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Ling Xin

The international physics community has hailed a Chinese-led discovery as a “triumph” after researchers presented the strongest evidence yet for the existence of the “glueball” – a mysterious particle made of pure force.
On Chinese social media, the conversation centred on whether the research could lead to a Nobel Prize.
The announcement was made last week at the International Conference on High Energy Physics in Natal, Brazil.
Made entirely of gluons – the tiny, massless carriers of the force that holds atomic nuclei together – glueballs were theoretically predicted in the early 1970s but remained one of the greatest unsolved puzzles of modern physics.
The breakthrough came after 15 years of work by researchers from 15 countries, who sifted through billions of measurements made at an underground collider in western Beijing to piece together the strongest evidence yet for the long-sought particle.


Colin Morningstar, a theoretical physicist at Carnegie Mellon University in Pittsburgh, Pennsylvania, called the study “an experimental triumph”.
Marek Karliner, a theoretical physicist at Tel Aviv University in Israel, said glueballs were among the most robust predictions of quantum theory but were extremely hard to confirm experimentally.
“There’s a very good chance they have nailed it,” he said.
Karliner said the Beijing Electron Positron Collider (BEPC) was uniquely suited to the search because it smashes electrons and their antimatter counterparts, positrons, at precisely the right energy to produce the particles used in the experiment.
Unlike larger colliders such as Europe’s Large Hadron Collider, where many different kinds of particles are created all at once, the BEPC produces far less unwanted clutter, allowing researchers to carry out the long checklist of tests needed to identify a glueball.
“There is a list of tests accepted by experts around the world,” Karliner said. “Very patiently, these people in Beijing have done them one after another. After all these years, it’s really an experimental tour de force.”
Atomic nuclei are made of protons and neutrons, which themselves are built from smaller particles called quarks. Quarks are held together by gluons, according to the Standard Model, the leading theory describing the universe’s fundamental particles.
Karliner likened quarks and gluons to steel balls connected by a rubber band. “If you cut off the balls, close the rubber band into a loop, and glue the two ends together, you’ll have just a rubber band,” he said. The looped rubber band represents a glueball – a particle made entirely of gluons, with no quarks inside.
Finding a glueball proved far more difficult than predicting its existence. Glueballs come in many different forms and are notoriously difficult to distinguish from ordinary particles with similar properties.
The search in Beijing began with huge numbers of short-lived particles created inside the BEPC. As they broke apart, they provided one of the world’s best opportunities for glueballs to form.
In 2011, researchers spotted something unusual among the debris. They called the new particle X(2370) – “X” because its identity was unknown, and “2370” because its mass is about 2,370 million electron volts, a standard unit for measuring particle masses. It soon became a leading glueball candidate.
Over the next 15 years, researchers worked through a checklist of tests, measuring the new particle’s mass, quantum numbers and, most recently, a final identifying feature known as a “flavour singlet”.
Together, the results built a complete chain of experimental evidence, ruling out the possibility that X(2370) was something else resembling the particle.
Even so, X(2370) is probably not a pure glueball. Instead, the researchers concluded that it was dominated by a glueball while also containing ordinary quark-based matter.
Scientists still do not know exactly how many gluons it contains. The simplest possibility was three, although more complicated combinations were also possible, Karliner said.
Morningstar said confirming one of the last major predictions of quantum theory would deepen scientists’ understanding of nature’s fundamental laws. “It could even lead to technological applications we never thought about before,” he said.
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Matter of pure force: US, Israeli physicists hail China’s glueball discovery as ‘triumph’

The Beijing Spectrometer III particle detector at the Beijing Electron Positron Collider in Beijing, where researchers have found the strongest evidence yet for the long-sought glueball. Photo: Handout

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The Beijing Spectrometer III of the Beijing Electron Positron Collider II has helped prove the existence of “glueballs”. Photo: Institute of High Energy Physics of Chinese Academy of Sciences

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Illustration: Handout

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China

Global experts praise team for solving 50-year-old puzzle, while Chinese social media buzzes with Nobel Prize predictions


Global experts praise team for solving 50-year-old puzzle, while Chinese social media buzzes with Nobel Prize predictions.


United States

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Brazil

Matter of pure force: US, Israeli physicists hail China’s glueball discovery as ‘triumph’

.css-1c6uqr6{color:inherit;font-weight:inherit;font-size:inherit;font-family:inherit;line-height:inherit;overflow-wrap:break-word;}Global experts praise team for solving 50-year-old puzzle, while Chinese social media buzzes with Nobel Prize predictions

Global experts praise team for solving 50-year-old puzzle, while Chinese social media buzzes with Nobel Prize predictions