Multiplicity is a powerful handle for probing QCD dynamics in pp, pA, and AA collisions, offering insights into collective behavior and correlations between the soft and hard sectors. Recent CMS results have revealed unexpectedly strong long-range correlations within high-multiplicity pp jets, pointing to a nontrivial dependence of jet substructure on internal multiplicity. In pA collisions, jet production also alters the overall event multiplicity, adding complexity to soft–hard correlations. 

In this talk, I will present a systematic study of jet multiplicity and substructure. We show that leading-logarithmic evolution significantly underestimates the probability of high-multiplicity jets and discuss theoretical improvements. We then develop a framework for energy-energy correlators conditioned on multiplicity to examine how soft fluctuations—quantified by charged particle multiplicity—correlate with jet substructure. These results provide new insights into the interplay between soft and hard components of jets. Finally, I will outline how this approach can be extended to investigate soft–hard correlations in small collision systems.