KNO scaling violation: the role of the fragmentation region

Abstract

This work is motivated by two independent considerations. The first is that KNO scaling is observed in the central rapidity region ($|η|<0.5$, for example) and violated when the fragmentation region is included ($|η|<3.4$, for example). The second is that, in the CGC framework, the $k_T$-factorization formalism is more appropriate to describe particle production in the central region and the hybrid formalism is more appropriate for the fragmentation region. The former contains small-x gluon interactions, where the latter introduce also collinear gluons and valence quarks in the dynamics. These two considerations put together raise the natural question: are collinear gluons and valence quarks responsible for KNO scaling violation? To answer this question we build a two-component CGC model which is able to reproduce the measured pseudorapidity distributions over all the available range as well as multiplicity distributions in pp collisions at LHC energies. With this model we find that KNO scaling occurs when the interaction is dominated by small-x gluons but is broken by the interactions including small-x gluons, collinear gluons and valence quarks.