Session H05: Snapshots of the Dynamics of the Nucleon

Glimpses of gluons in spatial imaging through DVCS

Generalized Parton Distribution (GPDs) functions describe the correlation between the spatial distribution of the quarks and its longitudinal momentum fraction. Their definition in the mid 1990's has revolutionized our approach to the description of the internal structure of the nucleon. The study of the GPDs together with the study of similar quantities are at the forefront of today hadronic physics enterprise. Deeply Virtual Compton Scattering (DVCS) off the nucleon ($\gamma^* N \rightarrow \gamma N $) is the simplest process which is sensitive to the GPDs. It has been the subject of intense focus at Hermes and JLab (Hall A and B). A suite of approved DVCS experiments is currently in preparation in Hall A and Hall C at Jefferson Lab. These experiments are the third phase of a successful approach to precise (about 5\%) measurement of absolute cross-section. The first generation of experiment showed the importance of precise measurement of absolute cross-section. In this talk, I will review the recently published results of the second generation of experiment. these accurate accurate measurements show an intriguing sensitivity to gluons, the carriers of the strong interaction.   

A glimpse of the proton spin through lattice

Deep-inelastic scattering experiments reveal that contrary to the naive quark model, the quark spin contribution to the proton spin is quite small, about 30\%. In an effort to search for the missing proton spin, recent analyses of the high-statistics 2009 STAR experiments at RHIC showed evidence of non-zero glue helicity in the proton. However, the results are limited by very large uncertainty in the x$<$0.05 region.\\ \\To address this issue from the theoretical side, we made the first lattice QCD calculation of the glue spin in the proton based on the LaMET framework, and the results suggested that a large part of the missing component of the proton spin comes from the gluon spin. The results on the other parts of the proton spin will be also presented.   

Glimpse on fluctuations in proton structure through heavy ion reactions

Hadronic and Heavy Ion collisions at relativistic energies provide a unique opportunity to explore the landscape of quantum chromodynamics (QCD) as a function of resolution scale and parton density. An interesting yet largely unexplored regime within this landscape is the regime of high-density saturated gluon matter that dominates the early stages of relativistic heavy ion (A$+$A) collisions. The dense gluonic matter eventually forms a medium that behaves hydrodynamically, expands, undergoes hadronization and forms detectable particles that have characteristic azimuthally anisotropic distributions, strongly correlated with the initial spatial geometry of the collision. Recently, precision data from the RHIC and LHC reveal striking similarities between the observed azimuthal anisotropies of particle production in high multiplicity light-heavy ion collisions (p$+$A) and heavy ion collisions. Such observations have provided hints that, at sufficiently high multiplicities, the system formed in p$+$A collisions can also behave hydrodynamically. In this talk, I discuss how the sub-nucleon scale fluctuations in the proton projectile, that lead to highly irregular collision regions, are essential to describe the observables in p$+$A collisions at the LHC. More specifically, a combined classical Yang-Mills and viscous fluid dynamic simulation shows that the experimental data on the azimuthal anisotropy of produced particles are sensitive to the hot-spots of initial energy density generated, for example, by the distribution of the three valence quarks inside a proton.