Observation of Beam Spin Asymmetries in the Process $e p \rightarrow e \pi^{+}\pi^{-}X$ with CLAS12

TL;DR

This paper reports the first observation of beam spin asymmetries in two-pion production during semi-inclusive deep inelastic scattering, providing new insights into parton distribution functions and fragmentation processes.

Contribution

It presents the first measurement of beam spin asymmetries in two-pion production, enabling extraction of the $e(x)$ distribution and revealing the helicity-dependent fragmentation function $G_1^ot$.

Findings

First observation of beam spin asymmetries in two-pion production.

Sign change around the $ ho$ mass indicating helicity dependence.

Provides data for extracting the $e(x)$ distribution.

Abstract

The observation of beam spin asymmetries in two-pion production in semi-inclusive deep inelastic scattering off an unpolarized proton target is reported. The data presented here were taken in the fall of 2018 with the CLAS12 spectrometer using a 10.6 GeV longitudinally spin-polarized electron beam delivered by CEBAF at JLab. The measured asymmetries provide the first opportunity to extract the parton distribution function $e(x)$, which provides information about the interaction between gluons and quarks, in a collinear framework that offers cleaner access than previous measurements. The asymmetries also constitute the first ever signal sensitive to the helicity-dependent two-pion fragmentation function $G_1^\perp$. A clear sign change is observed around the $\rho$ mass that appears in model calculations and is indicative of the dependence of the produced pions on the helicity of the fragmenting quark.