Using data samples collected with the BESIII detector operating at the BEPCII storage ring, the cross section of the inclusive process $e^{+}{e}^{-}\to \eta +X$, normalized by the total cross section of $e^{+}{e}^{-}\to \text{hadrons}$, is measured at eight center-of-mass energy points from 2.0000 to 3.6710 GeV. These are the first measurements with momentum dependence in this energy region. Our measurement shows a significant discrepancy compared to the existing fragmentation functions. To address this discrepancy, a new QCD analysis is performed at the next-to-next-to-leading order with hadron mass corrections and higher twist effects, which can explain both the established high-energy data and our measurements reasonably well.

• Received 1 February 2024
• Revised 26 May 2024
• Accepted 31 May 2024

DOI:https://doi.org/10.1103/PhysRevLett.133.021901

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Published by the American Physical Society