Vertex finding in pile-up rich events for p+p and d+Au collisions at STAR

Abstract

Primary vertex finding is an important part of accurately reconstructing events at STAR since many physics parameters, such as transverse momentum for primary particles, depend on the vertex location. Many analyses depend on trigger selection and require an accurate determination of where the interaction that fired the trigger occurred. Here we present two vertex finding methods, the Pile-Up Proof Vertexer (PPV) and a Minuit based vertex finder (MinuitVF), and their performance on the 2008 STAR p+p and d+Au data. PPV had been developed for use in p+p collisions and uses a 1D truncated log-likelihood method to determine the most probable z location of the vertex. MinuitVF had been developed for Au+Au events and finds the vertex by determining the location where the distance of closet approach (DCA) for all tracks is at a minimum. The heart of MinuitVF is the Minuit routine, which is a standard tool that finds the minimum value of a multi-parameter function. We will present the vertex finding efficiency for both forward and mid-rapidity triggers. A comparison to a hardware determination of the vertex will also be included