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Initial-state geometry and multiplicity distributions in pp and pPb collisions

R. Terra1, A. V. Giannini2,3, and F. S. Navarra1

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Phys. Rev. D 113, 094009 – Published 6 May, 2026

DOI: https://doi.org/10.1103/z423-ch4g

Abstract

This work investigates the possibility of accessing the initial geometric shape of the proton in proton-proton and proton-nucleus collisions at the LHC. In particular, we look for manifestations of the configuration in which the proton is made of three quarks linked by a Y-shape gluon string, called baryon junction. This initial state spatial configuration has been used in the past to describe data on baryon rapidity distributions, diffractive J/ψ production and multiplicity distributions in pp collisions. In spite of its success in explaining the data, the evidence of the baryon junction still needs confirmation. Further studies will be undertaken at the electron-ion collider. In this work we study multiplicity distributions measured in pp and pPb collisions. Different initial state geometries are used as input in a Monte Carlo event generator which implements the kT-factorization formalism of the color glass condensate with Kharzeev-Levin-Nardi unintegrated gluon distributions. The results show that the data on multiplicity distributions are good enough to discriminate between different initial state geometries. Moreover, they indicate that it is crucial to take into account the intrinsic fluctuations of the saturation scale.

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Phys. Rev. D 113, 094009– Published 6 May, 2026
  • Received 18 November 2025
  • Accepted 23 March 2026
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