Axion–plasmon conversion found in neutron stars
Dark matter constitutes approximately 85% of the matter in our universe, yet its fundamental nature remains one of the most profound mysteries in modern physics. Among the leading candidates are axions—hypothetical particles originally proposed to solve a fundamental symmetry problem in quantum chromodynamics. These elusive particles could simultaneously explain dark matter and resolve a decades-old puzzle in particle physics.
Neutron stars, particularly magnetars with their immensely powerful magnetic fields, have emerged as natural laboratories for axion detection. As dark matter axions stream through these extreme environments, they should convert into detectable radio waves through a process known as axion-photon conversion. This mechanism has formed the basis for numerous observational campaigns using the world's most sensitive radio telescopes.
In a recent research published in Physical Review Letters, Hugo Terças and Tito Mendonça from GoLP/IPFN, in collaboration with Robert Bingham from the Rutherford Appleton Laboratory, reveal a previously overlooked phenomenon that substantially alters this picture: they discovered that before axions can convert into detectable radio waves, a significant portion instead convert into plasma waves (plasmons) deep within the neutron star's magnetosphere. This 'silent' conversion pathway acts as an energy drain, diminishing the expected radio signals that astronomers search for.
The discovery of this overlooked effect forces a major reassessment of detection strategies, as the anticipated radio signals from axion conversion may be substantially fainter than previously predicted. This means current experimental constraints derived from non-observations may be too optimistic, and future searches must account for this damping effect. This work is interdisciplinary and connects fundamental particle and plasma physics with astrophysical observations.
The journey to detect dark matter requires not just powerful telescopes but also deep theoretical insights into how these elusive particles manifest in observable signals. The discovery of axion-plasmon conversion represents exactly this type of insight—one that moves us from looking for what we expect to see to understanding what we might actually find.
(Image credit: NASA/JPL-Caltech)
