Measurements of Beam Spin Asymmetries in $\pi^+\pi^0$ and $\pi^-\pi^0$ Dihadron Production at CLAS12

Abstract

Semi-Inclusive Deep Inelastic Scattering (SIDIS) is a powerful experimental tool for studying the internal structure and dynamics of the proton, revealing how quarks and gluons are distributed and interact within it. SIDIS describes a process where an electron scatters off one of the constituent quarks within the proton, causing it to undergo hadronization, creating multiple hadrons in the final state. Through factorization, the full process can be split into probabilistic components: one which describes the internal structure of the proton using Parton Distribution Functions (PDFs), and another which describes the hadronization process using Fragmentation Functions (FFs). These functions are non-perturbative quantities of Quantum Chromodynamics (QCD), meaning they cannot be calculated directly from first principles and must instead be extracted from experimental measurements.A common approach for accessing PDFs and FFs using SIDIS is to measure asymmetries. In this context, asymmetries correspond to subtle differences in the angular distribution of outgoing particles that arise when the spin orientation of the incoming beam or target is reversed. Because many of these effects only appear when spin is involved, they isolate specific, nuanced properties of the proton's spin-structure that are otherwise hidden in spin-averaged measurements. In practice, they show up as specific azimuthal modulations (e.g., $\sin\phi_R$, $\sin(\phi_h-\phi_R)$), whose amplitudes isolate convolutions of PDFs and FFs at leading and subleading twist. Non-zero asymmetries of these angular distributions can be traced back to unique combinations of PDFs and FFs, offering a way to probe them directly.In this work, we measure SIDIS by analyzing high energy electron-proton scattering events using the CLAS12 detector at Jefferson Lab. This study focuses on subset of SIDIS referred to as dihadron SIDIS, where pairs of hadrons---here $\pi^+\pi^0$ and $\pi^-\pi^0$---are observed. We analyzed these dihadrons using detector data collected during Fall 2018 and Spring 2019, where longitudinally polarized electrons from the CEBAF accelerator were incident on a liquid hydrogen target. A photon classifier using a Gradient Boosted Trees (GBTs) architecture was trained using Monte Carlo simulations to reduce the amount of false combinatorial background $\pi^0$'s. When deployed on experimental data, the model increases our dihadron statistics by up to five-fold compared to previous CLAS12 $\pi^0$ analyses. This work reports the first measurements of beam spin asymmetries for $\pi^+\pi^0$ and $\pi^-\pi^0$ dihadron production in SIDIS. The measured asymmetries offer new insights to the spin-dependent structure and dynamics within the proton, as well as the spin-dependent properties of quark fragmentation. Non-zero twist-3 $\sin\phi_R$ amplitudes are observed, providing sensitivity to the subleading twist PDF $e(x)$. The PDF $e(x)$ encodes quark-gluon correlations within the proton---a property that is otherwise inaccessible at leading twist. Additionally, this work measured significant twist-2 modulations carried by $\sin(\phi_h-\phi_R)$ and $\sin(2\phi_h-2\phi_R)$, providing experimental access to the helicity dihadron fragmentation function (DiFF) $G_1^{\perp}$. Because there is no equivalent quark helicity-dependent FF in single-pion SIDIS, the DiFF $G_1^\perp$ offers a unique lens into novel spin-dependent fragmentation. For instance, the twist-2 modulations observed in this study are enhanced by vector mesons created during fragmentation---a behavior predicted by phenomenological models. This study broadens our understanding of dihadron fragmentation, revealing new details about the flavor and charge dependence of hadronization.