Speaker
Description
The Mu3e experiment is on the frontier of charged lepton flavour violating searches and seeks to observe the rare muon decay $\mu^+ \rightarrow e^{+}e^{-}e^{+}$, as well as paving the way globally in terms of cutting-edge ultra low-mass detector technologies. Based at the Paul Scherrer Institute (PSI), a leading Swiss national laboratory near Zurich, the Mu3e experiment consists of four pixel tracking layers, as well as scintillator fibre and tile detectors.
The Mu3e outer pixel system is defined by three identical “stations”, one centred around the muon target, and one on either side known as the “recurl” stations. The central station will be built and operated first; and is scheduled to be installed in the beam area at PSI for data-taking in 2026. The basic module building block of the pixel tracker system is known as a ladder, which consists of between 17 and 18 70$\mu$m thick HVMAPS MuPix 11 pixel sensors in the case of the layer 3 and layer 4 (outer) pixel detector layers. In addition to the sensors, the ladders consist of an $\sim80 \mu$m thick aluminium/polyimide high-density interconnect (HDI) for the electrical connections and a mechanical support. Each of these layers are held together with minimal glue deposits, which are precisely controlled both in their placement location as well as the deposit amount. The radiation length of a 70$\mu$m Mu3e outer pixel ladder is approximately 0.1% $X/X_0$. This ultra-lightweight detector design is driven by the need to minimise the dominant effects of multiple scattering and to achieve precise momentum resolution of less than 1 MeV.
Electrical connections between the HDI, the sensors, and the read-out are achieved using single-point Tape Automated Bonding (spTAB). Over 1000 of these TAB bonds are required per ladder. Extensive electrical and thermo-mechanical tests have been undertaken to qualify the ladder objects as part of the QC/QA procedures before construction can commence. An innovative carbon-fibre geometry used solely for the outer pixel ladders has been custom-designed and will be presented in this talk. This mechanical support consists of 25$\mu$m uni-directional tow-spread carbon-fibre sheeting cured into a double-u shaped profile, with a 30-40% resin content and an 8$\mu$m kapton co-cured back layer for electrical isolation. The design considerations centre around the primary goal of ensuring maximum stiffness to protect the pixel sensors and improve ladder construction yield, whilst remaining extraordinarily lightweight. In total, 156 working ladders are required (52 per station), each weighing only $\sim 2.5$ grams. The Oxford Mu3e group is responsible for producing all ladders required for the outer pixel tracking system. An overview of the ladder-building procedure will be presented, which includes a snapshot of various thermo-mechanical studies that have been undertaken on prototype ladders in the lead-up to the production phase. Experience in handling 70$\mu$m thick silicon wafers as well as the performance of the MuPix 11 pixel sensors will also be presented.