Abstract
A new muon beamline, muon science innovative channel (MuSIC), was set up at the Research Centre for Nuclear Physics (RCNP), Osaka University, in Osaka, Japan, using the 392 MeV proton beam impinging on a target. The production of an intense muon beam relies on the efficient capture of pions, which subsequently decay to muons, using a novel superconducting solenoid magnet system. After the pion-capture solenoid the first $36^\circ$ of the curved muon transport line was commissioned and the muon flux was measured. In order to detect muons, a target of either copper or magnesium was placed to stop muons at the end of the muon beamline. Two stations of plastic scintillators located upstream and downstream from the muon target were used to reconstruct the decay spectrum of muons. In a complementary method to detect negatively-charged muons, the X-ray spectrum yielded by muonic atoms in the target were measured in a germanium detector. Measurements, at a proton beam current of 6 pA, yielded $(10.4 \pm 2.7) \times 10^5$ muons per Watt of proton beam power ($\mu^+$ and $\mu^-$), far in excess of other facilities. At full beam power (400 W), this implies a rate of muons of $(4.2 \pm 1.1) \times 10^8$ muons s$^{-1}$, amongst the highest in the world. The number of $\mu^-$ measured was about a factor of 10 lower, again by far the most efficient muon beam produced. The set up is a prototype for future experiments requiring a high-intensity muon beam, such as a muon collider or neutrino factory, or the search for rare muon decays which would be a signature for phenomena beyond the Standard Model of particle physics. Such a muon beam can also be used in other branches of physics, nuclear and condensed matter, as well as other areas of scientific research.
Highlights
High-intensity muon beams have applications in many areas of science, spanning high-energy particle physics to condensed matter physics and even areas of chemistry and biology
A new muon beam line, the muon science innovative channel, was set up at the Research Center for Nuclear Physics, Osaka University, in Osaka, Japan, using the 392 MeV proton beam impinging on a target
The production of an intense muon beam relies on the efficient capture of pions, which subsequently decay to muons, using a novel superconducting solenoid magnet system
Summary
High-intensity muon beams have applications in many areas of science, spanning high-energy particle physics to condensed matter physics and even areas of chemistry and biology. Intense muon beams are needed for the following experiments and areas of investigation. Rare muon decays such as charged lepton flavor violation (CLFV) have attracted much attention theoretically and experimentally [1,2]. Planned experiments of COMET [4] in Japan and Mu2e [5] in the United States, which will search for μ − e conversion with anticipated improvement of physics sensitivity of 104, need high-intensity muon beams of 1018 muons per year. The challenges of producing high-intensity muon beams are the need for a high-power proton beam; the efficient capture of pions produced at the target; and, given the large number of particles produced with a wide range of kinematic properties, effective methods to achieve a pure muon beam. The article reports on the commissioning of the beam line and a measure of the intensity of muons produced; the facility can be used for much of the variety of science discussed above
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