Measurements of Beam Spin Asymmetries of $\pi^\pm\pi^0$ dihadrons at CLAS12

TL;DR

This paper reports the first measurement of beam spin asymmetries in $ ightarrow$ dihadron production at CLAS12, revealing insights into quark-gluon correlations and dihadron fragmentation functions with significant experimental and analytical advancements.

Contribution

It introduces a novel measurement of dihadron asymmetries using advanced background reduction techniques, providing new data on twist-3 PDFs and isospin-dependent fragmentation functions.

Findings

Nonzero $ ext{sin}\u03c6_{R_ot}$ asymmetry observed.

First experimental evidence for isospin dependence of $G_1^ot$.

Large enhancement near $ ho$ mass for $ ext{sin}(2-2)$ modulation.

Abstract

A first measurement of beam spin asymmetries for $\pi^+\pi^0$ and $\pi^-\pi^0$ pairs in semi-inclusive deep inelastic scattering is reported. The asymmetries in the dihadron angular distributions were measured from the scattering of a 10.6 GeV longitudinally polarized electron beam off a proton target, using the CLAS12 detector at Jefferson Lab. A photon classifier using a Gradient Boosted Trees (GBTs) architecture was trained with Monte Carlo simulations to reduce the amount of false combinatorial background $\pi^0$s, increasing statistics by up to five-fold compared to previous CLAS12 $\pi^0$ analyses. A nonzero $\sin\phi_{R_\perp}$ asymmetry is observed. This measurement is sensitive to the underexplored collinear twist-3 PDF $e(x)$, which encodes quark-gluon correlations in the proton, and presents a new avenue for its point-by-point extraction. The asymmetries also provide the first experimental evidence for the isospin-dependence of the helicity-dependent dihadron fragmentation function $G_1^\perp$, revealed by a sign-difference between the $\pi^+\pi^0$ and $\pi^-\pi^0$ channels in the $\sin(\phi_h-\phi_{R_\perp})$ modulation. In contrast, a large, same-sign enhancement near the $\rho$ mass for the $\sin(2\phi_h-2\phi_{R_\perp})$ modulation is observed, matching spectator model predictions in $\pi^+\pi^-$ pairs.