   

TMDs and GPDs observables extraction

Over the last 20 years, an intense experimental activity has been dedicated to the measurement of observables towards a 3D description of the nucleon. Generalized parton distributions (GPDs) and transverse mommentum dependent (TMD) parton distributions provide information about the partonic orbital angular momentum which is an important piece to the solution of the proton spin puzzle.\\ GPDs are accessed by comparing model predictions with data cross-sections of DVCS of the E00-110 experiment at Jefferson Lab Hall A. The formulation of the DVCS cross-section in terms of helicity amplitude is parametrized with Compton Form Factors (CFFs) which are given by the convolution of GPDs. The CFFs, $\Re e{\cal H }$ and $\Re e{\cal E }$, appear at twist 2 approximation in the unpolarized beam target configuration and they are extracted with a local fit, the Kriesten-Liuti separation methods and simultaneous fit.\\ Similar techniques will be applied to extract one of the 8 different TMDs that describes the correlation between the momentum direction of the struck quark and the spin of its parent nucleon; the \emph{Sivers} functions. The future SpinQuest experiment, currently being constructed at Fermilab, will access the sea quark Sivers function using the Drell-Yan process.   

Deeply Virtual Neutral Pion Production Cross Section at CLAS12

Deeply virtual exclusive reactions provide unique channels to study both transverse and longitudinal properties of the nucleon simultaneously, allowing for a 3D image of nucleon substructure. The CLAS12 experiment in Jefferson Lab Hall B has recorded 400 $fb^{-1}$ of integrated luminosity of 10.6 GeV electrons incident on a liquid hydrogen target over a wide kinematic range relevant to these deeply virtual processes, and analysis of this data is currently in progress. This presentation will discuss the progress made on extracting an absolute cross section for one such exclusive process, deeply virtual $\pi^0$ production. This measurement is important as exclusive meson production has unique access to the chiral odd GPDs $\bar E_T$ and $H_T$, and is also a background for other exclusive processes such as DVCS, making the determination of this cross section crucial for other exclusive analyses.   

Lorentz invariance relation anomalies and intrinsic parton transverse momentum

We investigate the so-called Lorentz invariance relations for partonic correlation functions in light of renormalization and the treatment of ultraviolet divergences. We show that corrections to the Lorentz invariance relations are nontrivial even in very simple renormalizable quantum field theories. We also discuss the implications for phenomenological applications.   

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

The measurements of deeply virtual exclusive electroproduction processes are used to access and constrain the Generalized Parton Distributons from experimental observables. Among the variety of these exclusive reactions, $\pi^0$ electroproduction channel was shown to be particularly sensitive to the largely unknown chiral-odd GPDs $\bar E_T$ and $H_T$ which contain information on quark transverse spin densities in unpolarized and polarized nucleons. Recently, the CEBAF Large Acceptance Spectrometer (CLAS12) at Jefferson Lab carried out experimental measurements of longitudinally polarized 10.6 GeV electrons scattering on unpolarized liquid hydrogen target. This presentation will focus on the measurements of deeply virtual $\pi^0$ production (DV$\pi^0$P) from CLAS12 in a wide kinematic region with a photon virtuality $Q^2$ up to 8 GeV$^2$, extraction of beam spin asymmetries and the planned analysis in terms of underlying Generalized Parton Distributions (GPD). In this talk we will report the current status of the DV$\pi^0$P analysis and present preliminary results from CLAS12 data.   

Dilution factor calculation and its Contribution to SpinQuest Systematic Error

The contribution to the spin of a nucleon from its constituent partons is still under intense investigation. The SpinQuest experiment aims to add to the information available on sea-quarks by measuring their Sivers function. To separate the contributions of $\bar{u}$ and $\bar{d}$ quarks to the Sivers asymmetry, the experiment uses both NH$_3$ and ND$_3$ polarized targets, interacting with an incoming unpolarized 120 GeV/c proton beam. The dimuons from the Drell-Yan process are detected to analyze the azimuthal asymmetry. The incoming proton beam will also interact with other materials that are present in the experimental beam path, such as the target cell walls, the aluminum insert ladder, the microwave horn, liquid helium and nitrogen in the ammonia target. The figure of merit in our extracted Sivers function is directly dependent on both the magnitude of polarization and the interaction rate from these various unwanted materials resulting in a dilution factor. With the use of MCFM, a parton distribution based cross-section generator we can analyze the contributions from unmeasured cross-section from these various materials to find the degree of dilution and the corresponding kinematic sensitivity. This contribution to the experimental systematic error and its management.