RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-9. Instrumentation

Development of co-existing 129 Xe and 131 Xe nuclear spin masers
with active feedback scheme for the Xe atomic EDM search
T. Sato,∗1,∗2 Y. Ichikawa,∗1,∗2 T. Inoue,∗3,∗4,∗1 A. Uchiyama,∗4,∗1 A. Gladkov,∗1,∗5 A. Takamine,∗1 S. Kojima,∗2C. Funayama,∗2 S. Tanaka,∗2 Y. Sakamoto,∗2 Y. Ohtomo,∗2 C. Hirao,∗2 M. Chikamori,∗2 E. Hikota,∗2T. Furukawa,∗6 A. Yoshimi,∗7,∗1 C.P. Bidinosti,∗8 T. Ino,∗9 H. Ueno,∗1 Y. Matsuo,∗10,∗1 T. Fukuyama,∗11N. Yoshinaga,∗12 Y. Sakemi,∗13,∗4 and K. Asahi∗1,∗2
Astronomical observations reveal that the matter tophoton ratio in the Universe is many orders of magnitude larger than the prediction based on broadly accepted cosmological scenarios. The extra CP-violation,
which may come from physics beyond the Standard
Model (SM), will contribute to understanding of theorigin of matter. The electric dipole moment (EDM)
of a particle is one of the cleanest probes for newphysics, because the SM contribution to the EDM isundetectably small while most of proposed candidates
for new physics predict detectable sizes of EDM. There
are only three CP-violating eﬀective operators at thehadron level that can enter EDMs of diamagneticatoms.1) Therefore, a combination of measurements forat least three diﬀerent atoms is essential to distinguish
the new physics beyond the SM. The improvement of
the upper limits of the EDMs of diamagnetic atoms
has a signiﬁcant impact on the study of new physics inthe nucleon-nucleon and nucleon-electron interactions.
Thus, we aim to improve the current upper limit on
the Xe atomic EDM, |d(Xe)| < 4.1 × 10−27 ecm,2) bymore than one order of magnitude.
To detect the EDM of a Xe atom, the frequency ofspin precession in a static magnetic ﬁeld and an electric ﬁeld is measured. The presence of an EDM ofa size 10−28 ecm, for instance, in a 10 kV/cm electricﬁeld would change the frequency of the Xe spin precession on the level of nanohertz. In order to detect suchan extremely small frequency change, an experimental scheme, a nuclear spin maser with an active feedback was developed.3–5) The spin maser enables theelongation of the spin coherence time to an inﬁnitely
long duration, and thus, the statistical sensitivity canbe improved rapidly with time compared to repeatedshort measurements. In addition, in order to eliminate
the eﬀect of the drift in the magnetic ﬁeld, the 131 Xe
spin maser was introduced as a reference system (co-magnetometer). For this purpose, 3 He is often used2)
as its small EDM compared to that of Xe due to thesmall atomic number is suitable for the reference atom.
However, there remains a diﬃculty due to the diﬀerence in the magnitudes of the frequency shifts causedby the contact interaction with polarized Rb atoms.
This limitation can be eased by using 131 Xe. Since themagnitude of the polarized Rb interaction is almost
the same between 129 Xe and 131 Xe,6) the systematic
errors arising from the polarized Rb interaction will
be reduced dramatically.
The operation of the co-existing 129 Xe and 131 Xemasers with the active feedback scheme was realized
for the ﬁrst time, as shown in Fig. 1. By taking the frequency diﬀerence between the 129 Xe and 131 Xe masers,
the dependence of the frequency on the cell temperature, which virtually limits the measurement sensitivity in our 129 Xe/3 He masers, was mitigated significantly in the 129 Xe/131 Xe case. The study on theoptimization of the maser operation is ongoing. We
expect that further improvement in the maser stability can be achieved by increasing the magnitude of thepolarization and elongating the spin coherence times
for 129 Xe and 131 Xe.

Fig. 1. Maser signals obtained for (a) 129 Xe and (b) 131 Xe.
Beat signals from the lock-in ampliﬁers which were employed to improve the signal-to-noise ratio were observed at the reference frequency of 33.036 Hz for 129 Xeand 9.797 Hz for 131 Xe.

∗1RIKEN Nishina Center
Department of Physics, Tokyo Institute of Technology∗3 FRIS, Tohoku University∗4 CYRIC, Tohoku University∗5 Department of Physics, Kyungpook National University∗6 Department of Physics, Tokyo Metropolitan University∗7 RIIS, Okayama University∗8 Department of Physics, University of Winnipeg∗9 Institute of Material Structure Science, KEK∗10 Department of Advanced Sciences, Hosei University∗11 RCNP, Osaka University∗12 Department of Physics, Saitama University∗13 CNS, University of Tokyo∗2References
1) T. Chupp and M. Ramsey-Musolf: Phys. Rev. C 91,035502 (2014).
2) M. A. Rosenberry and T. E. Chupp: Phys. Rev. Lett.
86, 22 (2001).
3) A. Yoshimi et al.: Phys. Lett. A 304, 13 (2002).
4) A. Yoshimi et al.: Phys. Lett. A 376, 1924 (2012).
5) T. Inoue et al.: Eur. Phys. J. D 70, 129 (2016).
6) M. Bulatowicz et al.: Phys. Rev. Lett. 111, 102001(2013).

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