arXiv subjects

R. Iwai

Publications and source records attributed to R. Iwai.

9 recordsLinked to original sources

arXiv2024

Phase space compression of a positive muon beam in two spatial dimensions

We present the first demonstration of simultaneous phase space compression in two spatial dimensions of a positive muon beam, the first stage of the novel high-brightness muon beam under development by the muCool collaboration at the Paul Scherrer Institute. The keV-energy, sub-mm size beam would enable a factor 10$^5$ improvement in brightness for precision muSR, and atomic and particle physics measurements with positive muons. This compression is achieved within a cryogenic helium gas target with a strong density gradient, placed in a homogeneous magnetic field, under the influence of a complex electric field. In the next phase, the muon beam will be extracted into vacuum.

physics.acc-ph↗

arXiv2023

Improved Measurements of Muonic Helium Ground-State Hyperfine Structure at a Near-Zero Magnetic Field

Muonic helium atom hyperfine structure (HFS) measurements are a sensitive tool to test the three-body atomic system and bound-state quantum electrodynamics theory, and determine fundamental constants of the negative muon magnetic moment and mass. The world's most intense pulsed negative muon beam at the Muon Science Facility of the Japan Proton Accelerator Research Complex allows improvement of previous measurements and testing further $CPT$ invariance by comparing the magnetic moments and masses of positive and negative muons (second-generation leptons). We report new ground-state HFS measurements of muonic helium-4 atoms at a near-zero magnetic field, performed for the first time using a small admixture of CH$_{4}$ as an electron donor to form neutral muonic helium atoms efficiently. Our analysis gives $Δν$ = 4464.980(20) MHz (4.5 ppm), which is more precise than both previous measurements at weak and high fields. The muonium ground-state HFS was also measured under the same conditions to investigate the isotopic effect on the frequency shift due to the gas density dependence in He with CH$_{4}$ admixture and compared with previous studies. Muonium and muonic helium can be regarded as light and heavy hydrogen isotopes with an isotopic mass ratio of 36. No isotopic effect was observed within the current experimental precision.

physics.atom-ph↗

arXiv2022

Scintillation detectors with silicon photomultiplier readout in a dilution refrigerator at temperatures down to 0.2 K

We are developing a novel high-brightness atomic beam, comprised of a two-body exotic atom called muonium (M $ = μ^+ + e^-$), for next-generation atomic physics and gravitational interaction measurements. This M source originates from a thin sheet of superfluid helium (SFHe), hence diagnostics and later measurements require a detection system which is operational in a dilution cryostat at temperatures below 1 K. In this paper, we describe the operation and characterization of silicon photomultipliers (SiPMs) at ultra-low temperatures in SFHe targets. We show the temperature dependence of the signal shape, breakdown voltage, and single photon detection efficiency, concluding that single photon detection with SiPMs below 0.85 K is feasible. Furthermore, we show the development of segmented scintillation detectors, where 16 channels at 1.7 K and one channel at 170 mK were commissioned using a muon beam.

physics.ins-det↗

arXiv2021

Science Case for the new High-Intensity Muon Beams HIMB at PSI

In April 2021, scientists active in muon physics met to discuss and work out the physics case for the new High-Intensity Muon Beams (HIMB) project at PSI that could deliver of order $10^{10}$\,s$^{-1}$ surface muons to experiments. Ideas and concrete proposals were further substantiated over the following months and assembled in the present document. The high intensities will allow for completely new experiments with considerable discovery potential and unique sensitivities. The physics case is outstanding and extremely rich, ranging from fundamental particle physics via chemistry to condensed matter research and applications in energy research and elemental analysis. In all these fields, HIMB will ensure that the facilities S$μ$S and CHRISP on PSI's High Intensity Proton Accelerator complex HIPA remain world-leading, despite the competition of muon facilities elsewhere.

hep-ex↗

arXiv2020

Search for lepton flavour violating muon decay mediated by a new light particle in the MEG experiment

We present the first direct search for lepton flavour violating muon decay mediated by a new light particle X, $μ^+ \to \mathrm{e}^+\mathrm{X}, \mathrm{X} \to γγ$. This search uses a dataset resulting from $7.5\times 10^{14}$ stopped muons collected by the MEG experiment at the Paul Scherrer Institut in the period 2009--2013. No significant excess is found in the mass region 20--45 MeV/c$^2$ for lifetimes below 40 ps, and we set the most stringent branching ratio upper limits in the mass region of 20--40 MeV/c$^2$, down to $\mathcal{O}(10^{-11})$ at 90\% confidence level.

hep-ex↗

arXiv2020

Demonstration of Muon-Beam Transverse Phase-Space Compression

We demonstrate efficient transverse compression of a 12.5 MeV/c muon beam stopped in a helium gas target featuring a vertical density gradient and crossed electric and magnetic fields. The muon stop distribution extending vertically over 14 mm was reduced to a 0.25 mm size (RMS) within 3.5 $μ$s. The simulation including cross sections for low-energy $μ^+$-$\text{He}$ elastic and charge exchange ($μ^+\leftrightarrow $ muonium) collisions describes the measurements well. By combining the transverse compression stage with a previously demonstrated longitudinal compression stage, we can improve the phase space density of a $μ^+ $ beam by a factor of $ 10^{10} $ with $ 10^{-3} $ efficiency.

physics.acc-ph↗

arXiv2019

muCool: A novel low-energy muon beam for future precision experiments

Experiments with muons ($μ^{+}$) and muonium atoms ($μ^{+}e^{-}$) offer several promising possibilities for testing fundamental symmetries. Examples of such experiments include search for muon electric dipole moment, measurement of muon $g-2$ and experiments with muonium from laser spectroscopy to gravity experiments. These experiments require high quality muon beams with small transverse size and high intensity at low energy. At the Paul Scherrer Institute, Switzerland, we are developing a novel device that reduces the phase space of a standard $μ^{+}$ beam by a factor of $10^{10}$ with $10^{-3}$ efficiency. The phase space compression is achieved by stopping a standard $μ^{+}$ beam in a cryogenic helium gas. The stopped $μ^{+}$ are manipulated into a small spot with complex electric and magnetic fields in combination with gas density gradients. From here, the muons are extracted into the vacuum and into a field-free region. Various aspects of this compression scheme have been demonstrated. In this article the current status will be reported.

physics.acc-ph↗

arXiv2018

muCool: A next step towards efficient muon beam compression

A novel device to compress the phase space of a muon beam by a factor of $10^{10}$ with a $10^{-3}$ efficiency is under development. A surface muon beam is stopped in a helium gas target consisting of several compression stages, wherein strong electric and magnetic fields are applied. The spatial extent of the stopped muon swarm is decreased by means of these fields until muons with eV energy are extracted into vacuum through a small orifice. It was measured that a 20 cm long muon stop distribution can be compressed in longitudinal direction to sub-mm extent within 2 ${\rm μs}$. Additionally, a drift perpendicular to the magnetic field of the compressed low-energy muon swarm was successfully demonstrated, paving the way towards the extraction from the gas and re-acceleration of the muons.

physics.acc-ph↗

arXiv2018

The design of the MEG II experiment

The MEG experiment, designed to search for the mu+->e+ gamma decay at a 10^-13 sensitivity level, completed data taking in 2013. In order to increase the sensitivity reach of the experiment by an order of magnitude to the level of 6 x 10-14 for the branching ratio, a total upgrade, involving substantial changes to the experiment, has been undertaken, known as MEG II. We present both the motivation for the upgrade and a detailed overview of the design of the experiment and of the expected detector performance.

physics.ins-det↗