[Paper Review] Multiparton interactions, small-x processes and diffraction

F. Hautmann, H. Jung|arXiv (Cornell University)|Jan 1, 2017

High-Energy Particle Collisions Research1 citations

TL;DR

This paper investigates the interplay between multiparton interactions (MPI), small-x QCD dynamics, and diffraction in high-energy hadronic collisions. It connects MPI to gluon saturation and dijet production via Regge-theory-inspired diagrams and theoretical frameworks such as CCFM and high-energy factorization, showing that small-x effects significantly influence multi-particle final states and exclusive correlations, especially in LHC phenomenology.

ABSTRACT

The connection between multiparton interaction, diractive processesand saturation eects is discussed. The relation of the rise of the gluondensity at small longitudinal momentum fractions x with the occurrenceof saturation, diraction and multiparton interaction is being studiedboth experimentally and theoretically. We illustrate key ideas underlyingrecent progress, and stress the role of dierent theoretical approaches tosmall-x QCD evolution in investigations of multiparton interactions.

Motivation & Objective

• To clarify the theoretical connection between multiparton interactions (MPI), small-x dynamics, and diffraction in high-energy hadronic collisions.
• To examine how increasing parton densities at small x lead to saturation effects and enhanced MPI contributions.
• To assess the role of diffractive processes and saturation in shaping exclusive final-state correlations beyond single-parton scattering.
• To evaluate the impact of small-x evolution frameworks—especially CCFM and high-energy factorization—on MPI phenomenology.
• To provide a conceptual and theoretical foundation for interpreting multi-jet and multi-particle correlations at the LHC in terms of small-x QCD dynamics.

Proposed method

• Uses Regge-theory cut diagrams (AGK rules) to relate diffractive processes, saturation effects, and MPI in a unified framework.
• Applies the CCFM evolution equations to model transverse-momentum-dependent (TMD) splitting functions and non-Sudakov form factors in high-energy QCD.
• Integrates high-energy factorization formalisms that incorporate universal TMD splitting functions and finite TMD corrections to parton branching.
• Analyzes the role of unintegrated parton distributions and double-parton scattering (DPS) in dijet and multi-jet final states at small x.
• Compares theoretical predictions with experimental data on dijet azimuthal decorrelations, rapidity gaps, and jet multiplicities from LHC and HERA.
• Utilizes Monte Carlo event generators (e.g., Pythia) and theoretical models to interpret MPI signals in multi-differential cross sections and final-state correlations.

Experimental results

Research questions

• RQ1 How do multiparton interactions emerge from small-x dynamics and gluon saturation in high-energy hadronic collisions?
• RQ2 What is the role of diffractive processes in connecting MPI and saturation effects via Regge-theory cut diagrams?
• RQ3 How do TMD corrections and non-Sudakov form factors in CCFM evolution affect the structure of multi-jet final states at small x?
• RQ4 To what extent do small-x effects modify exclusive final-state correlations, even when inclusive cross sections remain unchanged?
• RQ5 How can high-energy factorization and CCFM evolution be used to model MPI contributions to dijet and multi-jet production at the LHC?

Key findings

• MPI contributions become significant at small x due to rising parton densities, affecting exclusive final-state structures even when inclusive rates are unchanged.
• The AGK cutting rules provide a theoretical framework linking diffractive processes, saturation effects, and double-parton scattering in Regge-theory diagrams.
• CCFM evolution with TMD corrections successfully describes multiplicity distributions and angular correlations in high-multiplicity final states at high energy.
• Finite TMD corrections to parton branching, including non-Sudakov form factors, are essential for modeling soft-gluon coherence beyond collinear ordering.
• High-energy factorization with universal TMD splitting functions improves the description of dijet production and azimuthal decorrelations in multi-jet final states.
• Theoretical models based on CCFM and high-energy factorization show good agreement with LHC data on dijet azimuthal decorrelations and rapidity gaps, supporting their relevance for MPI studies at small x.