Measurement of the charged-pion polarisability

TL;DR

The COMPASS experiment measured the pion's electric polarisability via pion Compton scattering, achieving the most precise value to date, which aligns with theoretical predictions but conflicts with some previous experimental results.

Contribution

This work provides the most precise experimental determination of the pion electric polarisability, using a novel method involving high-energy pion-nucleus interactions and quasi-real photon exchange.

Findings

Measured pion electric polarisability as (2.0 ± 0.6_stat ± 0.7_syst) × 10^{-4} fm^3.

Result agrees with chiral perturbation theory predictions.

Measurement is in tension with some previous dedicated experiments.

Abstract

The COMPASS collaboration at CERN has investigated pion Compton scattering, $\pi^-\gamma\rightarrow \pi^-\gamma$, at centre-of-mass energy below 3.5 pion masses. The process is embedded in the reaction $\pi^-\mathrm{Ni}\rightarrow\pi^-\gamma\;\mathrm{Ni}$, which is initiated by 190\,GeV pions impinging on a nickel target. The exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, $Q^2<0.0015$\,(GeV/$c$)$^2$. From a sample of 63\,000 events the pion electric polarisability is determined to be $\alpha_\pi\ =\ (\,2.0\ \pm\ 0.6_{\mbox{\scriptsize stat}}\ \pm\ 0.7_{\mbox{\scriptsize syst}}\,) \times 10^{-4}\,\mbox{fm}^3$ under the assumption $\alpha_\pi=-\beta_\pi$, which relates the electric and magnetic dipole polarisabilities. It is the most precise measurement of this fundamental low-energy parameter of strong interaction, that has been addressed since long by various methods with conflicting outcomes. While this result is in tension with previous dedicated measurements, it is found in agreement with the expectation from chiral perturbation theory. An additional measurement replacing pions by muons, for which the cross-section behavior is unambigiously known, was performed for an independent estimate of the systematic uncertainty.