Large multiplicity fluctuations and saturation effects in onium collisions
A. H. Mueller, G. P. Salam
TL;DR
The paper addresses how rare, high-multiplicity fluctuations and saturation arise in high-energy onium-onium scattering within the dipole/BFKL framework. It combines a transverse-dimensionless toy model with Monte Carlo simulations (OEDIPUS) to show that the central-m multiplicity cross-section tail decays as $e^{-c\sqrt{k}}$, extending beyond KNO expectations. Saturation is quantified through measures based on dipole overlap and interaction probabilities, and frame-change arguments reveal that including saturation yields cross sections that are effectively frame-independent by trading unitarity corrections for saturation effects. These results suggest that high-density QCD phenomena could be accessible at realistic energies and provide a framework for interpreting high-multiplicity signals in experiments.
Abstract
This paper studies two related questions in high energy onium-onium scattering: the probability of producing an unusually large number of particles in a collision, where it is found that the cross section for producing a central multiplicity proportional to $k$ should decrease exponentially in $\sqrt{k}$. Secondly, the nature of gluon (dipole) evolution when dipole densities become so high that saturation effects due to dipole-dipole interactions become important: measures of saturation are developed to help understand when saturation becomes important, and further information is obtained by exploiting changes of frame, which interchange unitarity and saturation corrections.