Mu3e Experiment Research Articles

Development of the scintillating fiber timing detector for the Mu3e experiment

We present the development and discuss the performance of a compact scintillating fiber detector for the Mu3e experiment for accurate time measurements at very high particle rates. Mu3e is a new experiment under preparation at the Paul Scherrer Institut (PSI) to search for charged Lepton Flavor Violation in the neutrinoless muon decay μ+→e+e−e+ with a projected sensitivity of 10−16 using the most intense continuous surface muon beam in the world. A very thin scintillating fiber detector (thickness <0.2% of a radiation length X0) with a time resolution of about 250 ps, efficiency around 97%, and spatial resolution of ∼100μm has been developed. The SciFi detector is read out with multi-channel silicon photomultiplier arrays at both ends to achieve the best timing performance. Different scintillating fiber types have been evaluated and various assembly procedures have been tested to achieve the best performance.

New physics in multi-electron muon decays

We study the exotic muon decays with five charged tracks in the final state. First, we investigate the Standard Model rate for μ+ → 3e+ 2e− 2ν (B\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{B} $$\\end{document} = 4.0 × 10−10) and find that the Mu3e experiment should have tens to hundreds of signal events per 1015μ+ decays, depending on the signal selection strategy. We then turn to a neutrinoless μ+ → 3e+ 2e− decay that may arise in new-physics models with lepton-flavor-violating effective operators involving a dark Higgs hd. Following its production in μ+ → e+hd decays, the dark Higgs can undergo a decay cascade to two e+e− pairs through two dark photons, hd→ γdγd→ 2(e+e−). We show that a μ+→ 3e+ 2e− search at the Mu3e experiment, with potential sensitivity to the branching ratio at the O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(10−12) level or below, can explore new regions of parameter space and new physics scales as high as Λ ∼ 1015 GeV.

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.

The Mu3e scintillating fiber detector R&D

The Mu3e experiment searches for a rare lepton flavor violating μ+→e+e+e− decay and it aims at reaching an ultimate sensitivity of 10−16 on the branching fraction of the μ+→e+e+e− decay, four orders of magnitude better than the current limit B(μ+→e+e+e−)<10−12. The experiment will be hosted at the Paul Scherrer Institute (Villigen, Switzerland) which delivers the most intense low momentum continuous muon beam in the world (up to few ×108μ/s).In order to achieve this unprecedent sensitivity new detector R&D have been performed. We will report about the Scintillating Fiber (SciFi) detector R&D aiming at a detector able to measure minimum ionizing particles with a highest as possible detection efficiency (>95%), timing resolutions well below 1 ns and spatial resolution of ≈100μm. The main challenge to address such a requirements is to keep the detector as thin as possible, to minimize the multiple scattering. Then the thickness of the detector must be below 0.4% of radiation length X0. It implies that the requirements listed above will be extracted measuring just a relative low number of photoelectrons. The results of several beam tests are given, proving that the requirements for the experiment has been addressed. These studies have been supported with detailed Monte Carlo simulations from the fiber through the photosensors up to the electronics and the data acquisition.

High-performance HV-CMOS sensors for future particle physics experiments — an overview

HV-CMOS (High Voltage-CMOS) sensors are emerging as a prime candidate for future tracking detectors that have extreme requirements on material budget, pixel granularity, time resolution and radiation tolerance. These sensors have the advantages of full monolithic structure, low manufacture cost, fast charge collection and high radiation tolerance. Confirmed and potential tracking applications in physics experiments include the Mu3e experiment, proton EDM searches, future upgrades of LHC (Large Hadron Collider) and CEPC (Circular Electron Positron Collider). The HV-CMOS group at Liverpool is doing generic R&D to push the boundaries of HV-CMOS sensors performance, especially in terms of single point resolution, fast-timing capability and radiation tolerance. This contribution gives an overview of the latest research by the Liverpool HV-CMOS group and presents the HV-CMOS prototypes developed in Liverpool.

Measurements of μ → 3e decay with polarised muons as a probe of new physics

Working within the Standard Model effective field theory approach, we examine the possibility to test charged lepton flavour violating (cLFV) new physics (NP) by angular measurements of the outgoing electron and positrons in the polarised μ+→e+e+e− decay. This decay is planned to be studied with a sensitivity to the branching ratio of Bμ→3e∼10−15 by the Mu3e experiment at PSI. To illustrate the potential of these measurements, we consider a set of eight effective operators generating the decay at the tree level, including dimension-five dipole operators and dimension-six scalar and vector four-fermion operators of different chiralities. We show that if the polarised μ+→e+e+e− decay is observed and is induced by a single operator, data from three angular observables – two P-odd asymmetries and one T-odd asymmetry – can allow to discriminate between all considered operators with the exception of scalar and vector operators of opposite chirality. For these two types of operators, the P-odd asymmetry is maximal in magnitude; they can be distinguished, in principle, by measuring the helicities of the outgoing positrons. The observation of a non-zero T-odd asymmetry would indicate the presence in the decay rate of a dipole and vector operator interference term that is T (CP) violating.

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.

Searching for the Muon Decay to Three Electrons with the Mu3e Experiment

Mu3e is a dedicated experiment designed to find or exclude the charged lepton flavor violating μ→ eee decay at branching fractions above 10−16. The search is pursued in two operational phases: Phase I uses an existing beamline at the Paul Scherrer Institute (PSI), targeting a single event sensitivity of 2·10−15, while the ultimate sensitivity is reached in Phase II using a high intensity muon beamline under study at PSI. As the μ→ eee decay is heavily suppressed in the Standard Model of particle physics, the observation of such a signal would be an unambiguous indication of the existence of new physics. Achieving the desired sensitivity requires a high rate of muons (108 stopped muons per second) along with a detector with large kinematic acceptance and efficiency, able to reconstruct the low momentum of the decay electrons and positrons. To achieve this goal, the Mu3e experiment is mounted with an ultra thin tracking detector based on monolithic active pixel sensors for excellent momentum and vertex resolution, combined with scintillating fibers and tiles for precise timing measurements.

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.

Test results of ATLASPIX3 — A reticle size HVCMOS pixel sensor designed for construction of multi chip modules

High voltage CMOS pixel sensors will be or are proposed to be used in several particle physics experiments for particle tracking like Mu3e experiment. ATLASPIX3 is the first full reticle size monolithic HVCMOS sensor for construction of multi-chip modules. The specifications for the use case have been taken from ATLAS pixel upgrade in fifth layer where it was a candidate for. The size of the chip is 2.0×2.1 cm2 with periphery at one side which makes the chip 3-side buttable. ATLASPIX3 has been implemented in a standard 180 nm HVCMOS process. Each pixel has an area of 150×50μm2 and contains a large charge collecting electrode implemented as deep n-well. The depleted volume around the n-well is enlarged by a high voltage bias and the usage of higher resistivity substrate. The readout electronics supports both triggered and triggerless readout with zero-suppression. ATLASPIX3 could be used for the construction of CMOS modules for particle tracking in experiments where high time resolution, high radiation tolerance, low power and low material budget are required. In the design phase, special attention has been paid to decreasing timing differences between pixels and the rate capability of the readout.

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.

Towards a new generation of Charged Lepton Flavour Violation searches at the Paul Scherrer Institut: The MEG upgrade and the Mu3e experiment

The MEG experiment has recently set a new upper limit on the branching ratio of the µ+ → e+γ decay, ℬ(µ+ → e+γ) &lt; 4.2 × 10−13 (at 90% confidence level) and un upgrade of the experiment (the MEGII experiment) is ongoing with the aim of improving the single event sensitivity (SES) by one order of magnitude with respect to the previous MEG experiment’s SES. The strong scientific motivation associated with the charged Lepton Flavour Violation (cLFV) searches pushes also towards searching for the complementary muon cLFV µ+ → e+e+e− decay with a completely new apparatus, the Mu3e experiment, aiming at a SES improved by at least three orders of magnitude with respect to the previous SINDRUM experiment’s SES (Mu3e phase I). An ultimate SES of few ×10−16 is foreseen requiring 109 µ/s (Mu3e phase II). Both experiments will be hosted at the Paul Scherrer Institut which currently delivers the most intense continuous low energy muon beam in the world up to few ×108 µ/s. The status of both the MEGII and Mu3e phase I experiments is given.

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.

Measurements with the technical prototype for the Mu3e tile detector

The tile detector is a dedicated timing detector developed for the Mu3e experiment, which is designed to search for the lepton-flavour violating (LFV) decay μ→eee with a target sensitivity of 10−16. In order to determine the vertex and coincident time of the three decay electrons, precise spatial and timing measurements are necessary, resulting in the requirement of a time resolution below 100 ps for the tile detector.The tile detector, which is currently being developed at the Kirchhoff Institute for Physics, Heidelberg University, employs plastic scintillator tiles and silicon photomultipliers which are read out by dedicated 32-channel mixed signal readout ASICs. Measurements using the first technical prototype of the tile detector in two testbeam campaigns, undertaken at the Deutsches Elektron-Synchrotron (DESY) in Hamburg, Germany in February and June 2018, show a preliminary single channel timing resolution of about 47 ps, which is well below the required resolution of 100 ps.Furthermore, the production and assembly procedures for the final detector system were defined based on the experience gained from the construction of the prototype. Thermal simulation studies using the prototype provided additional input for the finalisation of the detector structure.

The rare and forbidden: Testing physics beyond the standard model with Mu3e

The upcoming Mu3e experiment aims to search for the lepton flavour violating decay \boldsymbol{\muposeeemath} with an unprecedented final sensitivity of one signal decay in \boldsymbol{\num{e16}} observed muon decays by making use of an innovative experimental design based on novel ultra-thin silicon pixel sensors. In a first phase, the experiment is operated at an existing muon beam line with rates of up to \boldsymbol{\num{e8}} muons per second. Detailed simulation studies confirm the feasibility of background-free operation and project single event sensitivities in the order of \boldsymbol{\num{e-15}} for signal decays modelled in an effective field theory approach. The precise tracking of the decay electrons and large geometric and momentum acceptance of Mu3e enable searches for physics beyond the Standard Model in further signatures. Examples of which are searches for lepton flavour violating two-body decays of the muon into an electron and an undetected boson as well as for electron-positron resonances in \boldsymbol{\muposeeenunumath} which could result for instance from a dark photon decay. The Mu3e experiment is expected to be competitive in all of these channels already in phase I.

MEG II and Mu3e status and plan

The MEG experiment searching for lepton flavor violating muon decay, finished data taking successfully in 2013. The final sensitivity of the experiment was 5.3 × 10−13, and since the experiment did not find any signal, the upper limit of the branching ratio of the µ → e+γ was set to be 4.2 × 10−13 at 90% C.L.. The MEG II experiment will have a target sensitivity improved by an order of magnitude down to 6 ×10−14 with three years data taking. In 2018 after the detector integration, the muon beam data taking took place with a limited number of readout channels. In 2019, the engineering run and the physics run will be started. The Mu3e experiment will search for the lepton flavor violating decay µ+→ e+e−e+ with unprecedented sensitivity. Construction of the detector is going to start in 2020 and for the first phase a single event sensitivity of 2 × 10−15 is expected.

THE ADVANCED DESIGN AND TECHNOLOGICAL SOLUTIONS OF ULTRALIGHT DETECTOR MODULES FOR PHYSICAL EXPERIMENTS

One of the main tasks of improving the informative value of the experiments of high energy physics is to reduce the mass of matter in the detection volume to ensure reducing a probable impact on the parameters of particles under study. At the same time, the informative value of a research is planned to be increased, namely, the solvability of such studies, which, in turn, leads to a significant increase in information flows and the speed of data transmission and processing. The above-mentioned main tasks of improving available experiments and creating new ones in the sphere of physics can be solved due to the development of new design and technological solutions for detector modules, which are the basic cell of modern detection systems of international physics experiments. The constructive and technological approaches to creating detector modules mainly determine the fact whether the mass of the material and the speed of the entire detector system meet the set requirements. The above requirements while creating detector modules can be met if advanced semiconductor sensitive elements (sensors) and multi-layer switching elements with aluminium conductive layers are used. The subject matter of this study is the technology of creating detector modules with the low level of material mass in the detection volume using the advanced thin semiconductor element base. The goal of this work is to create and study ultra-light detector modules and their prototypes at high data transmission rates (over 1 Gbit/s). In order to achieve the goal, the following tasks are to be solved: the advanced HV-MAPS semiconductor sensors should be studied; the material of switching elements should be selected and this selection should be justified; the structure and technology for the detector modules assembly should be chosen. According to the results of the selection of the optimal method for the electrical interconnection of the module components, namely the assembly technology, the constructive and technological requirements for detector modules for the Mu3e experiment should be analyzed; the analysis will determine the structure and composition of the module. Taking into account the above design features, selected materials and technologies for selecting good materials and verifying this selection as well as the assembly technology, the mechanical experimental model of the detector module for Mu3e experiment and the prototype of an ultra-light multi-layer flexible board should be developed and made for studying the impact of signal transmission at speeds over 1 Gbit/s. Conclusions: when performing the work, the experimental models of detector modules and test multi-layer boards were developed, manufactured and studied; these models proved the expected results. The obtained results of studying the manufactured models make further work possible for using these approaches while creating innovative detector modules not only for Mu3e experiment but also for experiments with similar strict requirements for minimizing material mass in the detection volume and high speed of signal transmission, for example, upgrading/improving ATLAS experiment at Large Hadron Collider at CERN.

MuPix8 — Large area monolithic HVCMOS pixel detector for the Mu3e experiment

Abstract The requirements of the ultra thin pixel detectors for the Mu3e experiment at PSI can be achieved by the HVCMOS technology, which allows the design of fast monolithic detectors. The latest nearly full size prototype, MuPix8, has a size of about 1 × 2 cm 2 . The pixel readout circuitry was fully redesigned in comparison to the previous MuPix versions. MuPix8’s readout electronics implement a new concept with two comparators and two different operation modes. One mode uses two threshold voltages for time walk correction, the other is a ramp-ADC. First tests show a detection efficiency of 99.6% for 4 GeV electrons.

About heavy neutrinos: lepton-flavor violation in decays of charged leptons

The fundamental description of nature, beyond the Standard Model (SM), may include heavy neutrinos that mix and thus allow processes in which lepton flavor is not preserved. We investigate the impact of charged currents that couple heavy gauge bosons to heavy neutrinos and SM leptons on lepton-flavor-violating decays of SM leptons into three charged leptons, with no final-state neutrinos. We implement our expressions for the leading contributions to Br(lα → lβ lσ lσ), which hold for either Dirac or Majorana neutrinos, to the trilepton decay , of the muon, and so determine sets of masses of heavy neutrinos and the heavy gauge boson, within GeVs to few TeVs, that are consistent with the upper bounds provided by the SINDRUM Collaboration. We find, however, that constraints dictated by the upper bound on Br(μ → eγ), from the MEG Collaboration, are more stringent. We utilize such parameters to find that the contributions to tau decays are , well below bounds from B factories. The mixing of heavy and SM charged bosons is also investigated. We find that current experimental data from MEG and SINDRUM would allow mixing angles as large as ∼10−2, for a relatively light new charged boson, but the expected sensitivity of the Mu3e experiment would be capable of setting an upper bound on this angle as small as ∼10−4 if the mass of this boson is within the range of few TeVs.