Astrophysical and experimental constraints on the axion parameter space. The region labeled “Neutron Stars,” shown in red, represents the parameter space excluded by our study. The yellow line labeled “QCD Axion” indicates the most natural and theoretically motivated QCD axion models. This plot is a modified version of the one available on Credit: Gómez-Bañón et al.
Neutron stars, the remnants of massive stars after a supernova explosion, have often been the subject of studies aimed at testing and revealing exotic physics. Indeed, these stars are among the densest objects in the universe and therefore host extremely high temperatures and pressures.
Recent theoretical studies have explored the possibility that quantum chromodynamics (QCD) axions, elementary particles thought to emerge from the so-called Peccei-Quinn mechanism, influence the properties of neutron stars. By interacting with nucleons, these particles could alter the structure of neutron stars, which could have an impact on their cooling.
Building on this idea, researchers from the University of Alicante and the Technical University of Munich recently simulated the cooling processes of neutron stars and compared them to theoretical predictions to probe previously unexplored regions of the axion parameter space. Their article, published in Physical Examination Lettersimpose new constraints on lightweight QCD axions, which could inform future searches for these exotic particles.
“This study grew out of discussions about how a hypothetical particle like the QCD axion might influence the properties of nuclear matter,” Antonio Gómez-Bañón, first author of the paper, told Phys.org. “We realized that, if light enough, the QCD axion could change the size of a neutron star’s envelope, an outer layer that regulates its cooling.”
The main goal of Gómez-Bañón and colleagues’ recent work was to determine whether the influence of a QCD axion on the envelope of a neutron star could significantly accelerate the cooling of the star, which which would conflict with previous observations. To do this, they first examined how a QCD axion might affect the energy and pressure of nuclear matter within neutron stars.
The cooling curves of neutron stars affected by the presence of the QCD axion field are shown in color. The gray region illustrates the typical range of variability of ordinary neutron star cooling curves, consistent with observations. After 0.1 million years, three of the colored curves have luminosities several orders of magnitude lower than observations. Credit: Gómez-Bañón et al.
“Taking advantage of this understanding, we then solved the differential equations describing the balance of forces between the QCD axion field, nuclear matter and gravity within a neutron star,” explained Gómez-Bañón . “Our solutions showed that the neutron star envelope becomes considerably thinner for certain choices of axion parameters.”
Simulations and analyzes by Gómez-Bañón and colleagues suggest that when a neutron star’s envelope thins, the star becomes less isolated, causing it to cool more quickly. To further validate this prediction, they incorporated their balance equations into their neutron star cooling simulation and examined how the neutron star’s temperature changed over time.
“As expected, the cooling curves obtained from the simulation predicted cooler neutron stars than observations at a given age,” Gómez-Bañón said. “This gap allows us to impose new constraints on the QCD axion parameters.”
The simulations and analyzes carried out by this team of researchers excluded a new region in QCD axion parameter space. Additionally, their work introduces an alternative approach to setting constraints on these hypothetical particles, which draws on observations of neutron stars.
“Unlike previous limits based on axion emission and energy loss, our approach is based on how the QCD axion field changes the structure of the neutron star, compressing its envelope and accelerating its cooling,” added Gómez-Bañón.
“In our next studies, we plan to focus on finding astrophysical scenarios that could constrain the “QCD axion line,” a region of axion masses where many theoretically motivated models reside but which is difficult to probe. Excluding parts of this region would represent a significant step forward.”
More information:
Antonio Gómez-Bañón et al, Constraining light QCD axions with isolated neutron star cooling, Physical Examination Letters (2024). DOI: 10.1103/PhysRevLett.133.251002.
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