The Contribution of Event Shapes to Elliptic Flow in pp-Collisions at √s = 13 TeV at the LHC with ALICE

Abstract

Recent discoveries of collective behaviour in proton-proton collisions at ultra-relativistic energies measured at the Large Hadron Collider (LHC) have gained great interest, since they provide new insights on the behaviour of matter at extreme temperatures and densities. The newly observed collectivity is, to a great extent, similar to observations in Pb-Pb collisions where they indicate the existence of a Quark Gluon Plasma (QGP). The QGP transforms initial eccentricities in space to elliptic anisotropies in momentum space where it is measured as elliptic flow (v2). Elliptic flow in pp-collisions is measured around the same magnitude as in Pb-Pb collisions however, pp-collisions do not contain the elliptic overlap area that generates the spatial eccentricity in Pb-Pb collisions. This raises the question whether the observed elliptic flow in pp-collisions is generated by a QGP or can be generated by other underlying principles.

This thesis presents measurements on the contribution of event shapes to v2 using two and multi particle cumulants in pp-collisions at √s=13 TeV. Data is obtained from measurements at the LHC with the ALICE detector and from events simulated by two tunes of the PYTHIA8 model. A technique called event shape engineering is used to separate the inclusive events and study the multiplicity dependence of elliptic flow for events having low, mid and high sphericities. The collision data shows how measurements on v2 at low multiplicities are biased by low sphericity event shapes, most dominant in this range, and together with the consistent PYTHIA8 simulations show no QGP is required to generate v2 at low multiplicities in inclusive pp-collisions. The results on mid and high sphericity events show how the addition of colour reconnection in the PYTHIA8 model improves the performance for all multiplicities and thereby supports the theory that colour reconnection generates collective effects.