Traces of mysterious particle predicted decades ago may have been detected



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Evidence of a long-sought hypothetical particle could have been hiding from ordinary sight (x-rays) all this time.

The X-ray emission from a collection of neutron stars known as the Magnificent Seven is so excessive that it could come from axions, a type of particle long predicted, forged in the dense nuclei of these dead objects, scientists have shown.

If their findings are confirmed, this finding could help unravel some of the mysteries of the physical Universe – including the nature of the mysterious dark matter that holds everything together.

“The search for axions has been one of the major efforts of high-energy particle physics, both in theory and in experiments,” said astronomer Raymond Co of the University of Minnesota.

“We think axions might exist, but we haven’t discovered them yet. You can think of axions as ghost particles. They can be anywhere in the Universe, but they don’t interact strongly with us, so we have no observations of them yet. “

Axions are hypothetical particles of very low mass, first theorized in the 1970s to solve the question of why strong atomic forces follow what is called charge-parity symmetry, when most models say they don’t need it.

Axions are predicted by many string theory models – a proposed solution to the tension between general relativity and quantum mechanics – and axions of a specific mass are also a strong candidate for dark matter. Scientists therefore have a number of very good reasons to seek them out.

If they exist, axions should be produced inside stars. These stellar axions are not the same as dark matter axions, but their existence would imply the existence of other types of axions.

One way to look for axions is to look for excess radiation. Axions are expected to decay into pairs of photons in the presence of a magnetic field – so if more electromagnetic radiation than there should be is detected in a region where this decay is expected to take place, it could constitute proof of axions.

In this case, the excess hard X-radiation is exactly what astronomers found when looking at the Magnificent Seven.

These neutron stars – the collapsed nuclei of dead massive stars that died in a supernova – are not grouped together as a group, but share a number of traits in common. They are all isolated neutron stars of the middle age, a few hundred thousand years since the star’s death.

They all cool and emit low energy (soft) x-rays. They all have strong magnetic fields, billions of times stronger than those on Earth, strong enough to trigger axial decay. And they’re all relatively close, less than 1,500 light years from Earth.

This makes it an excellent laboratory for axion research, and when a team of researchers – led by author and senior physicist Benjamin Safdi of the Lawrence Berkeley National Laboratory – studied the Magnificent Seven with multiple telescopes, they identified X high-energy (hard) ray emission not expected for neutron stars of this type.

In space, however, many processes can produce radiation, so the team had to carefully consider other potential sources of emission. Pulsars, for example, emit harsh X-rays; but the other types of radiation emitted by pulsars, such as radio waves, are not present in the Magnificent Seven.

Another possibility is that unresolved sources near neutron stars could produce the emission of hard x-rays. But the datasets the team used, from two different X-ray space observatories – XMM-Newton and Chandra – indicated that the emission is coming from neutron stars. The team found that the signal was also not the result of a stack of soft X-ray emissions.

“We are quite confident that this excess exists, and very confident that there is something new about this excess,” Safdi said. “If we were 100% sure that what we are seeing is a new particle, it would be huge. It would be revolutionary in physics.”

This does not mean that the excess is a new particle. It could be a hitherto unknown astrophysical process. Or it could be something as simple as an artefact from telescopes or data processing.

“We are not claiming that we have made the discovery of the axion yet, but we are saying that the extra X photons can be explained by axions,” said Co. “This is an exciting discovery of the excess of photons X, and it’s an exciting possibility that is already consistent with our interpretation of axions. “

The next step will be to try to verify the discovery. If the excess is produced by the axions, then most of the radiation must be emitted at energies higher than those XMM-Newton and Chandra are able to detect. The team hopes to use a newer telescope, NASA’s NuSTAR, to observe the Magnificent Seven over a wider range of wavelengths.

Magnetized white dwarf stars could be another place to look for the emission of axions. Like the Magnificent Seven, these objects have strong magnetic fields and should not emit harsh x-ray emissions.

“It starts to be pretty convincing that it goes beyond the standard model if we see excess x-rays there as well,” Safdi said.

The research was published in Physical examination letters.

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