Session LI: Mini-Symposium: Nucleon Structure and Interactions III

Inclusive electron scattering off the proton with CLAS12

Electron scattering data off a proton target has been measured with the CLAS12 spectrometer in Hall B at Jefferson Laboratory in a wide kinematic range covering W up to 2.5~GeV and $Q^2$ up to 10~GeV$^2$. These results allow new opportunities to explore a wide range of physics including inclusive, semi-inclusive and fully exclusive reactions. Preliminary results aimed toward the extraction of the inclusive electroproduction cross section from CLAS12 data collected at a beam energy of 10.6 GeV from an unpolarized liquid-hydrogen target will be presented. The large acceptance and good PID of CLAS12 data offer a unique opportunity to measure the inclusive cross sections from the meson electroproduction threshold up to $W = 2.5$~GeV in many multidimensional bins of $W$ and $Q^2$. The coverage in $W$ and $Q^2$ is of particular importance for the extension of our knowledge of the nucleon parton distribution function from the data on the $F_2$ structure function in the resonance region from existing CLAS results on $\gamma p N^*$-electroexcitation amplitudes. These studies also offer valuable input for the exploration of quark-hadron duality   

Measurement of the Angular Distribution of Drell-Yan Production in p+Fe Interactions at 120 GeV Beam Energy

We report on progress towards a measurement of the angular distributions of Drell-Yan dimuons produced at the SeaQuest/E906 Fermilab experiment, using a 120 GeV proton beam on an Fe beam dump upstream of the dimuon spectrometer. To extract the Drell-Yan signal, a combinatoric background subtraction method was developed, after which the detector, trigger, and reconstruction efficiency is corrected using a Bayesian unfolding method. After the unfolding of the reconstructed data, we measure the angular coefficients $\lambda$, $\mu$, and $\nu$ of the Drell-Yan events in the Collins-Soper frame. Results for the angular distribution of proton-induced Drell-Yan process using an 800 GeV proton beam were reported by Fermilab experiment E-866, where the Lam-Tung relation, $1 - \lambda = 2 \nu$, was also validated; however, pion-induced Drell-Yan experiments, NA10 at CERN and E615 at Fermilab, demonstrated a clear violation of this relation. Results from the iron beam dump data at SeaQuest will provide another test of the validity of the Lam-Tung relation. In addition to the polarization of the virtual photon, the Boer-Mulders function for the Fe nucleus can also be extracted from the $\cos 2\phi$ behavior of the angular distribution.   

Measurements of $W^+/W^-$ cross-section ratio in $pp$ collisions at STAR

While the unpolarized valence quark ($d$ and $u$) distributions are well determined from DIS and $pp/p\bar{p}$ experiments, their sea quark counterparts, $\bar{d}$ and $\bar{u}$, are much less constrained, in particular, near the valence region.

Measurements of $W^+/W^-$ production ratio in $pp$ collider experiments, such as the STAR experiment at RHIC, are sensitive to the $\bar{d}/\bar{u}$ ratio at the leading order at a large $Q^2$ set by the $W$ mass.

Presented in this talk are the latest preliminary results of $W^+$ and $W^-$ cross-section ratio via lepton-decay tagging, using the STAR $pp$ collision data at a center-of-mass energy of $\sqrt{s} = 510\,\mathrm{GeV}$ collected in 2017, corresponding to an integrated luminosity of $350\,\mathrm{pb^{-1}}$.

The measurements cover the mid $(|\eta|<1)$ and intermediate rapidities $(1 < \eta < 2)$, probing the $\bar{d}/\bar{u}$ ratio within the proton momentum fraction range of $0.06 < x < 0.4$.   

$J/\psi$ Spin Alignment in 120 GeV $p$-Fe Interactions at Fermilab}

The $J/\psi$ is a charmonium bound state with spin 1 and has large branching ratio towards dilepton decay. The angular distribution of such decay particles from $J/\psi$ is affected by its spin alignment. The $J/\psi$ spin alignment, in turn, gives constraints to test and understand various production mechanisms. The SeaQuest, a fixed target experiment at Fermilab, has completed its data taking. The spectrometer of the experiment was designed to measure high energy muons, and uses a 500 cm long solid Iron (Fe) block as beam dump. While interactions in the target served the primary goal of probing the flavor structure of the nucleon, a wealth of data from interaction with the iron beam dump provides ample opportunity to study charmonium production as well. In this talk, we discuss the measurement of the spin alignment of $J/\psi$ produced in 120 GeV $p$-Fe interactions at the SeaQuest experiment.   

Extraction of Transverse Single Spin Asymmetry in $J/\psi$ Production in $p\vec{p}$ Interactions at 120 GeV Beam Energy

By studying the polarized proton collisions, the structure of the protons can be understood in great detail. Transverse single spin asymmetries are an imbalance of the production cross-section to one side of the protons spin relative to the other. SpinQuest is a fixed-target Drell-Yan experiment at Fermilab, using an unpolarized 120 GeV proton beam incident on a polarized solid ammonia target. Such measurements will allow us to test models for the internal transverse momentum and angular momentum structure of the nucleon. $J/\psi$ is a vector meson which is a $c\bar{c}$ bound state and it is predominantly produced by strong interaction via quark-antiquark annihilation and gluon fusion. A non-zero asymmetry provides information on the orbital angular momentum contribution of "sea-quarks" to the spin of the nucleon. Simulated data were generated using the SpinQuest/E1039 simulation framework. Gaussian Process Regression (GPR), which is a powerful technique used in machine learning, was used to predict the background under the $J/\psi$ invariant mass peak by fitting the Radial-basis function (RBF) kernel in side-band regions on either side of the $J/\psi$ peak. In this presentation, we discuss the extraction of generated asymmetry with statistical errors.   

Drell-Yan and charmonium production in p+p and p+d interactions at 120 GeV from the SeaQuest experiment

High-mass dimuon production in p + p and p + d interactions has been measured in the SeaQuest experiment with the 120 GeV proton beam at Fermilab. The first result of the Drell-Yan (p+d)/2(p+p) cross section ratio, based on the analysis of a fraction of the collected data, has provided new information on the light sea quark asymmetry. Analysis of the entire data set to extract the d̄/ū ratio is currently underway, and the status of this analysis will be reported. In addition to the Drell-Yan process, SeaQuest has also collected J/Ψ and Ψ’ production data, which are sensitive to the gluon distributions in the nucleons and provide information complementary to that of the Drell-Yan data. The status of the charmonium analysis will also be discussed.

    

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The so-called GPDs (Generalized Parton Distribution Function) provide access to the multidimensional partonic structure of the nucleon. DVCS (Deeply Virtual Compton Scattering), which has been measured in numerous experiments enable extraction of the GPDs at x = \xi. However, tomographic interpretations require measurements at other kinematic values for extrapolations to the zero skewness point. Double Deeply Virtual Compton Scattering (DDVCS), where both incoming and outgoing photon are highly virtual, provides GPDs at x != \xi. In this talk I will discuss the possibility of accessing the off diagonal elements of the GPDs through DDVCS experiment at Jefferson Lab Hall C, as a part of what new can be achieved in the 12 GeV era.   

Understanding the Physics and Practical Motivations and Feasibility for the Study of J/Ψ Meson Photoproduction Near Threshold from a Transversely Polarized Target at JLab

Current work is being expanded upon via simulation to understand the feasibility of a study of the J/Ψ meson near its production threshold when produced from a reaction with a transversely polarized target. The study of the J/Ψ is of general interest due to being the lightest of the heavy mesons, as a charmonium, and giving access to the extraction of gluon Generalized Parton Distributions (GPDs) for use in testing the universality and building a three-dimensional model of the nucleon. Study specifically in the threshold region allows for more interesting access into the area of "penta-quark" bound states while also accessing a region of wavering production mechanism dominance that can give insight into the very formation of charm quarks and the other heavy hadrons. Transversely polarizing the target gives the ability for the most interesting research due to giving space for an asymmetry to arise and be observed between the co-dominant production schemes so that an adequate map of the relationship between the two can be built. Photoproduction is specifically investigated due the practical effect that it has on the experiment by allowing access to more J/Ψ meson viably detected. This work and modelling is done through the development of Dr. Marie Boёr's event generator, DEEPSim, that can simulate isolated reactions with varying degrees of realism. Production of the J/Ψ meson has been successfully implemented, with a need to further develop some project specifics with more input from theory. This work is intended to serve as the basis for a dedicated experiment proposal at Jefferson Lab in the coming years upon refinement of the work.