Rare boson particle ‘triplets’ spotted by team for first time

Phenomenon had long been theorised but never seen before, Chinese-led research project reveals

An extremely rare event in the world of particles has taken place during a Chinese-led study at the Large Hadron Collider (LHC) near Geneva, Switzerland.

And the event has scored yet another victory for the Standard Model – our current best theory to describe how the basic building blocks of the universe interact.

Sifting through experimental data collected between 2016 and 2018, researchers from Peking University and their colleagues from around the world spotted the simultaneous appearance of three force-carrying particles, known as bosons, which had never been seen together before.

Such triplets were produced about 250 times, after protons were accelerated to close to the speed of light and smashed into each other billions of times inside the LHC’s 27km-long ring, the team reported in the journal Physical Review Letters last month.

“The probability for such an event to occur is 1/50 of the probability of detecting a Higgs boson,” paper co-author Li Qiang said.

“We are excited to play a leading role in such challenging work,” added Li, who has been part of the boson research group at Peking University since 2010.

The fundamental particles that make up everything in the world fall into two main types – fermions and bosons, Li said.

While fermions, such as protons and electrons, constitute ordinary matter, elementary bosons give rise to matter’s mass and different forces between particles.

For instance, a photon, or a particle of light, is a massless boson that mediates electromagnetic forces and helps us see and understand the universe.

The W boson, discovered in the 1980s, is much more massive – about 85 times heavier than a proton. It mediates the weak nuclear force and is responsible for some of the most common nuclear decays.

For their previous work, Li and his teammates used the LHC – the world’s largest and most powerful particle collider – to observe events where one or two bosons were generated.

In the new study, they detected a combination of three bosons which had never been seen together: two W bosons carrying opposite electric charge, plus one gamma photon.

While such a phenomenon had been theoretically predicted, it had never been observed before, Li said. As W bosons are very unstable, the triplets only existed for a very short time before the W bosons decayed into other particles, he added.

“Our study showed that all possible processes predicted by theory would happen and leave a trace – however rare and brief they might be,” he said.

We are excited to play a leading role in such challenging work LI QIANG, CO-AUTHOR OF REPORT IN THE JOURNAL PHYSICAL REVIEW LETTERS

Li was confident in the team’s findings because the observed significance was 5.6 standard deviations. It meant the chance for their observation to have resulted from a random fluctuation in the data set would be as low as one in a million. His team will now continue the search for multiple boson productions from proton collisions.

“Such experiments can only be done on the LHC at the moment, since they require extremely high collision energies,” he said.

Chinese scientists are playing an increasingly active role at the LHC, making up about 2 per cent of the thousands of scientists and engineers from all over the world who use the facility, Li said.

He also looks forward to using China’s own next-generation collider, known as the Circular Electron Positron Collider or Higgs Factory, which is set to be even larger and more powerful than the LHC and will cost 36 billion yuan (HK$38.9 billion) to build.

Construction could begin as soon as 2027.