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Our aim is to resolve the early stage of the formation of a discontinuity separating the inner cocoon formed by shocked pair plasma from the outer cocoon in relativistic jets and pair-plasma winds. An initially unmagnetized pair plasma impacts a background plasma permeated by a magnetic field perpendicular to the collision direction. The relative speed is 60% of the speed of light. The drift of electrons and positrons along the sharp, initially planar magnetic boundary destabilizes the boundary, fragmenting it into an ensemble of magnetic flux tubes. The magnetically reflected pair plasma then drives a strong, mildly relativistic shock mediated by the filamentation instability in front of the boundary. The heated and compressed pair plasma pushes the flux tubes deeper into the electron\u2013proton plasma. The electric fields induced by the moving flux tubes and by the unequal densities of electrons and positrons behind the boundary accelerate ambient protons. The simulation resolves the intrinsic spatial and temporal scales governing flux-tube formation and pair shock evolution, providing quantitative guidance for laboratory laser\u2013plasma experiments while directly informing models of astrophysical shocks.<\/jats:p>","DOI":"10.1038\/s41598-026-62899-0","type":"journal-article","created":{"date-parts":[[2026,7,22]],"date-time":"2026-07-22T15:55:08Z","timestamp":1784735708000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Early phase of shock formation in pair plasma colliding with electron\u2013proton plasma"],"prefix":"10.1038","volume":"16","author":[{"given":"M. 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Nature 371, 46. https:\/\/doi.org\/10.1038\/371046a0 (1994).","journal-title":"Nature"},{"key":"62899_CR3","doi-asserted-by":"publisher","first-page":"L81","DOI":"10.1086\/312733","volume":"536","author":"MC Weisskopf","year":"2000","unstructured":"Weisskopf, M. C. et al. Discovery of spatial and spectral structure in the x-ray emission from the crab nebula. Astrophys. J. 536, L81. https:\/\/doi.org\/10.1086\/312733 (2000).","journal-title":"Astrophys. J."},{"key":"62899_CR4","doi-asserted-by":"publisher","first-page":"455810401","DOI":"10.1017\/S002237781500046X","volume":"81","author":"G Sarri","year":"2015","unstructured":"Sarri, G. et al. Overview of laser-driven generation of electron-positron beams. J. Plasma Phys. 81, 455810401. https:\/\/doi.org\/10.1017\/S002237781500046X (2015).","journal-title":"J. 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