Proton Isovector Helicity Distribution on the Lattice at Physical Pion Mass.

Area of Science:

• Nuclear Physics
• Particle Physics
• Quantum Chromodynamics

Background:

• Understanding the proton's internal structure and spin is crucial in particle physics.
• Quark-helicity distributions provide insights into the spin contributions of quarks within the proton.

Purpose of the Study:

• To perform a state-of-the-art calculation of the isovector quark-helicity Bjorken-x distribution in the proton.
• To achieve high precision by employing advanced computational techniques and controlling systematic uncertainties.

Main Methods:

• Utilizing lattice quantum chromodynamics (QCD) ensembles at the physical pion mass.
• Computing quasidistributions at high proton momenta (2.2–3.0 GeV).
• Applying large-momentum effective theory (LaMET) to match lattice results to physical parton distributions.

Main Results:

• Achieving unprecedented precision in the calculation of the isovector quark-helicity distribution.
• Demonstrating agreement between the computed distribution and recent phenomenological analyses of experimental data.
• Observing that the up quark contribution is greater than the down quark contribution (Δu > Δd).

Conclusions:

• The study provides a highly precise, first-principles calculation of a key proton spin observable.
• The results validate the application of LaMET for extracting parton distributions from lattice QCD.
• This work offers valuable constraints for global analyses of spin-dependent parton distribution functions.