A new clue to the existence of a fifth fundamental force of nature may have emerged from a Hungarian laboratory.
Attila Krasznahorkay at the Atomki Institute of Nuclear Research, Hungary, and his colleagues have spent years studying the radioactive decay of beryllium-8, an unstable isotope. In 2016, they published details of an odd finding, suggesting it was being caused by a previously unknown particle.
Now the same researchers have found another anomaly, this time in an energy transition involving an excited state of the helium nucleus, and say that it points to the same particle.
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They calculate that the new particle has a mass of around 17 megaelectronvolts, or about 33 times that of an electron, and have called it X17 as a result.
The particle appears to carry energy away from an atomic nucleus, and then decay into an electron-positron pair, which the team鈥檚 experiment can detect.
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Normal physics predicts that the electron and positron should be emitted in roughly the same direction. But in the beryllium-8 experiment, the two particles diverged at an angle of 135 degrees, while the new helium result gives a similar detection bump at an angle of 115 degrees.
This difference is crucial because the helium transitions take place at a higher energy to the beryllium one. The higher the energy, the smaller the expected angle.
鈥淚f the bump is produced by a new particle, it has to move in exactly this way,鈥� says Jonathan Feng, a theoretical particle physicist at the University of California, Irvine, who investigated the original claim in 2016.
He proposed that the X17 particle must only be capable of interacting with neutrons, otherwise it would have been detected already in previous lab experiments. But for this to be true, the particle can鈥檛 be using any known force of nature. Instead it must be interacting through a force unrelated to gravity, electromagnetism or the two nuclear forces we know about.
Feng says that this latest result corroborates the first and has got people excited. 鈥淚 sent a one-line email to my collaborators with the link and one word: 鈥榃ow鈥�,鈥� he says.
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For his part, Krasznahorkay is now looking to other, independent groups to confirm whether the particle really exists. 鈥淚f the existence is settled, that would open up a new avenue in particle physics,鈥� he says. X17 could be the link between ordinary matter and dark matter.
But that independent corroboration is yet to come. The NA64 experiment at CERN looked for evidence of the X17 in 2017 and 2018. Although it has , Sergei Gninenko, the experiment鈥檚 spokesperson, told 快猫短视频 that the search hasn鈥檛 been exhaustive and that they will look again in light of this new result.
The Positron Annihilation into Dark Matter Experiment (PADME) in Frascati, Italy, is also on the hunt for the X17. Researchers there collected data this year and will run their experiment again next year with improved performance. PADME spokesperson Mauro Raggi says to expect the analysis around spring 2021.
鈥淭he beryllium-8 anomaly is a result published by a very qualified journal and the helium will soon have the same destiny,鈥� says Raggi. 鈥淲e need to seriously consider these two results. The question is whether it is connected to a new particle or not and this is what PADME may help to clarify.鈥�
Reference:听
