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Register a new userUltra-low material pixel layers for the Mu3e experiment
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Added by
Sebastian Dittmeier
Publication date
2016
fields
Physics
and research's language is
English
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The upcoming Mu3e experiment will search for the charged lepton flavour violating decay of a muon at rest into three electrons. The maximal energy of the electrons is 53 MeV, hence a low material budget is a key performance requirement for the tracking detector. In this paper we summarize our approach to meet the requirement of about 0.1 % of a radiation length per pixel detector layer. This includes the choice of thinned active monolithic pixel sensors in HV-CMOS technology, ultra-thin flexible printed circuits, and helium gas cooling.
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Mu3e is a novel experiment searching for charged lepton flavor violation in the rare decay $mu^+ rightarrow e^+e^-e^+$. Decay vertex position, decay time and particle momenta have to be precisely measured in order to reject both accidental and physics background. A silicon pixel tracker based on $50,mu$m thin high voltage monolithic active pixel sensors (HV-MAPS) in a 1 T solenoidal magnetic field provides precise vertex and momentum information. The MuPix chip combines pixel sensor cells with integrated analog electronics and a periphery with a complete digital readout. The MuPix7 is the first HV-MAPS prototype implementing all functionalities of the final sensor including a readout state machine and high speed serialization with 1.25 Gbit/s data output, allowing for a streaming readout in parallel to the data taking. The observed efficiency of the MuPix7 chip including the full readout system is $geq99%$ in a high rate test beam.
The MuPix7 chip is a monolithic HV-CMOS pixel chip, thinned down to 50 mu m. It provides continuous self-triggered, non-shuttered readout at rates up to 30 Mhits/chip of 3x3 mm^2 active area and a pixel size of 103x80 mu m^2. The hit efficiency depends on the chosen working point. Settings with a power consumption of 300 mW/cm^2 allow for a hit efficiency >99.5%. A time resolution of 14.2 ns (Gaussian sigma) is achieved. Latest results from 2016 test beam campaigns are shown.
The Mu3e experiment aims to find or exclude the lepton flavour violating decay $mu rightarrow eee$ at branching fractions above $10^{-16}$. A first phase of the experiment using an existing beamline at the Paul Scherrer Institute (PSI) is designed to reach a single event sensitivity of $2cdot 10^{-15}$. We present an overview of all aspects of the technical design and expected performance of the phase~I Mu3e detector. The high rate of up to $10^{8}$ muon decays per second and the low momenta of the decay electrons and positrons pose a unique set of challenges, which we tackle using an ultra thin tracking detector based on high-voltage monolithic active pixel sensors combined with scintillating fibres and tiles for precise timing measurements.
The Mu3e experiment searches for the lepton flavor violating decay $mu^+~rightarrow~e^+~e^+~e^-$ with an ultimate aimed sensitivity of 1 event in $10^{16}$ decays. This goal can only be achieved by reducing the material budget per tracking layer to $X/X_0 approx 0.1 %$. High-Voltage Monolithic Active Pixel Sensors (HV-MAPS) which are thinned to $50 mu m$ serve as sensors. Gaseous helium is chosen as coolant. Results of recent studies related to the sensor prototypes, the helium cooling, and module prototyping are presented. The recent chip submission MuPix10 has proven its functionality regarding efficiency and time resolution. The helium cooling system for the inner tracker could be verified using a full-scale prototype. A complete prototype equipped with MuPix10 chips will be tested inside the Mu3e magnet in summer 2021.
For experiments searching for rare signals, background events from the detector itself are one of the major limiting factors for search sensitivity. Screening for ultra-low radioactive detector material is becoming ever more essential. We propose to develop a gaseous Time Projection Chamber (TPC) with Micromegas readout for radio-screening purposes. The TPC records three-dimensional trajectories of charged particles emitted from a flat sample placed inside the active volume. The detector is able to distinguish the origin of an event and identify the particle types with information from trajectories, which improves the screening sensitivity significantly. For $alpha$ particles from the sample surface, we find that our proposed detector can reach a sensitivity of better than 100~$mu$Bq$cdot$m$^{-2}$ within two days.
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