High-intensity Pulsed Muon Beam Research Articles

Precise measurement of the hyperfine splitting in muonium with a high intensity pulsed muon beam at J-PARC

At J-PARC, the MuSEUM (Muonium Spectroscopy Experiment Using Microwave) collaboration aims to precisely measure the ground-state hyperfine splitting of muonium atoms arising from the muon and electron spins. The pulsed muon beam is stopped in a krypton gas cell to form muonium atoms. The transitions of spin states are induced with a microwave cavity, which are then measured by positron counters. After the previously performed successful measurements with a nearly-zero magnetic field, we are currently planning a measurement with the 2.9T magnetic field by measuring two Zeeman-split sub-levels, so that increased statistics will allow us to more precisely determine the transition frequency down to ∼1ppb. Moreover, a new microwave cavity with a unique geometry is being designed to perform the measurement at an even stronger field of 2.9T in the future.

The Mu2e Crystal Calorimeter: An Overview

The Mu2e experiment at Fermilab will search for the standard model-forbidden, charged lepton flavour-violating conversion of a negative muon into an electron in the field of an aluminium nucleus. The distinctive signal signature is represented by a mono-energetic electron with an energy near the muon’s rest mass. The experiment aims to improve the current single-event sensitivity by four orders of magnitude by means of a high-intensity pulsed muon beam and a high-precision tracking system. The electromagnetic calorimeter complements the tracker by providing high rejection power in muon to electron identification and a seed for track reconstruction while working in vacuum in presence of a 1 T axial magnetic field and in a harsh radiation environment. For 100 MeV electrons, the calorimeter should achieve: (a) a time resolution better than 0.5 ns, (b) an energy resolution <10%, and (c) a position resolution of 1 cm. The calorimeter design consists of two disks, each loaded with 674 undoped CsI crystals read out by two large-area arrays of UV-extended SiPMs and custom analogue and digital electronics. We describe here the status of construction for all calorimeter components and the performance measurements conducted on the large-sized prototype with electron beams and minimum ionizing particles at a cosmic ray test stand. A discussion of the calorimeter’s engineering aspects and the on-going assembly is also reported.

A search for atomic parity violation in muonic atoms using a high-intensity pulsed muon beam at J-PARC

An exotic atom consisting of a negative muon and a nucleus is a muonic atom. Atomic parity violation in muonic atoms provides a unique opportunity to determine the Weinberg angle and search for physics beyond the standard model at the low energy scale. We have proposed a new experiment to perform X-ray spectroscopy of muonic atoms using a high-intensity pulsed muon beam at J-PARC. A scintillator-based calorimeter and a lowdensity gaseous target were developed for the experiment. As a feasibility study, we measured the yield of muon atoms and searched for muonic atom’s metastability using low-density methane gas. An overview of the experiment and the test experiment results are presented.

New precise spectroscopy of the hyperfine structure in muonium with a high-intensity pulsed muon beam

A hydrogen-like atom consisting of a positive muon and an electron is known as muonium. It is a near-ideal two-body system for a precision test of bound-state theory and fundamental symmetries. The MuSEUM collaboration performed a new precision measurement of the muonium ground-state hyperfine structure at J-PARC using a high-intensity pulsed muon beam and a high-rate capable positron counter. The resonance of hyperfine transition was successfully observed at a near-zero magnetic field, and the muonium hyperfine structure interval of νHFS=4.463302(4)GHz was obtained with a relative precision of 0.9 ppm. The result was consistent with the previous ones obtained at Los Alamos National Laboratory and the current theoretical calculation. We present a demonstration of the microwave spectroscopy of muonium for future experiments to achieve the highest precision.

Performance evaluation of a silicon strip detector for positrons/electrons from a pulsed a muon beam

A high-intensity pulsed muon beam is becoming available at the at the Japan Proton Accelerator Research Complex (J-PARC). Many experiments to study fundamental physics using this high-intensity muon beam are proposed. An experiment to measure the muon magnetic moment anomaly (g−2) and the muon electric dipole moment (EDM) is one of these experiments and it requires a tracking detector for positrons from muon decay. Fine segmentation is required in a detector to tolerate the high rate of positrons. The time resolution is required to be much better than the muon anomalous spin precession period while a buffer depth of a front-end electronics needs to be much longer than the accelerated muon lifetime. Requirements of this detector also meet requirements of a measurement of the muonium hyperfine structure interval at the J-PARC and another experiment to measure the proton charge radius at Tohoku University. We have developed a single-sided silicon strip sensor with a 190 μm pitch, a front-end electronics with a sampling rate of 200 MHz and a buffer memory depth of 8192, and a data acquisition system based on DAQ-Middleware for the J-PARC muon g−2/EDM experiment. We have fabricated detector modules consisting of this sensor and the front-end electronics. Performance of fabricated detector modules was evaluated at a laboratory and a beam test using the positron beam at Tohoku University. The detector is confirmed to satisfy all requirements of the experiments except for the time walk, which will be solved by the next version of a front-end electronics.

Laboratory tests for MIR light detection and transport with specialty optical fibres

The FAMU (Fisica degli Atomi Muonici) experiment at the RIKEN RAL pulsed muon facility will measure the proton Zemach radius with high precision, thus contributing to the solution of the so-called “proton radius puzzle”. The situation is now confused and new measurements will help reduce the discrepancy as measured with electrons or muons. To this aim, FAMU will make use of a high-intensity pulsed muon beam at RIKEN-RAL impinging on a cryogenic hydrogen target and a tunable Mid-infrared (MIR) laser emitting at about 6.78 μm, to measure the hyperfine (HFS) splitting of the 1S state of muonic hydrogen. The injection of light into the cryogenic target and its monitoring via dedicated MIR sensors is an asset for the experiment. A possible solution is via specialty MIR fibres based on hollow core waveguides or polycrystalline fibres. Two dedicated setups based on pulsed QCL lasers, from Alpes Laser, emitting around 6.78 μm have been used to study the fibre attenuation while the high energy laser specifically developed for the FAMU project has been used to study the damage threshold at high energies. The alignment of MIR laser radiation into an optical cavity presents criticalities due to geometrical constraints. Its control requires the use of small and sensitive detectors. The results obtained with a quantum IR sensor with an integrated circuit for signal processing are reported.

Innovative amplifiers for muon spectroscopy experiments at RAL

The high intensity pulsed muon beam of ISIS facility at Rutherford Appleton Laboratory (RAL, U.K.) allows to perform high precision muonic X-ray spectroscopy experiments. In particular, FAMU and CHNET_TANDEM projects at RAL are dedicated, respectively, to a precise measurement of the Zemach radius of the proton through the measure of the hyperfine splitting of the ground state of the muonic hydrogen atom and to the optimization of analytical methods based on the muonic atom spectroscopy for non-destructive elementary characterization of archaeological samples. Custom electronic boards dedicated to the detector amplification stages have been designed and prototypized to provide fast high resolution for both experiments. The main features and performance of these custom boards are described in details in this work.

The proton radius puzzle

The FAMU (Fisica degli Atomi Muonici) experiment has the goal to measure precisely the proton Zemach radius, thus contributing to the solution of the so-called proton radius “puzzle”. To this aim, it makes use of a high-intensity pulsed muon beam at RIKEN-RAL impinging on a cryogenic hydrogen target with an high-Z gas admixture and a tunable mid-IR high power laser, to measure the hyperfine (HFS) splitting of the 1S state of the muonic hydrogen. From the value of the exciting laser frequency, the energy of the HFS transition may be derived with high precision (~ 10-5) and thus, via QED calculations, the Zemach radius of the proton. The experimental apparatus includes a precise fiber-SiPMT beam hodoscope and a crown of eight LaBr3 crystals and a few HPGe detectors for detection of the emitted characteristic X-rays. Preliminary runs to optimize the gas target filling and its operating conditions have been taken in 2014 and 2015-2016. The final run, with the pump laser to drive the HFS transition, is expected in 2018.

Superconducting curved transport solenoid with dipole coils for charge selection of the muon beam

At the J-PARC Muon Science Facility (MUSE) the Super-Omega muon beamline is now under construction in the experimental hall No. 2 of the Materials and Life Science Facility building. Muons up to 45MeV/c will be extracted with a large acceptance solid angle to produce the world highest intensity pulsed muon beam. This beamline comprises three parts, a normal-conducting capture solenoid, a superconducting curved transport solenoid and an axial focusing solenoid. Since only solenoids are used, both surface μ+ and cloud μ− are extracted simultaneously. To accommodate future experiments that would only require either μ+ or μ− beam, two dipole coils located on the straight section of the curved solenoid provide the muon charge selection by directing one of the beam onto the solenoid inner-wall. The design parameters, the construction status and the initial beam commissioning are reported.

Status of the Superomega Muon Beam Line at J-PARC

Abstract Superomega muon beamline is currently under construction at Experimental hall No. 2 of Materials and Life Science Facility at J-PARC in Tokai, Japan. The beamline has a large solid angle acceptance, and will produce the highest intensity pulsed muon beam in the world. The beamline is designed to capture both surface positive and cloud negative muons for simultaneous use in a variety of experiments. The expected rate of surface muons for this beamline is 4×108 μ+/s, and that for cloud muons is 107 μ−/s. The beamline consists of the normal-conducting capture solenoid, the superconducting curved transport solenoid and axial focusing solenoid. The construction of the capture solenoid has been completed and installed in March 2009, and the transport solenoid is now fabricated, and will be installed by the end of 2011. The conceptual design of the axial focusing solenoid is completed, and the mechanical design is underway.

The super omega muon beamline at J-PARC

The Materials and Life Science Facility (MLF) is currently under construction at J-PARC in Tokai, Japan. The facility houses both muon (graphite) and neutron (mercury) production targets, and is located at the end of the 3 GeV proton beamline. The Super Omega muon beamline (one of four secondary muon beamlines) extends at 45° into backward angles to transport the surface μ + and cloud μ − to experimental hall 2. Once completed, it will produce the highest intensity pulsed muon beam in the world, with 4×10 8 μ +/s and 10 7 μ −/s, which will be used for a variety of purposes such as experiments in muon-catalyzed fusion and the production of ultra-slow muons. The beamline consists of normal-conducting capture solenoids, superconducting transport solenoids, and a Dai Omega-type axial focusing magnet. Currently, the construction of the capture solenoids is nearing completion. The design of the transport solenoids is under way, and tests solenoids will be fabricated and tested during the summer of 2008. The axial focusing magnet, used to focus the muon beam on the experimental target, is under consideration with particular emphasis on its compatibility with the transport solenoids.

Development of a multistop TDC using a shift register for muon decay measurements in a pulsed beam

New multistop CAMAC TDC modules using shift registers have been developed and were successfully used to measure the time spectrum of electrons from μ → eνν decay in a high-intensity pulsed muon beam available at the UT-MSL/BOOM facility at KEK in Japan and after that also at Rutherford Appleton Laboratory. The time resolution of the TDC is as good as 16 ns. Among many features of the TDC, the combination of the shift register with fast memories is notable. By adopting this, the time range of the TDC can be extendable beyond the shift register depth without losing time resolution. The other feature is capability of making histograms in its own huge memories. The principle of operation and the performance of the TDC are described.

A new analog detection system with target-imaging capability for μSR measurements in a pulsed muon beam

We have developed a new method to measure μSR time spectra macroscopically in a high-intensity pulsed muon beam. Each macroscopic pulse from the Cherenkov radiation induced by decay positrons from stopped muons in a burst is measured by using a fast transient digitizer. This rate-unlimited method is effective in avoiding the distortion of the initial part of time spectra. The present counter system using Freon-12 at normal pressure is characterized by its target imaging capability and suitable threshold for muon-decay electrons ( E e > 10 MeV). We describe its principle, performance and data analyses.

Macroscopic Cherenkov imaging method for μSR measurements in a pulsed muon beam

We have developed a gas Cherenkov counter to measure μSR time spectra macroscopically in a high-intensity pulsed muon beam. We measured each macroscopic pulse from the Cherenkov radiation induced by decay positrons from muons using a fast transient digitizer. This counter using Freon-12 at normal pressure is characterized by its target imaging capability and suitable threshold for muon-decay electron (Ee>10MeV). We describe its principle and performance.

Macroscopic measurement of μSR time spectrum in a high-intensity pulsed muon beam

We have developed a macroscopic method of measuring a μSR time spectrum in a high-intensity pulsed muon beam. A single-shot pulse from a Cherenkov counter is analyzed by a fast transient digitizer. The principle and the performance of this rate-unlimited new method are described.