Session ML: Mini-Symposium: Nucleon Structure in the Valence Regime III

Precision (anti)neutrino scattering off nucleons and nuclei

A technique has been recently proposed to address the main limitations of past neutrino scattering experiments. In particular, it allows precise measurements of high statistics samples of (anti)neutrino-hydrogen interactions and of various nuclear targets. The planned high intensity LBNF beams give access to a broad mixture of measurements of electroweak parameters, QCD and hadron structure of nucleons and nuclei, nuclear physics, form factors, structure functions and cross-sections, as well as searches for new physics or verification of existing outstanding inconsistencies.   

Direct photon cross section and double helicity asymmetry at mid-rapidity in $\vec{p}$+$\vec{p}$ collisions at $\sqrt{s}$ = 510 GeV

Double helicity asymmetries $A_{LL}$ in hadron, jet and direct photon production in $\vec{p}$+$\vec{p}$ collisions at the Relativistic Heavy Ion Collider (RHIC) are sensitive to the gluon helicity contribution to the proton's spin. Unlike hadrons and jet, direct photon production provides clean access to the polarized gluon distribution since there is no hadronization. However, the small direct photon production cross section compared to that of $\pi^0$ and jet production has so far limited its utility in extracting the polarized gluon distribution. With recent increases in RHIC luminosity, we expect this limitation to be partially overcome and try to revisit this ``golden'' measurement of polarized gluons based on RHIC data from 2013. This analysis measures the direct photon cross section and $A_{LL}$ from the data collected employing the PHENIX detector at mid-rapidity ($|\eta| <$ 0.35). This will be the first direct photon cross section and $A_{LL}$ measurement in $\vec{p}$+$\vec{p}$ at $\sqrt{s}$ = 510 GeV with this detector. In this talk I will present the status of direct photon cross section and $A_{LL}$ analysis.   

Azimuthal Transverse Single-Spin Asymmetries of Charged Pions Within Jets from Polarized $pp$ Collisions at $\sqrt{s}$ = 200 GeV

The Collins effect involves the convolution of the quark transversity in the proton with the spin-dependent Collins fragmentation function, leading to azimuthal modulations of identified charged hadron yields about the jet axis. STAR has reported the first measurements of Collins asymmetries for charged pions in jets in polarized $pp$ collisions at $\sqrt{s}$ = 500 GeV and 200 GeV, based on data taken during 2011 and 2012. The results probe transversity for quark momentum fractions $0.1 \le x \le 0.4$ at $Q^2$ scales that are one to two orders of magnitude larger than similar measurements in semi-inclusive deep-inelastic scatterings. These hadron-in-jet measurements also provide a direct probe of the Collins fragmentation function and enable testing its evolution, universality and factorization breaking in the transverse momentum dependent formalism. New preliminary results for the Collins asymmetries of charged pions in jets from 2015 $pp$ collisions at $\sqrt{s}$ = 200 GeV with a much larger sample size and improved analysis procedures that lead to smaller systematic uncertainties will be presented.   

Constraining the Sea Quark Distributions Through W and Z Cross Sections and Cross-Section Ratios Measured at STAR

Although parton distribution functions (PDFs) have become more precise, there are still kinematic regions where more data are needed to help constrain global PDF extractions, such as the ratio of the sea quark distributions $\bar{d}$/$\bar{u}$ near the valence region. Furthermore, different measurements appear to suggest different high-$x$ behaviors of this ratio. The $W$ cross-section ratio ($W^+$/$W^-$) is sensitive to the unpolarized quark distributions at large $Q^2$. Such a measurement can be used to help constrain the $\bar{d}$/$\bar{u}$ ratio. The STAR experiment at RHIC is well equipped to measure the leptonic decays of $W$ and $Z$ bosons, in the pseudorapidity range $\left(-1.0 < \eta < 1.5 \right)$, produced in proton-proton collisions at $\sqrt{s}$ = 500/510 GeV. These cross sections and their ratios are sensitive to quark and antiquark distributions in the $x$-range $0.1 < x < 0.3$. This talk will present preliminary results from the 2011-2013 RHIC runs, which total about 350 pb$^{-1}$ of integrated luminosity. Measurements of the differential $W$ and $Z$ cross sections and the $W^+$/$W^-$ cross-section ratio as a function of the decay lepton's pseudorapidity, the $W/Z$ cross-section ratio and total $W$ and $Z$ cross sections will be shown.   

Constraining unpolarized PDFs using RHIC data

Weak boson productions in proton proton collisions at RHIC provide unique opportunities to study the internal structure of the nucleon. The cross sections and their ratios are sensitive to light quark distributions near the valence region at RHIC energies. We explore the impact of the recent measurements from the STAR collaboration in a global PDF analysis.   

Charge Symmetry Violation Quark Distribution via Precise Measurement using SIDIS in JLab Hall C

Charge symmetry in the parton distributions assumes the distribution of quarks in the proton are related to those in the neutron. Indirect experimental evidence constrains Charge Symmetry Violation (CSV) to be less than 9\%. In Quantum Chromo-Dynamics (QCD), charge symmetry is broken by the mass difference between up and down quarks. CSV in the valence region can be extracted from precision measurements of the cross section ratio of charged pion production in semi-inclusive deep inelastic scattering (SIDIS) from deuterium. The experiment E12-09-002 was conducted at Jefferson Lab in Hall C from fall 2018 to spring 2019 with upgraded 10.6 GeV electron beam. The experiment detected charged pions in coincidence with scattered electrons covering $Q^2$ from 4 to 5.5 GeV, $x$ for 0.35-0.65, and $z$ from 0.4-0.7. Some preliminary results and the current progress in data analysis will be discussed in this talk.   

Studies on Light Anti-Quark Flavor Asymmetry in the proton at the SeaQuest Experiment

The Fermilab E906 (SeaQuest) is an experiment aimed at studying the anti-quark distributions in nucleons and nuclei. The experiment used a 120 GeV proton beam extracted from the Main Injector at Fermilab. SeaQuest takes advantage of the Drell-Yan process to probe the anti-quark structure in the proton. In the Drell-Yan process, quark from one hadron annihilates with an anti-quark from another hadron, producing a virtual photon which eventually decays into dileptons. The SeaQuest forward spectrometer is designed to detect such dimuons generated by the Drell-Yan process. Ratio of cross-sections of the interaction of proton beam on liquid deuterium and hydrogen targets allows SeaQuest to map out dbar(x)/ubar(x) up to a region of ~0.45 in Bjorken-x, a region which hasn't been explored yet. Current status of the analysis will be reported in this talk.   

A new paradigm for QCD global analysis

Several decades of high-energy scattering experiments have given us intriguing, though limited, glimpses into the inner structure of protons and neutrons. With the 12~GeV nuclear physics program at Jefferson Lab underway, along with ongoing programs at RHIC and LHC and plans for a future Electron-Ion Collider (EIC) facility, we are at the threshold of imaging the nucleon's internal 3-dimensional quark and gluon structure in the theoretical framework of QCD. Compared with previous generations of experiments, the new facilities will deliver unprecedented quantities of high-precision data, posing new opportunities and challenges for accessing a variety of quantum correlation functions (QCFs), such as parton distribution functions (PDFs), parton to hadron fragmentation functions (FFs), transverse momentum dependent distributions (TMDs), and generalized parton distributions (GPDs). In this talk I will discuss the future directions for QCD global analysis.