Observation of Beam Spin Asymmetries in the Process ep→e^{'}π^{+}π^{-}X with CLAS12

Area of Science:

• Nuclear Physics
• Particle Physics
• Quantum Chromodynamics

Background:

• Semi-inclusive deep inelastic scattering (SIDIS) probes the internal structure of protons.
• Beam spin asymmetries provide sensitive probes of quark and gluon dynamics within hadrons.
• Previous measurements lacked the precision to cleanly extract certain parton distribution functions.

Purpose of the Study:

• To measure beam spin asymmetries in two-pion production off unpolarized protons.
• To extract the parton distribution function e(x) in a collinear framework.
• To probe the helicity-dependent two-pion fragmentation function G_{1}^{⊥} for the first time.

Main Methods:

• Utilized the CLAS12 spectrometer at Jefferson Lab (JLab).
• Employed a 10.6 GeV longitudinally spin-polarized electron beam.
• Analyzed data from two-pion production in SIDIS off a proton target.

Main Results:

• Reported the first measurement of beam spin asymmetries in this channel.
• Provided the first opportunity to extract the parton distribution function e(x) with cleaner access.
• Observed a signal sensitive to the helicity-dependent two-pion fragmentation function G_{1}^{⊥}.
• Detected a sign change around the rho meson mass, indicating pion helicity dependence.

Conclusions:

• The measurements offer crucial new information on the interactions between gluons and quarks.
• The results provide novel insights into the spin-dependent fragmentation of quarks into pions.
• This study advances our understanding of the nucleon's spin structure.