Hadronization models in General-Purpose Monte Carlo generators.

Quantum Chromodynamics (QCD) provides a remarkably successful description of the strong interaction at high energies, yet in the low-energy regime — precisely where quarks and gluons transition into the observable hadrons detected in experiments — it becomes inherently non-perturbative and analytically intractable. This transition, known as hadronization, remains one of the most profound unresolved challenges in high-energy physics. Since it cannot be derived from first principles, its description relies on phenomenological models implemented in Monte Carlo (MC) event generators, which form the indispensable bridge between theory and experiment at colliders such as the LHC. Two paradigms have dominated this field for decades: the Lund string model (PYTHIA), describing confinement through color flux tubes stretched between partons, and the cluster model (HERWIG), exploiting QCD preconfinement to group partons into color-singlet clusters that decay into hadrons. Each approach has complementary strengths and characteristic weaknesses, and both achieve agreement with data only after careful tuning of their free parameters.

The central motivation of this thesis is to advance the phenomenology of hadronization by systematically investigating these models and ultimately exploring the construction of a hybrid framework that combines the strengths of the Lund and cluster approaches. A crucial component of this work is the tuning of the model parameters — the effective degrees of freedom encapsulating our ignorance of non-perturbative QCD — against experimental data: Minimum Bias and Underlying Event measurements from LHC experiments, which probe soft QCD dynamics and multi-parton interactions, and event shape variables from LEP data, which provide a theoretically cleaner environment to isolate hadronization dynamics.

The project tasks:

1. Study characteristics (espec. mass) and creation mechanism of mesonic and baryonic clusters.

2. Understand the algorithm of transition from Herwig's partonic event generation to Pythis's hadronization,
   especially with respect to colour reconnection step.

3. Specify the role of proton remnants to cluster/string system for proton collisions.

4. Explore possibilities for creation a hybrid model combining both hadronization
   models by splitting the outgoing parton system into two groups.