EDM measurement in 129 Xe atom using dual active feedback nuclear spin maser

Transcription

1 Hyperfine Interact DOI /s EDM measurement in 129 Xe atom using dual active feedback nuclear spin maser T. Sato Y. Ichikawa Y. Ohtomo Y. Sakamoto S. Kojima C. Funayama T. Suzuki M. Chikamori E. Hikota M. Tsuchiya T. Furukawa A. Yoshimi C. P. Bidinosti T. Ino H. Ueno Y. Matsuo T. Fukuyama K. Asahi Received: 21 September 2014 / Accepted: 10 December 2014 Springer International Publishing Switzerland 2014 Abstract The technique of an active nuclear spin maser is adopted in the search for electric dipole moment in a diamagnetic atom 129 Xe. In order to reduce systematic uncertainties arising from long-term drifts of the external magnetic field and from the contact interaction between longitudinal polarized Rb atoms and 129 Xe spin, a 3 He comagnetometer with a Proceedings of the 5th Joint International Conference on Hyperfine Interactions and International Symposium on Nuclear Quadrupole Interactions (HFI/NQI 2014) Canberra, Australia, September 2014 T. Sato ( ) Y. Ohtomo Y. Sakamoto S. Kojima C. Funayama T. Suzuki M. Chikamori E. Hikota M. Tsuchiya K. Asahi Department of Physics, Tokyo Institute of Technology, Ookayama, Meguro, Tokyo , Japan sato@yap.nucl.ap.titech.ac.jp Y. Ichikawa H. Ueno RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama , Japan T. Furukawa Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo , Japan A. Yoshimi Research Core for Extreme Quantum World, Okayama University, Tsushimanaka, Kita, Okayama , Japan C. P. Bidinosti Department of Physics, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba, Canada T. Ino Institute of Material Structure Science, KEK, 1-1 Oho, Tsukuba, Ibaraki , Japan Y. Matsuo Department of Advanced Sciences, Hosei University, Kajino-cho, Koganei, Tokyo , Japan T. Fukuyama RCNP, Osaka University, 10-1 Mihogaoka, Ibaraki, Osaka , Japan

2 double-cell geometry was employed. The remaining shift, which turned out to show some correlation with the cell temperature, was mitigated by stabilizing the cell temperature. As a result, the frequency drift of the 129 Xe maser was reduced from 12 mhz to 700 μhz, and the determination precision of frequency of 8.7 nhz was obtained for a s measurement time using the double-cell geometry cell. Keywords Electric dipole moment Nuclear spin maser Fundamental symmetry 1 Introduction The search of a permanent electric dipole moment (EDM) is one of the most promising ways to explore new physics beyond the Standard Model (SM) of elementary particles. Since predicted values for the EDM in the SM are too small to detect in current experimental techniques, discovery of a finite value of EDM would strongly indicate existence of physics beyond the SM. At present, the most stringent upper limit on the EDM in diamagnetic atom is set by the 199 Hg experiment [1] as d( 199 Hg) < ecm. The EDM of a diamagnetic atom primarily arises from a CP-odd nuclear moment, the Schiff moment S [2], via P and T-odd nucleon-nucleon (NN) interactions [3]. Since the Schiff moment depends on the nuclear structure, the magnitude of EDM should be different for different nuclear species. Measurements of EDM for various nuclear species are necessary in order to clarify the nature of CP-odd NN forces. The objective of our study is to search for the EDM in a diamagnetic atom of 129 Xe, beyond the current upper limit of ecm [4]. In order to detect a frequency change in the Larmor precession of 129 Xe spin caused by the interaction between the EDM and applied electric field, a nuclear spin maser with active feedback is employed. The active feedback nuclear spin maser enables us to maintain the Larmor precession permanently [5, 6]. Thanks to the spin maser technique, the frequency precision of spin precession is expected to improve with a T 3/2 power law as the measurement time T increases. The active feedback nuclear spin maser is operated in the following way: With a static field B 0,the 129 Xe nuclear spin is polarized by means of the spin-exchange optical pumping using Rb vapor [7]. Once the spin precession of 129 Xe is started (either by thermal fluctuations or by an application of a radio frequency pulse), the spin precession is detected optically: Since the transverse component of 129 Xe spin induces a polarization of Rb atomic spin (re-polarization), the 129 Xe precession signal appears as a change in the transmission of a probe laser light through the Rb vapor. In order to maintain the spin precession, a feedback field B FB that rotates in a plain transverse to B 0 with the phase shifted by 90 is generated by a coil wound around the 129 Xe cell, to which a feedback current is supplied from a precession data processor. Both of the pumping effect and decoherence of the transverse spin force the polarization vector to align along the z-axis. However, the torque due to B FB and depolarization effect tilt the polarization vector from the z-axis. When the balance between these effects are realized, the spin precession of 129 Xe is maintained permanently, avoiding the decay of the transverse magnetization. Unique features of the active feedback nuclear spin maser are an optical detection of spin precession and an artificial generation of the maser feedback field. These allow the operation of a spin maser under magnetic fields as low as mg, in contrast to the conventional spin maser which requires G field strengths [8, 9]. The operation of the maser in such a low field has an advantage that the frequency uncertainty coming from the drift of the magnetic field is reduced. In the previous work, the determination precision of the maser

3 EDM measurement in 129 Xe atom using dual active feedback nuclear frequency reached δν = 9.3 nhz in a measurement time of s using a spherical cell [6]. More than an order of magnitude improvement in precision is required for the 129 Xe- EDM search in the ecm region, but it has been prevented by long-term drifts in the external magnetic field and by contact interactions between 129 Xe spins and polarized Rb atoms. In addition, in repeated measurements, the disturbances also led to a drifting of the maser frequency. In order to reduce drifts in the field, a comagnetometry using a 3 He maser was introduced. 3 He and 129 Xe masers are operated simultaneously in the same volume of the cell. The size of the EDM expected for a 3 He atom is negligible compared to that for a 129 Xe atom because of the small atomic number Z = 2. Thus the 129 Xe EDM can be deduced without disturbance from the magnetic field drift by observing the difference in phase evolution between the 129 Xe and 3 He masers. Moreover, in order to suppress the frequency shift due to contact interactions between the 129 Xe/ 3 He spin and a polarized Rb spin [10], a double-cell geometry has been adopted [4]. The frequency shift is proportional to P Rb [Rb], wherep Rb is the polarization and [Rb] is the number density of Rb atoms. In the double-cell geometry, the cell is divided into two cells: a pumping cell where the optical pumping of 129 Xe/ 3 He is performed and a probe cell where the masers are operated. The polarization P Rb in the probe cell is reduced, because the Rb polarization decays during its diffusion from the pumping cell to the probe cell, thus relieving the maser frequency shift. The first operation of the maser in the double-cell geometry was carried out for 129 Xe, yielding a precision of 4 μhz in a 10 5 s observation time using the double-cell geometry with 5 mm transfer tube [11]. Although the double-cell geometry was introduced, a correlation between the frequency drift and the cell temperature, that is, the number density of Rb atoms still remained. This was recognized as one of the crucial issues that limit the frequency precision [10]. Thus, the stabilization of the cell temperature should be important to improve the maser frequency precision. Although the incorporation of the 3 He comagnetometry with double-cell geometry should largely facilitate the reduction of systematic uncertainties, it might also bring side effects on the performance of the 129 Xe maser: The adoption of the double-cell geometry reduces the signal amplitude of 129 Xe maser, because the 129 Xe spins relax while they drift from the pumping cell to the probe cell; The reduction of the Rb polarization also leads to the reduction of the spin precession signal of 129 Xe [11]. In addition, the partial pressure of 129 Xe needs to be reduced by compromise with the size of 3 He maser signal. Note that the maser signal is proportional to the magnetization, or the product of the partial pressure and polarization of atoms. The polarization of 3 He decreases when 129 Xe coexists, due to spin-spin interaction between 129 Xe and 3 He and also to the exhaustion of the pumping laser power for polarizing the 129 Xe spins. The dependence of 3 He polarization on the 129 Xe partial pressure was studied by means of the adiabatic fast passage method of NMR [12]. As a result, the 3 He polarization reached 1.0(1) % under a 129 Xe partial pressure of 1 Torr at 80 C. This partial pressure of 129 Xe is 1/10 of that in the previous study with the double-cell geometry [11]. Thus, there arises a concern with possible attenuation of the signal amplitude of the 129 Xe maser. With the cell temperature being stabilized, the performance of the 129 Xe maser in a 129 Xe/ 3 He co-habiting cell of the double-cell geometry was studied.

4 Fig. 1 Schematic view of the experimental setup 2 Maser frequency under the temperature control Before running the 129 Xe maser, a free induction decay (FID) measurement of 129 Xe was carried out. The schematic view of the setup is shown in Fig. 1. The cell used in the measurement was made of GE180 glass and had the double-cell geometry. The pumping cell was a sphere 20 mm in diameter. The probe cell was a cylinder 10 mm long and 15 mm in diameter. A 15 mm long and 8 mm in inner diameter glass tube connected the pumping cell to the probe cell. The cell contained 1 Torr of 129 Xe, 425 Torr of 3 He and 100 Torr of N 2. The cell was installed in a box which had a partition made of silicone rubber to separate the upper and the lower spaces inside the box. Temperatures of the pumping cell located in the upper space and the probe cell in the lower space were stabilized individually by PID controlled heated airflows. In this measurement, we succeeded in maintaining the temperature at 106.4±0.1 C in the pumping cell and 90.2±0.2 C in the probe cell. A static magnetic field B 0 = 10 mg was applied to the cell. The box was placed at the center of the B 0 coils [13]. Surrounding the coils, a three-layer magnetic shield was installed. Each layer of the shield was a Permalloy cylinder of 2 mm wall thickness. Their sizes are, from the outermost to innermost layers, 800 mm in diameter and 1300 mm long, 600 mm in diameter and 1000 mm long, and 400 mm in diameter and 680 mm long. A 795-nm laser light from a TA-DFB laser (TOPTICA, TA-100) was circularly polarized and introduced along the B 0 axis to the pumping cell with a power of 1.0 W. A probe light from a DFB laser (TOPTICA, DL-100, 795 nm, 10 mw) passed through the probe cell in a direction perpendicular to the B 0 axis and was detected by a photodiode. The measured laser intensity signal was processed using a lock-in amplifier with a reference signal at a frequency chosen close to the 129 Xe spin precession. The resulting signal was recorded on a computer.

5 EDM measurement in 129 Xe atom using dual active feedback nuclear Fig. 2 Observed FID signal of 129 Xe Fig. 3 Observed maser signal of 129 Xe We succeeded in observing a 129 Xe FID signal even the partial pressure of 129 Xe was as lowas1torr,asshowninfig.2. The signal-to-noise ratio of about 30 was obtained, which was large enough to run the 129 Xe maser. Having obtained an FID signal with the high S/N ratio, we proceeded to the operation of the 129 Xe maser. The experimental setup was the same as in the FID measurement except for a feedback circuit for the maser operation. In the maser experiment, the resulting signal from the lock-in amplifier was processed by a feedback circuit to yield a signal which is proportional to the transverse magnetization of 129 Xe with a phase angle advanced by 90. This signal was applied to the drive coil to produce the feedback field B FB. The 129 Xe maser signal thus obtained is shown in Fig. 3. The time variation of the 100 s average maser frequency is plotted in Fig. 4, which reveals an overall drift of about 700 μhz in a s measurement time. This is an order of magnitude improvement from the previous work which adopted a double-cell geometry with 5 mm transfer tube [10]. The precision of the maser frequency in a one-shot measurement reached 8.7 nhz in a s measurement time as shown in Fig. 5. This is almost at the same level as that achieved with a 129 Xe maser using a usual spherical cell [6].

6 Fig. 4 Time evolution of the 129 Xe maser frequency. Each point is 100 s average maser frequency Fig. 5 Precision of the spin maser oscillation. Best precision was achieved at sas 8.7 nhz 3 Summary In order to search for the EDM in a 129 Xe atom, a method to measure the spin precession frequency using an active feedback spin maser of 129 Xe was studied. In order to cancel out the dominating components of the systematic uncertainties due to long-term field drifts and the contact interaction between 129 Xe spin and polarized Rb atoms, a 3 He comagnetometry with double-cell geometry was introduced in the 129 Xe maser system. With the stabilization of the cell temperatures, the 129 Xe FID and maser signals were observed for 1 Torr of Xe confined in a double-cell with a 15-mm long connecting tube. The temperature was kept within ± 0.1 C in the pumping cell and ± 0.2 C in the probe cell. Thanks to the stabilities thus attained, the drift in the 100 s-average maser frequency was within approximately 700 μhz during a s measurement time. As a result, the precision of the 129 Xe maser reached 8.7 nhz at a s measurement time. A study on the determination precision for the 3 He spin maser is planned.

7 EDM measurement in 129 Xe atom using dual active feedback nuclear Acknowledgments This work was partly supported by the JSPS KAKENHI (No and No ). One of the authors (T. Sato) would like to thank the JSPS Research Fellowships for Young Scientists for the support. References 1. Griffith, W.C., et al.: Improved Limit on the Permanent Electric Dipole Moment of 199 Hg. Phys. Rev. Lett. 102, (2009) 2. Schiff, L.I.: Measurability of Nuclear Electric Dipole Moments. Phys. Rev. 132, 2194 (1963) 3. Yoshinaga, N., et al.: Nuclear Schiff moments for the lowest 1/2 + states in Xe isotopes. Phys. Rev. C 87, (2013) 4. Rosenberry, M.A., et al.: Atomic Electric Dipole Moment Measurement Using Spin Exchange Pumped Masers of 129 Xe and 3 He. Phys. Rev. Lett. 86, 22 (2001) 5. Yoshimi, A., et al.: Nuclear spin maser with an artificial feedback mechanism. Phys. Lett. A 304, 13 (2002) 6. Yoshimi, A., et al.: Low-frequency 129 Xe nuclear spin oscillator with optical spin detection. Phys. Lett. A 376, 1924 (2012) 7. Happer, W., et al.: Optical Pumping. Rev. Mod. Phys. 44, 169 (1972) 8. Richards, M.G., et al.: The 3 He nuclear Zeeman maser. J. Phys. B 21, 665 (1988) 9. Chupp, T.E., et al.: Demonstration of a Two Species Noble Gas Maser. Phys. Rev. Lett. 77, 3971 (1994) 10. Ichikawa, Y., et al.: Search for electric dipole moment in 1 29Xe atom using active nuclear spin maser. Eur. Phys. J.: Web of Conf. 66, (2014) 11. Hikota, E., et al.: Active nuclear spin maser oscillation with double cell. Eur. Phys. J.: Web of Conf. 66, (2014) 12. Ohtomo, Y., et al.: Double-cell Geometry for 129 Xe/ 3 He co-magnetometry, Proceedings of the 2nd Conference on Advances in Radioactive Isotope Science (ARIS2014), submitted. 13. Sakamoto, Y., et al.: Development of high-homogeneity magnetic field coil for 129 Xe EDM experiment, Proceedings of the 5th Joint International Conference on Hyperfine Interactions and Symposium on Nuclear Quadrupole Interactions (HFI/NQI 2014) (this proceedings)