Master Thesis defense by Sarah Emilie Andersson

Title: Testing the effects of hadronic rescattering in proton-proton collisions at sqrt(s)= 900 GeV − 13 TeV over the widest possible pseudorapidity-range at the LHC

Abstract:

This study explorers the production of primary charged particles in proton-proton collisions at the Large Hadron Collider at CERN. The presented measurements are the charged-particle pseudorapidity density distributions (dNch/deta) over a wide pseudorapidity range in collisions at energies from sqrt(s) = 900 GeV to sqrt(s) = 13 TeV in the centre-of-mass system.

The purpose of this study is to develop a methodology that directly compares these experimental distributions with model predictions from the Pythia 8 model in a statistically sound fashion. Using this methodology, this thesis tests the hypothesis of hadronic rescattering as an alternative to the current null hypothesis with no such mechanism. It is found that we are not able to discard either hypothesis, largely due to large systematic uncertainties on the experimental measurements. Furthermore, model predictions exhibit narrower dNch /deta than seen in experimental data for the majority of the collision energies.

This indicates that model predictions struggle to describe experimental data adequately in the forward regions, where we also have the largest systematic uncertainties. Additionally, model predictions including hadronic rescattering yield a lower effective transverse momentum to mass ratio (a = pT /m), which suggests a collective behavior at mid-rapidity. This might be useful in future studies of particle behavior in larger collision systems, as it indicates that hadronic rescattering can contribute to the understanding of collective behavior in high energy collisions.