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  • Open Access

Small-x Factorization in the Target Fragmentation Region

Paul Caucal1,* and Farid Salazar2,3,4,5,†

  • *Contact author: caucal@subatech.in2p3.fr
  • †Contact author: farid.salazar@temple.edu
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Phys. Rev. Lett. 136, 081901 – Published 23 February, 2026

DOI: https://doi.org/10.1103/3b6j-th2m

Abstract

We consider the differential cross-section for single-inclusive jet production with transverse momentum P⊥ in deep inelastic scattering at small Bjorken xBj, mediated by a virtual photon with virtuality Q2. Unlike most studies at small x, which focus on particle production in the current fragmentation region, we investigate the kinematic regime where the jet is produced in the target fragmentation hemisphere of the Breit frame, and with P⊥≪Q. For a longitudinally polarized photon, we demonstrate that this cross section is not power suppressed in P⊥/Q and we derive a factorized expression in terms of extended quark and gluon jet fracture functions. Our formula, valid at next-to-leading order in αs at small x, is akin to the Altarelli-Martinelli identity for the longitudinal DIS structure function. Numerical estimates show that the extended quark jet fracture function is the most sensitive to saturation effects in large nuclei.

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Phys. Rev. Lett. 136, 081901– Published 23 February, 2026
  • Received 25 February 2025
  • Revised 1 January 2026
  • Accepted 3 February 2026
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