Pixel Sensor Evaluation and Track Fitting for the Mu3e Experiment

Abstract

Mu3e is a proposed new experiment that searches for the lepton flavour changing muon decay µ⁺ → e⁺e⁻e⁺ with a projected sensitivity of 1 in 10¹⁶ decays. This decay is extremely suppressed in the Standard Model and an observation would be a clear sign of new physics. The muons are stopped on a target and decay electrons are measured in a magnetic spectrometer with a pixel detector. A low material budget and special algorithms are required to reconstruct the low momentum, strongly curved tracks with high precision at high rates. In this work, I present the first systematic performance evaluation of the Mu3e high voltage monolithic active pixel sensor prototypes using test beam measurements at DESY. I showed that this novel technology allows sensor efficiencies above 99%. In addition, I characterized a new track fit based on hit triplets. This fast fit is optimized to reconstruct multiple scattering dominated tracks and is suitable for Mu3e online tracking. For a precise offline reconstruction, I adapted the advanced general broken lines fit. It allows an increase in momentum resolution of up to 20% compared to the best fast algorithm. The systematic pixel sensor and track fit evaluation with the corresponding software development is a cornerstone for Mu3e tracking.