2026/07/16 by Tobias Toll, Nahid Vasim
Physics and Astronomy · #hep-ph
5 pages, 2 figures
arxiv created 2026/07/29 · arxiv updated 2026/07/30
The transverse shape of the proton's small-x gluon distribution is determined from exclusive J/ψ photoproduction at HERA. We derive analytic expressions for the coherent and incoherent diffractive cross sections in the hotspot model at leading twist, enabling a global fit to all 104 available H1 and ZEUS data points, spanning three decades in t, with χ2/\rm ndf=0.77. The gluonic hotspots are resolved into a perturbative Gaussian core of size 0.105(2) fm surrounded by a nonperturbative exponential halo of range 0.220(15) fm, in agreement with the gluon-field correlation length of the QCD vacuum and with the core-halo structure of flux tubes recently determined on the lattice. This shape is independent of xI P and of the assumed number of hotspots or hotspot repulsion. The gluonic geometry evolve only through a slow transverse diffusion of the hotspot centres with α'\rm eff.=0.046(28)~\rm GeV-2, while the incoherent cross section at small |t| is dominated by shot-noise fluctuations at the order of one gluon per hotspot.