The signature of a hypothetical particle of dark matter found in a group of stars?

On Earth, it is mainly in particle accelerators that the quest for new particles is played out. It had a brilliant result in July 2012 with the discovery of the Higgs boson. Since then, however, physicists have been less inclined to party: their experiments to uncover other, more exotic particles like Weakly Interacting Massive Particles, or WIMPs have so far failed, despite a few alerts. It is a great pity because the latter were ideal candidates for understanding what dark matter is made up of, this elusive matter essential to explain the behavior of galaxies. Suddenly, specialists, never disillusioned, turn to other hypotheses and other candidate particles. The axion is one of them. And if its name reminds you of a brand of detergent, that’s normal! It is indeed for this reason that it was named thus: it was supposed to “clean up” a major problem of a new theory, quantum chromodynamics. But now she too is missing even if a signal recorded in the experiment tank Xenon1T leaves some doubt. Most recently, it is towards the stars that the hopes of theorists have been directed since they think they have detected, finally, indirect traces of this particle.

The axion, a very useful particle

The axion is a fundamental particle like the electron, the photon or the neutrinos. However, unlike these particles, the axion is hypothetical. This means that we do not yet know if it exists in nature. This is what we are trying to find out. There are, however, theoretical reasons to suspect that such a particle might exist. Axions can solve long-standing anomalies related to the behavior of neutrons in the presence of electric fields“, explains to Science and the Future Benjamin Safdi, theoretical physicist at Lawrence-Berkeley National Laboratory (Berkeley Lab) in California. The proof of the existence of the axion would also help to solve the string theory and this particle constitutes a candidate of choice to explain the nature of the dark matter. “If axions exist, they would be bosons, like the Higgs boson. Unlike the Higgs boson, however, they are said to be ultralight, like neutrinos. Axions would also interact very weakly with ordinary matter, as neutrinos do“, he adds.

To track down the axions, he and his team looked at the “7 Mercenaries” a group of neutron stars (which are actually 8) located 1140 light years from Earth. “Neutron stars, like the 7 Mercenaries, are known to cool down by emitting neutrinos. However, axions behave in a very similar way to neutrinos. In particular, if axions exist in nature, they could also be emitted by neutron stars“says the researcher. The 7 mercenaries also have several advantages: they are the closest known neutron stars and they have intense magnetic fields (which is a necessary condition for the axions to materialize). Better still: they are “silent and boring“. Understand by this that they emit little radiation unlike some pulsars and especially not in the range of X-rays studied by this research which is published in the journal Physical Review Letters.

Excess X-rays

The cluster of stars was observed with the Chandra and XMM-Newton space telescopes, both of which revealed an excess of high energy x-rays. However, the theory predicts that the axions which are produced by collisions inside the hot nuclei of stars can spontaneously convert into X photons by crossing very powerful magnetic fields like those of the 7 Mercenaries, this is called the inverse Primakoff effect. The excess observed here cannot be caused by objects that would be located behind these stars and whose signature would be visible in the telescope data. Does this mean that the researchers claim to have detected axions? No. They are confident that this excess of X-rays does exist and that it reveals something new: “no other explanation for the excess has yet been offered, but that does not mean that it does not exist, neither involving another exotic particle, nor even some other process within the Standard Model. Perhaps the first alternative explanation to mind would be one involving neutrinos, since neutrinos are also produced in abundance in the nuclei of neutron stars.“specifies Benjamin Safdi. However the neutrinos are not supposed to transform into photons under the conditions which reign around the stars with neutrons. In fact, very few known or even theoretical particles would have this behavior. Which leaves the room free to the axions.

To confirm their existence, however, further studies in other environments are needed. Thus, the next stage could focus around the white dwarfs – these stars which represent the last phase of the evolution of stars with a mass similar to that of the Sun – which also have very strong magnetic fields and which are not considered important sources of X-rays. The observation of an excess identical to that found near neutron stars would argue in favor of axions or at least for an explanation outside the Standard Model.

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