New Spin Structure Constraints on Hyperfine Splitting and Proton Size

TL;DR

This paper improves the understanding of proton structure effects on hyperfine splitting by using new spin structure data, reducing theoretical uncertainties and resolving previous tensions between data-driven and theoretical evaluations.

Contribution

The paper provides updated evaluations of proton polarizability effects using new data, significantly reducing uncertainties in hyperfine splitting predictions and resolving longstanding discrepancies.

Findings

Reduced uncertainty in hyperfine splitting calculations.

Resolved tension between data-driven and theoretical evaluations.

Enhanced precision in proton structure measurements.

Abstract

The 1S hyperfine splitting in hydrogen is measured to an impressive ppt precision and will soon be measured to ppm precision in muonic hydrogen. The latter measurement will rely on theoretical predictions, which are limited by knowledge of the proton polarizability effect $\Delta_\text{pol}$. Data-driven evaluations of $\Delta_\text{pol}$ have long been in significant tension with baryon chiral perturbation theory. Here we present improved results for $\Delta_\text{pol}$ driven by new spin structure data, reducing the long-standing tension between theory and experiment and halving the dominating uncertainty in hyperfine splitting calculations.