Measurements of Beam Spin Asymmetries of $π^\pmπ^0$ dihadrons at CLAS12

Abstract

A first measurement of beam spin asymmetries for $π^+π^0$ and $π^-π^0$ 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 $π^0$s, increasing statistics by up to five-fold compared to previous CLAS12 $π^0$ analyses. A nonzero $\sinφ_{R_\perp}$ 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 $π^+π^0$ and $π^-π^0$ channels in the $\sin(φ_h-φ_{R_\perp})$ modulation. In contrast, a large, same-sign enhancement near the $ρ$ mass for the $\sin(2φ_h-2φ_{R_\perp})$ modulation is observed, matching spectator model predictions in $π^+π^-$ pairs.

Figures (3)

• Figure 1: CLAS12 diphoton invariant mass spectrum. The legacy selection (blue) keeps photons with $E_\gamma>0.6$ GeV, whereas the new Gradient Boosted Tree vetoes photons with classifier score $p<0.78$ (red).
• Figure 2: The coordinate system for dihadron production. The scattering plane is defined by the incoming and outgoing lepton $\mathbf{\ell}$ and $\mathbf{\ell}'$, the $\mathbf{q}\times\mathbf{P_h}$ plane by the virtual photon $\mathbf{q}$ and total hadron momentum $\mathbf{P_h}$, and the dihadron plane by $\mathbf{q}$ and $\mathbf{R}_T$. The azimuthal angles $\phi_h$ and $\phi_{R_\perp}$ are given by angles spanned between two planes.
• Figure 3: Results for the $A_{LU}^{{\sin\phi_{R_\perp}}}$ (top row), $A_{LU}^{{\sin(\phi_h-\phi_{R_\perp})}}$ (middle row) and $A_{LU}^{{\sin(2\phi_h-2\phi_{R_\perp})}}$ (bottom row) as a function of $x$, $z$, $M_h$, $P_{h\perp}$ and $x_F$ for $\pi^+\pi^0$ (blue circles) and $\pi^-\pi^0$ (green squares) dihadrons. The error bars represent the total uncertainty. The boxes' extents show the systematic uncertainties for each point.