Ⅲ-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 49 (2016)

Development of RF cavity for MuSEUM experiment in a zeromagnetic field
K. S. Tanaka∗1,∗2 on behalf of J-PARC MuSEUM Collaboration∆ex
Mu = 4.463302765(53) GHz (12 ppb) .

(1)

The total precision was limited by the statistical uncertainty. We will achieve a precision more than 10 times
better than that of the latest experiment by using thehigh intensity muon beam provided at J-PARC/MLF.
We plan to measure muonium HFS in a zero magneticﬁeld at D Line3) and in a high magnetic ﬁeld at H Line.
The measurement procedure of the experiment is asfollows:

In this paper, the development of the RF cavity forthe zero ﬁeld experiment is described.
Figure 1 shows the cavity for zero ﬁeld experiment.
The inner diameter of the cylindrical cavity for thezero ﬁeld experiment is tuned so that the resonancefrequency of the TM110 mode matches the transition
frequency of muonium HFS of 4.463 GHz. Frequency
sweep is enabled in the range of 20 MHz by the insertion system of a tuning bar made of conductive material.
A microwave is delivered by a coaxial cable and coupled into the cavity by the input loop. The microwave
power in the cavity is picked up by a loop coil and measured by the thermal power sensor (R&S NRP-Z51).
The measured power is feedback to the signal genera∗1∗2

RIKEN Nishina Center
Graduate School of Arts and Sciences, University of Tokyo- 226 -Fig. 1. Photo of the cavity for zero field experiment. The
inner radius is 41 mm and the axial length is 230 mm.
muon
displacement (mm)-20
0 20tuning bar

RF antenna82 mm230 mm• Muons with their spins polarized to the upstream
direction are provided from J-PARC/MLF muon
beam line.
• RF cavity located in the center of the magnet containing pure Krypton gas. Muons stop by collisions in the gas and polarized muonium are formedby electron capture process with Krypton.
• High momentum decay positrons are emitted preferentially in the direction of the muon spin. By
driving the transitions with an applied microwave
magnetic ﬁeld, the muon spin can be ﬂipped
and the angular distribution of high momentum positrons is changed from predominantly upstream to downstream with respect to the beamdirection.

tor to stabilize the input microwave power at a level of0.02 % which brings no systematic uncertainty.
Figure 2 shows the dependence of resonance frequency on the displacement of the tuning bar. The
red line shows the transition frequency of the hyperﬁne splitting and the red band shows the sweep rangeof the measurement. As shown in this ﬁgure, tunabilityof the cavity is enough for the measurement.
Thus, the RF system for the zero ﬁeld experiment isready. We will start the ﬁrst measurement of muonium
HFS in a zero ﬁeld in 2016.

frequency (GHz)The MuSEUM (Muonium Spectroscopy Experiment
Using Microwave) group is planning to measure theenergy of the ground state hyperﬁne structure (HFS) ofmuonium at J-PARC/MLF1) . Muonium is a hydrogenlike bound state that consists only of leptons, and its
HFS is a good probe for testing the QED theory. The
latest experiment at LAMPF obtained the following
value:2)

4.485
4.48
4.475
4.47
4.465

sweep range
HFS value

4.46
4.455
- 30-25- 20- 15- 10-50510 15
displacement (mm)Fig. 2. The left figure shows the cross-sectional view ofthe cavity. The tuning bar is inserted from the sidesurface to tune the resonance frequency of the cavity.
Right figure shows the relation between the resonancefrequency and the displacement of the tuning bar. The
red line shows a transition frequency of HFS and the
red band is a sweep range to observe the resonance lineshape.

References
1) K. S. Tanaka on behalf of MuSEUM Collaboration.:
RIKEN Accel. Proog. Rep. 47, 277 (2014).
2) W. Liu et al., Phys. Rev. Lett. 82, 711 (1999).
3) S. Kanda on behalf of MuSEUM Collaboration.: in thisreport.