Session MA: New insights in the spin structure of the nucleon

Proton structure - a modern review

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, and plans being made for a future Electron-Ion Collider facility, we are at the threshold of imaging the nucleon’s three-dimensional structure through its quark and gluon quantum probability distributions. Extracting the quantum distributions from the experimental data is very challenging, however, because of the inverse problem: the measured cross sections are given by convolutions of the quantum probability distributions with process-dependent hard coefficients that are perturbatively calculable from Quantum Chromodynamics. While most previous analyses have been based on the maximum likelihood approach, it has become evident that Bayesian likelihood methods are needed, using Monte Carlo sampling techniques to thoroughly explore the parameter space associated with the quantum probability distributions. I this talk I will review recent developments in tackling the inverse problem for extracting quantum distributions from experimental measurements.   

New results from RHIC

The Relativistic Heavy Ion Collider is the world's only high-energy polarized proton-proton collider, providing access to hard scattering processes at $\sqrt{s}$ = 62, 200, and 500 GeV. Measurements with longitudinal beam polarizations have given new insights into the helicity structure of the proton, while measurements with transverse polarizations have provided new ways to probe polarized parton distribution functions in the collinear and transverse momentum dependent frameworks. \\ In this talk, I will highlight recent results from the RHIC Spin program addressing its main physics questions: How do gluon spins contribute to the proton spin? What is the landscape of the polarized quark-sea in the nucleon? And what can transverse spin effects teach us about the structure of the proton and properties of QCD? Finally, I will give an outlook on the forward upgrade of the STAR detector and spin physics opportunities beyond the year 2021.   

Insights from and Prospects for Belle (II) and JLab

Semi-inclusive deep inelastic scattering (SIDIS), where a lepton scatters off a quasi-free quark in the nucleon which subsequently hadronizes, is one of the most important tools to probe the spin structure of the nucleon. A recent upgrade of the JLab facility to be able to deliver a polarized electron beam up to 12 GeV enables high precision studies of the valence region of the nucleon, where spin effects are significant. Since quarks cannot be detected directly, detailed knowledge of the hadronization process, described by fragmentation functions, is required to infer their properties from the hadrons detected in the final state. To this end, a program to determine fragmentation functions from $e^+e^-$ annihilation has been underway at the Belle experiment and will continue at its successor, the new Belle II experiment. This talk will discuss recent developments and future plans at JLab and Belle (II) relevant for our understanding of the spin structure of the nucleon.