First trial of the muon acceleration for J-PARC muon g-2/EDM experiment

Muon acceleration is an important technique in exploring the new frontier of physics. A new measurement of the muon dipole moments is planned in J-PARC using the muon linear accelerator. The low-energy (LE) muon source using the thin metal foil target and beam diagnostic system were developed for the world’s first muon acceleration. Negative muonium ions from the thin metal foil target as the LE muon source was successfully observed. Also the beam profile of the LE positive muon was measured by the LE-dedicated beam profile monitor. The muon acceleration test using a Radio-Frequency Quadrupole linac (RFQ) is being prepared as the first step of the muon accelerator development. In this paper, the latest status of the first muon acceleration test is described.

1. Introduction E821 experiment of Brookhaven National Laboratory measured the muon (g − 2) µ with the precision of 0.54 ppm and reported the discrepancy of more than 3 σ for the muon (g − 2) µ [1] from the theoretical prediction. More precise measurement is required in order to search the physics beyond the standard model. The J-PARC E34 experiment which aims to precisely measure the muon anomalous magnetic moment (g − 2) µ and electric dipole moment (EDM) is planned with brand new techniques [2]. The goals of the precision are 0.1 ppm for the muon (g − 2) µ and 1 × 10 −21 e·cm for the muon EDM. The muon (g − 2) µ and EDM will be measured at the same time under the no electric focusing in the muon storage ring, using the ultra-cold muons (UCMs) which are produced by the new muon cooling method and the muon linear accelerator (Muon linac). The electric focusing isn't necessary for UCMs, because UCMs have a very small transverse momentum compared with the surface muon beam. The surface muons having a large emittance are stopped the muoniums (Mu: µ + e − ) production target and become the thermal Mu. Mu are ionized by the muon ionization laser after Mu are evaporated from the surface of the production target to the vacuum. The ultra-slow muons (USMs) with the room

Low-energy muon source
The LE muon source with energy less than 5.6 keV is required in order to test the muon accelerator, since the design input energy of the initial RF accelerator (RFQ) is 5.6 keV. Negative muoniums (Mu − : µ + e − e − ) that are generated by the muons passing through a thin metal foil is a good candidate of the epi-thermal muon source for the muon acceleration, because the energy dispersion of the Mu − is smaller than one of the decelerated positive muons from the metal foil [3]. Using the LE muon source like Mu − , the muon accelerator can be tested even before the ionization laser is ready.
The beam parameters of the Mu − should be measured in order to use the Mu − as the LE muon source for the muon acceleration.  The beam profile should be measured in order to measure the beam emittance. The beam emittance can be measured by the Q scan method [4]. In the Q scan method, a quadrupole is set in beam profile of the LE positive muons was measured first as shown in Figure 4.. In conclusion, the beam diagnostic system including the beam line and LE BPM was demonstrated for the muon acceleration test.

First muon acceleration
The first trial of the muon acceleration using the muon source and the LE beam diagnostic system is planed at the muon test beam line (D line) in the J-PARC Material and Life science Facility (MLF). The prototype RFQ ( Figure 5.) of the J-PARC linac will be used for the muon acceleration test [5]. The input energy of the prototype RFQ is 5.6 keV. The structure of the prototype RFQ is similar to the first half of the RFQ which will be used for the muon linac. The surface muons are decelerated by the thin metal foil and reaccelerated to 5.6 keV by the SOA lens. Muons with 5.6 keV energy are accelerated to about 100 keV by the prototype RFQ. The diagnostic beam line after the prototype RFQ consists of the two quadrupole magnets, a bending magnet and LE BPM. Figure 6. shows the latest drawing for the muon acceleration test at the D line. Figure 7. shows the simulation result by PARMTEQM assuming the developed LE muon source at the D line [6]. Detailed particle tracking simulations based on the actual setting are being developed. The setup will be constructed in this summer. The first trial of the muon acceleration is planned  in this autumn.

Summary
The world's first muon acceleration will open the door of new frontier of accelerator physics and particle physics. The LE muon source to be used for the commissioning of the muon accelerator was developed. The diagnostic system of the LE muons was also prepared and the measurement of the LE positive muon beam profile was successfully done. The first trial of the muon acceleration with the prototype RFQ will be demonstrated in this year.

Acknowledgement
We would like to deeply thank to RIKEN Advanced Meson Science Laboratory and J-PARC linac group for their significant supports .