Speaker
Description
Small collision systems, such as high-energy proton-proton (pp) collisions, have attracted considerable attention following the observation of QGP-like phenomena, including strangeness enhancement and collective flow. These observations have motivated extensive studies of the underlying event (UE) to better understand the microscopic origin of particle production in high-multiplicity events. In particular, identifying observables that reliably characterize UE activity across different theoretical models has become an important challenge.
In this study, we investigate the validity of the transverse activity classifier, RT, as a proxy for UE activity using multiple event generators. Previous studies using PYTHIA demonstrated a strong correlation between RT and the number of multiple parton interactions (NMPI), suggesting that RT is an effective classifier of UE activity within the MPI framework. In this work, we compare the sensitivity of RT with that of charged-particle multiplicity measured at midrapidity (∣η∣<0.8) and forward rapidity (2.8<η<5.1, −3.7<η<−1.7) using PYTHIA8 and EPOS4. Since EPOS does not provide an observable equivalent to NMPI, we use the Core vs. Corona (CVC) fraction to characterize the hydrodynamic contribution to the underlying event. While RT shows a strong correlation with NMPI in PYTHIA, no comparable correlation is observed with CVC in EPOS, indicating that the interpretation of RT depends on the underlying event model.
