Scientists are making progress in exploring new areas of physics beyond the Standard Model

“The sky is high and the earth is vast, and the universe is infinite.” This famous sentence of Wang Bo in the “Preface to the Pavilion of King Teng” aptly describes the awe and yearning of human beings for the infinite universe. Spin, which is one of the most fundamental properties of elementary particles. So is it possible that a small spin on the Earth’s surface, such as polarized 3He, could sense the presence of a celestial body such as the Sun due to a new type of interaction that goes beyond the Standard Model? Recently, the neutron polarization team of the Institute of Nuclear Physics and Chemistry, Chinese Academy of Engineering Physics studied this problem by analyzing the experimental data of polarization 3He+129Xe. In the astronomical range of force ranges (~1012 m), state-of-the-art detection accuracy is given for new interactions outside the three standard models. For scalar-pseudoscalar (SP)-type interactions, the new limit surpasses the previous most stringent combined astronomical-laboratory limit for the first time, and is ~70 times higher; For vector-axis-vector (VA) interactions, the accuracy of the existing limit is improved by 12 orders of magnitude; For axial-axional (AA) type interactions, the first limit for astronomical distance is given. The findings were published online Aug. 29 in the journal Physical Review Letters.

In addition to studying neutron polarization technology, the Mianyang Neutron Polarization Team (3He Innovation Team) has long been committed to developing precision measurement methods based on polarized neutrons and polarized 3He, and exploring new spin-related interactions beyond the standard model. The propagators of these new interactions may be axions and axion-like particles. In the 70s of the last century, in order to solve the CP (charge conjugation-cosmogeneum) problem in strong interactions, Pescei and Quinn proposed a theory that included a new U(1) symmetry and spontaneously broke it; Weinberg and Wilczek found that the theory could lead to the creation of a new scalar particle, the axion. Many subsequent theories, including supersymmetric spontaneous breaking chord theory, predicted the existence of lightweight, weakly coupled particles with similar properties to axions, axion-like particles. They are also considered possible candidates for cold dark matter. Therefore, the study of axions and axion-like particles has become a hot spot in recent years at the intersection of some of the most important questions in contemporary physics and astronomy (Figure 1).

Figure 1: (Stills from The Big Bang Theory Season 7 Episode 21) In recent years, axions have become a frontier and hot research field

New particles may transfer spin-related new interactions between Standard Model particles, so exploring this new interaction provides a possible avenue to find these particles. According to existing theories, the energy scale of Peccei-Quinn symmetry breaking can be arbitrarily high, which means that axion-like particles can have arbitrarily light mass and arbitrarily weak coupling. As a result, the force ranges of new interactions they may transmit can range from the nanoscale up to astronomical distances. In recent years, many experiments have placed strict limits on these new spin-related interactions at the micrometer to kilometer scale. However, at the astronomical scale, taking SP-type interactions as an example, only the joint limitation of SP-type interaction is indirectly derived through astronomical observation of PP-type restriction and laboratory measurement restriction of SS-type, and there is no separate experiment to limit SP-type interaction.

If this new interaction exists, objects like the Sun will have an impact on the precession frequency of polarimetric 3He in laboratories on Earth’s surface. This effect is represented in the laboratory coordinate system as an effective magnetic field that varies by the frequency of the Earth’s rotation, and this effective magnetic field can be determined with great precision by quantum precision measurement. The neutron polarization research team cleverly regarded the Sun (~1030 kg) and Moon (~1022 kg) as huge sources of non-polarized mass, using the 3He+129Xe comagnetometer in the Earth Laboratory to measure the equivalent magnetic field generated by the new interaction, thereby limiting the new spin-related interaction between these two mass sources and polarized neutrons in the comagnetometer. Earth-polarized 3He will sense the radiofrequency field that autotransforms with the Earth due to new interactions. Thus, in a laboratory coordinate system, tiny signals generated by new interactions can be extracted from the noise background by a common-magnetometer polarized 3He+129Xe. The working principle of this commonmagnetometer is also the basis for quantum gyroscopes. The research team used the existing 3He+129Xe comagnetometer to measure the cosmic background field data caused by the Lorentz break[Phys. Rev. Lett. 112(11):110801, (2014).]An analysis was carried out to determine that the upper limit of the equivalent magnetic field generated by the new interaction at a 95% confidence level is 0.023 fT. Based on this result, the research team proposed the latest limits of SP-type, VA type and AA-type interactions at astronomical scales. As shown in Figure 2, for SP-type interactions, for the first time, the strictest astronomical and laboratory joint restrictions were broken; The limiting accuracy for VA-type interactions is improved by 12 orders of magnitude; For AA-type interactions, limits were proposed for the first time at astronomical distances. More importantly, this new approach opens up entirely new possibilities for exploring new spin-related interactions on astronomical scales. The work was published on the preprint website arXiv on February 15, 2023[arXiv:2302.09096], which has attracted the attention of the scientific community at home and abroad. Since then, several similar studies have sprung up.

Figure 2: SP-type interaction intensity gNSgnP, the solid line in the figure is one of the main results of the polarization team’s work, the astronomy-laboratory joint limit is shown by the dotted line on the left, and the existing results exceed the astronomy-laboratory joint results by ~70 times.

This study once again witnessed an important breakthrough by the neutron polarization team of the Mianyang Research Reactor in China in the frontier of innovation. “This is a clever approach” and “the paper is well written” are the very generous comments given by PRL’s reviewers about the originality and quality of the work. Previously, the team has made several key advances in exploring new areas of physics beyond the Standard Model using neutron polarization technology and precision measurement methods. The results have been published in Physical Review Letters, including[Phys. Rev. Lett. 129, 051802 (2022)] 、[Phys. Rev. Lett. 115, 182001 (2015)]and[Phys. Rev. Lett. 110, 082003 (2013)]and other articles. These important achievements are the result of the team’s many years of research in neutron polarization technology and inert gas nucleus polarization technology. Polarized inert gases have important applications in neutron scattering, medical imaging, inertial navigation, field weakening detection, new physics detection, and other fields. Syn-magnetometers, i.e. mixing two polarized inert gas magnetometers, suppress systematic errors caused by fluctuations in the main magnetic field and are therefore extremely sensitive, which makes them extremely promising for detecting new spin-related interactions. In addition, for the key component of the magnetometer, the inert gas special glass chamber, the team has accumulated extensive technical experience in manufacturing, annealing, cleaning and filling.

(Ming) Wen Zheng Ming Xing Book “Preface to the Pavilion of King Teng”

However, as quoted at the beginning of the second half of the “Preface to the Pavilion of King Teng”, “Joy and sorrow come, and there are many things that are good and false.” In this endless universe, what we understand and know may only be the tip of the iceberg. Despite this endless possibility, human beings have never stopped exploring, and the journey of scientific researchers is like a sea of stars, which is endless.

The first author of the paper is doctoral student Wu Liangyong, the second author is Dr. Zhang Kaiyuan, who is new to the team, and the corresponding author is researcher Yan Haiyang, the leader of the neutron polarization innovation team. This research was supported by the National Natural Science Foundation of China (U2230207 and U2030209) and the Key Research and Development Program of the Ministry of Science and Technology (2020YFA0406001 and 2020YFA0406002). (Source: Science Network)

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