Intense Muon Beam Research Articles

Beamline design for multipurpose muon beams at CSNS EMuS

A new muon beam facility, called the Experimental Muon Source (EMuS), was proposed for construction at the China Spallation Neutron Source (CSNS). The design of the complex muon beamlines for the EMuS baseline scheme, which is based on superconducting solenoids, superferric dipoles and room-temperature magnets, is presented herein. Various muon beams, including surface muons, decay muons and low energy muons, have been developed for multipurpose applications. The optics design and simulation results of the trunk beamline and branch beamlines are presented. With a proton beam power of 25 kW at a standalone target station that consists of a conical graphite target and high-field superconducting solenoids, the muon beam intensity in the trunk beamline varies from 107/s for surface muons to 1010/s for high-momentum decay muons. And at the endstations, these values vary from 105/s for surface muons to 108/s for decay muons.

MEG II physics and detector performance

In the panorama of the state-of-the-art searches for extremely rare Charged Lepton Flavor Violating (CLFV) processes, the Mu-E-Gamma (MEG) experiment is definitely a reference point in the intensity frontier of modern physics research, setting the best upper limit on the μ + → e + γ decay. The upgrade of MEG, MEG II, wants to give further impetus to the CLFV searches with muons. MEG II relies on a series of upgrades: on the photon side we point up improvements of the γ detector resolutions and acceptance; on the positron side we rely on completely brand new detectors with better acceptance, efficiency and performances; on the Trigger and Data Acquisition (DAQ) side we are able to exploit a higher muon beam intensity despite the increased number of read out channels thanks to a new and optimized electronics. After three years of commissioning, in 2021 the MEG II experiment finally entered the physics data taking phase. An overview of the MEG II physics and experimental contexts is presented, together with the current detector performances based on data. Thanks to the new experimental apparatus the final sensitivity goal is expected to be one order of magnitude better than the first phase of MEG.

The PSI meson target facility and its upgrade IMPACT-HIMB

The high intensity proton accelerator complex (HIPA) at the Paul Scherrer Institute (PSI) delivers a 590 MeV CW proton beam with currents up to 2.4 mA (1.4 MW). Besides two spallation targets for thermal/cold neutrons (SINQ) and for ultracold neutrons (UCN), the beam feeds two meson production targets Target M and Target E. The targets consist of graphite wheels of effective thickness 5 mm (M) and 40/60 mm (E). The target stations M and E are of quite different design; however, both of them rotate at 1 Hz to dissipate the heat (20 kW/mA for the 40 mm target E) efficiently. Recent progress was made by a new type of bearings and a new target geometry able to increase the rate of surface muons by up to 50 %. This is also foreseen for the upgrade of the target station M within the High Intensity Muon Beam (HIMB) initiative aiming to increase the surface muons available for experiment by two orders of magnitude. HIMB is part of IMPACT (Isotope and Muon Production with Advanced Cyclotron and Target Technology), an application for the Swiss Roadmap of Research Infrastructure.

Beamline design and optimisation for high intensity muon beams at PSI

The High Intensity Muon Beams (HIMB) project at the Paul Scherrer Institute (PSI) will provide muon intensities of the order of 1010 muons/s for particle physics and material science experiments, two orders of magnitude higher than the state of the art, which is currently available also at PSI. In particle transport simulations for the HIMB, we use G4beamline with measured + cross-sections and with variance reduction. We also use the codes COSY INFINITY, TRANSPORT, and TURTLE for some studies. We perform asynchronous Bayesian optimisation of the beamlines on a computing cluster using G4beamline and the optimisation package DeepHyper. We performed numerous studies for the design of the HIMB, and we produced various results, including the muon transmission, beam phase space, polarisation, and momentum spectrum.

Beam monitoring detectors for High Intensity Muon Beams

We present two novel beam monitors designed for use in current PSI muon beams and upgradable for the High Intensity Muon Beam (HIMB) project beamlines: the scintillating fibre (SciFi) detector, a grid of scintillating fibres coupled to SiPMs, and the MatriX detector, a matrix of plastic scintillators coupled to silicon photomultipliers (SiPMs), both detectors having particle ID capability. The advantage of these highly segmented detectors is the capability to withstand the high beam rate and to perform total beam rate measurements. Furthermore, the use of the SiPM as a photosensor enables operation of these detectors in high magnetic field environments (up to 1.25T). Both detectors have been tested in the beam at PSI and with continuous beam rates up to 108μ+/s. The performance of these detectors as measured on the beamlines are presented.

Simulation studies for upgrading a high-intensity surface muon beamline at Paul Scherrer Institute

The $\ensuremath{\mu}\mathrm{E}4$-LEM beamline at Paul Scherrer Institute (PSI, Switzerland) is a special muon beamline combining the hybrid-type surface muon beamline $\ensuremath{\mu}\mathrm{E}4$ with the low-energy muon (LEM) facility and delivers ${\ensuremath{\mu}}^{+}$ with tunable energy up to 30 keV for low-energy muon spin rotation (LE-$\ensuremath{\mu}\mathrm{SR}$) experiments. We investigate a possible upgrade scenario for the surface muon beamline $\ensuremath{\mu}\mathrm{E}4$ by replacing the last set of quadrupole triplets with a special solenoid to obtain 1.4 times the original beam intensity on the LEM muon moderator target. In order to avoid the muon beam intensity loss at the LEM spectrometer due to the stray magnetic field of the solenoid, three kinds of solenoid models have been explored and the stray field of the solenoid at the LEM facility is finally reduced to the magnitude of the geomagnetic field. A more radical design, ``Super-$\ensuremath{\mu}\mathrm{E}4$,'' has also been investigated for further increasing the brightness of the low-energy muon beam, where we make use of the current $\ensuremath{\mu}\mathrm{E}4$ channel and all sets of quadrupole triplets are replaced by large aperture solenoids. Together with the new slanted muon target E, at least 2.9 times the original muon beam intensity can be expected in the Super-$\ensuremath{\mu}\mathrm{E}4$ beamline. Our work demonstrates the feasibility of upgrading surface muon beamlines by replacing quadrupole magnets with normal-conducting solenoids, resulting in higher muon rates and smaller beam spot sizes.

Roles of resonant muonic molecule in new kinetics model and muon catalyzed fusion in compressed gas

Muon catalyzed fusion (muCF) in which an elementary particle, muon, facilitates the nuclear fusion between the hydrogen isotopes has been investigated in a long history. In contrast to the rich theoretical and experimental information on the muCF in cold targets, there is relatively scarce information on the high temperature gas targets of deuterium-tritium mixture with high-thermal efficiency. We demonstrate new kinetics model of muCF including three roles of resonant muonic molecules, (i) changing isotopic population, (ii) producing epi-thermal muonic atoms, and (iii) inducing fusion in-flight. The new kinetics model reproduces experimental observations, showing higher cycle rate as the temperature increasing, over a wide range of target temperatures (T<800 K) and tritium concentrations. Moreover, it can be tested by measurements of radiative dissociation X-rays around 2 keV. High energy-resolution X-ray detectors and intense muon beam which are recently available are suitable to reveal these dynamical mechanism of muCF cycles. Towards the future muCF experiments in the high-temperature gas target we have clarified the relationship between the fusion yield and density-temperature curve of adiabatic/shock-wave compression.

Characterization of a Continuous Muon Source for the Non-Destructive and Depth-Selective Elemental Composition Analysis by Muon Induced X- and Gamma-rays

The toolbox for material characterization has never been richer than today. Great progress with all kinds of particles and interaction methods provide access to nearly all properties of an object under study. However, a tomographic analysis of the subsurface region remains still a challenge today. In this regard, the Muon Induced X-ray Emission (MIXE) technique has seen rebirth fueled by the availability of high intensity muon beams. We report here a study conducted at the Paul Scherrer Institute (PSI). It demonstrates that the absence of any beam time-structure leads to low pile-up events and a high signal-to-noise ratio (SNR) with less than one hour acquisition time per sample or data point. This performance creates the perspective to open this technique to a wider audience for the routine investigation of non-destructive and depth-sensitive elemental compositions, for example in rare and precious samples. Using a hetero-structured sample of known elements and thicknesses, we successfully detected the characteristic muonic X-rays, emitted during the capture of a negative muon by an atom, and the gamma-rays resulting from the nuclear capture of the muon, characterizing the capabilities of MIXE at PSI. This sample emphasizes the quality of a continuous beam, and the exceptional SNR at high rates. Such sensitivity will enable totally new statistically intense aspects in the field of MIXE, e.g., elemental 3D-tomography and chemical analysis. Therefore, we are currently advancing our proof-of-concept experiments with the goal of creating a full fledged permanently operated user station to make MIXE available to the wider scientific community as well as industry.

Towards a New μ→eγ Search with the MEG II Experiment: From Design to Commissioning

The MEG experiment represents the state of the art in the search for the Charged Lepton Flavour Violating μ+→e+γ decay. With its first phase of operations at the Paul Scherrer Institut (PSI), MEG set the most stringent upper limit on the BR (μ+→e+γ)≤4.2×10−13 at 90% confidence level, imposing one of the tightest constraints on models predicting LFV-enhancements through new physics beyond the Standard Model. An upgrade of the MEG experiment, MEG II, was designed and it is presently in the commissioning phase, aiming at a sensitivity level of 6×10−14. The MEG II experiment relies on a series of upgrades, which include an improvement of the photon detector resolutions, brand new detectors on the positron side with better acceptance, efficiency and performances and new and optimized trigger and DAQ electronics to exploit a muon beam intensity twice as high as that of MEG (7×107 μ+/s). This paper presents a complete overview of the MEG II experimental apparatus and the current status of the detector commissioning in view of the physics data taking in the upcoming three years.

The Search for μ+ → e+γ with 10–14 Sensitivity: The Upgrade of the MEG Experiment

The MEG experiment took data at the Paul Scherrer Institute in the years 2009–2013 to test the violation of the lepton flavor conservation law, which originates from an accidental symmetry that the Standard Model of elementary particle physics has, and published the most stringent limit on the charged lepton flavor violating decay μ+→e+γ: BR(μ+→e+γ) &lt;4.2×10−13 at 90% confidence level. The MEG detector has been upgraded in order to reach a sensitivity of 6×10−14. The basic principle of MEG II is to achieve the highest possible sensitivity using the full muon beam intensity at the Paul Scherrer Institute (7×107 muons/s) with an upgraded detector. The main improvements are better rate capability of all sub-detectors and improved resolutions while keeping the same detector concept. In this paper, we present the current status of the preparation, integration and commissioning of the MEG II detector in the recent engineering runs.

A search for charged lepton flavor violation in the Mu2e experiment

The Mu2e experiment will search for the neutrino-less conversion of a muon into an electron in the field of an aluminum nucleus. An observation would be the first signal of charged lepton flavor violation and de facto evidence for new physics beyond the Standard Model. The clean signature of the conversion process offers an opportunity for a powerful search: Mu2e will probe four orders of magnitude beyond current limits, with real discovery potential over a wide range of well-motivated new physics models. This goal requires an integrated system of solenoids that will create the most intense muon beam in the world, and suppression of all possible background sources. The Mu2e components are currently being constructed, with the experiment planned to begin operations in the Fermilab Muon Campus within the next few years.

Nuclear isotope production by ordinary muon capture reaction

Muon capture isotope production using ordinary muon (μ) capture reaction is used to efficiently produce various kinds of nuclear isotopes for both fundamental and applied science studies. The large capture probability of μ into a nucleus, together with the high intensity μ beam, make it possible to produce nuclear isotopes in the order of 109−10 per second depending on the muon beam intensity. Radioactive isotopes produced by muon capture isotope production technique are complementary to those produced by photon and neutron capture reactions and are used for various science and technology applications. Muon capture isotope production on NatMo by using the MuSIC μ beam at RCNP, Osaka University is presented to demonstrate the feasibility of the technique. The nuclear isotopes produced by this technique are evaluated by using a pre-equilibrium and equilibrium proton neutron emission model. Radioactive 99Mo isotopes and the metastable 99mTc isotopes, which are used extensively in medical science, are produced by muon capture reaction on NatMo and 100Mo.

Radiation hardness study for the COMET Phase-I electronics

Radiation damage on front-end readout and trigger electronics is an important issue in the COMET Phase-I experiment at J-PARC, which plans to search for the neutrinoless transition of a muon to an electron. To produce an intense muon beam, a high-power proton beam impinges on a graphite target, resulting in a high-radiation environment. We require radiation tolerance to a total dose of 1.0kGy and 1MeV equivalent neutron fluence of 1.0×1012neqcm−2 including a safety factor of 5 over the duration of the physics measurement. The use of commercially-available electronics components which have high radiation tolerance, if such components can be secured, is desirable in such an environment. The radiation hardness of commercial electronic components has been evaluated in gamma-ray and neutron irradiation tests. As results of these tests, voltage regulators, ADCs, DACs, and several other components were found to have enough tolerance to both gamma-ray and neutron irradiation at the level we require.

Development of a data acquisition system for the μSR spectrometer prototype at the China Spallation Neutron Source

An experimental muon source (EMuS) to provide a very intense muon beam has been studied at the China Spallation Neutron Source (CSNS) since 2007 and will be operational in the next few years. R&D efforts including a 128-channel muon spin rotation, relaxation, and resonance (μSR) spectrometer prototype are also ongoing. A data acquisition (DAQ) system was been developed to run the μSR spectrometer prototype on the EMuS. It includes front-end electronics (FEEs), time-to-digital converter (TDC) modules, and a DAQ software. The raw data acquired by the DAQ software were converted into a specific format and submitted to specialized analysis software for analysis. The DAQ system was tested on the ISIS muon beam, and the test results proved its performance.

Full system of positron timing counter in MEG II having time resolution below 40 ps with fast plastic scintillator readout by SiPMs

A positron timing counter (TC) with 30–40 ps time resolution for 50 MeV/c positrons required by the MEG II experiment has been developed. We employed a highly segmented design with 512 scintillator plates (120 × 40 × 5 mm3 and 120 × 50 × 5 mm3) read out by 6-SiPM-arrays at the both ends. Pile up is reduced by the segmented design and multi-counter measurement improves the overall timing resolution. All the single counters were assembled and their resolutions were measured to be below 100 ps in a pre-test with a 90Sr source. The construction and installation were completed in 2017. An engineering run was performed at the end of 2017 at the πE5 muon beam line in PSI, which supplies the most intense muon beam (7 × 107 stops/s on a target) in the world. We successfully operated the full system of TC in the MEG II environment and achieved a time resolution below 40 ps with more than 6 counter hits. The overall resolution for the signal positrons is estimated to be 38.5 ps by weighting the obtained resolutions with the number of hits distribution of the signal positrons obtained by a Monte Carlo simulation. The time resolution of the TC is improved by a factor of 2 with respect to the MEG experiment.

The Future Prospects of Muon Colliders and Neutrino Factories

The potential of muon beams for high energy physics applications is described along with the challenges of producing high quality muon beams. Two proposed approaches for delivering high intensity muon beams, a proton driver source and a positron driver source, are described and compared. The proton driver concepts are based on the studies from the Muon Accelerator Program (MAP). The MAP effort focused on a path to deliver muon-based facilities, ranging from neutrino factories to muon colliders, that could span research needs at both the intensity and energy frontiers. The Low EMittance Muon Accelerator (LEMMA) concept, which uses a positron-driven source, provides an attractive path to very high energy lepton colliders with improved particle backgrounds. The recent study of a 14-TeV muon collider in the LHC tunnel, which could leverage the existing CERN injectors and infrastructure and provide physics reach comparable to the 100[Formula: see text]TeV FCC-hh, at lower cost and with cleaner physics conditions, is also discussed. The present status of the design and R&amp;D efforts towards each of these sources is described. A summary of important R&amp;D required to establish a facility path for each concept is also presented.

The full lepton flavor of the littlest Higgs model with T-parity

We re-examine lepton flavor violation (LFV) in the Littlest Higgs model with T-parity (LHT) including the full T-odd (non-singlet) lepton and Goldstone sectors. The heavy leptons induce two independent sources of LFV associated with the couplings necessary to give masses to the T-odd mirror fermions and to their partners in right-handed SO(5) multiplets, respectively. The latter, which have been neglected in the past, can be decoupled from gauge mediated processes but not from Higgs mediated ones and must therefore also be included in a general analysis of LFV in the LHT. We also further extend previous analyses by considering on-shell Z and Higgs LFV decays together with the LFV processes at low momentum transfer. We show that current experimental limits can probe the LHT parameter space up to global symmetry breaking scales f ∼ 10 TeV. For lower f values ≳ 1 TeV, μ − e transitions require the misalignment between the heavy and the Standard Model charged leptons to be ≲ 1%. Future LFV experiments using intense muon beams should be sensitive to misalignments below the per mille level. For τ LFV transitions, which could potentially be observed at Belle II and the LHC as well as future lepton colliders, we find that generically they can not discriminate between the LHT and supersymmetric models though in some regions of parameter space this may be possible.

Application of passive wedge absorbers for improving the performance of precision-science experiments

A scheme is discussed for momentum selection and momentum-spread reduction for muon-based experiments. The concept relies on placing a wedge absorber at a point along a beam transport system with nonzero dispersion. The technique has direct relevance to precision-science experiments such as the Fermilab Muon g-2 Experiment as it can enhance the muon beam intensity and therefore minimize the statistical uncertainty of the anomalous magnetic moment measurement. This paper presents a theoretical and numerical study about the orientation, material, geometrical parameters, and performance of this wedge. Results suggest a considerable increase in muon intensity for the Muon g-2 Experiment, when the optimal wedge is introduced along the beam path.

Experimental searches for muon decays beyond the Standard Model

The study of muon properties and decays played a crucial role in the early years of particle physics and contributed over decades to build and consolidate the Standard Model. At present, searches for muon decays beyond the Standard Model are performed by exploiting intense beams of muons, and plans exist to upgrade the present facilities or build new ones, which would open new prospects for the quest of new physics in this sector. In this paper I review the present status of the search for muon decays beyond the Standard Model, with a special attention to the most conventional muon lepton flavor violation experiments, but also considering more exotic scenarios and future outlooks.

The Mu3e experiment at PSI

Mu3e will search for charged Lepton Flavor Violation in the neutrinoless muon decay \mu^+ \rightarrow e^+ e^- e^+μ+→e+e−e+ with a sensitivity down to 10^{-16}10−16 (90% C.L.) using the world most intense continuous muon beam at PSI. This search requires a large acceptance detector capable of coping with rates of up to 2 \times 10^92×109 stopped muons per second with excellent momentum, spatial, and time resolution. The Mu3e detector is based on thin monolithic active silicon pixel sensors for tracking in conjunction with scintillating fibers and tiles for timing measurements. The Mu3e apparatus is under constructions and first data is expected in 2020.