RIKEN Accel. Prog. Rep. 50 (2017)

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

Background study with negative muons in RIKEN-RAL for the laser
spectroscopy of hyperﬁne splitting energy in muonic hydrogen
M. Sato,∗1 S. Aikawa,∗2,∗3 K. Ishida,∗2 M. Iwasaki,∗2,∗3 S. Kanda,∗4 Y. Ma,∗2 Y. Matsuda,∗5 K. Midorikawa,∗6Y. Oishi,∗7 S. Okada,∗8 N. Saito,∗6 A. Takamine,∗2 K. Tanaka,∗9 H. Ueno,∗2 S. Wada,∗6 and M. Yumoto∗6∗1∗2∗3∗4∗5∗6∗7∗8∗9Condensed Molecular Materials Laboratory, RIKEN
RIKEN Nishina Center
Department of Physics, Tokyo Institute of Technology
Department of Physics, The University of Tokyo
Graduate School of Arts and Science, The University ofTokyo
RIKEN Center for Advanced Photonics
Institute of Materials Structure Science, KEK
AMO Physics Laboratory, RIKEN
Cyclotron and Radioisotope Center, Tohoku University- 244 -For the study of the background level, the timing
spectrum after muon stoppage was measured in theRIKEN-RAL PORT-4 beam line. In the beam time inMarch 2016, we acquired data using an existing μSR
spectrometer in PORT-4 (CHRONUS), which consistsof segmented plastic scintillation counters. Each segment was viewed using a photomultiplier tube withwavelength shifting fiber.
ﬁber4)4). The timing spectrum obtained by CHRONUS is shown by the red line in Fig.
1. The inset shows a close-up of the prompt timing
which shows peaks caused by electrons and muons ina double pulse. In the spectrum, there is an unknown
long-tail component which is the severe background of
the measurement. To suppress this background, weprepared another type of counters in the successive
beam time in May 2016. It has two-layered plastic scintillation counters with photomultiplier tubes on bothsides. We acquired the timing spectrum with the coincidence of the two counters. As shown by the blueline in Fig. 1, the long-tail component was drasticallysuppressed. The reduction factor is greater than 10−4 ,which is suﬃcient for the measurement. The origin of
the component is still not understood, but it may becaused by neutral particles coming along with a negative muon pulse. Further investigation will be conducted in RIKEN-RAL, and the design of the electron
counter will be ﬁnalized based on these studies.
105

μ- μ-105
Counts per 16 ns104

104
103

CountsWe are planning a new measurement of the hyperﬁnesplitting energy in ground-state muonic hydrogen in
the RIKEN-RAL muon facility. This project was motivated by the proton radius puzzle, which is an unsettled
problem on the root-mean-square charge radius of protons. Conventionally, the charge radius has been determined by electron-proton scattering and atomic hydrogen spectroscopy as compiled in CODATA.
CODATA1)1). However,the charge radius determined from the Lamb shift inmuonic hydrogen has a value smaller by 4 %, which cor2,3)
responds to 7 times the standard deviationdeviation.2,3). Thusfar, there has been no deﬁnitive explanation for thispuzzling discrepancy.
In our proposed experiment, we will determine theproton Zemach radius, which is a convolution of theproton charge and magnetic moment distributions.
Since the Zemach radius can be determined from thehyperﬁne splitting energy in hydrogen-like atoms, wederive it from muonic hydrogen which is a Coulomb
bound system consisting of a negative muon and a proton. Then, we can obtain information on the magneticstructure inside the proton by probing with muons.
For laser spectroscopy, muon spin is re-polarized bya circularly polarized laser in the laser-induced transition between hyperﬁne sub-levels. The polarizedmuons have a spatial asymmetry in the electron emission from the muon decay (μ− → e− + νe + νμ ). The
resonance can be identiﬁed by detecting the spatial
distribution of electrons.
In the experiment, pulsed negative-muons arestopped in a hydrogen target to form muonic hydrogen.
The pulsed laser used to induce the transition betweenhyperﬁne sublevels is irradiated with a delay of 1 μs
from muon stoppage to suppress background events
caused by muon stoppage due to materials in otherthan the hydrogen target. Since the negative muon
capture rate is proportional to Z 4 , muons stopped inhigh-Z materials around the target are quickly captured. In this manner, we aim to achieve a reduction factor greater than 10−4 at 1 μs compared to theprompt timing events at the time of muon stoppage.

e- e-103
102
101-2

CHRONUS

102
10-101234Time [μs]coincidence of 2 counters105101520Time [μs]Fig. 1. Time spectra with negative muons obtained byCHRONUS (red) and coincidence counters (blue).

References
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84, 1527 (2012).
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3) A. Antognini et al., Science 339, 417 (2013).
4) D. Tomono et al., J. Phys. Conf. Ser. 225, 012056(2010).