Multiplicity Distributions and the Frontier between Soft and Hard Physics
H. R. Martins-Fontes, F. S. Navarra
TL;DR
High-level goal: explain shoulder structures in pp multiplicity distributions at the LHC by testing a two-component model grounded in QCD. Approach: use a k_T factorization framework to split soft and semihard contributions with a separation scale Λ, and fit a double NB distribution $P(n)=λ[αP(n,⟨n⟩_s,k_s)+(1-α)P(n,⟨n⟩_{sh},k_{sh})]$ with $dN/dy = dN_s/dy + dN_{sh}/dy$ and $⟨n⟩ = λ[α⟨n⟩_s+(1-α)⟨n⟩_{sh}]$. Key results show that $α$ decreases with energy, the NB parameters $k_s$ and $k_{sh}$ evolve with energy, the fits are robust to the separation scale Λ, and KNO scaling is sensitive to the pseudorapidity window, indicating distinct QCD dynamics across central and forward regions. Significance: provides a QCD-motivated, empirically robust framework linking soft and semihard production to multiplicity structure and paves the way for refined forward-physics modeling and deeper tests of QCD dynamics.
Abstract
The multiplicity distributions measured in proton proton collisions at the LHC exhibit interesting new features. One of them is the appearance of substructures, such as the so-called "shoulder" at large multiplicities. The most natural interpretation of this behavior is the existence of two production mechanisms. The final result is then a superposition of two distributions. In a previous publication we assumed that the two production mechanisms are soft and semihard partonics scatterings. In this work we further discuss this assumption and, in particular, we study the dependence of the results on the scale which separates soft from hard events.