To flow or not to flow : a study of elliptic flow and nonflow in proton-proton collisions in ALICE

The standard model of particle physics describes all known elementary particles and the forces between them. The strong force, which binds quarks inside hadrons and nucleons inside nuclei, is described by the theory of Quantum Chromodynamics. This theory predicts a new state of matter at extreme temperatures and densities: the Quark Gluon plasma. The ALICE experiment at the Large Hadron Collider near Geneva was build to study this QGP by looking at collisions of the most heavy stable ions: lead (Pb) ions. In such collisions one hopes to achieve sufficient energy density for the creation of a QGP. One of the signatures of QGP formation in high energy heavy ion collisions is the presence of collective behaviour in the system formed during the collision. This collectivity manifests itself in a common velocity in all produced particles: a collective flow. The most dominant contribution to collective flow is elliptic flow, which originates from the anisotropic overlap region of the two nuclei in non-central collisions and is visible in the azimuthal distribution of the produced particles. Elliptic flow is related to the equation of state of the system and its degree of thermalisation. The analysis of elliptic flow is complicated by the presence of correlations between particles from other sources, summarised in the term nonflow. Several analysis methods have become available over the years and have been implemented for elliptic flow analysis within the ALICE computing framework. These methods have different sensitivities to these nonflow correlations. Because the centre of mass energy at the LHC is so high, predictions have been made of collective behaviour even in proton-proton collisions. These predictions are very divers and give values between 0 and 0.2 for elliptic flow using different models. To constrain these predictions proton-proton data, recorded with the ALICE experiment at the LHC in the 2010 7 TeV proton-proton run, was studied. In proton-proton collisions large nonflow correlations are certainly present and might mask the elliptic flow correlation. The nonflow correlations have to be suppressed sufficiently such that the elliptic flow signal becomes detectable. Therefor an analysis method was choosen that can suppress nonflow correlations by increasing the separation in pseudorapidity of two subevents. This method is called the scalar product method. How much nonflow is suppressed is shown to depend on the pseudorapidity range of the nonflow. The dependence on the pseudorapidity gap size between the subevents, in 7 TeV proton-proton collisions, points to a strong nonflow component, because the signal decreases with increasing gap size. The corresponding Monte Carlo data set shows the same dependence, while it only includes nonflow correlations. This enforces the conclusion that nonflow is the dominant or the only correlation in 7 TeV proton-proton data at the LHC. The conclusion from this analysis is that elliptic flow in 7 TeV proton-proton collisions with at least 10 particles is less than 0.05. Predictions of a higher elliptic flow for these events can be excluded. To exclude or confirm lower predicted values the nonflow contribution has to be further reduced.