Master Thesis defense by Jonathan Baltazar

Title: Aspects of the τ Lepton Lifetime Measurement at the FCCee

Abstract:

The τ-lepton, is key to testing lepton universality within the Standard Model of particle physics. The proposed Future Circular Collider at CERN will produce an unprecedented amount of ττ pairs in clean experimental conditions, and offers the potential for highly precise lifetime mea- surements. Achieving this, particularly to a relative precision at the 10−5 level, requires careful management of systematic uncertainties, with a focus on minimising biases arising from vertex detector alignment. This study focuses on different τ-lepton lifetime measurement techniques, and addresses the impact of detector alignment on the measurement.

With centrally produced events, we determine the lifetime of the τ using a novel method analysing 3 × 3 τ decays by determining the spatial distance between decay vertices. We achieve a relative statistical uncertainty of 0.2% on this measurement. Our findings suggest that to achieve the desired 10−5 relative precision on the lifetime, contributions from different event topologies must be considered.

Analysis of vertex detector alignment showed a global minimal bias with a relative uncertainty of 2 · 10−5 on the flight length. We found that the measurement was unaffected by simulated shifts of the vertex staves, given that the azimuthal range is unbroken and equally weighed. We observe that angular localised misalignments can cause disparities in flight length measurements in the affected region, but these effects cancel out over the entire azimuthal range. This demonstrates that the systematic uncertainty arising from alignment in the vertex detector can be minimised.