



A statistical anomaly in the particle separation angle

In their experiment on beryllium-8, the researchers found an abnormal number of particle pairs separating at an angle of 140 ° (angular correlation). Credits: AJ Krasznahorkay et al. 2019

New experimental evidence for a potential fifth fundamental force

In their new experiment involving an excited helium nucleus, the researchers again observed an abnormal angular correlation for the electron-positron pairs, but this time around 115 °. Credits: AJ Krasznahorkay et al. 2019

According to the Standard Model, the Universe is governed by four elementary interactions: electromagnetism, weak and strong nuclear interactions, and gravity. For the first three, mediating particles, the bosons, have been identified. As for gravity, the existence of a graviton potential is still debated. But the Standard Model might not be complete. This is proposed by a team of Hungarian physicists whose recent experimental results suggest, according to their analysis, that there could be a fifth fundamental force. In 2016, the team had already highlighted some experimental evidence for a potential fifth elemental boson.The same team has now observed a second example of this potential fifth elemental force and the particle - called X17 - that carries it. If the discovery is confirmed, learning more about X17 could help us better understand the forces that govern our universe, but could also help physicists solve the problem of dark matter. However, these latest results have not yet been validated by peers, so they must be taken with extreme caution.Attila Krasznahorkay and colleagues at the Hungarian Nuclear Research Institute suspected something strange in 2016 after analyzing how an excited beryllium-8 atom emits light when it disintegrates. If this light is energetic enough, it is transformed into an electron and a positron, which move away from each other at a predictable angle.On the basis of the law of conservation of energy, as the energy of the light producing the two particles increases, the angle between them should decrease. Statistically speaking, at least. But, that's not quite what Krasznahorkay and his team saw. Among the different angles observed, there was an unexpected increase in the number of electrons and positrons separating at an angle of 140 degrees.The study seemed serious enough and quickly attracted the attention of other researchers around the world, who suggested that a whole new particle could be responsible for the anomaly. And its characteristics suggested that it must be a completely new type of fundamental boson.This new boson can not be one of the fundamental bosons already known, considering its distinctive mass of 17 megaelectronvolts (MeV) - about 33 times that of an electron, and its very short life (10-14 seconds ).All these data therefore seem to indicate the existence of a fifth force. However, the discovery of a new particle, and a fortiori a new boson, requires several careful examinations and experimental repeatability before being announced.To do this, the Krasznahorkay team decided to hunt down this potential new boson in another experiment, moving from the decay of beryllium-8 to a change in the state of an excited helium nucleus.Similar to their previous discovery, the researchers found pairs of electrons and positrons separating at an angle that does not fit the currently accepted models. This time the number was closer to 115 degrees.The team calculated that the helium nucleus could also have produced an ephemeral boson with a mass of just under 17 megaelectronvolts. While the 2016 experience has been accepted in the very serious journal Physical Review Letters , this latest study has not yet been peer-reviewed. It is however freely available on the pre-publication server arXiv .Many other experiments, including those conducted by independent teams, will have to be conducted, and the initial results will have to be corroborated before any official announcement can be made.The results obtained by the Krasznahorkay team are still far from sufficient. However, if the discovery of a fifth boson was to be confirmed, it could announce a revolution in the field of dark matter. A number of experiments on dark matter actually looking for a 17 MeV peak.