The hunt for X17: a Chinese quest for the fifth force in physics

Stephen Chen

Qiao Congfeng is looking for a quiet place to look for a “fifth force”.

The physics professor at the University of the Chinese Academy of Sciences is one of a group of researchers on the hunt for X17 but they need somewhere tranquil to do it.

“I just had a chat with people at the Beijing Spectrometer Experiment to see if we could detect it at their facility,” Qiao said.

“It might be possible, but the job would be very challenging because the background noise could be too high. We need a very quiet environment to catch the phenomenon.”

Qiao and his team are looking for a particle that could open up a new frontier in physics, taking science beyond the four known forces – gravity, electromagnetism, and the strong and weak nuclear interactions – to a fifth.

It’s all Qiao and his colleagues have talked about since scientists at the Institute for Nuclear Research at the Hungarian Academy of Sciences caused a buzz in the science community earlier this month by publishing a paper about evidence they had found supporting the existence of this fifth force.

The work replicated results from a 2016 experiment on beryllium – except this time the experiment was on helium, strengthening their case for the existence of what has been called the X17 particle.

Researchers say that the new force particle might be the key to unlocking some of the most fundamental mysteries about our universe, such as the dark matter, leading to the discovery of new physics.

“Provided, of course, it really exists,” Qiao added.

The story of X17 dates back to 2016.

In a seemingly mundane experiment, Attila Krasznahorkay and his colleagues from the Institute of Nuclear Research in Hungary detected an unusual bleep.

They charged an atom of beryllium to a high energy level and let it decay. According to the standard model of physics, the beryllium atom would release excess energy in the form of light, which would then transform into an electron and a positron that would repel each other at a predictable angle.

But the angle turned out to be off the predicted range. The Hungarian team recorded a strange spike in their data which, after some calculations, suggested it could be produced by an unknown force carried by an unknown particle.

They dubbed the particle X17 because its energy falls in the X-ray zone.

They published the results in the journal Physical Review Letters.

If correct, the work would be a shoo-in for a Nobel Prize. But it could also be an error of instruments. The mainstream physics community remains sceptical because nobody else has been able to reproduce the results.

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Late last month, the Hungarian team released another paper on preprint service arXiv, saying that they had detected the same phenomenon on the atom of helium.

The paper has not been peer-reviewed but already it is generating ripples around the globe, including in China.

Qiao’s team is one of the research groups trying to make sense of the strange phenomenon detected by the Hungarian team. They have published three papers on the topic, including one that explored the feasibility of detecting a fifth force in China’s particle colliders. The Hungarian team quoted Qiao’s work in their latest paper.

Qiao said they could not immediately comment on the new results on helium because they needed more time to analyse the data and consider the physical explanations, but the progress was definitely worthy of attention.

To China, X17 is not just a question of physics, but of money. The country has some of the world’s biggest, most advanced particle physics facilities in the pipeline, including the 30 billion yuan (US$4.26 billion) Circular Electron Positron Collider which is projected to be three times as big as Europe’s Large Hadron Collider but just half the price.

But if X17 could be detected at a relatively low-energy range, it would affect the design and goal of the giant, high-energy machines.

“It doesn’t mean the large facilities will be unnecessary. Quite the opposite,” Qiao said.

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“If the fifth force exists, it must be a universal natural phenomenon. There will be new theories to explain its role in both low-energy and high-energy ranges. It means we will need more powerful, supersensitive machines to study the new physics.”

However, some scientists remain sceptical and say the present buzz about X17 will be short-lived.

Professor Cao Jun, researcher with the Institute of High Energy Physics in Beijing, said the experiment by the Hungarian team was fairly simple but so far no other laboratory in the world has reproduced the results.

“I don’t think it worth too much attention until there is more solid evidence,” Cao said.

An Haipeng, assistant professor of physics at Tsinghua University, said the standard model of physics had been tested and proven in many experiments. So even if the fifth force and the X17 particle existed, it would be more as an extension to the standard model than disruption, he said.

But many researchers agreed that the dawn of new physics could be over the horizon.

A team of Chines physicists, for instance, had recently proven with experiment that ghostly neutrinos which previously were thought to be weightless actually had mass.

This phenomenon could not be explained by established theories, and the team won a US$1 million Future Science Prize this month for taking physics into uncharted territory.

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