The Mianyang Neutron Polarization team is making progress in the field of new physical exploration beyond the Standard Model

Recently, the neutron polarization team of the Institute of Nuclear Physics and Chemistry of the Chinese Academy of Engineering Physics used a quantum magnetometer array based on the polarized alkali metal Rb to measure the abnormal magnetic field caused by the new interaction outside the Standard Model with an accuracy of ~30aT (3×10-17T) through exquisite experimental design and effective noise suppression. Compared with previous detection results of new interactions with a force range of ~10 m, the team’s measurements of va (vector-axis vector) interactions outside the Standard Model have improved in accuracy by ~20,000 times, while the AA (axial arrow-axis vector) interactions have increased by ~140 times. The results were published online on July 29 in the journal Physiological Review Letters.

The Mianyang Neutron Polarization Team has long been committed to using polarized spins such as polarized neutrons and polarized 3He to detect new spin-related interactions propagated by axion-like particles. Axion originated in 1977. Peccei and H. Quinn proposed a new theory (later known as the PQ mechanism or PQ theory) to solve the problem of CP conservation in strong interactions. At the heart of the theory is the introduction of a new U(1) integral symmetry and its spontaneous defilement. Subsequently, S. Weinberg and F. Wilczek et al. noticed at about the same time that the theory would lead to the production of a new type of light-mass boson, which Wilczek named axion. After the PQ theory and axion were proposed, it immediately became the most elegant solution to the strong CP problem of quantum chromodynamics. Some theorists, including Wilczek, believe that the axion will soon be detected. However, more than forty years later, human beings still have not found the shaft. The particle’s extreme difficulty in detecting has led humans to associate it with another unsolved mystery of fundamental physics, dark matter. Particles outside the Standard Model with similar properties to axions are called Axion Like Particles and are among the candidates for cold dark matter.

Thus, the detection of axoid subparticles is intertwined with the most important problems in particle physics, astrophysics, and fundamental physics. If present, all indications are that axion-like particles have only a very weak coupling to ordinary matter particles. Their significantly different property from ordinary particles is that they can propagate new spin-related interactions. Axion-like particles are very light—de Broglie wavelengths are long, so one way to detect this is to measure the effect of the axion-like bosonic field produced by ordinary matter on polarized spins.

Using an array of quantum magnetometers based on the polarized alkali metal Rb, the neutron polarization team averaged the data over ~10 days and obtained results with an accuracy of ~30aT (3×10-17T) for the abnormal magnetic field measurement. Based on this measurement accuracy, the team probed the vector-axial vector (VA) and axial-axial vector (AA) type interactions coupled between the outer nucleons and polarized electrons in the Standard Model, and obtained a new experimental upper limit. Previously, the highest detection accuracy of these two interactions was published in a paper by a research group at Los Alamos National Laboratory in the United States[PRL 121,091802(2018)]and[Nat. Commun.10.2215(2019)]given.

Figure 1: The accuracy of type VA interaction detection is about 20,000 times higher than the previous results.

Polarized alkali metal atoms and polarized inert gas nuclei not only enable neutron polarization and polarization analysis (the most advanced and mainstream choice in neutron scattering applications today), but also the basis for quantum precision measurement techniques such as quantum gyroscopes and extremely weak magnetic field measurements. Such precision measurement methods and technologies are not only widely used in the fields of defense and civilian defense, but also the basic physical means for human beings to explore the ultimate truth. In 1900, Lord Kelvin famously said in a lecture before the birth of relativity and quantum mechanics: “There is nothing new in physics today, and all that remains is an increasingly precise measurement.” (There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.) “Whether in defense, civilian or the search for truth, physics researchers have long instinctively sought the highest measurement accuracy.

Figure 2: Lord Kelvin.

Being able to measure a physical quantity more precisely and accurately through exquisite experimental design and perfect noise cancellation technology is the ultimate pursuit and dream of every experimental physicist. In this study, the neutron polarization team applied four quantum magnetometers and detected anomalous magnetic fields in an array manner, which improved statistical accuracy by a factor of 2; Then the two mass sources are modulated at high speed (600RMP) in a rotating manner, which is not only technically easier to achieve than the translational mode, but also greatly reduces the interference of 1/f noise at a modulation frequency of up to 20Hz – the modulation frequency of the previous experiment was only ~0.3Hz; Four array magnetometers are carefully arranged, and the abnormal magnetic field caused by the new interaction is extracted as an addition and subtraction averaging method, and the measured method shows that the common mode noise is 95% inhibition rate; Turn the direction of rotation of the mass source by flipping every 660 seconds and use[+1,-3,+3,-1]The drift removal algorithm further eliminates the slow drift of the experimental system baseline and the common mode noise of the time domain; In addition to abandoning DC to avoid 1/f noise, signal extraction also adopts a weighted average method for fundamental frequency and three multiplier signals to further improve the signal-to-noise ratio; The data processing method adopts high-precision numerical integration throughout the process, which effectively avoids the interference of high-frequency noise. Although there is still a gap with international counterparts in some key devices and materials, the neutron polarization team has finally achieved an order of magnitude improvement in measurement accuracy through more ingenious experimental design.

Figure 3: Schematic diagram of the team experimental device of the Institute of Nuclear Physics and Chemistry of the Chinese Academy of Engineering Physics.

The first author of the paper is Dr. Wu Keyan, and the corresponding author is Researcher Yan Haiyang, head of the neutron polarization innovation team. The research is supported by the National Natural Science Foundation of China (U2030209) and the Key R&D Program of the Ministry of Science and Technology (2020YFA406001 and 2020YFA0406002). (Source: Science Network Author: Xue Yanmei)

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