Searching for Axions in Polarized Gas
• Physics 17, 157
Using the collisions of a flowing gas, the researchers have made a careful search for unusual spin-dependent interactions, placing new constraints on the so-called axion.
The standard model of basic components and interactions has now been around for almost half a century. It has passed test after test in particle accelerators. However, many parts of the model are poorly understood, and it is now clear that the particles in the standard model make up only about 5% of the visible energy of the Universe. This situation naturally encourages researchers to look for new particles and interactions that fall outside this model. One way to do this research is to prepare a gas of polarized atoms and look for changes in this polarization that may appear in new physics. Haowen Su of the University of Science and Technology of China and colleagues used two separate samples of xenon gas to examine spin-dependent interactions. [1] (Figure 1). The results place constraints on axions—a candidate for dark matter—within a preferred mass band called the axion window.
Research into new spin-dependent interactions has exploded over the past decade. A special relationship with quantum mechanics strictly enforces the mathematical method for such interactions, and the main variables that can be changed are the coupling strength and the spatial distance. Since the nature of these interactions is general to many species, it is possible to carry out experimental research for new signatures of interactions, even if there is no particular theory of physics that goes beyond the usual conditions.
Another possible type of spin-dependent interaction is that mediated by an axion, a very luminous (but not heavy) particle that can make up some of the missing dark matter in the Universe. Axion was originally planned as a way to solve a particle-physics mystery: there seems to be no time-reversal violation in the strong interaction. [2]. The axion model does not explain the mass of this hypothetical particle, but the simplest version of the theory suggests that the mass of the axion is most likely between 10 µeV/c.2 and 1 meV/c2the so-called axion window [3–5].
Using a variety of instruments (from microwave ovens to radio telescopes), researchers have searched for the signatures of axions that have been detected over a wide range of masses—often outside the window. of axion. Another way to search for axions is to study spin-dependent effects similar to the magnetic interaction. An advantage of this strategy is that experiments can now investigate small spin-dependent effects with very high sensitivity. This attitude arises from the well-understood effects of quantum entanglement in weak state systems and from new ideas and technologies from the development of quantum information and quantum sensing.
Su and colleagues designed their experiments to take advantage of this spin-based technology to probe spin-dependent interactions mediated by axions in a large array that includes the axion window. [1]. Specifically, the researchers looked for interactions between axion and xenon-129 (129Xe) atoms that were separated by spin-exchange optical pumping [6]. This well-organized process involves mixing 129Xe atoms with rubidium-87 (87Rb) atoms and exposing the compound to circularly polarized light. The light excites the outermost electrons of the rubidium atoms, but has no effect on the closed electrons surrounding the xenon atoms. However, the optically pumped rubidium atoms act as a spin-angular-momentum pool that gradually polarizes the spin-1/2 nuclei of 129Xe atoms by atomic collisions, thus creating an interactive assembly of atoms with electronic and nuclear polarization. The closed electron shell of xenon separates its polarized nucleus from external disturbances enough to maintain its polarization during the required measurement.
The team used two polarized cells 87Rb–129Xe ensembles are separated by a distance of 60 mm. A magnetic field applied to one cell (called the source) caused the spins of the xenon atoms to tilt by 90° and move forward around their polarization axis at a specific rate of an atom called Larmor waves. This state of the magnetic moment of the xenon atoms produced an output signal related to the normal interaction of the magnetic field. But progress can produce a second signal from an unusual rate-dependent interaction. Both of these signals can influence the polarization of the atoms in the second cell (called sensor), but the researchers surrounded the two cells with a shield that greatly suppressed the effect from the normal interaction of gravity. Using a laser probe, they monitored the polarization of the atoms in the sensor cell, looking for deviations that would reveal the presence of a strange interaction.
Polarized 87Rb–129Xe ensembles have been designed and used by many researchers for many purposes, including the investigation of spin-dependent interactions. The work of Su and colleagues is characterized by their use of two new techniques – magnetic amplification and signal templates – which have helped them improve their understanding of the search for unusual interactions. which depend on the spin by 2 orders of magnitude more than the existing state. art. The magnetic amplification comes from the resonance frequency of xenon’s Larmor frequency – the atoms of the xenon source “emit” at this frequency, and the atoms of the xenon sensor “receive” at this frequency. Any signal received by the xenon atoms is then transmitted via atom-atom collisions to nearby rubidium atoms, whose spin state is measured with a laser probe. To detect this potential signal, the researchers use a signal template, which is a filter based on the signature expected from an unusual spin-dependent interaction. This template method is similar to the data analysis methods used by LIGO to detect weak gravitational signals.
The researchers found no sign of axions in their data, which allows them to set new limits on the axion fusion energy in the micro-electron-volt region. The new limits are interesting because they extend into the axion window and improve upon the previous limits by a factor of 50 to 10 µeV. [7]. However, the sensitivity of the measurement is not yet sufficient to rule out “window” axions such as dark matter. However, the new developments presented by Su and colleagues can be used in future experiments of polarized gas to search for other effects that may arise from cosmological axion sources, such as dark matter decay. . [8] and binary black holes [9].
References
- H. Su and al.“New constraints on axion-mediated spin interactions using magnetic fields,” Phys. Pastor Lett. 133191801 (2024).
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- S. Borsanyi and al.“Axion mass calculation based on high-temperature lattice quantum chromodynamics,” Nature 53969 (2016).
- G. Ballesteros and al.“Unifying inflation and axion, dark matter, baryogenesis, and the seesaw mechanism,” Phys. Pastor Lett. 118071802 (2017).
- VB Klaer and GD Moore, “The dark-matter axion mass,” J. Cosmol. Astropart. Phys. 2017049 (2017).
- TG Walker and W. Happer, “Spin-exchange optical pumping of noble-gas nuclei,” Pastor Mod. Phys. 69629 (1997).
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- S. However and al.“Search for topological defect dark matter with the global network of optical magnetometers,” Nat. Phys. 171396 (2021).
- C. Dailey and al.“Quantum sensor networks as exotic field telescopes for multimessenger astronomy,” Nat. Astron. 5150 (2020).
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