High Voltage Monolithic Active Pixel Research Articles

Search for the lepton flavor violating decay μ + → e + e - e + with the Mu3e experiment at PSI

Mu3e is an experiment under construction at the Paul Scherrer Institute (Switzerland) aiming to find or exclude the charged lepton flavor violating decay μ + → e + e - e + at branching fractions above 10-16. A first running phase using an existing beamline targets a single event sensitivity of 2 · 10-15, while the ultimate sensitivity is reached in a second phase with a new high intensity muon beamline. Achieving such sensitivity requires a high muon rate along with a detector of large kinematic acceptance able to reconstruct the low momenta of the decay positrons and electrons with minimal material budget. The Mu3e experiment is mounted with ultra thin tracking detectors based on high-voltage monolithic active pixel sensors, combined with scintillating fibers and tiles for precise timing. This paper presents an overview of the experimental requirements, the technical design, and the construction readiness of Mu3e.

Performance Studies of the ATLASpix HV-MAPS Prototype for Different Substrate Resistivities

The ATLASpix high-voltage monolithic active pixel sensor (HV-MAPS) was designed as a technology demonstrator for the ATLAS ITk Upgrade and the CLIC tracking detector. In this contribution new results from laboratory-based energy calibration measurements using fluorescence X-rays are presented for the ATLASpix_Simple matrix. These findings are complemented by new results from test-beam studies with inclined tracks, in which the active charge collection depth is determined.

Characterisation of HV-MAPS ATLASPix3 and its applications for future lepton colliders

HV-MAPS are a novel type of CMOS depleted active pixel sensors for ionizing particles, implemented in standard CMOS processes, that have been proposed in several future particle physics experiments for particle tracking. In depleted monolithic sensors, the sensor element is the n-well/p-substrate diode. The sensor matrix and the readout are integrated in one single piece of silicon and the electronics is embedded in shallow wells inside deep n-wells, isolated from the substrate. High voltage biasing increases the depth of the depletion region, improving sensor properties as signal amplitude, charge collection speed and radiation tolerance. ATLASPix3 is the first full reticle size high voltage Monolithic Active Pixel CMOS sensor, designed to meet the specifications of the outer layers of the ATLAS inner tracker (ITk). Its thin design, the excellent position resolution, high readout rate and high radiation tolerance make ATLASPix3 an ideal candidate for large-area tracking detector R&D of future collider experiments such as the Circular Electron Positron Collider (CEPC) silicon tracker.

The Mu3e detector

The Mu3e experiment searches for the lepton flavour violating decay μ → eee with an ultimate sensitivity of 1 event in 1016 decays. This goal can only be achieved by reducing the material budget per tracking layer to X/X 0 ≈ 0.1%. For this purpose, the tracking detector is based on High Voltage Monolithic Active Pixel Sensors (HV-MAPS) thinned to 50 µm. The powering and data transmission is performed by means of polyimide-aluminum HDIs, which serve as mechanical support as well. The detector concept is presented, focusing on the technical aspects of the pixel tracker and the associated challenges.

The Mu3e experiment

The experiment aims for a single event sensitivity of 2\cdot 10^{-15}2⋅10−15 on the charged lepton flavour violating \mu^+\rightarrow e^+ e^+ e^-μ+→e+e+e− decay. The experimental apparatus, a light-weight tracker based on custom High-Voltage Monolithic Active Pixel Sensors placed in a 1 T magnetic field is currently under construction at the Paul Scherrer Institute, where it will fully use the intense 10^88\mu^+μ+/s beam available. A final sensitivity of 1 \cdot 10^{-16}1⋅10−16 is envisioned for a phase II experiment, driving the development of a new high-intensity continuous muon source which will deliver >10^99\mu^+μ+/s to the experiment.

Technical design of the phase I Mu3e experiment

The Mu3e experiment aims to find or exclude the lepton flavour violating decay μ→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 2⋅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 108 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 Data Acquisition

The Mu3e experiment aims to find or exclude the lepton flavor violating decay μ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> → e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> with a sensitivity of one in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> muon decays. The first phase of the experiment is currently under construction at the Paul Scherrer Institute (PSI, Switzerland), where beams with up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> muons per second are available. The detector will consist of an ultra-thin pixel tracker made from High-Voltage Monolithic Active Pixel Sensors (HV-MAPS), complemented by scintillating tiles and fibers for precise timing measurements. The experiment produces about 100Gbit/s of zero-suppressed data, which are transported to a filter farm using a network of field programmable gate arrays (FPGAs) and fast optical links. On the filter farm, tracks and three-particle vertices are reconstructed using highly parallel algorithms running on graphics processing units, leading to a reduction of the data to 100 Mbyte/s for mass storage and offline analysis. This article introduces the system design and hardware implementation of the Mu3e data acquisition and filter farm.

The MuPix sensor for the Mu3e experiment

The Mu3e experiment will search for the charged lepton flavor violating decay μ+→e+e−e+ with an ultimate sensitivity of one in 1016 decays (in phase II). To reach this sensitivity, a momentum resolution of 0.5MeV/c and a vertex resolution of about 200 μm is required. The particle rate is given by more than 109 muon stops per second. To achieve the specifications, High Voltage Monolithic Active Pixel Sensors (HVMAPS) are used for all layers of the tracking system. The chip developed for Mu3e is called MuPix. In the last years, several prototypes have been designed and tested. By the end of 2019, the next large MuPix – MuPix10 – was submitted and represents a final version prototype. MuPix10 is based on a 180 nm HVCMOS process. The matrix consists of 250×256 pixels with a size of 80×80 μm2. The total chip size is 20.66×23.18mm2. All hits are read out using a continuous readout system. The signal is amplified in the pixel and transmitted as an analogue signal to the readout cell located in the chip periphery, where it is discriminated by two comparators.

Performance of the large scale HV-CMOS pixel sensor MuPix8

The Mu3e experiment is searching for the charged lepton flavour violating decay\\break μ+→ e+ e− e+, aiming for an ultimate sensitivity of one in 1016 decays. In an environment of up to 109 muon decays per second the detector needs to provide precise vertex, time and momentum information to suppress accidental and physics background. The detector consists of cylindrical layers of 50 μm thin High Voltage Monolithic Active Pixel Sensors (HV-MAPS) placed in a 1 T magnetic field. The measurement of the trajectories of the decay particles allows for a precise vertex and momentum reconstruction. Additional layers of fast scintillating fibre and tile detectors provide sub-nanosecond time resolution. The MuPix8 chip is the first large scale prototype, proving the scalability of the HV-MAPS technology. It is produced in the AMS aH18 180 nm HV-CMOS process. It consists of three sub-matrices, each providing an untriggered datastream of more than 10 MHits/s. The latest results from laboratory and testbeam characterisation are presented, showing an excellent performance with efficiencies >99.6 % and a time resolution better than 10 ns achieved with time walk correction.

Irradiation study of a fully monolithic HV-CMOS pixel sensor design in AMS 180 nm

High-Voltage Monolithic Active Pixel Sensors (HV-MAPS) based on a 180 nm HV-CMOS process have been proposed to realize thin, fast and highly integrated pixel sensors. The MuPix7 prototype, fabricated in the commercial AMS H18 process, features a fully integrated on-chip readout, i.e. hit-digitization, zero suppression and data serialization. MuPix7 is the first fully monolithic HV-CMOS pixel sensor that has been tested for the use in high irradiation environments like HL-LHC. We present results from laboratory and test beam measurements of MuPix7 prototypes irradiated with neutrons (up to 5.0 × 1015 neq/cm2) and 24 GeV protons (up to 7.8 × 1015 protons/cm2) and compare the performance with non-irradiated sensors. At sensor temperatures of about 8 °C efficiencies of ≥90% at noise rates below 40 Hz per pixel are measured for fluences of up to 1.5 × 1015 neq/cm2. A time resolution better than 22 ns, expressed as Gaussian σ, is measured for all tested settings and sensors, even at the highest irradiation fluences. The data transmission at 1.25 Gbit/s and the on-chip PLL remain fully functional.

Efficiency and timing performance of the MuPix7 high-voltage monolithic active pixel sensor

The MuPix7 is a prototype high voltage monolithic active pixel sensor with 103×80μm2 pixels thinned to 64 μm and incorporating the complete read-out circuitry including a 1.25 Gbit/s differential data link. Using data taken at the DESY electron test beam, we demonstrate an efficiency of 99.3 % and a time resolution of 14 ns. The efficiency and time resolution are studied with sub-pixel resolution and reproduced in simulations.

Status of the Mu3e detector

Mu3e is an experiment searching for charged lepton flavour violation in the decay μ+ → 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 μm thin High-Voltage Monolithic Active Pixel Sensors (HV-MAPS) in a 1 T magnetic field provides precise vertex and momentum information. A scintillating fibre detector and a scintillating tile detector provide sub-nanosecond time information. The status of the Mu3e detector is presented, summarizing the development of HV-MAPS chips, the pixel detector modules as well as the timing detectors.

The MuPix Telescope: A Thin, High-Rate Tracking Telescope

The MuPix Telescope is a particle tracking telescope, optimized for tracking low momentum particles at high rates. It is based on the novel High-Voltage Monolithic Active Pixel Sensors (HV-MAPS), designed for the Mu3e tracking detector. The telescope represents a first application of the HV-MAPS technology and also serves as test bed of the Mu3e readout chain. The telescope consists of up to eight layers of the newest prototypes, the MuPix7 sensors, which send self-triggered data via fast serial links to FPGAs, where the data is time-ordered and sent to the PC. A particle hit rate of 1 MHz per layer could be processed. Online tracking is performed with a subset of the incoming data. The general concept of the telescope, chip architecture, readout concept and online reconstruction are described. The performance of the sensor and of the telescope during test beam measurements are presented.

The MuPix system-on-chip for the Mu3e experiment

Mu3e is a novel experiment searching for charged lepton flavor violation in the rare decay μ+→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 50μm thin high voltage monolithic active pixel sensors (HV-MAPS) in a 1T 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 1.25Gbit/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 ⩾99% in a high rate test beam.

The MuPix high voltage monolithic active pixel sensor for the Mu3e experiment

Mu3e is a novel experiment searching for charged lepton flavor violation in the rare decay μ → eee. In order to reduce background by up to 16 orders of magnitude, decay vertex position, decay time and particle momenta have to be measured precisely. A pixel tracker based on 50 μm thin high voltage monolithic active pixel sensors (HV-MAPS) in a magnetic field will deliver precise vertex and momentum information. Test beam results like an excellent efficiency of >99.5% and a time resolution of better than 16.6 ns obtained with the MuPix HV-MAPS chip developed for the Mu3e pixel tracker are presented.

The ultra-lightweight support structure and gaseous helium cooling for the Mu3e silicon pixel tracker

The Mu3e experiment searches for charged lepton flavor violation in the rare decay μ→eee. In order to reach a sensitivity of better than 10−16, more than 109 muon decays per second have to be observed over a running time of one year. Precise determination of particle momentum, vertex position and time are necessary for background suppression. These requirements can be met by combining an ultra-lightweight tracker based on High-Voltage Monolithic Active Pixel Sensors (HV-MAPS) with a timing system which consists of a scintillating fiber detector and a tile hodoscope. As the momentum of particles from muon decay at rest is below 53 MeV/c, the silicon pixel tracker resolution is dominated by multiple Coulomb scattering. This leads to extreme requirements for the material budget of the tracking detector of below 0.1% of a radiation length per layer. Even though the target power consumption of the HV-MAPS detector is as low as 150 mW/cm2, the detector cooling must be very efficient and at the same time avoid adding material inside the active tracking volume.

The Mu3e Experiment

The Mu3e experiment is designed to search for charged lepton flavour violation in the process μ+→e+e−e+ with a branching ratio sensitivity of 10−16. This requires a detector capable of coping with rates of up to 2⋅109 muons/s whilst being able to reduce backgrounds from accidental coincidences and the process μ+→e+e−e+ν¯μνe to below the 10−16 level. The use of 50μm thin high-voltage monolithic active pixel sensors in conjunction with an innovative tracking concept provide the momentum and position resolution necessary, whilst the timing resolution is provided by a combination of scintillating fibres and tiles. The recently approved Mu3e experiment will be located at the Paul Scherrer Institute in Switzerland and is currently preparing for detector construction.

A tracker for the Mu3e experiment based on high-voltage monolithic active pixel sensors

The Mu3e experiment searches for the lepton flavour violating decay μ+→e+e−e+, aiming for a branching fraction sensitivity of 10−16. This requires an excellent momentum resolution for low energy electrons, high rate capability and a large acceptance. In order to minimise multiple scattering, the amount of material has to be as small as possible. These challenges can be met with a tracker built from high-voltage monolithic active pixel sensors (HV-MAPS), which can be thinned to 50μm and which incorporate the complete read-out electronics on the sensor chip. To further minimise material, the sensors are supported by a mechanical structure built from 25μm thick Kapton foil and cooled with gaseous helium.

A novel experiment searching for the lepton flavour violating decay μ → eee

Since the discovery of neutrino oscillations it is known that lepton flavour is not conserved. Lepton flavour violating processes in the charged lepton sector have so far however eluded detection; as they are heavily suppressed in the standard model of particle physics, an observation would be a clear signal for new physics and help to understand the source of neutrino masses and CP violation. We propose a novel experiment searching for the decay μ → eee with the aim of ultimately reaching a sensitivity of 10−16, an improvement by four orders of magnitude compared to previous experiments. The technologies enabling this step are thin high-voltage monolithic active pixel sensors for precise tracking at high rates with a minimum of material and scintillating fibres for high resolution time measurements.