Observation of Beam Spin Asymmetries in the Process ep →e′π+π-X with CLAS12

(CLAS Collaboration)

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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 G1. A clear sign change is observed around the ρ mass that appears in model calculations and is indicative of the dependence of the produced pions on the helicity of the fragmenting quark.

Original languageEnglish
Article number152501
Number of pages6
JournalPhysical Review Letters
Issue number15
Publication statusPublished - 12 Apr 2021

Bibliographical note

Funding Information:
We acknowledge the outstanding efforts of the staff of the Accelerator and the Physics Divisions at Jefferson Lab in making this experiment possible. This work was supported in part by the U.S. Department of Energy, the National Science Foundation (NSF), the Italian Istituto Nazionale di Fisica Nucleare (INFN), the French Centre National de la Recherche Scientifique (CNRS), the French Commissariat pour l′Energie Atomique, the UK Science and Technology Facilities Council, the National Research Foundation (NRF) of Korea, the Helmholtz-Forschungsakademie Hessen für FAIR (HFHF) and the Ministry of Science and Higher Education of the Russian Federation. The Southeastern Universities Research Association (SURA) operates the Thomas Jefferson National Accelerator Facility for the U.S. Department of Energy under Contract No. DE-AC05-06OR23177. T. H. thanks the Department of Energy for support under Grant No. DE-FG02-96ER41003. The work of C. D. and A. V. is supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under Awards No. DE-SC0019230 and No. DE-AC05-06OR23177.

Publisher Copyright:
© 2021 authors. Published by the American Physical Society.


  • hep-ex

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