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Dynamic Focusing to Suppress Emittance Transfer in Crab-Crossing Flat Beam Collisions

Derong Xu1,*, J. Scott Berg1, Michael M. Blaskiewicz1, Yue Hao2, Yun Luo1, Christoph Montag1, Sergei Nagaitsev1, Boris Podobedov1, Vadim Ptitsyn1 et al.

Ferdinand Willeke1 and Binping Xiao1

  • *Contact author: dxu@bnl.gov
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Phys. Rev. Lett. 136, 135001 – Published 1 April, 2026

DOI: https://doi.org/10.1103/rf7b-96z1

Abstract

Flat hadron beam collisions, though expected to enhance peak luminosity by about an order of magnitude, have not yet been demonstrated. We identify a new mechanism in which the hourglass-induced synchrobetatron resonance, when coupled to unavoidable machine fluctuations, drives emittance transfer and degrades the flat-beam condition. This effect becomes a critical limitation for maintaining flat hadron beams on operational timescales. Using Electron-Ion Collider (EIC) parameters, we show that it imposes stringent tolerances on machine noise. We further demonstrate that a dynamic focusing scheme, implemented by combining sextupoles with crab cavities, suppresses the resonance and relaxes these constraints, providing a practical path toward robust flat-beam collisions in future lepton-hadron colliders.

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Phys. Rev. Lett. 136, 135001– Published 1 April, 2026
  • Received 27 June 2025
  • Revised 4 December 2025
  • Accepted 11 February 2026
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