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

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 $\sin {\varphi }_{R_{\perp }}$ 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 $\sin ({\varphi }_h-{\varphi }_{R_{\perp }})$ modulation. In contrast, a large, same-sign enhancement near the $\rho$ mass for the $\sin (2{\varphi }_h-2{\varphi }_{R_{\perp }})$ modulation is observed, matching spectator model predictions in ${\pi }^{+}{\pi }^{-}$ pairs.