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COMPASS publishes the hint of sign change in Drell-Yan Sivers function

Post Date:2017-09-13

Our understanding of the partonic structures of protons has been deepened from the one-dimension longitudinal momentum distributions to multi-dimensional distributions with additional dependency on transverse size or transverse momentum. Among the transverse momentum dependent parton distributions (TMDs), Sivers function describes the correlation between the transverse spin of the parent proton and the transverse momentum of the probed quark. A nonzero Sivers function is considered to be the strong evidence of quark orbital motion. The effects of a nonzero Sivers function have been observed in the semi-inclusive deep inelastic scattering (SIDIS) process.
A fundamental prediction based on the factorization of Quantum Chromodynamics (QCD) is a sign change of Sivers functions between SIDIS and Drell-Yan processes due to their opposite time directions of the strong interaction involved. This milestone test has been recently accomplished by the COMPASS collaboration in 2015 transversely-polarized Drell-Yan run. The observed sign of Sivers functions is found to be consistent with the fundamental prediction of QCD and this finding is published in Physics Review Letters on September 12, 2017 (PRL 119, 112002 (2017)).
The IPAS COMPASS group has significantly contributed to the success of this measurement in terms of instrumentation, data-taking and data analysis. The IPAS team are Wen-Chen Chang, Ming-Lee Chu, Chia-Yu Hsieh, Yu-Shiang Lian, Chih-Hsun Lin, Takahiro Sawada and Yu-Sheng Teng. A second-year polarized Drell-Yan run in 2018 will enhance the statistical significance of the measurement.

https://www.phys.sinica.edu.tw/files/bpic20170921115404am_pic20170913044243pm_COMPASS_2017.jpg

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Journal Links: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.112002

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