Martha Constantinou (Temple University): The proton spin puzzle from Lattice QCD

Quantum Chromodynamics (QCD) is the theory of the strong interactions that binds quarks and gluons to form the nucleons, the fundamental constituents of the visible matter. Understanding nucleon structure is considered a milestone of hadronic physics and new facilities are planned devoted to its study. A future Electron-Ion-Collider proposed by the scientific community will greatly deepen our knowledge on the fundamental constituents of the visible world. To achieve this goal, a synergy between the experimental and theoretical sectors is imperative, and Lattice QCD is in a unique position to provide input from first principle calculations. Over the last years Lattice QCD has made significant progress yielding results that can be compared to experimental measurements with controlled systematics. In this talk we will discuss recent progress in nucleon structure from Lattice QCD using state-of-the-art simulations with pion mass tuned at its physical value. Emphasis will be given on quantities that have implication on the proton spin in order to address the question: “Where does the spin of proton come from”? Along the line of understanding this long-standing puzzle we will also highlight developments on the evaluation of the gluon momentum fraction.

(CFNS host: Andrey Tarasov)

Stefan Floerchinger (University of Heidelberg): Thermal excitation spectrum from entanglement in an expanding quantum string

A surprising result in $e^+ e^-$ collisions is that the particle spectra from the string formed between the expanding quark-antiquark pair have thermal properties even though scatterings appear not to be frequent enough to explain this. We address this problem by considering the finite observable interval of a relativistic quantum string in terms of its reduced density operator by tracing over the complement region. We show how quantum entanglement in the presence of a horizon in spacetime for the causal transfer of information leads locally to a reduced mixed-state density operator. For very early proper time $\tau$, we show that the entanglement entropy becomes extensive and scales with the rapidity. At these early times, the reduced density operator is of thermal form, with an entanglement temperature $T_\tau=\hbar/(2\pi k_B \tau)$, even in the absence of any scatterings.

(CFNS host: Christos Kallidonis)